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Variations in asexual reproduction

Variations in asexual reproduction


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It has always been said that sexual reproduction produces offsprings which are superior to their parents, due to the variations which they acquire causing them to survive better in their environment. That's because we can think of meiosis occurring at some level of their life cycle resulting in the variations in the final offspring. But what when we talk about bisexual organisms, might they be plants, animals or any other life form? Can't there be variations in their offsprings if they produce them asexually? It's like when they undergo gamete formation in different male and female structure present in that single parent, they'll surely undergo meiosis(if the parent is not haploid undoubtedly) forming the gametes which are dissimilar in their genetic makeup. When these fertilize, shouldn't they show variations?


The question is rooted in a misunderstanding of what sexual reproduction is and is not.

Any good intro book to evolution has a chapter on the diversity of reproductive systems. Consider for example Evolution: Making Sense of Life. If you speak french, the book évolution biologique has a very chapter on the diversity of reproductive systems. If you want something more complete and more advanced, you can have a look at The Evolution of Sex Determination.

It has always been said that sexual reproduction produces offsprings which are superior to their parents, due to the variations which they acquire causing them to survive better in their environment.

Who said that? The common simplistic idea and sex increases mean fitness because it increases genetic diversity is both wrong because sex does not necessarily increase genetic diversity and wrong because increase in genetic diversity does not necessarily result in increasing mean fitness.

You can have a look at the work of Nick Barton, Sally Otto and many others on the subject. The above misconception is well explained in the introduction of this talk from S. Otto's

[… ] But what when we talk about bisexual organisms, might they be plants, animals or any other life form?

I'm not sure what you meant here. You probably did not want to use the term bisexual. Maybe you meant dioceous.

Please note that sexual reproduction does not mean dioecy. Sexual reproduction does not even mean anisogamy and doe snot necessarily involved more than one individual (see selfing).

Can't there be variations in their offsprings if they produce them asexually?

There is a whole range of reproduction mode and saying sexually vs asexually is making gross categories that make it sometimes hard to really answer the question. It is also hard to know what you mean by "variations" here. But shortly speaking:

Asexual reproduction does not necessarily mean total absence of recombination (see for example Narra and Ochman 2006) and definitely does not mean absence of mutations (see for example Sniegowski et al. 1997).

It's like when they undergo gamete formation in different male and female structure present in that single parent, they'll surely undergo meiosis(if the parent is not haploid undoubtedly) forming the gametes which are dissimilar in their genetic makeup. When these fertilize, shouldn't they show variations?

If there is fusion, then there is sexual reproduction, even if it is selfing and even is they don't have genders (anisogamy).


The problem here is simply that you are considering self-fertilization as an example of assexual reproduction, when it's not the case.

Here is an oversimplified explanation, that may help you:

Sexual reproduction is a form of reproduction in which there is genetic recombination (unlike assexual reproduction).

Regarding the origin of the gametes, sexual reproduction can be classified as:

  1. Cross fertilization: each gamete comes from a different individual.
  2. Self-fertilization: both gametes come from the same individual.

Contrary to common belief, most hermaphrodite animals (what you call bisexual) and most monoecious plants perform cross fertilization, not self-fertilization, but some of them do perform self-fertilization. However, self-fertilization is a type of sexual reproduction, not assexual.

Therefore, next time you read about a hermaphrodite (like the tapeworm Taenia, for instance) doing self-fertilization, remember: that is sexual reproduction, not asexual.


  • Reproduction (or procreation) is the biological process by which new &ldquooffspring&rdquo are produced from their &ldquoparents&rdquo.
  • Asexual reproduction yields genetically-identical organisms because an individual reproduces without another.
  • In sexual reproduction, the genetic material of two individuals from the same species combines to produce genetically-different offspring this ensures mixing of the gene pool of the species.
  • Organisms that reproduce through asexual reproduction tend to grow exponentially and rely on mutations for DNA variation, while those that reproduce sexually yield a smaller number of offspring, but have larger genetic variation.
  • reproduction: the act of producing new individuals biologically
  • clone: a living organism produced asexually from a single ancestor, to which it is genetically identical

Introduction

Many higher organisms have both sexual and asexual means of letting themselves be represented in future generations. In plants partial asexuality is normally due to a mixture of sexual reproduction by seed and asexual reproduction by specialized structures such as runners or bulbils, whereas in animals partial asexuality often follows from the failure of cyclical parthenogens to go through the sexual stage successfully.

The present article considers the pattern of selectively neutral genetic variation expected in organisms with a stable mixture of sexual and asexual reproduction. Asexuality has classically been regarded as a factor that reduces genetic variation. As a general notion, this is, of course, not correct. Asexual species often harbour a wealth of variation ( Ellstrand & Roose, 1987 Hebert, 1987 Suomalainen et al., 1987 Asker & Jerling, 1992 ) coming from new mutations as well as remnant sexuality and/or multiple origins. But how genetically variable do we expect a predominantly asexual organism to be?

From models of infinitely large populations is known ( Marshal & Weir, 1979 ), that asexuality as such does not affect the equilibrium genotype frequencies of neutral alleles in organisms practicing at least some outbreeding sexuality. The only role asexuality plays in such reproductively mixed conditions is to slow down the rates at which the multilocus equilibrium values are attained ( Marshal & Weir, 1979 ). Asexuality differs in this respect from, say, inbreeding, in that it does not change the genetic structure of a population in any specific, unidirectional way.

Although this insight is valuable, one can ask how relevant it is for actual populations of limited size in which genetic drift cannot be ignored. Two immediate questions arise for such populations. The first concerns the degree of allelic variation expected at loci at which neutral mutations occur. Asexuality affects this variation in two different ways. On the one hand, asexuality implies that sampling drift acts at the level of individuals as well as on the level of genes, which will lead to an increased homogenization at the population level. On the other hand, asexuallity tends to ‘lock up’ gene combinations in fixed pairs that are inherited together, which will tend to increase the divergence between gene copies. By use of a coalescence argument in Model 1 below, I show how these processes interact and describe the expected pattern of pairwise allelic divergence in a population with a fixed degree of asexuality. The results are obtained under the assumption that the investigated population is large and that it has had a constant size for a long time.

The second question concerns genotypic variation. With the advent of methods for multilocus genotyping (isozymes, RAPD, AFLP and other techniques), the genotypic, ‘clonal’, variation in populations of organisms with different degrees of asexuality has become easy to investigate empirically. It has then been found that apparently asexual organisms often harbour considerable genotypic variability. To assess the basis for this variation – is it because of low levels of sexuality or to factors such as somatic mutations, balancing selection, or perpetual re-creation of the asexual form? – the expected level of neutral genotypic variation in a limited population with partial asexuality must be known for comparison. This question is investigated in Model 2, which is less restricted than Model 1 with respect to the underlying assumptions made. Numerical examples are used to describe the most important results, many of which are already directly or indirectly known.

It is assumed in both models that the organism investigated has its dominant, size-limited life-stage at the higher ploidy level (the interesting question of genetic variation in mosses, for example, thus not being considered) and that infinitely many gametes are produced during sexual reproduction. Unless otherwise indicated, sexual reproduction is assumed to occur through outbreeding. Generations are assumed to follow each other in discrete and separate steps.

Earlier theoretical analyses of fully or partially asexual organisms (see, e.g. Lokki, 1976a,b Pamilo, 1987 Brookfield, 1992 ) have dealt primarily with the degree of variation expected for particular genetic markers. My aim is to generalize the type of situations considered and to clarify the interactions between the evolutionary processes involved. The approximations used in the first model apply to very long time scales in which an equilibrium in the population is established between mutation and drift, whereas the second model deals with shorter time scales for which reorganization by recombination of the variation already available is of greater importance. The results are of relevance for the question of what method to use for estimating past and current levels of sexuality/asexuality from population data. However, this is a complex topic (see various approaches in Marshall & Brown, 1974 Stoddard & Taylor, 1988 Maynard Smith & Smith, 1998 Mes, 1998 Maynard Smith, 1999 Ceplitis, 2000 ) that needs further developments. The aim of the present paper is limited to a description of the qualitative and quantitative interactions between the major parameters that govern neutral evolution in partially asexual populations.


Materials and methods

Cockroaches used in our experiments were randomly collected from large laboratory colonies of several thousand individuals. We maintained both the experimental cockroaches and the cockroaches from the stock colonies in incubators at 28 °C, ambient humidity (approximately 50%) and a 12 : 12 photoperiod. We provided food (dry dog chow) and water ad libitum and replaced both frequently to ensure that the food and water were fresh. We collected females that we allowed the opportunity to reproduce asexually as sexually immature, last instar nymphs. We isolated these females into individual 11 × 11 × 3-cm plastic containers and maintained them in female-only incubators. These females therefore never had the opportunity to encounter a male. We also collected sexually reproducing females as last instar nymphs and then we randomly mated them after they had reached sexual maturity. We reared individual family groups in 19 × 13 × 10-cm plastic containers. For all experiments, we stored all individuals whole at −80 °C until we scored their genotypes.

Except where noted, genetic variation was assessed by starch gel electrophoresis. We performed standard horizontal starch gel electrophoresis using tris-citrate buffer I (pH 6.7) and a morpholine buffer (pH 8.0). We screened 16 loci for allelic variation. We identified four polymorphic loci in the first experiment – glucose-6-phosphate dehydrogenase (G6PDH), glucose dehydrogenase (GCDH), isocitrate dehyrogenase (IDH), and phosphoglucomutase (PGM). In the second experiment, we found two additional polymorphic loci after additional screening – glycerol-3-phosphate dehydrogenase (G3PDH) and glutamate oxaloacetate transaminase (GOT). We tested all allozyme loci for Mendelian inheritance by scoring the genotypes of parents and offspring from five sexually reproducing families and all loci segregated in a Mendelian fashion.

Experiment 1: mode of parthenogenesis

Our first experiment was designed to determine the mode of parthenogenesis and we used starch gel electrophoresis to compare the genotypes of parthenogenetically reproducing females and their offspring. Both sexual and parthenogenetic female offspring are diploid, with 2n=36 chromosomes ( 5 ) and in this species all offspring produced parthenogenetically are female ( 31 ). Automixis and apomixis are two general mechanisms of parthenogenesis that do not involve changes in ploidy between parthenogenetic offspring and their mothers ( 32 ). Automictic parthenogenesis involves meiosis and a variety of mechanisms to restore diploidy ( 32 ). Regardless of the details of the restoration of diploidy, which are numerous ( 15 ), with this mechanism some or all of the offspring can be genetically different from their mothers and typically show increased (or total) homozygosity. This depends (in part) on the amount of crossing over that occurs. Apomictic parthenogenesis is a form of reproduction where the reduction division of meiosis is suppressed ( 32 ). Offspring produced by this mechanism are genetically identical to their mothers and heterozygosity is maintained. Thus, if female reproduce through apomictic parthenogenesis we expected offspring to be heterozygous at all loci where their mothers were heterozygous. If we observe no heterozygosity or a reduction in heterozygosity, we can conclude that a mechanism of parthenogenesis related to automixis is occurring. Offspring with genotypes identical to their mothers support, but does not prove, an apomictic mechanism of parthenogenesis.

We isolated 100 female nymphs from the mass colonies to identify those who could reproduce parthenogenetically. All had moulted to the adult stage and we maintained these females with fresh food and water ad libitum until they reproduced parthenogenetically or died. Fourteen females reproduced parthenogenetically, nine of which were collected with their offspring and frozen. We scored the genotype of each mother and her offspring at G6PDH, GCDH, IDH and PGM. To compare with our laboratory population, we sampled 183 randomly chosen individuals from mass colonies. Parthenogenetic reproduction does not occur within females that have mated ( 5 ). In addition, the opportunity to remain unmated long enough to reproduce asexually is unlikely in mass colonies and sexually produced clutches have a slightly male-biased sex ratio ( 4 ). Thus, we assumed that the females we isolated from mass colonies were sexually produced. We used a goodness-of-fit test to determine if genotypic frequencies were in Hardy–Weinberg equilibrium. We also calculated observed and expected heterozygosity, and the extent of inbreeding (F), for all loci in the population.

Experiment 2: heterosis and reproductive potentials

In the second experiment, we altered our experimental design to compare females that could reproduce asexually with those that could not, rather than to the population at large. For this experiment, we isolated 95 virgin females from the mass colonies as last instar nymphs. We again maintained these females until they reproduced parthenogenetically (n=21) or died (n= 74). We collected females and their parthenogenetically produced offspring and immediately froze them. Females that never reproduced were collected and frozen at their death. We checked all individuals daily for offspring and/or mortality. In this experiment, we scored the original four polymorphic loci plus an additional two loci which we found to be polymorphic (GOT and G3PDH). We again tested for Hardy–Weinberg equilibrium and calculated, observed and expected heterozygosities for the whole sample, for those females that reproduced asexually and for those that never reproduced. We also calculated inbreeding coefficients for the population (F) and levels of inbreeding observed in the subpopulations compared with the expectation for the total population (FIS) to illustrate how heterozygosity varied among the different groups of females. Finally, we compared genotypic frequencies of nonreproducing females with the parthenogenetically reproducing females to determine if the parthenogenetically reproducing females represented a random sample of females.

Experiment 3: quantitative genetic variation and reproductive potentials

We conducted a third experiment to investigate whether the ability to reproduce parthenogenetically ran in families. Although our previous work suggested that sisters were more likely to produce viable parthenogens ( 4 ), here we assessed the development of embryos by dissection for a more direct measure of fitness. This is a more sensitive assay because it avoids confounding parthenogenetic ability with parthenogen viability.

For this experiment, we used a full-sib study to determine if variation in the extent of successful parthenogenetic reproduction could have a genetic basis. In the context of experiments examining potential developmental constraints on parthenogenetic reproduction ( 5 ), we collected virgin females and manually removed egg cases from the females’ brood pouch. Because N. cinerea gives live birth, it is not possible to determine pregnancy of females without removing oothecae. We then determined the extent of development of embryos by dissection and inspection under a compound microscope. Among sexually reproducing females, all embryos are at the same stage of development in a single ootheca. Among asexually reproducing females, however, there is variation within oothecae in the development of embryos. Normal development can be determined by inspection of embryo morphology ( 5 ) and of the 80 females with oothecae, we found 11 families that had at least one sister that had at least one apparently normal asexually produced embryo. We scored the total number of eggs and the number of normal embryos within an ootheca from all females who were in a family with at least one sister producing normal embryos. This scoring was made blind to family membership. We used ANOVA to determine whether family membership influenced the percentage of viable asexually produced embryos. A broad-sense heritability was calculated using a weighted family mean ( 18 ).


Multiple Choice

What is a likely evolutionary advantage of sexual reproduction over asexual reproduction?

  1. sexual reproduction involves fewer steps
  2. less chance of using up the resources in a given environment
  3. sexual reproduction results in greater variation in the offspring
  4. sexual reproduction is more cost-effective

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Which type of life cycle has both a haploid and diploid multicellular stage?

  1. an asexual life cycle
  2. diploid-dominant
  3. haploid-dominant
  4. alternation of generations

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Which event leads to a diploid cell in a life cycle?

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Asexual reproduction

Algae

…female gametes (sex cells), by asexual reproduction, or by both ways.

Animals

Asexual reproduction (i.e., reproduction not involving the union of gametes), however, occurs only in the invertebrates, in which it is common, occurring in animals as highly evolved as the sea squirts, which are closely related to the vertebrates. Temporary gonads are common among lower animals…

Apicomplexans

Asexual reproduction is by binary or multiple fission (schizogony).

Echinoderms

Asexual reproduction in echinoderms usually involves the division of the body into two or more parts (fragmentation) and the regeneration of missing body parts. Fragmentation is a common method of reproduction used by some species of asteroids, ophiuroids, and holothurians, and in some…

Fungi

Typically in asexual reproduction, a single individual gives rise to a genetic duplicate of the progenitor without a genetic contribution from another individual. Perhaps the simplest method of reproduction of fungi is by fragmentation of the thallus, the body of a fungus. Some…

Growth and development

…in plants that reproduce by vegetative division, the breaking off of a part that can grow into another complete plant. The possibilities for debate that arise in these special cases, however, do not in any way invalidate the general usefulness of the distinctions as conventionally made in biology.

Major references

In asexual reproduction the new individual is derived from a blastema, a group of cells from the parent body, sometimes, as in Hydra and other coelenterates, in the form of a “bud” on the body surface. In sponges and bryozoans, the cell groups from which new…

Multicellular organisms also reproduce asexually and sexually asexual, or vegetative, reproduction can take a great variety of forms. Many multicellular lower plants give off asexual spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or multinucleate. In some cases the reproductive body is multicellular, as in…

…higher plants also reproduce by nonsexual means. Bulbs bud off new bulbs from the side. Certain jellyfish, sea anemones, marine worms, and other lowly creatures bud off parts of the body during one season or another, each thereby giving rise to populations of new, though identical, individuals. At the microscopic…

Plants

Both homosporous and heterosporous life histories may exhibit various types of asexual reproduction (vegetative reproduction, somatic reproduction). Asexual reproduction is any reproductive process that does not involve meiosis or the union of nuclei, sex cells, or sex organs. Depending on the type of…

Asexual reproduction involves no union of cells or nuclei of cells and, therefore, no mingling of genetic traits, since the nucleus contains the genetic material (chromosomes) of the cell. Only those systems of asexual reproduction that are not really modifications of sexual reproduction are considered…

Population ecology

In sexual populations, genes are recombined in each generation, and new genotypes may result. Offspring in most sexual species inherit half their genes from their mother and half from their father, and their genetic makeup is therefore different from either parent or any other…

Protozoans

Asexual reproduction is the most common means of replication by protozoans. The ability to undergo a sexual phase is confined to the ciliates, the apicomplexans, and restricted taxa among the flagellated and amoeboid organisms. Moreover, sexual reproduction does not always result in an immediate increase…

Spores

Spores are agents of asexual reproduction, whereas gametes are agents of sexual reproduction. Spores are produced by bacteria, fungi, algae, and plants.


Genetic variation in organisms with sexual and asexual reproduction

The genetic variation in a partially asexual organism is investigated by two models suited for different time scales. Only selectively neutral variation is considered. Model 1 shows, by the use of a coalescence argument, that three sexually derived individuals per generation are sufficient to give a population the same pattern of allelic variation as found in fully sexually reproducing organisms. With less than one sexual event every third generation, the characteristic pattern expected for asexual organisms appear, with strong allelic divergence between the gene copies in individuals. At intermediary levels of sexuality, a complex situation reigns. The pair-wise allelic divergence under partial sexuality exceeds, however, always the corresponding value under full sexuality. These results apply to large populations with stable reproductive systems. In a more general framework, Model 2 shows that a small number of sexual individuals per generation is sufficient to make an apparently asexual population highly genotypically variable. The time scale in terms of generations needed to produce this effect is given by the population size and the inverse of the rate of sexuality.


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Evolutionary Dynamics and Consequences of Parthenogenesis in Vertebrates

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Evolutionary Dynamics and Consequences of Parthenogenesis in Vertebrates

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NCERT Exemplar Class 12 Biology Chapter 1 Reproduction in Organisms

Multiple Choice Questions
Single Correct Answer Type

1. A few statements describing certain features of reproduction are given below
i. Gametic fusion takes place
ii. Transfer of genetic material takes place
iii. Reduction division takes place
iv. Progeny have some resemblance with parents
Select the options that are true for both asexual and sexual reproduction from the options given below:
(a) i and ii (b) ii and iii
(c) ii and iv (d) i and iii
Answer. (c) Transfer of genetic material and progeny have some resemblance with parents are the phenomenon common in’both asexual and sexual reproduction while gametic fusion and reduction division takes place in sexual reproduction only.

2. The term ‘ clone ’ cannot be applied to offspring formed by sexual reproduction because
(a) Offspring do not possess exact copies of parental DNA
(b) DNA of only one parent is copied and passed on to the offspring
(c) Offspring are formed at different times
(d) DNA of parent and offspring are completely different
Answer. (a)
• In asexual reproduction, a single individual (parent) is capable of producing offspring which are not only identical to one another but are also exact copies of their parent. The term clone is used to describe such morphologically and genetically similar individuals.
• In sexual reproduction because of the fusion of male and female gametes (either by same individual or by different individual of the opposite sex), sexual reproduction results in offspring that are not identical to the parents or amongst themselves.

3. Amoeba and Yeast reproduce asexually by fission and budding respectively, because they are
(a) Microscopic organisms
(b) Heterotrophic organisms
(c) Unicellular organisms
(d) Uninucleate organisms
Answer. (c) Many single-celled organisms reproduce by binary fission (e.g., Amoeba, Paramecium), where a cell divides into two halves and each rapidly grows into an adult.
In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which eventually gets separated and mature into new yeast organism (cells). Budding is also found in Hydra.

4. A few statements with regard to sexual reproduction are given below
i. Sexual reproduction does not always require two individuals
ii. Sexual reproduction generally involves gametic fusion
iii. Meiosis never occurs during sexual reproduction
iv. External fertilisation is a rule during sexual reproduction
Choose the correct statements from the options below:
(a) i and iv , (b) i and ii
(c) ii and iii (d) i and iv
Answer. (b)
• Sexual reproduction requires male and female gametes (either by same individual or by different individual of the opposite sex).
• Sexual reproduction generally involves gametic fusion.
• Meiosis occurs during sexual reproduction in dipoloid organisms.
• External fertilisation is not a rule during sexual reproduction, internal fertilization also takes place

5. A multicellular, filamentous alga exhibits a type of sexual life cycle in which the meiotic division occurs after the formation of zygote. The adult filament of this alga has
(a) Haploid vegetative cells and diploid gametangia
(b) Diploid vegetative cells and diploid gametangia
(c) Diploid vegetative cells and haploid gametangia
(d) Haploid vegetative cells and haploid gametangia
Answer. (d) Adult filament of a multicellular, filamentous alga have haplontic life cycle in which the meiotic division occurs after the formation of zygote. So, the filament of this alga have haploid vegetative cells and haploid gametangia.

6. The male gametes of rice plant have 12 chromosomes in their nucleus. The chromosome number in the female gamete, zygote and the cells of the seedling will be, respectively,
(a) 12,24,12 . (b) 24,12,12
(c) 12,24,24 (d) 24,12,24
Answer. (c) Gametophytic structure (n) of rice plant contain 12 chromosomes and sporophytic structure (2n) of rice contain 24 chromosomes.
Female gamete (n) =12,
Zygote (2n) = 24,
The cells of the seedling (2n) = 24.

7. Given below are a few statements related to external fertilization. Choose the correct statements.
i. The male and female gametes are formed and released simultaneously.
ii. Only a few gametes are released into the medium.
iii. Water is the medium in a majority of organisms exhibiting external fertilization.
iv. Offspring formed as a result of external fertilization have better chance of survival than those formed inside an organism.
(a) iii and iv (b) i and iii
(c) ii and iv (d) i and iv .
Answer. (b) In most aquatic organisms, such as a majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water), i.e., outside the body of the organism. This type of gametic fusion is called external fertilisation. Organisms exhibiting external fertilisation show great synchrony between the sexes and release a number of gametes into the surrounding medium (water) in order to enhance the chances of syngamy. This happens in the bony fishes and frogs where a large number of offspring are produced. A major disadvantage is that the offspring are extremely vulnerable to predators threatening their survival up to adulthood.

8. The statements given below describe certain features that are observed in the pistil of flowers.
i. Pistil may have many carpels
ii. Each carpel may have more than one ovule
iii. Each carpel has only one ovule
iv. Pistil have only one carpel
Choose the statements that are true from the options below:
(a) i and ii (b) i and iii
(c) ii and iv (d) iii and iv
Answer. (a)
• Pistil may have many carpels (multicapillary pistil like Papaver)
• Each carpel may have more than one ovule (like Watermelon,.Papaya etc.)

9. Which of the following situations correctly describe the similarity between an angiosperm egg and a human egg?
i. Eggs of both are formed only once in a lifetime
ii. Both the angiosperm egg and human egg are stationary
iii. Both the angiosperm egg and human egg are motile transported
iv. Syngamy in both results in the formation of zygote
Choose the correct answer from the options given below:
(a) ii and iv (b) iv only
(c) iii and iv (d) i and iv
Answer. (b) Syngamy in both results in the formation of zygote is similarity between an angiosperm egg and a human egg.

10. Appearance of vegetative propagules from the nodes of plants such as sugarcane and ginger is mainly because
(a) Nodes are shorter than intemodes
(b) Nodes have meristematic cells
(c) Nodes are located near the soil
(d) Nodes have non-photosynthetic cells
Answer. (b) Appearance of vegetative propagules from the nodes of plants such as sugarcane and ginger is mainly because nodes have meristematic cells. Examples of vegetative propagules: (i) Leaf buds of bryophyllum, (ii) Eyes of potato, (iii) Bulbifof Agave, (iv) Offset of water hyacinth, (v) Rhizome of ginger.

11. Which of the following statements, support the view that elaborate sexual reproductive process appeared much later in the organic evolution?
i. Lower groups of organisms have simpler body design
ii. Asexual reproduction is common in lower groups
iii. Asexual reproduction is common in higher groups of organisms
iv. The high incidence of sexual reproduction in angiosperms and vertebrates
Choose the correct answer from the options given below:
(a) i, ii and iii (b) i, iii and iv
(c) i, ii and iv (d) ii, iii and iv
Answer. (c) Elaborate sexual reproductive process appeared much later in the organic evolution because of
• Lower groups of organisms have simpler body design.
• Asexual reproduction is common in lower groups of organisms.
• High incidence of sexual reproduction in angiosperms and vertebrates.

12. Offspring formed by sexual reproduction exhibit more variation than those formed by asexual reproduction because
(a) Sexual reproduction is a lengthy process
(b) Gametes of parents have qualitatively different genetic composition
(c) Genetic material comes from parents of two different species
(d) Greater amount of DNA is involved in sexual reproduction
Ans. (b)
• Offspring formed by sexual reproduction exhibit more variation than those formed by asexual reproduction because gametes of parents have qualitatively different genetic composition.
• In asexual reproduction due to involvement of only one parent, so there is no chance of variation.

13. Choose the correct statement from amongst the following:
(a) Dioecious (hermaphrodite) organisms are seen only in animals.
(b) Dioecious organisms are seen only in plants.
(c) Dioecious organisms are seen in both plants and animals.
(d) Dioecious organisms are seen only in vertebrates.
Answer. (c) Dioecious organisms are seen in both plants (like papaya) and animals (like cockroach).

14. There is no natural death in single celled organisms like Amoeba and bacteria because
(a) They cannot reproduce sexually
(b) They reproduce by binary fission
(c) Parental body is distributed among the offspring
(d) They are microscopic
Answer. (c) There is no natural death in single celled organisms like Amoeba and bacteria because the parental body is distributed among the offspring.

15. There are various types of reproduction. The type of reproduction adopted by an organism depends on
(a) The habitat and morphology of the organism
(b) Morphology of the organism
(c) Morphology and physiology of the organism
(d) The organism’s habitat, physiology and genetic make up
Answer. (d) The organism’s habitat, its internal physiology and several other factors (genetic make up) are collectively responsible for how it reproduces. When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual.

16. Identify the incorrect statement.
(a) In asexual reproduction, the offspring produced are morphologically and genetically identical to the parent.
(b) Zoospores are sexual reproductive structures.
(c) In asexual reproduction, a single parent produces offspring with or without the formation of gametes.
(d) Conidia are asexual structures in Penicillium.
Answer. (b) Zoospores are asexual reproductive structures.

17.Which of the following is a post-fertilisation event in flowering plants?
(a) Transfer of pollen grains
(b) Embryo development
(c) Formation of flower
(d) Formation of pollen grains
Answer. (b)

18. The number of chromosomes in the shoot tip cells of a maize plant is 20. The number of chromosomes in the micro spore mother cells of the same plant shall be
(a) 20 (b) 10 (c) 40 (d) 15
Answer. (a) Shoot tip cells of a maize plant is a sporophytic structure (2n) and microspore mother cells of maize plant is also a sporophytic structure (2n). So, microspore mother cells (MMC) contain 20 chromosomes.

Very Short Answer Type Questions
1. Mention two inherent characteristics of Amoeba and yeast that enable them to reproduce asexually.
Answer. a. They are unicellular organisms.
b. They have a very simple body structure.

2. Why do we refer to’offspring formed by asexual method of reproduction as clones?
Answer. Offspring formed by asexual reproduction are called clones because they are morphologically and genetically similar to the parent.

3. Although potato tuber is an underground part, it is considered as a stem. Give two reasons.
Answer. a. The tuber has nodes and intemodes (as stem),
b. Leafy shoots appear from the nodes.

4. Between an annual and a perennial plant, which one has a shorter juvenile phase? Give one reason.
Answer. An annual has a shorter juvenile phase. Since its entire life cycle has to be completed in one growing season, its juvenile phase is shorter.

5. Rearrange the following events of sexual reproduction in the sequence in which they occur in a flowering plant: embryogenesis, fertilisation, gametogenesis, pollination.
Answer. Gametogenesis, Pollination, Fertilisation, Embryogenesis

6. The probability of fruit set in a self-pollinated bisexual flower of a plant is far greater than a dioecious plant. Explain.
Answer. There is assured fruit set in self pollinated bisexual flower even in the absence of pollinators. In dioecious plants, there is male and female flowers present on different plants, so external pollinating agent is required for pollination.

7. Is the presence of large number of chromosomes in an organism a hindrance to sexual reproduction? Justify your answer by giving suitable reasons.
Answer. Presence of large number of chromosomes in an organism is not a hindrance to sexual reproduction. Butterfly has 380 chromosomes but it can reproduce sexually.

8. Is there a relationship between the size of an organism and its life span? Give two examples in support of your answer.
Answer. Life spans of organisms are not necessarily correlated with their sizes. The sizes of crows and parrots are not very different yet their life spans show a wide difference. Live span of crow is 15 year and of parrot is 140 years. A mango tree has a much shorter life span as compared to a peepal tree.

9. In the figure given below, the plant bears two different types of flowers marked ‘A’ and ‘B Identify the types of flowers and state the type of pollination that will occur in them.

Answer. ‘A’ is chasmogamous flower while ‘B’ is cleistogamous flower. A bisexual flower which normally open is called chasmogamous flower. Cleistogamous flowers do not open at all.
Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma.
In a normal flower which opens and exposes the anthers and stigma complete autogamy is rather rare. Chasmogamous flower may show autogamy, geitonogamy or xenogamy.

10. Give reasons as to why cell division cannot be a type of reproduction in multicellular organisms.
Answer. Cell division cannot be a type of reproduction in multicellular organisms because cell division only increases the number of cells in an organism which leads to the growth of body.

11. In the figure given below, mark the ovule and pericarp.

12. Why do gametes produced in large numbers in organisms exhibit external fertilisation?
Answer. Organisms exhibiting external fertilisation release a number of gametes into the surrounding medium (water) in order to enhance the chances of syngamy because there are few’ chances of fusion between male and female gametes.

13. Which of the followings are monoecious and dioecious organisms?
a. Earthworm ——————–
b. Chara ——————–
c. Marchantia ——————-
d. Cockroach ——————–
Answer. a. Earthworm—Monoecious
b. Chara—Monoecious
c. Marchantia—Dioecious
d. Cockroach—Dioecious

14. Match the organisms given in Column ‘A’ with the vegetative propagules given in column ‘B’.

Answer. Bryophyllum—leaf buds Agave—bulbils Potato—eyes
Water hyacinth—offset

15. What do the following parts of a flower develop into after fertilisation?
a. Ovary
b. Ovules
Answer. a. Ovary—Fruit
b. Ovules—Seeds

Short Answer Type Questions
1. In haploid organisms that undergo sexual reproduction, name the stage in the life cycle when meiosis occurs. Give reasons for your answer.
Answer. Meiosis takes place during its post-zygotic stage. Since the organism is haploid, meiosis cannot occur during gametogenesis.

2. The number of taxa exhibiting asexual reproduction is drastically reduced in higher plants (angiosperms) and higher animals (vertebrates) as compared with lower groups of plants and animals. Analyse the possible reasons for this situation.
Answer. Both angiosperms and vertebrates have a more complex structural organisation. They have evolved very efficient mechanism of sexual reproduction. Since asexual reproduction does not create new genetic pools in the offspring and consequently hampers their adaptability to external conditions, these groups have resorted to reproduction by the sexual method.

3. Honeybees produce their young ones only by sexual reproduction. Inspite of this, in a colony of bees we find both haploid and diploid individuals. Name the haploid and diploid individuals in the colony and analyse the reasons behind their formation.
Solution.
• The colony of honey bees has three types of members: (i) Diploid queen are fertile females, (ii) Worker bees are sterile females and (iii) Drones are haploid males.
• An offspring formed from the union of a sperm and an egg develops as a female (queen or worker), and an unfertilized egg develops as a male (drone) by means of parthenogenesis. This means that the males have half the number of chromosomes than that of a female.

4. With which type of reproduction do we associate the reduction division? Analyse the reasons for it.
Answer. Reduction division (meiosis) is associated with sexual reproduction. The reasons for this are:
a. Since sexual reproduction involves the fusion of two types of gametes (male and female), they must have haploid number of chromosomes.
b. The cell (meiocyte) which gives rise to gametes often has diploid number of chromosomes and it is only by reducing the number by half that we can get haploid gametes.
c. Reduction division also ensures maintenance of constancy of chromosome number from generation to generation.

5. Is it possible to consider vegetative propagation observed in certain plants like Bryophyllum, water hyacinth, ginger etc., as a type of asexual reproduction? Give two/three reasons.
Answer. Vegetative propagation is considered as a type of asexual reproduction because
(i) This is uniparental.
(ii) Clone formation takes place.
(iii) There is no fertilisation.

6. ‘Fertilisation is not an obligatory event for fruit production in certains plants’. Explain the statement.
Answer. Yes, it is observed in parthenocarpic fruits. The ‘seedless fruits’ that are available in the market such as pomegranate, grapes etc. are in fact good examples. Flowers of these plants are sprayed by a growth hormone that induces fruit development even though fertilisation has not occurred. The ovules of such fruits, however, fail to develop into seeds.

7. In a developing embryo, analyse the consequences if cell divisions are not followed by cell differentiation.
Answer. During embryogenesis, zygote undergoes cell-division (mitosis) and cell differentiation. While cell divisions increase the number of cells in the developing embryo Cell differentiation helps groups of cells to undergo certain modifications to form specialised tissues and organs to form an organism.
If cell divisions are not followed by cell differentiation then there will be no formation of tissues or organs, so a new organisms cannot be formed.

8. List the changes observed in an angiosperm flower subsequent to pollination and fertilisation.
Answer. Post-fertilisation modifications

9. Suggest a possible explanation why the seeds in a pea pod are arranged in a row, whereas those in tomato are scattered in the juicy pulp.
Answer. In a fruit, seed arrangement depends on type of placentation. Pea and tomato shows different placentation. Pea shows marginal placentation while tomato shows axile placentation.

10. Draw the sketches of a zoospore and a conidium. Mention two dissimilarities between them and alt least one feature common to both structures.
Answer.

11. Justify the statement ‘Vegetative reproduction is also a type of asexual reproduction’.
Answer. Vegetative propagation is also a type of asexual reproduction because
(i) This is uniparental.
(ii) Clone formation takes place.
(iii) There is no fertilisation.
(iv) There is no gamete formation.

Long Answer Type Question
1. Enumerate the differences between asexual and sexual reproduction. Describe the types of asexual reproduction exhibited by unicellular organisms.
Answer.

The types of asexual reproduction exhibited by unicellular organisms:
• Many single-celled organisms reproduce by binary fission (e.g., Amoeba, Paramecium), where a cell divides into two halves and each rapidly grows into an adult.
• In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which eventually gets separated and mature into new yeast organism (cells).

2. Do all the gametes formed from a parent organism have the same genetic composition (identical DNA copies of the parental genome)? Analyse the situation with the background of gametogenesis and provide or give suitable explanation.
Answer. The gametes of a parent do not have the same genetic composition because they do not have identical copies of DNA. In the pachytene and diplotene stages of meiosis-I, the phenomenon of crossing over and chiasma formation take place between homologous chromosomes. This shifts segments of DNA from one chromatid to another (homologous chromosomes) in a random manner resulting in several new combinations of DNA sequences. As a result, when meiotic division is completed, gametes possess DNA with varying degree of variations.

3. Although sexual reproduction is a long drawn, energy-intensive complex form of reproduction, many groups of organisms in Kingdom Animalia and Plantae prefer this mode of reproduction. Give at least three reasons for this.
Answer. a. Sexual reproduction brings about variation in the offspring.
b. Since gamete formation is preceded by meiosis, genetic recombination occurring during crossing over (meiosis-I), leads to a great deal of variation in the DNA of gametes.
c. The organism has better chances survival in a changing environment.

4. Differentiate between (a) oestrus arid menstrual cycles (b) ovipary and vivipary. Cite an example for each type.
Answer. Differences between oestrus and menstrual cycles


5. Rose plants produce large, attractive bisexual flowers but they seldom produce Suits. On the other hand a tomato plant produces plenty of fruits though they have small flowers. Analyse the reasons Tor failure of fruit formation in rose.
Answer. Failure of fruit formation in rose may be due to several reasons. Some of the likely reasons are
a. Rose plants may not produce viable pollen.
b. Rose plants may not have functional egg.
c. Rose plants may have abortive ovules.
d. Being hybrids, the meiotic process may be abnormal resulting in non-viable gametes. ‘
e. There may be self-incompatibility.
f. There may be internal barriers for pollen tube growth and/or fertilisation.

I think you got complete solutions for this chapter. If You have any queries regarding this chapter, please comment on the below section our subject teacher will answer you. We tried our best to give complete solutions so you got good marks in your exam.


Reproduction Buddies

In this game, you have to help your organisms make it to through the puzzle to the finish line. Each organism will reproduce sexually or asexually. You will have to figure out how your organisms reproduce. Their offspring will help you cross the obstacles to make it to the final flag. You have to make it to the flag before time runs out.

Pre-Game Discussion Questions:

  • What is sexual reproduction and asexual reproduction? What are the key differences?
  • What are some different forms of asexual reproduction we see in organisms?

Post-Game Discussion Questions:

  • What is the difference between fragmentation and budding?
  • How are the genes split up in sexual reproduction?

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Related Resources

Reproduction

Reproduction is the production of offspring. There are two main forms: sexual and asexual reproduction. In sexual reproduction, an organism combines the genetic information from each of its parents and is genetically unique. In asexual reproduction, one parent copies itself to form a genetically identical offspring. Sea turtles are an example of an animal that reproduces sexually, a volvox (green algae) is an example of an organism that reproduces asexually, and a brittle star can reproduce in either way. Help your students understand the sexual and asexual reproduction with these classroom resources.

Defender: Natural Selection

This game will teach you about forms of adaptation and the role they play in natural selection. You will have to defend your territory by placing animals with different adaptations on your land. Your knowledge about adaptations will help you earn extra points! When you place the right animals to defend your territory, you will win the game.

Genetic Variation

Genetic variation is the presence of differences in sequences of genes between individual organisms of a species. It enables natural selection, one of the primary forces driving the evolution of life.

Genes

Genes are units of hereditary information. A gene is a section of a long molecule called deoxyribonucleic acid (DNA).

Related Resources

Reproduction

Reproduction is the production of offspring. There are two main forms: sexual and asexual reproduction. In sexual reproduction, an organism combines the genetic information from each of its parents and is genetically unique. In asexual reproduction, one parent copies itself to form a genetically identical offspring. Sea turtles are an example of an animal that reproduces sexually, a volvox (green algae) is an example of an organism that reproduces asexually, and a brittle star can reproduce in either way. Help your students understand the sexual and asexual reproduction with these classroom resources.

Defender: Natural Selection

This game will teach you about forms of adaptation and the role they play in natural selection. You will have to defend your territory by placing animals with different adaptations on your land. Your knowledge about adaptations will help you earn extra points! When you place the right animals to defend your territory, you will win the game.

Genetic Variation

Genetic variation is the presence of differences in sequences of genes between individual organisms of a species. It enables natural selection, one of the primary forces driving the evolution of life.

Genes

Genes are units of hereditary information. A gene is a section of a long molecule called deoxyribonucleic acid (DNA).


Watch the video: Variation. Genetics. Biology. FuseSchool (July 2022).


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