In haploid organisms that undergo sexual reproduction, name the stage in the life cycle when meiosis occurs. Give reasons for your answer.
Meiosis can take place only in a diploid stage (post-zygotic stage) because the zygote is the only diploid cell in the life cycle of such organisms. This meiosis an case of haploid organisms will occur of the fertilisation.
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.
Higher plants (angiosperms) and higher animals (vertebrates) have a more complex structural organisation as compared to the lower groups of plants and animals. They have evolved very efficient mechanism of sexual reproduction. These groups have resorted to reproduction by the sexual method for the following reasons
(i) To ensure healthy progeny
(ii) To produce genetically varied offsprings that adapt to changes in environment and surviveal in all climatic conditions.
(iii) It ensures the genetic recombination that results in variation which gives rise to evolution.
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.
(i) Sterile diploid females as workers
(ii) One fertile diploid female as queen
(iii) Fertile haploid males as drones.
In case of honeybees, both haploid and diploid individuals formad as a result of incomplete (cyclic) parthenogenesis, i.e., both sexual reproduction and parthenogenesis. Fertilised eggs (zygote) give rise to queen and workers (both are females) by the process of sexual reproduction and unfertilised eggs (ova) develop into drones (males) by the process of parthenogenesis.
With which type of reproduction do we associate the reduction division? Analyse the reasons for it.
Reduction division (meiosis) is associated with sexual reproduction. The reasons are
(i) Since, sexual reproduction involves the fusion of two types of gametes male and female), they must have haploid number of chromosomes.
(ii) 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.
(iii) Reduction division also ensures maintenance of constancy of chromosome number from generation to generation.
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.
The formation of new plants from vegetative units (vegetative propagules) such as buds, tubers, rhizomes. etc., is called vegetative propagation (vegetative reproduction). It can be considered as a type of asexual reproduction as it involves the production of new individuals.
(i) by a single parent
(ii) without the formation and fusion of gametes
(iii) without resulting in any genetic or morphological variations.