Significance of Parthenogenesis

Parthenogenesis as a form reproduction has established itself in a wide variety of organism, both plants and animals. In certain respects parthenogenesis has advantages over sexual reproduction. The significance of parthenogenesis lies in the following aspects.

(1) Control of the sex ratio. When the queen bee is fertilized by the drone, the spermatozoa are deposited in the seminal receptacles. A single mating is sufficient to fertilize the eggs for the lifetime of the queen. The queen controls fertilization in some manner not known, and thus controls the sex ratio. Fertilized eggs develop into diploid females (queen or workers), while unfertilized eggs develop parthenogenetically into haploid males.

(2) Rapid breeding. In aphids, females reproduce in summer by diploid parthenogenesis. Several generations of females are formed. Thus parthenogenesis enables rapid increase in numbers in a season when vegetation (on which the aphids feed) is abundant. The rapid increase in numbers is due to the fact that the entire population consists of breeding females. Also, the delay resulting from the time spent in finding a mate, and mating is eliminated.

(3) Relation to hybridization. When a hybrid is formed between two species, the chromosomes are to some extent unmatched. This results in difficulties during maturation divisions of meiosis, because chromosome pairing becomes difficult. Normal reproduction by fertilization may become impossible in such cases. Diploid prthenogenesis is, therefore, the solution, because no maturation divisions are undergone during the formation of the egg. New species arising as a result of hybridization have the combined haploid number of chromosomes of the two parents.

(4) Relation to polyploidy. When polyploids like triploids (3n) and pentaploids (5n) are formed, the odd number of chromosome sets makes normal meiosis difficult. This usually results in sterility. If parthenogenetic development is adopted it can bring about escape from sterility. Thus there is a correlation between polyploidy and obligatory parthenogenesis. Polyploidy is more common in plants than in animals. In the latter group it is usually accompanied by parthenogenesis.

(5) Variation. In total parthenogenesis, fertilization and meiosis are both eliminated. Thus there are no chances of genetic recombination. Since all the descendants have identical heredity, there is practically no variation. Species having total parthenogenesis can thrive because they are well adapted to a particular environment. In these species genetic variability has been sacrificed to preserve genotypes that have proved their value in survival. However, any changes in the environment could have an adverse effect on the species. Thus it is desirable that variation should be introduced in a population to counteract the effects of changes in environment. This can only be done by introducing sexual reproduction. Some species have adopted cyclic parthenogenesis, in which a sexual generation is introduced between one or more parthenogenetic generations.