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Maturation Division in Parthenogenesis

A natural consequence of parthenogenesis is that the maturation division is affected in many cases. Fertilization and meiosis are complementary processes. The doubling of chromosomes by fertilization is counterbalanced by halving in meiosis. In parthenogenetic forms, maturation division is modified in several ways.

(1) In aphids, which undergo diploid parthenogenesis, reduction of the chromosome number is avoided by their undergoing only one polar division. In Phyllaphis, eggs that require fertilization undergo two polar divisions and produce two polar bodies. During this process the chromosome number is halved from six to three. Parthenogenesis eggs, on the other hand, give off only one polar body, and do not undergo any chromosome reduction. They retain the diploid number of six chromosomes.

(2) In many Hymenoptera the eggs undergo two maturation divisions and produce haploid ova. If the ova are fertilized they develop into diploid females; if not they form haploid males. These males produce haploid sperms. Obviously spermatogenesis has to be ameiotic (without meiosis) to avoid halving the chromosomes number further. This is achieved by suppression of the first maturation division.

(3) In a few Hymenoptera two maturation divisions are undergone but the eggs remain diploid because both divisions are equational. These diploid eggs usually produce females.

(4) In the gall flies (Cynipidae), eggs which produce females undergo no polar divisions and remain diploid. Male-producing eggs undergo two polar divisions and produce haploid eggs.

(5) In the brine shrimp Artemia there may be two polar divisions. After the second division, however, the second polar nucleus migrates back and fuses with the egg nucleus. Thus, in effect, the egg is fertilized by its own polar nucleus.

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