Cyclical Parthenogenesis

Frequently parthenogensis occurs along with sexual reproduction and is said to be cyclical. Cyclical parthenogenesis shows several variations in the alternation of sexual and parthenogenesis generations.

(1) Diploid parthenogenesis, open type. In aphids the fertilized eggs develop into females. These females reproduce by diploid parthenogenesis and give rise to many female generations during the summer. Late in summer there appear parthenogenetic stem mothers. These are of two types, one producing males and the other producing females. The male sexual forms arise through the loss of one X chromosome during maturation division. They therefore contain one chromosome less than the female sexual forms. Both males and females arise parthenogenetically and produce spermatozoa and ova, respectively. Bisexual reproduction occurs, and the zygote is formed. the fertilized eggs survive winter and hatch during spring to produce another batch of parthenogenetic females. In aphids parthenogenetic development is of the diploid type only. It is also of the open type, as many parthenogenetic generations appear before the sexual forms.

(2) Diploid parthenogenesis, closed type. In Phylloxera fallax and most phylloxerans, parthenogenesis is of the diploid type. Also, since the zygote directly gives rise to the stem mother without intervening parthenogenetic generations, it is of the closed type.

(3) Diploid and haploid (mixed) parthenogenesis, open type. In rotifers, both diploid and haploid parthenogenesis occur in the same life cycle. In spring and summer many generation of females are produced by diploid parthenogenesis. In autumn the sexual females lay haploid eggs, some of which are smaller than the rest. The small eggs develop parthenogenetically into degenerate haploid males which produce spermatozoa. The larger eggs are haploid ova. When these ova are fertilized by the spermatozoa, zygotes are formed which develop thick shells. These zygotes can resist unfavorable environmental conditions of winter. In the following spring they develop into parthenogenetic females.

(4) Diploid and haploid (mixed) parthenogensis, closed type. In the gall fly Neuroterus, fertilized eggs produce, as usual, parthenogenetic females. Some of the females directly produce females by diploid parthenogenesis, others produce males by haploid parthenogenesis. The diploid females and the haploid males are both sexual forms, which produce ova and spermatozoa, respectively. Both diploid and haploid parthenogenesis are found in the same cycle.