Obligatory Parthenogenesis

In a few species of animals, parthenogenesis is the only method of reproduction. This type of parthenogenesis is called obligatory or complete or total parthenogenesis. In this form of parthenogenesis meiosis is either totally lacking or if followed by a compensatory process. Populations having total parthenogenesis consist almost entirely of females. Males when present are very rare and are genetically nonfunctional. In cases where there is no meiosis, segregation and recombination of genes is absent. Thus the populations show no variations, and are in effect clones.

In obligatory theylytoky meiosis may be totally eliminated or may be present with a later compensating process. In apomictic thelytoky there is no meiosis. It may or may not be replaced by one or two mitotic divisions, although sometimes there may be two. In automictic thelytoky meiosis is present and results in halving of the chromosomes number. However, the somatic number is restored by fusion of the ovum with one of the polar bodies, or by fusion of the cleavage nuclei. Since there is no meiosis in apomictic thelytoky, there is no synapsis and crossing over. Hence there is no genetic combination and variation. In automatic thelytoky, however, occurrence of meiosis provides a greater possibility of genetic variation.

Obligatory thelytoky has not proved very successful in evolution because of its limitations in producing variations. Population in which there is little variation tends to be eliminated when environmental conditions become adverse. Obligatory parthenogenesis has arisen sporadically in animals, and there are about a thousand species known. In the rose gall fly Rhodities rosae and in the phasmid Bacillus rossii males are extremely rare. In the gall fly Cynips kollari, the praying mantis Brunneria borelis and some rotifers males have never been found. Theses species reproduce exclusively by unfertilized eggs.