CYTOGENETICS OF MICROBE-ASSOCIATED PARTHENOGENESIS AND ITS CONSEQUENCES FOR GENE FLOW IN TRICHOGRAMMA WASPS

Cytogenetics and gene flow were studied in microbe-associated partheno genetic (thelytokous) forms of three species of the genus Trichogramma (T. pretiosum, T. deion and T. nr. deion). The chromosome behaviour i n newly laid eggs indicated that the mechanism allowing restoration of diploidy in unfertilized thelytokous eggs was a segregation failure o f the two sets of chromosomes in the first mitotic anaphase. This resu lts in a nucleus containing two sets of identical chromosomes. The mec hanism is known as gamete duplication and results in complete homozygo sity. This was confirmed by investigation of the segregation pattern o f allozymes in the offspring of heterozygous thelytokous females. Cont rary to the generally assumed genetic isolation of thelytokous lines, thelytokous females of these species can mate and will use the sperm t o fertilize some of their eggs. These fertilized eggs give rise to fem ales whose genome consists of one set of chromosomes from each parent. Egg fertilization and the resulting syngamy of the sperm and egg pron ucleus apparently precludes the gamete duplication that would have tak en place if the egg had remained unfertilized. Most field populations of Trichogramma contain both parthenogenetic (thelytokous) and sexual (arrhenotokous) forms. In the two field populations that we studied th ere was evidence for high levels of gene flow from the sexual (arrheno tokous) fraction to the parthenogenetic (thelytokous) fraction of the population. The implications of the cytogenetic mechanism of parthenog enesis, i.e. gamete duplication, for the mechanism of sex determinatio n in Hymenoptera are discussed.