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

Cytogenetics and gene flow were studied in microbe-associated parthenogenetic (thelytokous) forms of three species of the genus Trichogramma (T. pretiosum, T. deion and T. nr. deion). The chromosome behaviour in newly laid eggs indicated that the mechanism allowing restoration of diploidy in unfertilized thelytokous eggs was a segregation failure of the two sets of chromosomes in the first mitotic anaphase. This results in a nucleus containing two sets of identical chromosomes. The mechanism is known as gamete duplication and results in complete homozygosity. This was confirmed by investigation of the segregation pattern of allozymes in the offspring of heterozygous thelytokous females. Contrary to the generally assumed genetic isolation of thelytokous lines, thelytokous females of these species can mate and will use the sperm to fertilize some of their eggs. These fertilized eggs give rise to females whose genome consists of one set of chromosomes from each parent. Egg fertilization and the resulting syngamy of the sperm and egg pronucleus apparently precludes the gamete duplication that would have taken 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 there was evidence for high levels of gene flow from the sexual (arrhenotokous) fraction to the parthenogenetic (thelytokous) fraction of the population. The implications of the cytogenetic mechanism of parthenogenesis, i.e. gamete duplication, for the mechanism of sex determination in Hymenoptera are discussed.