EVIDENCE FOR FEMALE HETEROGAMETY IN 2 TERRESTRIAL CRUSTACEANS AND THEPROBLEM OF SEX-CHROMOSOME EVOLUTION IN ISOPODS

Female heterogamety (WZ type) has been demonstrated in the terrestrial isopods Oniscus asellus (Oniscidae) and Eluma purpurascens (Armadilli diidae), by making crosses between two genetic females (one of them ex perimentally reversed into a functional neo-male). The WW individuals generated by such crosses were viable and fertile females. These data, plus the frequent monomorphism of sex chromosomes and the coexistance of two heterogamety types (XX/XY and WZ/ZZ), indicate that sex chromo some differentiation in isopods is at a primitive stage. The evolution of sex chromosomes in this group of crustaceans is discussed, and it is suggested that this evolution has been disturbed by parasitic sex f actors.