SEX DETERMINATION IN THE FLY MEGASELIA-SCALARIS, A MODEL SYSTEM FOR PRIMARY STEPS OF SEX-CHROMOSOME EVOLUTION

The fly Megaselia scalaris Loew possesses three homomorphic chromosome pairs; 2 is the sex chromosome pair in two wild-type laboratory stock s of different geographic origin (designated ''original'' sex chromoso me pair in this paper). The primary male-determining function moves at avery low rate to other chromosomes, thereby creating new Y chromosom es. Random amplified polymorphic DNA markers obtained by polymerase ch ain reaction with single decamer primers and a few available phenotypi c markers were used in testcrosses to localize the sex-determining loc i and to define the new sex chromosomes. Four cases are presented in w hich the primary male-determining function had been transferred from t he original Y chromosome to a new locus either on one of the autosomes or on the original X chromosome, presumably by transposition. In thes e cases, the sex-determining function had moved to a different locus w ithout an obvious cotransfer of other Y chromosome markers. Thus, with Megaselia we are afforded an experimental system to study the otherwi se hypothetical primary stages of sex chromosome evolution. An initial molecular differentiation is apparent even in the new sex chromosomes . Molecular differences between the original X and Y chromosomes illus trate a slightly more advanced stage of sex chromosome evolution.