A GENERAL-METHOD FOR IDENTIFYING MAJOR HYBRID MALE-STERILITY GENES INDROSOPHILA

The genes responsible for hybrid male sterility in species crosses are usually identified by introgressing chromosome segments, monitored by visible markers, between closely related species by continuous backcr osses. This commonly used method, however, suffers from two problems. First, it relies on the availability of markers to monitor the introgr essed regions and so the portion of the genome examined is limited to the marked regions. Secondly, the introgressed regions are usually lar ge and ir is impossible to tell if the effects of the introgressed reg ions are the result of single (or few) major genes or many minor genes (polygenes). Here we introduce a simple and general method for identi fying putative major hybrid male sterility genes which is free of thes e problems. In this method, the actual hybrid male sterility genes (ra ther than markers), or tightly linked gene complexes with large effect s, are selectively introgressed from one species into the background o f another species by repeated backcrosses. This is performed by select ively backcrossing heterozygous (for hybrid male sterility gene or gen es) females producing fertile and sterile sons in roughly equal propor tions to males of either parental species. As no marker gene is requir ed for this procedure, this method can be used with any species pairs that produce unisexual sterility. With the application of this method, a small X chromosome region of Drosophila mauritiana which produces c omplete hybrid male sterility (aspermic testes) in the background of D . simulans was identified. Recombination analysis reveals that this re gion contains a second major hybrid male sterility gene linked to the forked locus located at either 62.7+/-0.66 map units or at the centrom ere region of the X chromosome of D. mauritiana.