RAPID EVOLUTION OF A COADAPTED GENE-COMPLEX - EVIDENCE FROM THE SEGREGATION DISTORTER (SD) SYSTEM OF MEIOTIC DRIVE IN DROSOPHILA-MELANOGASTER

Segregation Distorter (SD) is a system of meiotic drive found in natur al populations of Drosophila melanogaster. Males heterozygous for an S D second chromosome and a normal homologue (SD+) produce predominantly SD-bearing sperm. The coadapted gene complex responsible for this tra nsmission advantage spans the second chromosome centromere, consisting of three major and several minor interacting loci. To investigate the evolutionary history of this system, we surveyed levels of polymorphi sm and divergence at six genes that together encompass this pericentro meric region and span seven map units. Interestingly, there was no dis cernible divergence between SD and SD+ chromosomes for any of these mo lecular markers. Furthermore, SD chromosomes harbored much less polymo rphism than did SD+ chromosomes. The results suggest that the SD syste m evolved recently, swept to appreciable frequencies worldwide, and ca rried with it the entire second chromosome centromeric region (roughly 10% of the genome). Despite its well-documented genetic complexity, t his coadapted system appears to have evolved on a time scale that is m uch shorter than can be gauged using nucleotide substitution data. Fin ally, the large genomic region hitchhiking with SD indicates that a mu ltilocus, epistatically selected system could affect the levels of DNA polymorphism observed in regions of reduced recombination.