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 natural populations of Drosophila melanogaster. Males heterozygous for an SD second chromosome and a normal homologue (SD +) produce predominantly SD-bearing sperm. The coadapted gene complex responsible for this transmission 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 polymorphism and divergence at six genes that together encompass this pericentromeric region and span seven map units. Interestingly, there was no discernible divergence between SD and SD + chromosomes for any of these molecular markers. Furthermore, SD chromosomes harbored much less polymorphism than did SD + chromosomes. The results suggest that the SD system evolved recently, swept to appreciable frequencies worldwide, and carried with it the entire second chromosome centromeric region (roughly 10% of the genome). Despite its well-documented genetic complexity, this coadapted system appears to have evolved on a time scale that is much shorter than can be gauged using nucleotide substitution data. Finally, the large genomic region hitchhiking with SD indicates that a multilocus, epistatically selected system could affect the levels of DNA polymorphism observed in regions of reduced recombination.

DISTURBANCE OF ECLOSION SEQUENCE IN HYBRID LEPIDOPTERA

Some interspecific and interpopulation Lepidoptera and Orthoptera hybrids show a syndrome of developmental abnormalities referred to here as the “sequence effect.” In normal within population broods of Lepidoptera males develop slightly faster than females, but in crosses showing the sequence effect, females in one direction of the cross develop faster and in the reciprocal cross much more slowly than their male siblings. In some cases the environmentally induced diapause of the females may differ strikingly from that of their male siblings. Development rate and diapause in these species appear to be controlled by a sex-linked coadapted gene complex. Expression of the sequence effect may result from a loss of a species- or population-specific balance between regulatory and secretory portions of this complex, resulting in hormonal abnormalities which are more likely to be expressed in the heterogametic sex (females in Lepidoptera, males in Orthoptera).