Evolution of sexual isolation in laboratory populations: fitness differences between mating types and the associated hybrid incompatibilities.
Drosophila melanogaster is known to have two races in the incipient stages of speciation that exhibit strong asymmetric premating isolation: Zimbabwe (Z) and cosmopolitan (M). In a study examining the phenotypic and genotypic evolution after secondary contact, we found that despite strong sexual selection favoring the Z-type behavior, it is the M-type behavior that comes to dominate hybrid populations. This article examines the fitness costs associated with the Z-type behavior. We have discovered that these costs are great enough to explain the failure of the Z-type behavior to prosper. Here we report that Z-type females produce approximately half the number of offspring that M-type females produce. Furthermore, crosses between populations have revealed that Z-type females mated to M-type males have approximately 20% fewer offspring than the reciprocal crosses because of an inability of M-type sperm to successfully fertilize Z-type eggs. Hybrid crosses also exhibit much-reduced numbers of viable offspring in addition to reduced hybrid male fertility. These fitness effects suggest that multiple mechanisms of postmating isolation have evolved concurrently with the divergence in behavior.
Alipaz
Julie A
JA
Committee on Evolutionary Biology, University of Chicago, Chicago, Illinois 60637, USA. jalipaz@stanford.edu
Karr
Timothy L
TL
Wu
Chung-I
CI
D000818:Animals; D003433:Crosses, Genetic; D004331:Drosophila melanogaster / Q000145:classification / Q000235:genetics / Q000502:physiology*; D005260:Female; D005298:Fertility; D049810:Genetic Speciation; D005838:Genotype; D006824:Hybridization, Genetic; D008297:Male; D010641:Phenotype; D012726:Sexual Behavior, Animal; D015030:Zimbabwe