EDMANDS, S.; University of Southern California: Mechanisms underlying reproductive isolation in the copepod Tigriopus californicus

Much of what we know of the genetics of speciation comes from studies of Drosophila. The copepod Tigriopus californicus provides an alternative model. This species is well suited for breeding studies due its hardiness and short generation time (~ 3 weeks). It is particularly good for studies of reproductive isolation because reproductively compatible populations span a huge range of genetic distances (0-22% mitochondrial DNA divergence) allowing study of the progression from what is usually termed “outbreeding depression” to what is usually termed “speciation”. Work on the species to date has shown that reproductive isolation is entirely postzygotic rather than prezygotic, that incompatibilities are largely intrinsic rather than extrinsic, that fitness reductions begin in the second generation of hybridization, and that negative gene interactions extend beyond single chromosomes. Hybridization between populations over a range of genetic and geographic distances shows F1 hybrid vigor even in some of the widest crosses, and F2 hybrid breakdown which increases with distance. Replicate hybrid populations maintained for one year (up to 15 generations) show greater hybridity than expected, suggesting that hybrid inferiority in early generations gives way to hybrid superiority in later generations. Work in progress focuses on the genetic basis of hybrid incompatibilities, and shows that most of the species twelve chromosomes harbor loci involved in gene interactions that significantly affect hybrid fitness.