|Tempo of post-zygotic isolation in teleosts
Daniel Bolnick and I are investigating the rate at which reproductive isolation evolves in teleost fishes. It is already well known that the strength of reproductive isolation is greater for more divergent taxa: this pattern was noted by Darwin (1859, pp 257) and formalized by more recent studies such as Coyne and Orr’s (1989) analysis of pre- and post-zygotic isolation in Drosophila. We were inspired to begin this research because we identified three major questions that remain unresolved. First, the actual time-scale for isolation is often unclear, as comparative studies of reproductive isolation have generally used molecular markers that are not particularly clock-like, usually lack appropriate fossil calibrations, and use a genetic distance measure as a proxy for absolute age. Second, little is known about the relative rates at which different components of post-zygotic isolation evolve. Third, few studies have identified variables that explain the residuals around the age-isolation regression line and hence may represent factors that modify the rate of speciation.
Research has focused on the North American endemic freshwater fish clade Centrarchidae that are particularly suited for such analyses. First, Dan Bolnick, Peter Wainwright, and I have generated a highly resolved six-gene phylogeny containing all species in the clade. Second, using six fossil calibrations and penalized likelihood, we have derived robust estimates of divergence times for every node in the phylogeny. Third, artificial spawning techniques in centrarchids are well established and allow the PIs to generate artificial hybrids and measure multiple facets of post-zygotic isolation: fertilization rates, embryo, larval, and juvenile survival rates, and adult sex ratios and fertility.
This research has potential to provide the most detailed comparative study to date of the rates of reproductive isolation, the relative importance of different mechanisms of post-zygotic isolation, and factors that might modify the rate of speciation.
Published papers from our research on centrarchids have made important contributions to the study of reproductive isolation. For example, we have shown that hybrid embryo viability decreased at a rate that is much slower than other taxa investigated to date. Also, divergence time and body size disparity explains a significant portion of the variation in hybrid viability among centrarchid clades, indicating that gradual accumulation of genetic incompatibilities of small effect and ecological divergence (as measured by body size disparity) may work in concert in facilitating speciation.