Saturday, July 23, 2011

Quantitative genetics in stickleback: implications for reproductive isolation

We now have a study online in JEB that examines how differences between lake and stream stickleback ecotypes in foraging-related traits (gill raker number and aspects of body shape) are inherited quantitative genetically. The study is a line cross analysis of different lab-reared pure and hybrid lines derived from the well-known ecotype pair residing in Misty Lake and its inlet stream on Vancouver Island. We find that gill raker number is inherited additively (e.g., F1 hybrids are intermediate between the pure ecotypes), whereas there is strong dominance in body depth. That is, F1s and other hybrids between the lake and stream types display the body depth typical of the lake ecotype. We argue that this difference in inheritance has implications for the speciation process between the ecotypes: given divergent selection on gill raker number, hybrids should be at a disadvantage relative to pure types in both the lake and the stream. By contrast, asymmetric introgression from the stream to the lake (but not vice versa) should be easier for body depth because hybrids resemble the lake resident and should therefore not be selected against. It would now be great to have direct information on the strength of selection acting on these phenotypes, and to look into the molecular basis of their divergence.

In addition to these biological findings, the paper has some methodological relevance because we found that a geometric morphometric (relative warp) approach to shape analysis yielded results qualitatively different from an approach based on traditional distance traits. Given that relative warps are principal components extracted from shape variables and that principal component analysis creates artifacts (see Berner 2011, Oecologia), we hypothesize in the paper that the patterns identified through relative warp analysis are artificial to some extent. We have now confirmed this with simulated data; a formal analysis should come soon.

...And who does not agree relative warps indeed look artiFISHal:
If interested, check out "Quantitative genetic inheritance of morphological divergence in a lake-stream stickleback ecotype pair: implications for reproductive isolation".

Daniel and Joost

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