Axial variation in the threespine stickleback: relationship to Hox gene expression.

Despite mounting evidence that key developmental regulator genes are involved in significant macroevolutionary changes, there have been few studies demonstrating the functional significance of variation in such genes for the generation of population-level variation. In this study we examined and compared the expression domains of three Hox gene homeobox sequences in embryos derived from two morphologically distinct populations of the threespine stickleback, Gasterosteus aculeatus. We found within-population variation in the location of anterior limits, particularly in more 5' Hox genes whose anterior expression domains showed graded distributions of transcripts over several somites. However, despite considerable and statistically significant differences in the anteroposterior pattern of the axial and median skeletons between the two stickleback populations, this phenotypic variation was not found to be correlated with any of the variation in Hox gene expression. The possible functional significance of the combinatorial Hox code in fish species is discussed with respect to the buffering of development in fluctuating environments, and it is argued that population and quantitative genetic perspectives should also be taken into account in considering the function and evolution of Hox genes.