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

Despite mounting evidence that key developmental regulator genes are involv ed in significant macroevolutionary changes, there have been few studies de monstrating the functional significance of variation in such genes for the generation of population-level variation. In this study we examined and com pared the expression domains of three Hox gene homeobox sequences in embryo s derived from two morphologically distinct populations of the threespine s tickleback, Gasterosteus aculeatus. We found within-population variation in the location of anterior limits, particularly in more 5' Hox genes whose a nterior expression domains showed graded distributions of transcripts over several somites. However, despite considerable and statistically significan t differences in the anteroposterior pattern of the axial and median skelet ons 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 fis h species is discussed with respect to the buffering of development in fluc tuating environments, and it is argued that population and quantitative gen etic perspectives should also be taken into account in considering the func tion and evolution of Hox genes.