The role of chromosome rearrangements in speciation and reproductive isolation remains a debated topic, although certain conditions associated with divergence should promote their emergence. We examined chromosome changes in two Lake Whitefish (Coregonus clupeaformis) lineages that recently diverged after the colonization of post-glacial lakes following allopatry. A dwarf limnetic form evolved repeatedly from the normal benthic form, becoming reproductively isolated. We applied conventional and molecular cytogenetic methods to three such pairs of sympatric Lake Whitefish populations, in order to i) test the null hypothesis of no difference in chromosomal rearrangements between pure lineages and ii) test the hypothesis of chromosomal instability in healthy and unfit backcross embryos.
While the dwarf and normal karyotypes were highly similar, more detailed cytogenetic analyses revealed an extensive polymorphism partly shared by the three species pairs. Multivariate analyses on cytogenetic markers revealed ongoing genomic rearrangements between these incipient species, consistent with chromosomal divergence initiated in allopatry. Our results also support the hypothesis of mitotic instability in healthy backcross embryos. We found extensive aneuploidy in unfit backcross embryos, indicating meiotic breakdown in their F1 parent.
Thus, intra-chromosomal differentiation in the Lake Whitefish system may contribute to divergence by destabilizing mitotic and meiotic chromosome segregation in hybrids. Importantly, the fine chromosome structures detected here, such as centromeres, repetitive elements and heterochromatin remain difficult to sequence and assemble. Therefore, cytogenetics is highly complementary to genomics, and should be integrated with population and quantitative genetic studies towards a better understanding of speciation.