Meiotic drive against chromosome fusions in butterfly hybrids

Species frequently differ in karyotype, but heterokaryotypic individuals may suffer from reduced fitness. Chromosomal rearrangements like fissions and fusions can thus serve as a mechanism for speciation between incipient lineages but their evolution poses a paradox. How does underdominant rearrangements evolve? One solution is the fixation of underdominant chromosomal rearrangements through genetic drift. However, this requires small and isolated populations. Fixation is more likely if a novel rearrangement is favored by a transmission bias, such as meiotic drive. Here, we investigate transmission ratio distortion in hybrids between two wood white ( Leptidea sinapis ) butterfly populations with extensive karyotype differences. Using data from two different crossing experiments, we uncover a transmission bias favoring the fused state at chromosome with unknown polarization in one experiment and a transmission bias favoring the unfused state of derived fusions in both experiments. The latter result support a scenario where chromosome fusions can fix in populations despite counteracting effects of meiotic drive. This means that meiotic drive not only can promote runaway chromosome number evolution and speciation, but also that this transmission bias can be a conservative force acting against karyotypic change and the evolution of reproductive isolation. Based on our results, we suggest a mechanistic model for why derived fusions may be opposed by meiotic drive and discuss factors contributing to karyotype evolution in Lepidoptera.