Data from: Chromosomal rearrangements directly cause underdominant F1 pollen sterility in Mimulus lewisii-M. cardinalis hybrids

Chromosomal rearrangements can contribute to the evolution of postzygotic reproductive isolation directly, by disrupting meiosis in F1 hybrids, or indirectly, by suppressing recombination among genic incompatibilities. Because direct effects of rearrangements on fertility imply fitness costs during their spread, understanding the mechanism of F1 hybrid sterility is integral to reconstructing the role(s) of rearrangements in speciation. In hybrids between monkeyflowers Mimulus cardinalis and M. lewisii, rearrangements contain all quantitative trait loci (QTLs) for both premating barriers and pollen sterility, suggesting that they may have facilitated speciation in this model system. We used artificial chromosome doubling and comparative mapping to test whether heterozygous rearrangements directly cause underdominant male sterility in M. lewisii-M. cardinalis hybrids. Consistent with a direct chromosomal basis for hybrid sterility, synthetic tetraploid F1s showed highly restored fertility (83.4% pollen fertility) relative to diploids F1s (36.0%). Additional mapping with M. parishii-M. cardinalis and M. parishii-M. lewisii hybrids demonstrated that underdominant male sterility is caused by one M. lewisii-specific and one M. cardinalis-specific reciprocal translocation, but that inversions had no direct effects on fertility. We discuss the importance of translocations as causes of reproductive isolation, and consider models for how underdominant rearrangements spread and fix despite intrinsic fitness costs.染色体重排（chromosomal rearrangements）可通过两种途径助力合子后生殖隔离（postzygotic reproductive isolation）的演化：一是直接破坏F1杂种（F1 hybrids）的减数分裂过程，二是间接抑制基因不相容性（genic incompatibilities）位点间的重组。由于重排对育性的直接作用会在其扩散过程中带来适合度代价（fitness costs），因此解析F1杂种不育的机制，对于重建重排在物种形成（speciation）过程中的作用至关重要。在猴面花属的红花猴面花（Mimulus cardinalis）与刘易斯猴面花（M. lewisii）的杂种中，染色体重排涵盖了交配前隔离（premating barriers）与花粉不育（pollen sterility）的全部数量性状位点（quantitative trait loci, QTLs），这表明重排可能推动了该模式系统中的物种形成过程。本研究通过人工染色体加倍（artificial chromosome doubling）与比较定位（comparative mapping）技术，检验刘易斯猴面花-红花猴面花杂种中的杂合染色体重排（heterozygous rearrangements）是否直接导致了下显性雄性不育（underdominant male sterility）。与杂种不育的直接染色体基础假说一致，人工合成的四倍体F1杂种（tetraploid F1s）相较于二倍体F1杂种（diploid F1s），育性得到大幅恢复——二倍体F1的花粉育性仅为36.0%，而四倍体F1的花粉育性可达83.4%。通过对帕里什猴面花（M. parishii）-红花猴面花以及帕里什猴面花（M. parishii）-刘易斯猴面花杂种的额外定位实验证实，下显性雄性不育由一个刘易斯猴面花特异性相互易位（reciprocal translocation）与一个红花猴面花特异性相互易位共同导致，而倒位（inversions）并未对育性产生直接影响。本研究讨论了相互易位作为生殖隔离诱因的重要性，并探讨了尽管存在内在适合度代价，下显性染色体重排仍能得以扩散并固定的相关模型。