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）可通过两种途径参与合子后生殖隔离（postzygotic reproductive isolation）的演化：一是直接破坏F1杂种（F1 hybrid）的减数分裂过程，二是通过抑制基因不相容位点间的重组发挥间接作用。由于染色体重排对育性的直接效应会在其扩散阶段带来适合度成本，因此解析F1杂种不育的机制，对于重构染色体重排在物种形成（speciation）中的作用具有核心意义。
在猴面花属（Mimulus）的红花猴面花（Mimulus cardinalis）与刘易斯猴面花（Mimulus lewisii）的杂种群体中，染色体重排涵盖了预交配障碍与花粉不育相关的全部数量性状位点（quantitative trait loci, QTLs），这提示其可能在这一经典模式系统中推动了物种形成进程。
本研究借助人工染色体加倍（artificial chromosome doubling）与比较作图（comparative mapping）技术，验证了杂合染色体重排是否会直接引发刘易斯猴面花-红花猴面花杂种的显性不足型雄性不育（underdominant male sterility）。实验结果与杂种不育的直接染色体基础假说一致：合成四倍体F1杂种的花粉育性（83.4%）相较于二倍体F1杂种（36.0%）得到了显著恢复。
通过对帕里什猴面花（Mimulus parishii）分别与红花猴面花、刘易斯猴面花的杂种开展额外作图分析，本研究发现显性不足型雄性不育由一个刘易斯猴面花特有的相互易位（reciprocal translocation）与一个红花猴面花特有的相互易位共同介导，但倒位（inversions）对育性并无直接影响。
最后，我们讨论了相互易位作为生殖隔离成因的重要性，并探讨了尽管存在内在适合度成本，显性不足型染色体重排仍能得以扩散并固定的多种演化模型。