Department of Ecology, Environment, and Plant Sciences, Science for Life Laboratory, Stockholm University, Stockholm, Sweden.
Laboratory of Systematic Botany and Phytogeography, School of Biology, Aristotle University of Thessaloniki, Thessaloniki, Greece.
Mol Biol Evol. 2022 Jan 7;39(1). doi: 10.1093/molbev/msab349.
Fertilization in angiosperms involves the germination of pollen on the stigma, followed by the extrusion of a pollen tube that elongates through the style and delivers two sperm cells to the embryo sac. Sexual selection could occur throughout this process when male gametophytes compete for fertilization. The strength of sexual selection during pollen competition should be affected by the number of genotypes deposited on the stigma. As increased self-fertilization reduces the number of mating partners, and the genetic diversity and heterozygosity of populations, it should thereby reduce the intensity of sexual selection during pollen competition. Despite the prevalence of mating system shifts, few studies have directly compared the molecular signatures of sexual selection during pollen competition in populations with different mating systems. Here we analyzed whole-genome sequences from natural populations of Arabis alpina, a species showing mating system variation across its distribution, to test whether shifts from cross- to self-fertilization result in molecular signatures consistent with sexual selection on genes involved in pollen competition. We found evidence for efficient purifying selection on genes expressed in vegetative pollen, and overall weaker selection on sperm-expressed genes. This pattern was robust when controlling for gene expression level and specificity. In agreement with the expectation that sexual selection intensifies under cross-fertilization, we found that the efficacy of purifying selection on male gametophyte-expressed genes was significantly stronger in genetically more diverse and outbred populations. Our results show that intra-sexual competition shapes the evolution of pollen-expressed genes, and that its strength fades with increasing self-fertilization rates.
被子植物的受精过程涉及花粉在柱头的萌发，随后花粉管伸长穿过花柱，将两个精子细胞输送到胚囊。当雄性配子体为受精而竞争时，性选择可能会在整个过程中发生。花粉竞争中的性选择强度应该受到沉积在柱头的基因型数量的影响。由于自交增加了交配伙伴的数量，以及种群的遗传多样性和杂合性，因此它应该降低花粉竞争中的性选择强度。尽管交配系统的转变很普遍，但很少有研究直接比较不同交配系统种群中花粉竞争过程中的性选择的分子特征。在这里，我们分析了来自具有不同交配系统的自然种群的拟南芥的全基因组序列，以测试从异交向自交的转变是否会导致与花粉竞争中涉及的基因的性选择一致的分子特征。我们发现，在营养花粉中表达的基因受到有效净化选择的证据，并且精子表达的基因的选择总体上较弱。当控制基因表达水平和特异性时，这种模式是稳健的。与性选择在异交下加剧的预期一致，我们发现，在遗传上更多样化和杂交的种群中，雄性配子体表达基因的净化选择效力明显更强。我们的研究结果表明，种内竞争塑造了花粉表达基因的进化，而且随着自交率的增加，其强度会减弱。
