Sweigart Andrea L, Mason Amanda R, Willis John H
Department of Biology, Duke University, Durham, North Carolina 27708, USA.
Evolution. 2007 Jan;61(1):141-51. doi: 10.1111/j.1558-5646.2007.00011.x.
Understanding the process by which hybrid incompatibility alleles become established in natural populations remains a major challenge to evolutionary biology. Previously, we discovered a two-locus Dobzhansky-Muller incompatibility that causes severe hybrid male sterility between two inbred lines of the incompletely isolated wildflower species, Mimulus guttatus and M. nasutus. An interspecific cross between these two inbred lines revealed that the M. guttatus (IM62) allele at hybrid male sterility 1 (hms1) acts dominantly in combination with recessive M. nasutus (SF5) alleles at hybrid male sterility 2 (hms2) to cause nearly complete hybrid male sterility. In this report, we extend these genetic analyses to investigate intraspecific variation for the hms1-hms2 incompatibility in natural populations of M. nasutus and M. guttatus, performing a series of interspecific crosses between individuals collected from a variety of geographic locales. Our results suggest that hms2 incompatibility alleles are common and geographically widespread within M. nasutus, but absent or rare in M. guttatus. In contrast, the hms1 locus is polymorphic within M. guttatus and the incompatibility allele appears to be extremely geographically restricted. We found evidence for the presence of the hms1 incompatibility allele in only two M. guttatus populations that exist within a few kilometers of each other. The restricted distribution of the hms1 incompatibility allele might currently limit the potential for the hms1-hms2 incompatibility to act as a species barrier between sympatric populations of M. guttatus and M. nasutus. Extensive sampling within a single M. guttatus population revealed that the hms1 locus is polymorphic and that the incompatibility allele appears to segregate at intermediate frequency, a pattern that is consistent with either genetic drift or natural selection.
理解杂种不相容等位基因在自然种群中确立的过程仍然是进化生物学面临的一项重大挑战。此前，我们发现了一种双位点的多布赞斯基-穆勒不相容现象，它导致了不完全隔离的野花物种半边莲（Mimulus guttatus）和鼻状半边莲（M. nasutus）的两个近交系之间出现严重的杂种雄性不育。这两个近交系之间的种间杂交表明，杂种雄性不育1（hms1）位点上半边莲（IM62）的等位基因与杂种雄性不育2（hms2）位点上隐性的鼻状半边莲（SF5）等位基因结合时起主导作用，导致几乎完全的杂种雄性不育。在本报告中，我们扩展了这些遗传分析，以研究鼻状半边莲和半边莲自然种群中hms1 - hms2不相容现象的种内变异，对从各种地理区域收集的个体进行了一系列种间杂交。我们的结果表明，hms2不相容等位基因在鼻状半边莲中很常见且在地理上广泛分布，但在半边莲中不存在或很罕见。相比之下，hms1位点在半边莲中具有多态性，且不相容等位基因在地理上似乎受到极大限制。我们仅在彼此相距几公里的两个半边莲种群中发现了hms1不相容等位基因存在的证据。hms1不相容等位基因的有限分布目前可能限制了hms1 - hms2不相容现象作为半边莲和鼻状半边莲同域种群之间物种屏障的潜力。在单个半边莲种群内进行的广泛采样显示，hms1位点具有多态性，且不相容等位基因似乎以中等频率分离，这种模式与遗传漂变或自然选择一致。
