Department for Evolutionary Biology, Max Planck Institute for Developmental Biology, Tübingen, 72076, Germany.
Department of Biological Sciences, University of New Orleans, 2000 Lakeshore Drive, New Orleans, LA70148, USA.
BMC Evol Biol. 2017 Feb 23;17(1):57. doi: 10.1186/s12862-017-0873-7.
The nematode species Pristionchus pacificus has an androdioecious mating system in which populations consist of self-fertilizing hermaphrodites and relatively few males. The prevalence of males in such a system is likely to depend on the relative pros and cons of outcrossing. While outcrossing generates novel allelic combinations and can therefore increase adaptive potential, it may also disrupt the potentially beneficial consequences of repeated generations of selfing. These include purging of deleterious alleles, inheritance of co-adapted allele complexes, improved hermaphrodite fitness and increased population growth. Here we use experimental and population genetic approaches to test hypotheses relating to male production and outcrossing in laboratory and natural populations of P. pacificus sampled from the volcanic island of La Réunion.
We find a significant interaction between sampling locality and temperature treatment influencing rates of spontaneous male production in the laboratory. While strains isolated at higher altitude, cooler localities produce a higher proportion of male offspring at 25 °C relative to 20 or 15 °C, the reverse pattern is seen in strains isolated from warmer, low altitude localities. Linkage disequilibrium extends across long physical distances, but fails to approach levels reported for the partially selfing nematode species Caenorhabditis elegans. Finally, we find evidence for admixture between divergent genetic lineages.
Elevated rates of laboratory male generation appear to occur under environmental conditions which differ from those experienced by populations in nature. Such elevated male generation may result in higher outcrossing rates, hence driving increased effective recombination and the creation of potentially adaptive novel allelic combinations. Patterns of linkage disequilibrium decay support selfing as the predominant reproductive strategy in P. pacificus. Finally, despite the potential for outcrossing depression, our results suggest admixture has occurred between distinct genetic lineages since their independent colonization of the island, suggesting outcrossing depression may not be uniform in this species.
扁形动物门秀丽隐杆线虫的交配系统为雌雄同体-雌雄同体-雄体系统，其中种群由自交的雌雄同体和相对较少的雄体组成。在这样的系统中，雄体的出现概率可能取决于杂交的相对利弊。虽然杂交产生了新的等位基因组合，因此可以增加适应潜力，但它也可能破坏自交产生的潜在有益后果，包括清除有害等位基因、继承共同适应的等位基因复合物、提高雌雄同体的适应性和增加种群增长。在这里，我们使用实验和群体遗传学方法来检验与实验室和从火山岛留尼汪岛采集的自然种群中秀丽隐杆线虫的雄性产生和杂交相关的假设。
我们发现采样地点和温度处理之间存在显著的相互作用，影响实验室中自发产生雄性的速度。虽然来自高海拔、较凉爽地点的菌株在 25°C 下产生较高比例的雄性后代，而在 20°C 或 15°C 下则相反，但来自较温暖、低海拔地点的菌株则相反。连锁不平衡延伸到很长的物理距离，但未能达到部分自交的线虫秀丽新杆线虫报道的水平。最后，我们发现了不同遗传谱系之间混合的证据。
实验室中雄性产生率的升高似乎发生在与自然种群所经历的环境条件不同的条件下。这种升高的雄性产生可能导致更高的杂交率，从而推动有效重组率的增加，并产生潜在适应的新等位基因组合。连锁不平衡模式的衰减支持自交是秀丽隐杆线虫的主要繁殖策略。最后，尽管存在杂交衰退的可能性，但我们的结果表明，自独立殖民该岛以来，不同遗传谱系之间已经发生了混合，这表明在这个物种中，杂交衰退可能并不均匀。
