Weeks Stephen C, Marcus Vivien, Crosser Bobbi R
Department of Biology, University of Akron, Akron, Ohio, 44325-3908.
Evolution. 1999 Apr;53(2):472-483. doi: 10.1111/j.1558-5646.1999.tb03782.x.
The observation that offspring produced by the mating of close relatives are often less fit than those produced by matings between unrelated individuals (i.e., inbreeding depression) has commonly been explained in terms of the increased probability of expressing deleterious recessive alleles among inbred offspring (the partial dominance model). This model predicts that inbreeding depression should be limited in regularly inbreeding populations because the deleterious alleles that cause inbreeding depression (i.e., the genetic load) should be purged by regularly exposing these alleles to natural selection. We indirectly test the partial dominance model using four highly inbred populations of an androdioecious crustacean, the clam shrimp Eulimnadia texana. These shrimp are comprised of males and hermaphrodites, the latter capable of either self-fertilizing or mating with a male (i.e., outcrossing between hermaphrodites is impossible). Hermaphrodites are further subdivided into monogenics (produced via self-fertilization) and amphigenics (produced via self-fertilization or outcrossing). Electrophoretic evidence suggests significant differences in heterozygosity among populations, but that selfing rates were not statistically different (average s = 0.67). Additional electrophoretic analyses reveal that three previously described sex-linked loci (Fum, Idh-1, and Idh-2) are all tightly linked to each other, with crossing over on the order of 1% per generation. Although selfing rates are clearly high, we present evidence that early inbreeding depression (hatching rates, juvenile survival, and age at sexual maturity) exists in all four populations. For all of these factors, inbreeding depression was inferred by the positive correlation of multilocus heterozygosity and fitness. Cumulative inbreeding depression (8) is between 0.41 and 0.47 across all populations, which appears to be too low to limit the effects of purging via identity disequilibrium. Instead, we suggest that the maintenance of inbreeding depression in these populations is due to the observed linkage group, which we suggest contains a large number of genes including many related to fitness. Segregation of such a large linkage group would explain our observations of the predominance of amphigenic hermaphrodites in our field samples and of survival differences between monogenics and amphigenics within selfed clutches. We propose that a modified form of the overdominance model for inbreeding depression operating at the level of linkage groups maintains the observed levels of inbreeding depression in these populations even in the face of high rates of selfing.
近亲交配产生的后代通常比非近亲个体交配产生的后代适应性更差（即近亲繁殖衰退），这一现象通常根据近亲繁殖后代中有害隐性等位基因表达概率增加来解释（部分显性模型）。该模型预测，近亲繁殖衰退在经常近亲繁殖的种群中应受到限制，因为导致近亲繁殖衰退的有害等位基因（即遗传负荷）应通过定期使这些等位基因接受自然选择而被清除。我们使用一种雌雄同体的甲壳类动物——德克萨斯蚌虾（Eulimnadia texana）的四个高度近亲繁殖种群间接检验部分显性模型。这些虾由雄性和雌雄同体组成，后者能够自我受精或与雄性交配（即雌雄同体之间不可能进行异体受精）。雌雄同体进一步细分为单亲生殖型（通过自我受精产生）和双亲生殖型（通过自我受精或异体受精产生）。电泳证据表明种群之间的杂合度存在显著差异，但自交率在统计上没有差异（平均s = 0.67）。额外的电泳分析表明，三个先前描述的性连锁基因座（Fum、Idh - 1和Idh - 2）彼此紧密连锁，每代的交换率约为1%。尽管自交率明显很高，但我们提供的证据表明，所有四个种群中都存在早期近亲繁殖衰退（孵化率、幼体存活率和性成熟年龄）。对于所有这些因素，通过多位点杂合度与适应性的正相关来推断近亲繁殖衰退。所有种群的累积近亲繁殖衰退（δ）在0.41至0.47之间，这似乎太低，无法限制通过同型不平衡进行清除的影响。相反，我们认为这些种群中近亲繁殖衰退的维持是由于观察到的连锁群，我们认为该连锁群包含大量基因，包括许多与适应性相关的基因。这样一个大的连锁群的分离可以解释我们在野外样本中观察到的双亲生殖型雌雄同体占优势以及自交卵块中单亲生殖型和双亲生殖型之间的生存差异。我们提出，在连锁群水平上起作用的近亲繁殖衰退的一种修正形式的超显性模型，即使面对高自交率，也能维持这些种群中观察到的近亲繁殖衰退水平。
