Haag C R, Ebert D
Département de Biologie, Université de Fribourg, Fribourg, Switzerland.
J Evol Biol. 2007 May;20(3):881-91. doi: 10.1111/j.1420-9101.2007.01313.x.
The genetic basis of fitness reduction associated with inbreeding is still poorly understood. Here we use associations between allozyme genotypes and fitness to investigate the genetic basis of inbreeding depression in experimental outdoor populations of the water flea, Daphnia magna. In Daphnia, a phase of clonal reproduction follows hatching from sexually produced resting eggs, and changes in genotype frequencies during the clonal phase can be used to estimate fitness. Our experiment resembles natural colonization of ponds in that single clones colonize an empty pool, expand asexually and produce sexual offspring by selfing (sisters mate with their clonal brothers). These offspring diapause and form populations consisting of selfed sibships in the following spring. In 12 of 13 experimental populations, genotypes of selfed hatchlings after diapause conformed to Mendelian expectations. During the subsequent ca. 10 asexual generations, however, genotype frequencies changed significantly at 19 of 27 single loci studied within populations, mostly in favour of heterozygotes, with heterozygosity at multiple loci affecting the change in genotype frequency multiplicatively. Because variance in heterozygosity among siblings at a given marker reflects only heterozygosity in the chromosomal region around this marker, our results suggest that selection at fitness-associated loci in the chromosomal regions near the markers were responsible for these changes. The genotype frequency changes were more consistent with selection acting on linked loci than on the allozymes themselves. Taken together, the evidence for abundant selection in the chromosomal regions of the markers and the fact that changes in genotype frequencies became apparent only after several generations of clonal selection, point to a genetic load consisting of many alleles of small or intermediate effects, which is consistent with the strong genetic differentiation and repeated genetic bottlenecks in the metapopulation from which the animals for this study were obtained.
与近亲繁殖相关的适合度降低的遗传基础仍未得到充分理解。在此，我们利用等位酶基因型与适合度之间的关联，来研究大型溞实验室外种群中近亲繁殖衰退的遗传基础。在大型溞中，从有性生殖产生的休眠卵孵化后会经历一个克隆繁殖阶段，克隆阶段基因型频率的变化可用于估计适合度。我们的实验类似于池塘的自然定殖过程，即单个克隆体定殖到一个空池塘中，进行无性繁殖扩张，并通过自体受精（姐妹与克隆兄弟交配）产生有性后代。这些后代进入滞育状态，并在次年春天形成由自体受精同胞亲缘个体组成的种群。在13个实验种群中的12个中，滞育后自体受精幼体的基因型符合孟德尔预期。然而，在随后约10个无性繁殖世代中，在种群内研究的27个单基因座中的19个，基因型频率发生了显著变化，大多有利于杂合子，多个基因座的杂合性对基因型频率变化有相乘效应。由于给定标记处同胞之间杂合性的差异仅反映该标记周围染色体区域的杂合性，我们的结果表明，标记附近染色体区域中与适合度相关基因座上的选择导致了这些变化。基因型频率的变化更符合对连锁基因座而非对等位酶本身起作用的选择。综合来看，标记染色体区域存在大量选择的证据，以及基因型频率变化仅在几代克隆选择后才明显这一事实，表明存在由许多小效应或中等效应等位基因组成的遗传负荷，这与本研究中所用动物来源的集合种群中强烈的遗传分化和反复出现的遗传瓶颈是一致的。
