Bush Robin M, Smouse Peter E
Department of Ecology and Evolutionary Biology, University of California, Irvine, CA, 92717, USA.
Center for Theoretical and Applied Genetics, Cook College, Rutgers University, New Brunswick, NJ, 08903-0231, USA.
Evolution. 1991 May;45(3):481-498. doi: 10.1111/j.1558-5646.1991.tb04325.x.
Reports of positive associations between allozymic heterozygosity and measures of fitness are routine, but it has not been possible to distinguish between the two preeminent explanations of the phenomenon, dominance and overdominance. We tested several of the assumptions of these hypotheses in our study of the relationship between electrophoretic genotype and three life history traits in loblolly pines (Pinus taeda L.). Traits examined included the survival and growth of selfed and outcrossed progeny of 45 maternal trees, and maternal fecundity, measured as the number of surviving progeny per mother tree. Inbreeding depression was severe; the relative fitness of the selfed progeny was only 8% that of the outcrossed progeny. We found a heterozygote fecundity advantage, which should have resulted in an excess of rare alleles in the progeny. Instead, there was evidence of severe survival selection against rare alleles in both heterozygous and homozygous forms. The deficit of rare alleles averaged 69 and 50% in the selfed and outcrossed progeny, respectively. The one allele in the sample that we should have suspected of being maintained by overdominance (a PGI2 mid-frequency allele) appeared to be overdominant for outcrossed height growth and probably for fecundity as well. Multiple-locus genotype explained very little of the variation in growth, however, and rather than seeing evidence for overdominance as a force in maintaining most of the observed polymorphism, we were left to explain, in the face of the severe survival selection, why the rare alleles were present at all. Projection of the stand into the future through computer simulation showed how balancing selection acting on differential growth, fecundity, and mortality among genotypes could, over the life of the stand, account for the maintenance of the rare alleles in the population.
关于等位酶杂合性与适合度指标之间存在正相关关系的报道屡见不鲜，但一直无法区分该现象的两种主要解释：显性和超显性。在我们对火炬松（Pinus taeda L.）电泳基因型与三个生活史性状之间关系的研究中，我们检验了这些假说的若干假设。所研究的性状包括45株母树自交和异交后代的存活与生长情况，以及以每株母树存活后代数量衡量的母本繁殖力。近亲繁殖衰退严重；自交后代的相对适合度仅为异交后代的8%。我们发现了杂合子繁殖力优势，这本应导致后代中稀有等位基因过剩。然而，有证据表明，无论是杂合形式还是纯合形式的稀有等位基因，在存活选择方面都受到严重影响。在自交和异交后代中，稀有等位基因的缺失平均分别为69%和50%。样本中我们本应怀疑由超显性维持的一个等位基因（一种PGI2中频等位基因），似乎对异交树高生长以及可能对繁殖力而言是超显性的。然而，多位点基因型对生长变异的解释微乎其微，而且在面对严重的存活选择时，我们并未看到超显性作为维持大多数观察到的多态性的一种力量的证据，反而要解释为何稀有等位基因竟然存在。通过计算机模拟对林分未来进行预测，结果表明，在林分的生命周期内，作用于不同基因型间生长、繁殖力和死亡率差异的平衡选择如何能够解释种群中稀有等位基因的维持。
