Schmidt P S, Rand D M
Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912, USA.
Evolution. 2001 Jul;55(7):1336-44. doi: 10.1111/j.0014-3820.2001.tb00656.x.
In the northern acorn barnacle, Semibalanus balanoides, genotype frequencies of three genetic markers were tracked over time in four types of intertidal habitats. These habitats were selected to represent natural variation in several environmental parameters, specifically the degree of physical stress experienced by barnacles. Frequencies for one allozyme locus (Gpi) and a presumably neutral mtDNA marker were homogeneous among habitats in each temporal sample. Similarly, no temporal stratification in genotype frequencies was evident across the five sampling intervals: from planktonic larvae sampled in March to juveniles collected at the end of June. In contrast to the Gpi and mtDNA loci, Mpi genotypes significantly changed in frequency in two habitats in the high intertidal zone. On exposed substrate, the Mpi-FF homozygote increased in frequency, whereas the alternative homozygote, Mpi-SS, significantly decreased in frequency. Barnacles that were protected from environmental stress at high intertidal heights by the Ascophyllum nodosum algal canopy demonstrated the opposite pattern. In both habitats, the change in frequency of the heterozygote was intermediate to that of the homozygous genotypes. Furthermore, these patterns of genotype-by-environment association reflected a pulse of genotype-specific mortality that occurred over a two-week interval subsequent to metamorphosis from the larval to the adult form. These data indicate that each Mpi homozygote is the highest fitness genotype in some portion of the intertidal environment. Using the Levene (1953) model to evaluate the spatial variation in genotypic fitness, the stable maintenance of the Mpi polymorphism is predicted under certain subsets of conditions. Environmental heterogeneity in the intertidal zone translates to spatial variation in selection pressures, which may result in the active maintenance of the Mpi polymorphism in this species.
在北方藤壶（Semibalanus balanoides）中，在四种潮间带栖息地对三个遗传标记的基因型频率随时间进行了跟踪。选择这些栖息地是为了体现几个环境参数的自然变化，特别是藤壶所经历的物理压力程度。在每个时间样本中，一个等位酶位点（Gpi）和一个可能为中性的线粒体DNA标记的频率在各栖息地之间是均匀的。同样，在五个采样间隔（从3月采集的浮游幼虫到6月底采集的幼体）中，基因型频率没有明显的时间分层。与Gpi和线粒体DNA位点不同，在高潮间带的两个栖息地中，Mpi基因型的频率发生了显著变化。在暴露的基质上，Mpi - FF纯合子的频率增加，而另一种纯合子Mpi - SS的频率则显著下降。在高潮间带高度受到墨角藻藻幕保护免受环境压力的藤壶则表现出相反的模式。在这两个栖息地中，杂合子频率的变化介于纯合基因型之间。此外，这些基因型与环境关联的模式反映了从幼虫变态为成虫形态后的两周间隔内发生的特定基因型死亡率脉冲。这些数据表明，每个Mpi纯合子在潮间带环境的某些部分是适应性最高的基因型。使用Levene（1953）模型评估基因型适应性的空间变化，预测在某些条件子集下Mpi多态性会稳定维持。潮间带的环境异质性转化为选择压力的空间变化，这可能导致该物种中Mpi多态性的积极维持。
