Husband Brian C, Barrett Spencer C H
Department of Botany, University of Toronto, Toronto, Ontario, M5S 3B2, Canada.
Evolution. 1992 Dec;46(6):1875-1890. doi: 10.1111/j.1558-5646.1992.tb01175.x.
Populations of the tristylous, annual Eichhornia paniculata are markedly differentiated with respect to frequency of mating types. This variation is associated with evolutionary changes in mating system, from predominant outcrossing to high self-fertilization. To assess the potential influence of genetic drift acting on this variation, we estimated effective population size in 10 populations from northeastern Brazil using genetic and demographic methods. Effective size (N ) was inferred from temporal changes in allele frequency at two to eight isozyme loci and also calculated using five demographic variables: 1) the number of flowering individuals (N); 2) temporal fluctuations in N; 3) variance in flower number; 4) frequency of mating types; and 5) selfing rate. Average N based on isozyme data was 15.8, range 3.4-70.6, and represented a fraction (mean N /N = 0.106) of the census number of individuals (mean N = 762.8; range: 30.5-5,040). Temporal variation in N and variance in flower number each reduced N to about a half of N whereas mating type frequencies and selfing rate caused only small reductions in N relative to N. All estimates of N based on demographic variables were considerably larger than those obtained from genetic data. The two kinds of estimates were in general agreement, however, when all demographic variables were combined into a single measure. Monte Carlo simulations indicated that effective size must be fewer than about 40 for drift to overcome the frequency-dependent selection that maintains the polymorphism for mating type. Applying the average N /N value to 167 populations censused in northeastern Brazil indicated that 72% had effective sizes below this number. This suggests that genetic drift is likely to play a dominant role in natural populations of E. paniculata.
三型花柱的一年生凤眼蓝种群在交配型频率方面存在显著差异。这种变异与交配系统的进化变化相关，从主要的异交转变为高度自交。为了评估遗传漂变对这种变异的潜在影响，我们使用遗传和人口统计学方法估计了巴西东北部10个种群的有效种群大小。有效大小（Ne）是根据两到八个同工酶位点等位基因频率的时间变化推断出来的，也使用五个人口统计学变量进行计算：1）开花个体数量（N）；2）N的时间波动；3）花数量的方差；4）交配型频率；5）自交率。基于同工酶数据的平均Ne为15.8，范围为3.4 - 70.6，占个体普查数量（平均N = 762.8；范围：30.5 - 5040）的一部分（平均Ne/N = 0.106）。Ne的时间变化和花数量的方差各自将Ne降低到约为N的一半，而交配型频率和自交率相对于N仅使Ne有小幅降低。基于人口统计学变量的所有Ne估计值都远大于从遗传数据获得的估计值。然而，当将所有人口统计学变量合并为一个单一指标时，这两种估计总体上是一致的。蒙特卡罗模拟表明，为了使漂变克服维持交配型多态性的频率依赖性选择，有效大小必须小于约40。将平均Ne/N值应用于巴西东北部普查的167个种群表明，72%的种群有效大小低于这个数字。这表明遗传漂变可能在凤眼蓝的自然种群中起主导作用。
