Winn Alice A, Gross Katherine L
Department of Biological Science B-142, Florida State University, 32306-2043, Tallahassee, FL, USA.
W.K. Kellogg Biological Station and Department of Botany, Michigan State University, 49060, Hickory Corners, MI, USA.
Oecologia. 1993 Feb;93(1):55-62. doi: 10.1007/BF00321191.
Studies of seed-weight variation across altitudinal and latitudinal gradients have led to conflicting hypotheses regarding the selective value of this traint in relation to the length of the growing season. Growing-season length may also influence the evolution of seed number, and population differentiation in seed weight may be constrained by a negative genetic correlation between seed weight and seed number within populations. We examined variation in seed weight and an estimate of seed number (flower number) and the covariance of these traits among populations of Prunella vulgaris at five latitudes between northern Michigan and South Carolina. We measured seed weight and flower number in native habitats and in a common environment to determine the extent to which patterns observed in the field reflect genetic differentiation. We observed no genetically based variation in seed weight across the latitudinal gradient, although genetic variation among populations within a latitude was observed. In contrast to the lack of variation in seed weight, flower number increased clinally from northern Michigan to Tennessee in a common environment. Population mean flowering date in a common environment was successively later from north to south. Later-flowering individuals appear to achieve a larger size before flowering and consequently possess more resources for seed production. This difference may account for the greater flower production of late-flowering, southern populations. Independence of population mean seed weight and flower number across the latitudinal gradient suggests that population differentiation in seed weight has not been constrained by a trade-off between seed size and number within populations.
关于种子重量在海拔和纬度梯度上的变化研究，就这一性状与生长季节长度相关的选择价值产生了相互矛盾的假说。生长季节长度也可能影响种子数量的进化，并且种群内种子重量的分化可能受到种子重量与种子数量之间负遗传相关性的限制。我们研究了密歇根州北部至南卡罗来纳州五个纬度上的夏枯草种群中种子重量的变化、种子数量（花的数量）的估计值以及这些性状之间的协方差。我们在原生栖息地和共同环境中测量了种子重量和花的数量，以确定在野外观察到的模式在多大程度上反映了遗传分化。尽管在一个纬度内的种群间观察到了遗传变异，但我们未观察到种子重量在纬度梯度上存在基于遗传的变化。与种子重量缺乏变化形成对比的是，在共同环境中，花的数量从密歇根州北部到田纳西州呈渐变增加。在共同环境中，种群的平均开花日期从北到南依次变晚。开花较晚的个体在开花前似乎能长得更大，因此有更多资源用于种子生产。这种差异可能解释了开花较晚的南方种群花产量更高的原因。纬度梯度上种群平均种子重量和花的数量相互独立，这表明种群在种子重量上的分化并未受到种群内种子大小和数量之间权衡的限制。
