Levitan D R, Irvine S D
Department of Biological Science, Florida State University, Tallahassee, 32306-1100, USA.
Evolution. 2001 Dec;55(12):2479-83. doi: 10.1111/j.0014-3820.2001.tb00762.x.
Organisms with external fertilization are often sperm limited, and in echinoids, larger eggs have a higher probability of fertilization than smaller eggs. This difference is thought to be a result of the more frequent sperm-egg collisions experienced by larger targets. Here we report how two components of egg target size, the egg cell and jelly coat, contributed to fertilization success in a selection experiment. We used a cross-sectional analysis of correlated characters to estimate the selection gradients on egg and jelly-coat size in five replicate male pairs of the sand dollar Dendraster excentricus. Results indicated that eggs with larger cells and jelly coats were preferentially fertilized under sperm limitation in the laboratory. The selection gradients were an average of 922% steeper for egg than for jelly-coat size. The standardized selection gradients for egg and jelly-coat size were similar. Our results suggest that fertilization selection can act on both egg-cell and jelly-coat size but that an increase in egg-cell volume is much more likely to increase fertilization success than an equal change in jelly-coat volume. The strengths of the selection gradients were inversely related to the correlation of egg traits across replicate egg clutches. This result suggests the importance of replication in studies of selection of correlated characters.
进行体外受精的生物往往精子受限，在海胆类动物中，较大的卵比较小的卵受精概率更高。这种差异被认为是较大目标物经历更多精卵碰撞的结果。在此我们报告在一项选择实验中，卵细胞和卵胶膜这两个卵目标大小的组成部分是如何对受精成功产生影响的。我们采用相关性状的横断面分析来估计五对重复雄性太平洋饼海胆（Dendraster excentricus）中卵和卵胶膜大小的选择梯度。结果表明，在实验室精子受限的情况下，细胞和卵胶膜较大的卵更易受精。卵大小的选择梯度平均比卵胶膜大小的选择梯度陡922%。卵和卵胶膜大小的标准化选择梯度相似。我们的结果表明，受精选择可作用于卵细胞大小和卵胶膜大小，但卵细胞体积增加比同等变化的卵胶膜体积更有可能提高受精成功率。选择梯度的强度与重复卵块中卵性状的相关性呈负相关。这一结果表明了在相关性状选择研究中重复实验的重要性。
