Lynch Michael, Spitze Ken, Crease Teresa
Department of Ecology, Ethology, and Evolution, Shelford Vivarium, University of Illinois, 606 East Healey Street, Champaign, IL, 61820.
Evolution. 1989 Dec;43(8):1724-1736. doi: 10.1111/j.1558-5646.1989.tb02622.x.
In the midwestern United States the Daphnia pulex complex consists of a mosaic of sexual and asexual populations, providing a useful model system for studying the evolutionary forces underlying the maintenance of sex. One asexual and two sexual populations were surveyed for genetic variation for isozymes, mitochondrial DNA, and life-history characters. While the sexual populations exhibited substantial levels of genetic variance for fitness characters, no variation was detected in the asexual population at any level. However, a parallel survey among asexual clones derived from other ponds revealed large amounts of quantitative variation among clones, even among those with the same molecular profile. As a group, the asexuals are more variable for life histories than are the sexual populations. The molecular data indicate a relatively recent origin for the extant asexual D. pulex. The polyphyletic origin of these clones, combined with their microevolutionary potential, provides an explanation for their broad geographic distribution. The distribution of sex in the complex cannot be explained with the standard models that assume an invariant asexual population in reproductive isolation from the parental species. Although the frequency of asexuality may be driven by the spread of a sex-limited meiosis suppressor through sexual populations, the complete displacement of sexuality may be prevented by ecological distinctions between the two classes of individuals. On average, the asexuals are larger but produce smaller clutches than the sexuals.
在美国中西部，蚤状溞复合体由有性和无性种群组成，为研究维持有性生殖的进化力量提供了一个有用的模型系统。对一个无性种群和两个有性种群的同工酶、线粒体DNA和生活史特征的遗传变异进行了调查。虽然有性种群在适合度特征上表现出相当程度的遗传变异，但在无性种群的任何水平上都未检测到变异。然而，对来自其他池塘的无性克隆进行的平行调查显示，即使在具有相同分子特征的克隆之间，克隆之间也存在大量的数量变异。总体而言，无性个体在生活史上比有性种群更具变异性。分子数据表明，现存的无性蚤状溞起源相对较近。这些克隆的多系起源及其微进化潜力，为它们广泛的地理分布提供了解释。该复合体中有性生殖的分布无法用标准模型来解释，这些模型假设无性种群不变且与亲本物种生殖隔离。虽然无性生殖的频率可能由性限减数分裂抑制因子在有性种群中的传播所驱动，但两类个体之间的生态差异可能会阻止有性生殖的完全取代。平均而言，无性个体比有性个体更大，但产卵量更小。
