Crease Teresa J, Stanton David J, Hebert Paul D N
Department of Biology, Washington University, St. Louis, MO, 63130.
Department of Biological Sciences, University of Windsor, Windsor, ON, N9B 3P4, CANADA.
Evolution. 1989 Aug;43(5):1016-1026. doi: 10.1111/j.1558-5646.1989.tb02547.x.
Allozyme studies of the cladoceran Daphnia pulex have shown that most populations reproduce by obligate parthenogenesis, although some cyclically parthenogenetic populations remain throughout the southern portion of its range. Clonal diversity within the obligate parthenogens is extremely high and has been attributed to the polyphyletic origin of asexuality. Specifically, it has been proposed that the clonal diversity in the obligate parthenogens was generated via the spread of a sex-limited meiosis suppressor through populations of a cyclically parthenogenetic ancestor. In this study, analysis of polymorphism of restriction-endonuclease sites in the mitochondrial genome, in conjunction with allozyme analysis, was used to determine whether obligate parthenogenesis has a monophyletic or polyphyletic origin in D. pulex. An allozyme survey of 77 populations from Ontario and Michigan was first conducted to determine breeding systems and levels of clonal diversity (Hebert et al., 1989). Mitochondrial-DNA variation was then surveyed in one isolate of each clone from each population reproducing by obligate parthenogenesis and in 2-4 isolates from each population reproducing by cyclic parthenogenesis. Seventeen restriction enzymes were used in this analysis. Thirty-five mitochondrial genotypes were found among the 36 obligate clones (as identified by allozyme analysis), while 17 mitochondrial genotypes were identified among 40 cyclic isolates from 14 populations. Five mitochondrial genotypes were found in both groups. Parsimony and phenetic-clustering methods were used to construct trees showing the genetic relationship among mitochondrial genotypes. The results clearly show that obligate parthenogenesis had a polyphyletic origin in this species. The close relationship between cyclic and obligate parthenogens in the Great Lakes region suggests that many obligate clones have recently been derived from cyclic populations and that the generation of clones is still occurring in this area. Patterns of clonal diversity based on the joint consideration of allozyme and mitochondrial-DNA data are discussed.
对枝角类水蚤的等位酶研究表明，尽管在其分布范围的南部仍存在一些周期性孤雌生殖的种群，但大多数种群通过专性孤雌生殖进行繁殖。专性孤雌生殖群体内的克隆多样性极高，这被归因于无性生殖的多系起源。具体而言，有人提出专性孤雌生殖群体中的克隆多样性是通过一种性别限制的减数分裂抑制因子在周期性孤雌生殖祖先的种群中传播而产生的。在本研究中，结合等位酶分析，对线粒体基因组中限制性内切酶位点的多态性进行分析，以确定专性孤雌生殖在水蚤中是单系起源还是多系起源。首先对来自安大略省和密歇根州的77个种群进行了等位酶调查，以确定繁殖系统和克隆多样性水平（赫伯特等人，1989年）。然后对每个通过专性孤雌生殖繁殖的种群中的每个克隆的一个分离株以及每个通过周期性孤雌生殖繁殖的种群中的2 - 4个分离株进行线粒体DNA变异调查。本分析使用了17种限制性酶。在36个专性克隆中（通过等位酶分析鉴定）发现了35种线粒体基因型，而在来自14个种群的40个周期性分离株中鉴定出了17种线粒体基因型。在两组中都发现了5种线粒体基因型。使用简约法和表型聚类方法构建树形图，以显示线粒体基因型之间的遗传关系。结果清楚地表明，专性孤雌生殖在该物种中具有多系起源。五大湖地区周期性和专性孤雌生殖者之间的密切关系表明，许多专性克隆最近源自周期性种群，并且该地区仍在发生克隆的产生。基于对等位酶和线粒体DNA数据的联合考虑，讨论了克隆多样性的模式。
