Paland Susanne, Colbourne John K, Lynch Michael
Department of Biology, Indiana University, Bloomington, Indiana 47405, USA.
Evolution. 2005 Apr;59(4):800-13.
Asexual taxa are short-lived, suggesting that transitions to asexuality represent evolutionary dead-ends. However, with high rates of clonal origin and coexistence of asexuals and sexuals via selective asymmetries, asexuality may persist in the long term as a result of a dynamic equilibrium between clonal origin and extinction. Few such systems have been studied in detail. Here, we investigate the evolutionary history of asexual lineages of Daphnia pulex, which are derived from sexual relatives via the inheritance of a dominant female-limited meiosis-suppressing locus and inhabit ponds throughout northeastern North America (NA). Our extensive sampling and subsequent phylogenetic analysis using mitochondrial sequence data reveals a young and genetically diverse asexual assemblage, reflecting high rates of clonal origin due to the contagious nature of asexuality. Yet, asexuality is restricted to two phylogroups (B and C) with historical and/or present associations with northeastern NA and is absent from a northwestern phylogroup (A), supporting a recent northeastern origin of asexuality in this species. Furthermore, macrogeographic patterns of genetic variability indicate that phylogroups B and C recolonized northeastern NA from opposite directions, yet their presently overlapping geographic distributions are similarly divided into an eastern asexual and a western sexual region. We attribute these patterns to a recent contagious spread of asexuality from a northeastern source. If environment-mediated selective asymmetries play no significant role in determining the outcome of competitive interactions between sexuals and asexuals, regions of contact may be setting the stage for continued asexual conquests.
无性类群寿命较短，这表明向无性状态的转变代表着进化的死胡同。然而，由于无性繁殖的高克隆起源率以及无性个体和有性个体通过选择性不对称共存，无性状态可能会由于克隆起源与灭绝之间的动态平衡而长期持续存在。很少有这样的系统得到详细研究。在这里，我们研究了蚤状溞无性谱系的进化历史，这些无性谱系是通过一个显性的雌性限制减数分裂抑制位点的遗传从有性亲属衍生而来的，并且栖息于北美东北部（NA）的各个池塘中。我们广泛的采样以及随后使用线粒体序列数据进行的系统发育分析揭示了一个年轻且遗传多样的无性组合，这反映了由于无性状态的传染性而导致的高克隆起源率。然而，无性状态仅限于与北美东北部有历史和/或当前关联的两个系统发育类群（B和C），而在西北系统发育类群（A）中不存在，这支持了该物种无性状态最近起源于东北部的观点。此外，遗传变异的宏观地理模式表明，系统发育类群B和C从相反方向重新定殖到北美东北部，然而它们目前重叠的地理分布同样被划分为东部无性区域和西部有性区域。我们将这些模式归因于无性状态最近从东北部来源的传染性传播。如果环境介导的选择性不对称在决定性个体和无性个体之间竞争相互作用的结果中不起重要作用，那么接触区域可能正在为无性状态的持续征服奠定基础。
