Simon Fraser University, Biological Sciences, Burnaby, BC V5A 1S6, Canada.
Curr Biol. 2011 Jul 12;21(13):1129-34. doi: 10.1016/j.cub.2011.05.026. Epub 2011 Jun 16.
Asexuality is rare in animals in spite of its apparent advantage relative to sexual reproduction, indicating that it must be associated with profound costs [1-9]. One expectation is that reproductive advantages gained by new asexual lineages will be quickly eroded over time [3, 5-7]. Ancient asexual taxa that have evolved and adapted without sex would be "scandalous" exceptions to this rule, but it is often difficult to exclude the possibility that putative asexuals deploy some form of "cryptic" sex, or have abandoned sex more recently than estimated from divergence times to sexual relatives [10]. Here we provide evidence, from high intraspecific divergence of mitochondrial sequence and nuclear allele divergence patterns, that several independently derived Timema stick-insect lineages have persisted without recombination for more than a million generations. Nuclear alleles in the asexual lineages displayed significantly higher intraindividual divergences than in related sexual species. In addition, within two asexuals, nuclear allele phylogenies suggested the presence of two clades, with sequences from the same individual appearing in both clades. These data strongly support ancient asexuality in Timema and validate the genus as an exceptional opportunity to attack the question of how asexual reproduction can be maintained over long periods of evolutionary time.
尽管无性繁殖相对于有性繁殖具有明显的优势，但在动物中却很少见，这表明它必然伴随着巨大的代价[1-9]。一个预期是，新的无性系获得的繁殖优势将随着时间的推移而迅速被侵蚀[3,5-7]。已经进化和适应无性繁殖的古老无性生物类群将是这一规则的“令人震惊”的例外，但通常很难排除所谓的无性生物采用某种形式的“隐性”性繁殖，或者比从与性相关的亲缘物种的分歧时间估计的更早地放弃了性繁殖[10]。在这里，我们提供了证据，证明几个独立衍生的 Timema 竹节虫谱系已经在没有重组的情况下持续了超过一百万代。在无性谱系中的核等位基因显示出比相关有性物种更高的个体内差异。此外，在两个无性生物中，核等位基因系统发育表明存在两个分支，同一个体的序列出现在两个分支中。这些数据有力地支持了 Timema 中的古老无性繁殖，并验证了该属是一个特殊的机会，可以用来解决无性繁殖如何能够在长时间的进化过程中得以维持的问题。
