Schwander Tanja, Crespi Bernard J
Department of Biological Sciences, Simon Fraser University, Burnaby BC.
Evolution. 2009 Jan;63(1):84-103. doi: 10.1111/j.1558-5646.2008.00524.x. Epub 2008 Sep 18.
Transitions from sexual reproduction to parthenogenesis may occur along multiple evolutionary pathways and involve various cytological mechanisms to produce diploid eggs. Here, we investigate routes to parthenogenesis in Timema stick insects, a genus comprising five obligate parthenogens. By combining information from microsatellites and karyotypes with a previously published mitochondrial phylogeny, we show that all five parthenogens likely evolved spontaneously from sexually reproducing species, and that the sexual ancestor of one of the five parthenogens was probably of hybrid origin. The complete maintenance of heterozygosity between generations in the five parthenogens strongly suggests that eggs are produced by apomixis. Virgin females of the sexual species were also able to produce parthenogenetic offspring, but these females produced eggs by automixis. High heterozygosity levels stemming from conserved ancestral alleles in the parthenogens suggest, however, that automixis has not generated the current parthenogenetic Timema lineages but that apomixis appeared abruptly in several sexual species. A direct transition from sexual reproduction to (at least functional) apomixis results in a relatively high level of allelic diversity and high efficiency for parthenogenesis. Because both of these traits should positively affect the demographic success of asexual lineages, spontaneous apomixis may have contributed to the origin and maintenance of asexuality in Timema.
从有性生殖到孤雌生殖的转变可能沿着多条进化途径发生，并且涉及各种细胞学机制以产生二倍体卵。在此，我们研究了Timema竹节虫的孤雌生殖途径，该属包含五个专性孤雌生殖物种。通过将微卫星和核型信息与先前发表的线粒体系统发育相结合，我们表明所有五个孤雌生殖物种可能都是从有性生殖物种自发进化而来的，并且五个孤雌生殖物种之一的有性祖先可能是杂交起源。五个孤雌生殖物种在代与代之间完全保持杂合性，这强烈表明卵是通过无融合生殖产生的。有性物种的未交配雌虫也能够产生孤雌生殖后代，但这些雌虫通过自体融合产生卵。然而，孤雌生殖物种中源于保守祖先等位基因的高杂合性水平表明，自体融合并未产生当前的孤雌生殖Timema谱系，而是无融合生殖在几个有性物种中突然出现。从有性生殖直接转变为（至少是功能性的）无融合生殖会导致相对较高水平的等位基因多样性和孤雌生殖的高效率。由于这两个特征都应该对无性谱系的种群成功产生积极影响，因此自发的无融合生殖可能有助于Timema中无性状态的起源和维持。
