“蛭形轮虫”被认为已经以无性方式存在和分化了数百万年，这很奇怪，因为有性生殖的丧失对后生动物来说被普遍认为是走进了一条演化上的死胡同。此前人们仍怀疑它们也许偶尔会进行有性生殖。但在这项研究中，Olivier Jaillon及同事对一种名叫“Adineta vaga”的“蛭形轮虫”的基因组进行了测序，发现其结构与传统减数分裂（与有性生殖相关的细胞分裂类型）不匹配。其基因组已经历了丰富的基因转换，这可能限制了在没有减数分裂时有害突变的积累。多达8%的基因可能来自非后生动物，可能是通过横向基因转移获得的。这些发现为无性演化提供了肯定证据，支持关于“蛭形轮虫”从古以来进行无性生殖的假说。（生物谷Bioon.com）
生物谷推荐英文摘要：
Nature doi: 10.1038/nature12326
Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga
Jean-Francois Flot, Boris Hespeels, Xiang Li, Benjamin Noel, Irina Arkhipova, Etienne G. J. Danchin, Andreas Hejnol, Bernard Henrissat, Romain Koszul, Jean-Marc Aury, Valérie Barbe, Roxane-Marie Barthélémy, Jens Bast, Georgii A. Bazykin, Olivier Chabrol, Arnaud Couloux, Martine Da Rocha, Corinne Da Silva, Eugene Gladyshev, Philippe Gouret, Oskar Hallatschek, Bette Hecox-Lea, Karine Labadie, Benjamin Lejeune, Oliver Piskurek et al.
Loss of sexual reproduction is considered an evolutionary dead end for metazoans, but bdelloid rotifers challenge this view as they appear to have persisted asexually for millions of years. Neither male sex organs nor meiosis have ever been observed in these microscopic animals: oocytes are formed through mitotic divisions, with no reduction of chromosome number and no indication of chromosome pairing. However, current evidence does not exclude that they may engage in sex on rare, cryptic occasions. Here we report the genome of a bdelloid rotifer, Adineta vaga (Davis, 1873), and show that its structure is incompatible with conventional meiosis. At gene scale, the genome of A. vaga is tetraploid and comprises both anciently duplicated segments and less divergent allelic regions. However, in contrast to sexual species, the allelic regions are rearranged and sometimes even found on the same chromosome. Such structure does not allow meiotic pairing; instead, we find abundant evidence of gene conversion, which may limit the accumulation of deleterious mutations in the absence of meiosis. Gene families involved in resistance to oxidation, carbohydrate metabolism and defence against transposons are significantly expanded, which may explain why transposable elements cover only 3% of the assembled sequence. Furthermore, 8% of the genes are likely to be of non-metazoan origin and were probably acquired horizontally. This apparent convergence between bdelloids and prokaryotes sheds new light on the evolutionary significance of sex.
