Molecular Genetic Studies of Bdelloid Rotifers

蜈蚣类轮虫是常见的淡水无脊椎动物，隶属于4科19属350种。雄株、雌雄同体和减数分裂在该纲中都是未知的。卵是由卵母细胞通过两次有丝分裂产生的，没有染色体配对，也没有染色体数量的减少。对分别代表3个不同家族的4个不同的瓢虫物种的研究表明，蜈蚣轮虫的个体基因组包含所研究的四个基因hsp82、tbp、tpi和RNApolIII中的每一个的高度差异的基因拷贝。这一不寻常的发现可能是由于长期没有减数分裂造成的，在减数分裂过程中，以前的等位基因发生了很大的差异，因此细胞核不再包含紧密同源的染色体集。或者，这可能意味着，尽管在形态、细胞遗传学和分子上观察到了相反的情况，蜈蚣是减数分裂的二倍体或多倍体，具有不寻常的基因复制倾向，并具有偶尔的和未被识别的减数分裂周期，保持染色体组之间的同源性。为了确定蜈蚣是否为减数分裂的二倍体(或多倍体)，将以基因组粘粒和YAC克隆为探针，用荧光原位杂交技术标记蜈蚣中期染色体。同源染色体，如果存在，将通过它们的同源标记模式来揭示。尽管蜈蚣类轮虫中没有同源染色体对，这表明它们不是减数分裂的二倍体或多倍体，但这样的结果可能会留下它们是雌性单倍体的可能性，这在后生动物中是前所未有的，并且同性配子和减数分裂发生在一些罕见的和未被识别的有性时期。这种非传统的有性繁殖，以及任何其他形式的基因交换，都会产生重组的基因类型。为了研究这种可能性，我们将通过测定大量采集自古老冰川池塘的粉红瓢虫和四角大嘴蚤的核苷酸序列，来筛选天然瓢虫种群，寻找线粒体基因COI与核基因hsp82和tpi重组的证据。除了为任何形式的重组提供灵敏的测试外，种群研究还将提供可能是古代无性类群所特有的蛭形轮虫的遗传多样性的信息。将对轮虫纲轮虫纲无融合生殖的可能性进行严酷的测试，在没有减数分裂或异花配子的情况下繁殖。证明蜈蚣是无性进化的，这将挑战主流观点，即有性繁殖是进化成功的关键，并将提供一种独特的系统来检验假说，这些假说试图解释动植物中几乎普遍存在的有性繁殖维持。

Bdelloid rotifers are common fresh water invertebrates classified among 4 families, 19 genera and some 350 species. Males, hermaphrodites, and meiosis are unknown within the Class. Eggs are produced from oocytes by two mitotic divisions with no chromosome pairing and no reduction in chromosome number. Studies of four different bdelloid species, representing three different bdelloid families, have shown that individual genomes of bdelloid rotifers contain highly diverged gene copies of each of four genes examined: hsp82, tbp, tpi, and RNApolIII. This unusual finding could result from a long standing absence of meiosis during which former alleles have greatly diverged, so that nuclei no longer contain closely homologous chromosome sets. Alternatively, it could mean that despite morphological, cytogenetic and molecular observations to the contrary, bdelloids are meiotic diploids or polyploids with an unusual predilection for gene duplication and with an occasional and unrecognized meiotic cycle that maintains homology between chromosome sets. In order to determine whether bdelloids are meiotic diploids (or polyploids), bdelloid metaphase chromosomes will be labeled by fluorescent in situ hybridization, using genomic cosmid and YAC clones as probes. Homologous chromosomes, if present, will be revealed by their homologous patterns of labeling. While the absence of homologous chromosome pairs in bdelloid rotifers would show that bdelloids are not meiotic diploids or polyploids, such a result would leave the possibility that they are female haploids, a condition unprecedented among metazoans, and that syngamy and meiosis occur in some infrequent and unrecognized sexual phase. Such unconventional sexual reproduction, as well as any other form of genetic exchange, would generate recombinant genotypes. In order to investigate this possibility, natural bdelloid populations will be screened for evidence of recombination between the mitochondrial gene COI and the nuclear genes hsp82 and tpi by determining the nucleotide sequences of these genes in numerous individuals of the bdelloids Philodina roseola and Macrotrachela quadricornifera, collected from an ancient glacial pond. In addition to providing a sensitive test for any form of recombination, the population studies will provide information on the genetic diversity of bdelloid rotifers that may be unique to ancient asexual taxa.Rigorous tests of the possibility that the Class Bdelloidea of the Phylum Rotifera has evolved apomictically, reproducing without meiosis or syngamy, will be undertaken. Demonstration that bdelloids have evolved asexually would challenge the prevailing view that sexual reproduction is essential for evolutionary success and would provide a unique system to test hypothesis that seek to account for the almost universal maintenance of sexual reproduction in animals and plants.