Molecular Genetic Studies of Bdelloid Rotifers

蛭形轮虫是常见的淡水无脊椎动物，分为4科、19属、约350种。本纲中不知道雄性、雌雄同体和减数分裂。卵子是由卵母细胞通过两次有丝分裂产生的，没有染色体配对，染色体数目也没有减少。对代表三个不同蛭形轮虫家族的四个不同蛭形轮虫物种的研究表明，蛭形轮虫的个体基因组包含四个基因中的每一个的高度分化的基因拷贝：hsp82，tbp，tpi和RNApolIII。这一不寻常的发现可能是由于减数分裂的长期缺乏，在此期间，以前的等位基因已经大大分化，因此细胞核不再包含紧密同源的染色体组。或者，这可能意味着，尽管形态学，细胞遗传学和分子观察相反，蛭形虫是减数分裂二倍体或多倍体，具有不寻常的基因复制偏好，偶尔和未识别的减数分裂周期，保持染色体组之间的同源性。为了确定蛭形类是否是减数分裂二倍体（或多倍体），蛭形中期染色体将通过荧光原位杂交标记，使用基因组粘粒和YAC克隆作为探针。同源染色体，如果存在的话，将通过它们的同源标记模式显示出来。虽然蛭形轮虫中没有同源染色体对表明蛭形轮虫不是减数分裂二倍体或多倍体，但这样的结果将留下它们是雌性单倍体的可能性，这是后生动物中前所未有的情况，并且合配和减数分裂发生在一些罕见和未被识别的有性阶段。这种非传统的有性生殖，以及任何其他形式的基因交换，将产生重组基因型。为了研究这种可能性，自然蛭种群将筛选线粒体基因COI和核基因hsp82和TPI之间的重组的证据，通过确定这些基因的核苷酸序列在许多个体的蛭Philodina roseola和Macrotrachela quadricornifera，收集从一个古老的冰川池塘。除了为任何形式的重组提供敏感的测试外，种群研究还将提供古代无性分类群所特有的蛭形轮虫遗传多样性的信息，将对轮虫门蛭形轮虫纲无融合生殖进化的可能性进行严格的测试，即不进行减数分裂或合配生殖。蛭形类动物无性进化的证明将挑战有性生殖对进化成功至关重要的流行观点，并将提供一个独特的系统来测试试图解释几乎普遍存在的动物和植物有性生殖维持的假设。

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.