Mark Welch David B, Cummings Michael P, Hillis David M, Meselson Matthew
Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biology Laboratory, 7 MBL Street, Woods Hole, MA 02543, USA.
Proc Natl Acad Sci U S A. 2004 Feb 10;101(6):1622-5. doi: 10.1073/pnas.2136686100. Epub 2004 Jan 27.
Rotifers of the asexual class Bdelloidea are unusual in possessing two or more divergent copies of every gene that has been examined. Phylogenetic analysis of the heat-shock gene hsp82 and the TATA-box-binding protein gene tbp in multiple bdelloid species suggested that for each gene, each copy belonged to one of two lineages that began to diverge before the bdelloid radiation. Such gene trees are consistent with the two lineages having descended from former alleles that began to diverge after meiotic segregation ceased or from subgenomes of an alloploid ancestor of the bdelloids. However, the original analyses of bdelloid gene-copy divergence used only a single outgroup species and were based on parsimony and neighbor joining. We have now used maximum likelihood and Bayesian inference methods and, for hsp82, multiple outgroups in an attempt to produce more robust gene trees. Here we report that the available data do not unambiguously discriminate between gene trees that root the origin of hsp82 and tbp copy divergence before the bdelloid radiation and those which indicate that the gene copies began to diverge within bdelloid families. The remarkable presence of multiple diverged gene copies in individual genomes is nevertheless consistent with the loss of sex in an ancient ancestor of bdelloids.
无性生殖的蛭形轮虫纲轮虫很特别，其已被研究的每个基因都有两个或更多不同的拷贝。对多个蛭形轮虫物种中的热休克基因hsp82和TATA框结合蛋白基因tbp进行系统发育分析表明，对于每个基因，每个拷贝都属于两个谱系之一，这两个谱系在蛭形轮虫辐射之前就开始分化。这样的基因树与两个谱系分别源自减数分裂分离停止后开始分化的前等位基因，或源自蛭形轮虫异源多倍体祖先的亚基因组的情况是一致的。然而，最初对蛭形轮虫基因拷贝分化的分析仅使用了一个外群物种，且基于简约法和邻接法。我们现在使用最大似然法和贝叶斯推断方法，并且对于hsp82基因，使用了多个外群，试图构建更可靠的基因树。在此我们报告，现有数据无法明确区分将hsp82和tbp拷贝分化的起源置于蛭形轮虫辐射之前的基因树，以及那些表明基因拷贝在蛭形轮虫家族内部开始分化的基因树。不过，单个基因组中存在多个分化的基因拷贝这一显著现象，与蛭形轮虫古老祖先中性别丧失的情况是一致的。
