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arXiv 0912.4472q-bio.PE
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结合无根基因树分布判别有根种树

Identifying the Rooted Species Tree from the Distribution of Unrooted Gene Trees under the Coalescent

Elizabeth S. Allman, James H. Degnan, John A. Rhodes

摘要:

基因树是代表从多个种群中取样的基因的祖先的进化树。物种树代表个体的种群--每个个体都有许多基因--分裂成新的种群或物种。聚结过程,它模拟种群内基因拷贝的祖先,经常被用来模拟给定一个固定物种树的基因树的概率分布。这个多物种结合模型提供了一个框架,遗传学家推断物种树的基因树,使用最大似然或贝叶斯方法。因为合并模型是一个随时间变化的分支过程,所以所有的树通常都被假定为在这个设置中扎根。然而,传统的系统发育方法推断的基因树往往是无根的。我们研究了多物种联合模型下无根基因树的概率。我们发现,当有4个物种,每个物种的一个基因采样,无根基因树拓扑结构的分布确定无根物种树拓扑结构和一些,但不是全部,在物种树的边缘(分支长度)的信息。在这种情况下,根在物种树上的位置是不可识别的。然而,对于5个或更多的物种,每个物种的一个基因采样,我们表明,无根基因树拓扑结构的分布确定了根物种树拓扑结构和所有的内部分支长度。对于任何一个物种,如果从其上取样了一个以上的基因,则该物种树中通向一片叶子的任何悬垂枝的长度也是可以识别的。

英文摘要:

Gene trees are evolutionary trees representing the ancestry of genes sampled from multiple populations. Species trees represent populations of individuals -- each with many genes -- splitting into new populations or species. The coalescent process, which models ancestry of gene copies within populations, is often used to model the probability distribution of gene trees given a fixed species tree. This multispecies coalescent model provides a framework for phylogeneticists to infer species trees from gene trees using maximum likelihood or Bayesian approaches. Because the coalescent models a branching process over time, all trees are typically assumed to be rooted in this setting. Often, however, gene trees inferred by traditional phylogenetic methods are unrooted. We investigate probabilities of unrooted gene trees under the multispecies coalescent model. We show that when there are 4 species with one gene sampled per species, the distribution of unrooted gene tree topologies identifies the unrooted species tree topology and some, but not all, information in the species tree edges (branch lengths). The location of the root on the species tree is not identifiable in this situation. However, for 5 or more species with one gene sampled per species, we show that the distribution of unrooted gene tree topologies identifies the rooted species tree topology and all its internal branch lengths. The length of any pendent branch leading to a leaf of the species tree is also identifiable for any species from which more than one gene is sampled.

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