Science2013-12-15 7:05 AM

The Genome of the Ctenophore Mnemiopsis leidyi and Its Implications for Cell Type Evolution 梳状栉水母的基因组和其对细胞类型演变的意义

Abstract An understanding of ctenophore biology is critical for reconstructing events that occurred early in animal evolution. Toward this goal, we have sequenced, assembled, and annotated the genome of the ctenophore Mnemiopsis leidyi. Our phylogenomic analyses of both amino acid positions and gene content suggest that ctenophores rather than sponges are the sister lineage to all other animals. Mnemiopsis lacks many of the genes found in bilaterian mesodermal cell types, suggesting that these cell types evolved independently. The set of neural genes in Mnemiopsis is similar to that of sponges, indicating that sponges may have lost a nervous system. These results present a newly supported view of early animal evolution that accounts for major losses and/or gains of sophisticated cell types, including nerve and muscle cells. 对栉水母具有生物学上的理解是重构动物进化早期所发生事件的关键。为了实现这个目标,我们已经对梳状栉水母的基因组进行了排序组装和标注。其中对氨基酸位点和基因含量进行的分子系统分析表明比起海绵动物类,栉水母类和其他动物有更接近的姊妹血缘关系。双侧对称的中胚层细胞类型中具有的许多基因都是梳状栉水母缺失的,这表明这些细胞类型是独立进化出来的。梳状栉水母的神经基因系统和海绵动物的相似,这表明海绵动物可能在进化中失去了一套神经系统。这些研究结果表明了最近备受支持的有关早期动物进化的观点,该观点认为动物进化早期导致了包括神经和肌肉细胞在内的复杂细胞类型的丢失或增加。 Introduction An understanding of ctenophore biology is critical for reconstructing events that occurred early in animal evolution. The phylogenetic relationship of ctenophores (comb jellies) to other animals has been a source of long-standing debate. Until recently, it was thought that Porifera (sponges) was the earliest diverging animal lineage, but recent reports have instead suggested Ctenophora as the earliest diverging animal lineage. Because ctenophores share some of the same complex cell types with bilaterians (such as neural and mesodermal cells), the phylogenetic position of ctenophores affects how we think about the early evolution of these cell types. 介绍 对栉水母具有生物学上的理解是重构动物进化早期所发生事件的关键。梳状栉水母与其他动物间的亲缘关系一直是长期争论的根源。一直以来多孔动物(海绵类动物)被认为是动物分化最古老的代表,直到最近,新的研究发现声明栉水母(梳状栉水母)才具有动物分化最古老的血统。因为栉水母与两侧对称动物具有部分相同的复杂细胞类型(比如神经细胞和中胚层细胞),栉水母的系统进化地位让我们改变了对这些细胞类型早期进化的看法。 Methods We have sequenced, annotated, and analyzed the 150-megabase genome of the ctenophore Mnemiopsis leidyi. We have performed detailed phylogenetic analyses on these new data using both sequence matrices and information on gene content. We conducted extensive genomic inventories on signaling pathway components and genes known to be critical to neural and mesodermal cell types, among others. 方法 我们已经对梳状栉水母的150兆碱基基因组进行了排序,标注和分析。通过使用有关基因含量的序列矩阵和信息,我们对这些新数据进行了具体的系统发育分析。我们也对相关信号传导构件和基因进行了广泛的基因编制清查。相对于其他细胞类型来说,这些信号传导构件和基因对神经和中胚层细胞类型尤其关键。 Results Our phylogenetic analyses suggest that ctenophores are the sister group to the rest of the extant animals. We find that the sets of neural components present in the genomes of Mnemiopsis and the sponge Amphimedon queenslandica are quite similar, suggesting that sponges have the necessary genetic machinery for a functioning nervous system but may have lost these cell types. We also find that, although Mnemiopsis has most of the genes coding for structural components of mesodermal cells, they lack many of the genes involved in bilaterian mesodermal specification and, therefore, may have independently evolved these cell types. 结果 我们所做的系统发育分析表明栉水母是现存动物的姊妹类群。我们还发现梳状水母和大堡礁海绵的基因中有非常相似的神经组成成分,这表明海绵动物具有保证神经系统正常运作的必要遗传机制,但很可能已经失去了这些细胞类型。同时我们也发现,虽然梳状水母具有中胚层细胞的结构性构成成分中的大部分基因编码,但他们仍然缺少许多对中胚层双侧对称起规定作用的基因,因此他们可能单独进化出了这些细胞类型。 Discussion These results present a newly supported view of early animal evolution that accounts for major losses and/or gains of sophisticated cell types, including nerve and muscle cells. This evolutionary framework, along with the comprehensive genomic resources made available through this study, will yield myriad discoveries about our most distant animal relatives, many of which will shed light not only on the biology of these extant organisms but also on the evolutionary history of all animal species, including our own. 讨论 这些结果呈现了一个最近备受支持的有关动物进化早期导致包括神经和肌肉细胞在内的复杂细胞类型的主要丢失和增加的观点。伴随着通过该研究获得的综合基因资源,这个进化构架将催生出有关我们最遥远的动物远亲的无数发现,他们中的许多将不光清楚显示出现存有机物的生物学意义,也将为包括人类本身在内的全体动物物种进化历史提供宝贵线索。

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