| 其他摘要 | Horizontal gene transfer (HGT), which is also called lateral gene transfer, refers to movement of genetic information between more or less distantly related organisms and plays an important role in organismal evolution and genome novelty. Up to now, massive horizontal gene transfer events have been reported, and much more HGT are found in prokaryotes than in eukaryotes. HGT is still traditionally considered to be rare in multicellular eukaryotes such as animals. However, recently, many genes of miscellaneous algal origins were discovered in choanoflagellates. Considering that choanoflagellates are the existing closest relatives of animals, we speculated that ancient HGT might have occurred on the unicellular ancestor of animals and have had impact on the long-term evolution of animals. Currently, the methods of detecting and evaluating cases of HGT include phylogenetic analysis, intron analysis, base composition analysis, etc. Of all the methods, phylogenetic incongruence is the most commonly used and considered to be the gold standard for indentifying HGT. So, here, we chose phylogenetic analysis as the main approach to perform research. Through candidate screening, phylogenetic and domain analyses, we identified 14 gene families, which include 92 genes, in the tunicate Ciona intestinalis that are likely derived from miscellaneous photosynthetic eukaryotes. Almost all of these gene families are distributed in diverse animals, suggesting that they were mostly acquired by the common ancestor of animals. Their miscellaneous origins also suggest that these genes are not derived from a particular algal endosymbiont. In addition, most genes identified in our analyses are functionally related to molecule transport, cellular regulation and methylation signaling, suggesting that the acquisition of these genes might have facilitated the intercellular communication in the animal ancestor. Because eukaryotic photosynthesis is derived from a primary endosymbiosis with a cyanobacterium as well as secondary and tertiary endosymbioses with miscellaneous algae, these algal/cyanobacterial genes in eukaryotes are frequently interpreted as relicts of earlier algal/cyanobacterial endosymbionts. However, our findingsprovide additional evidence that algal genes in aplastidic eukaryotes are not exclusively derived from historical plastids and thus important for interpreting the evolution of eukaryotic photosynthesis. Most importantly, our data represent the first evidence that more anciently acquired genes might exist in animals and that ancient HGT events have played an important role in animal evolution. |
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