| Meiosis is a key mechanism of eukaryotes' adaptive evolution. Protozoa--a kind of single-celled eukaryotes, occupied a very important niche in the whole ecosystem.As a large group of subkingdom of protozoa, ciliates have a very high degree of differentiation, whose meiotic mechanism research could provide important clues for the study of the origin and evolution of eukaryotic meiosis.Fifteen genome data of ciliates can be acquired online, and nine of them have been published. These species were divided into two classes on taxonomy:Oligohymenophorea and Spirotrichea. Some of them are of free water life, and the other are parasitic or facultative parasitic. Moreover, there are fourteen meiotic genes were reported in Tetrahymena thermophila. These genes are involved with several processes on meiosis, including chromosome cohesion, double-strand break formation,double-stranded DNA invasion, double-strand break repair, homologous recombination, homologous chromosome separation etc. In this study, we take those fifteen sequenced ciliates as our research object, and use those genes to study the molecular evolution mechanism that based on the identification of homologous genes.The aim is to explore the evolution characteristics of meiotic genes of ciliates, and provide clues for understanding the origin and evolution of meiosis. The results were as follows:(1) Specificity of meiotic genes lost and duplication were found among variety of ciliates based on the similar research(BLASTP). Such as, T. epidokyrea duplicated the Mre11 and Sgs1 genes, Pseudocohnilembus persalinus duplicated the Hop2 and Sgs1 genes, Paramecium tetraurelia duplicated the Mre11, Rad51, Msh4 and Sgs1 genes, Oxytricha trifallax and Stylonychia lemnae duplicated the Mre11, Rad51,Hop2, Mnd1 and Sgs1 genes also. While, Ichthyophthirius multifiliis lost the Msh4,Msh5 and Mus81 genes, P. persalinus, O. trifallax and S. lemnae lost the Rec8 and Esp1 genes, P. tetraurelia lost the Dmc1 and Rec8 gene. The result of genes lost often lead to the defect of function in biology, while the effects of the genes duplication mainly embodied in two aspects. One is the doses of the compensation effect, the other one is the functional separation among two copies during the process of evolution, at some extent.(2) Homologous recombination of ciliates were lacked the components of ZMM(ZIP1, ZIP2, ZIP3, ZIP4, MSH4, MSH5 and MER3) protein complex, but relied on the function of Mus81 gene, after identified the homologous genes of ciliates and analyzed the functional and biological processes of meiotic genes. They do not form the synaptonemal complex, but form the special D-ring structure in the process of homologous recombination. Ciliate is consistent with the prevalence of a Mus81-dependent class II crossover pathway that is considered secondary in most model eukaryotes. Anyway, the class II crossover pathway have the characteristics of simple regulation network and earlier in their origin.(3) All the dN/dS values were less than one,by calculating the dN and dS values of meiotic genes through PAML. Indicating that all the meiotic genes were under the very strong purify selection during the process of evolution, and these genes were very conversed.(4) The phylogeny analysis showing some of duplicated genes were vary widely on the evolutionary distance. Such as, the Mre11 and Sgs1 genes of T. epidokyrea, the Hop2 and Sgs1 genes of Pseudocohnilembus persalinus, the Mre11, Rad51, Hop2,Mnd1 and Sgs1 of Oxytricha trifallax and Stylonychia lemnae. One reasons for this phenomenon is that the duplication events were occurred for a very long time,probably earlier than the speciation. The other reasons is that one copy of these duplicated genes were escaped from selection, resulting in the rapid evolution of genes. Moreover, due to the differential of evolution and the variety of evolution rates of meiotic genes, thus occurring some paradox development relationship between the gene trees and special tree at local. |
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