Origin And Evolution Analysis Of MicroRNA Family Under The Recapitulation Theory In Metazoans

MicroRNA (miRNA) is a kind of endogenously expressed small noncoding RNA with transcripts ranging about22nt in length. Ever since the detection of miRNAs in1993, many studies have been undertaken of their function, structure, prediction and evolution. In metazoans, miRNA plays an important role in cell differentiation and body patterning. Thus, the occurrence and variations of miRNAs are likely to be associated with the evolution of body plans, as well as phenotypic variation within related species. Discovering the evolution process of miRNAs would not only solve the question of when and how these miRNAs appeared or change, it would also provide a basis for functional studies.Current studies on miRNA evolution usually use two types of analysis methods. The first type solves the problems from a molecular perspective using functional shifts comparisons, sequence clustering and miRNA-transcription factor-genes co-evaluation analysis. The other method uses statistics. By counting the numbers of miRNAs and comparing the same miRNA family members in different species, researchers can speculate on the evolutionary histories of certain miRNAs or miRNA families. So far, almost all studies have been based on experimental data mining. The collection of experimental data is hard and time-consuming work, and the data are usually incomplete. Newly added or replaced miRNAs frequently appear during updates of databases such as miRbase, which could influence the study of miRNA evolution when using molecular technologies such as sequencing and clustering.We decided to study miRNA evolution from the perspective of the recapitulation theory. The recapitulation theory, also called the embryological parallelism or biogenetic law, is a biological hypothesis that states that in developing from the embryo to the adult, animals go through stages resembling or representing successive stages in the evolution of their remote ancestors. More simply, this theory is based on two important notions.1. The organisms have the same origin and common ancestor; and2. Generally, higher organisms have inherited developmental stages from their lower progenitors. MiRNAs, as "micro-managers" in metazoan organisms, have very important roles in different tissues and organs. Thus, we believe that the origins of some miRNA families may be related to the evolution of tissues and organs. The origins and evolution of miRNA families may follow the recapitulation theory. We generated a list of "tissue-specific" miRNA families (Placenta-specific:miR-515family, Epididymis-specific:miR-743family and miR-891family, Liver-specific: miR-122family and Testis-specific:miR-212family) and a list of "germ-layer specific" miRNA families (Endoderm-specific:miR-192family and miR-217family, Mesoderm-specific:miR-155family, miR-187family, miR-196family, miR-205family, miR-208family, miR-449family and miR-499family, Ectoderm-specific: miR-124family, miR-128family, miR-137family, miR-219family, miR-330family, miR-454family, miR-488family, miR-542family, miR-628family and miR-9family) using human miRNA expression data, and then confirmed the origin time of these miRNA families with respect to the origin time of their specific tissues, organs or germ layers. This allowed us to construct an origin and evolution map of the miRNA families. This study aims at solving some questions on miRNA families’orgin and evolution, and give some inspration to detailed miRNA families’studies.