title: Rapid evolution of mammalian X-linked testis microRNAs.
abstract: BACKGROUND
  MicroRNAs (miRNAs), which are small, non-coding RNAs approximately 21-nucleotides in length, have become a major focus of research in molecular biology. Mammalian miRNAs are proposed to regulate approximately 30% of all protein-coding genes. Previous studies have focused on highly conserved miRNAs, but nonconserved miRNAs represent a potentially important source of novel functionalities during evolution.
  
  
  RESULTS
  An analysis of the chromosome distribution of miRNAs showed higher densities of miRNAs on the X chromosome compared to the average densities on autosomes in all eight mammalian species analyzed. The distribution pattern did not, however, apply well to species beyond mammals. In addition, by comparing orthologous human and mouse miRNAs, we found that X-linked miRNAs had higher substitution rates than autosomal miRNAs. Since the highest proportion of X-linked miRNAs were found in mouse testis, we tested the hypothesis that testis miRNAs are evolving faster on the X chromosome than on autosomes. Mature X-linked testis miRNAs had an average substitution rate between mouse and human that was almost 25-fold higher than mature testis miRNAs on autosomes. In contrast, for mature miRNAs with precursors not expressed in testis, no significant difference in the substitution rate between the X chromosome and autosomes was found. Among mammals, the rapid evolution of X-linked testis miRNAs was also observed in rodents and primates.
  
  
  CONCLUSIONS
  The rapid evolution of X-linked testis miRNAs implies possible important male reproductive functions and may contribute to speciation in mammals.
authors.affiliation: Laboratory of Reproductive Medicine, Department of Histology and Embryology, Nanjing Medical University, Nanjing 210029, PR China. guo_xuejiang@njmu.edu.cn
authors.forename: Xuejiang
authors.initials: X
authors.lastname: Guo
authors.forename: Bing
authors.initials: B
authors.lastname: Su
authors.forename: Zuomin
authors.initials: Z
authors.lastname: Zhou
authors.forename: Jiahao
authors.initials: J
authors.lastname: Sha
mesh_terms: D000818:Animals; D019143:Evolution, Molecular; D050172:Genes, X-Linked; D006801:Humans; D008297:Male; D051379:Mice; D035683:MicroRNAs / Q000235:genetics*; D009690:Nucleic Acid Conformation; D016415:Sequence Alignment; D017423:Sequence Analysis, RNA; D013737:Testis; D014960:X Chromosome / Q000235:genetics*
chemical_list: D035683:MicroRNAs