The variability of goat microRNA genes is strongly shaped by functional constraints

BackgroundMicroRNAs (miRNAs) are a type of small non-coding RNAs involved in the post-transcriptional repression of target mRNA transcripts, and responsible for the fine-tuning of numerous molecular mechanisms regulating cell metabolism. In goats, multiple miRNAs are involved in coordinating the expression of networks of genes with key roles on the phenotypic variation of milk and meat traits. Although a comprehensive set of goat miRNAs has been annotated, their levels of polymorphism have not been characterized yet. Such information would be relevant in order to explore the effects of miRNA variants on phenotypes of economic interest in goats. ResultsBy using whole-genome sequencing data from 770 domestic goats with African, Asian, and European origins, we have identified polymorphic sites located within miRNA genes as well as in their flanking regions. In doing so, we have found that miRNA polymorphisms are rare (median alternative allele frequency of 0.46%) and that the distribution of polymorphic sites within and around miRNA loci is uneven. Remarkably, the stem, loop and neighbouring regulatory regions of precursor miRNA hairpins show a significantly higher polymorphism density compared to the miRNA seed, which determines the binding affinity to target mRNAs. Moreover, we have detected a differential segregation of miRNA variants across and within continental regions, with an enriched segregation of putatively high impact polymorphisms, i.e. those located in the seed and other biologically relevant regions of miRNA genes, in isolated goat populations with a low census and elevated content of runs of homozygosity. ConclusionGoat miRNA genes display low levels of variation particularly in the seed region, likely due to the action of strong purifying selection removing mutations with potential effects on gene regulatory networks linked to miRNA function. Moreover, miRNA polymorphisms tend to be more abundant in goat breeds with high levels of homozygosity, likely because purifying selection is less efficient in populations of limited size. The information provided in the current work could be useful to identify miRNA polymorphisms contributing to phenotypic variation through the disruption of gene regulatory networks in domestic goats, as well as to assess their potential impact on adaptation and fitness.