New Insights into the Origin and Evolution of Mysmenid Spiders (Araneae, Mysmenidae) Based on the First Four Complete Mitochondrial Genomes.

The mitochondrial genome (mitogenome) is recognized as an effective molecular marker for studying molecular evolution and phylogeny. The family Mysmenidae is a group of widely distributed and covert-living spiders, of which the mitogenomic information is largely unclear. In this study, we obtained the first four complete mitogenomes of mysmenid spiders (one aboveground species: , and three cave-dwelling species: , and ). Comparative analyses revealed that their lengths ranged from 13,771 bp () to 14,223 bp (), containing a standard set of 37 genes and an A + T-rich region with the same gene orientation as other spider species. The mitogenomic size of was more similar to that of mitogenomes than that of , which might indicate the convergent evolution of cave spiders. High variability was detected between the genera and . The A + T content, the amino acid frequency of protein-coding genes (PCGs) and the secondary structures of tRNAs showed large differences. and contained almost identical truncated tRNAs, and their intergenic spacers and overlaps exhibited high uniformity. The two species also possessed a higher similarity of start/stop codons for PCGs than the two species. In selective pressure analysis, compared to , had much higher Ka/Ks values, which implies that selection pressure may be affected by habitat changes. In our study, the phylogenetic analysis based on the combination of 13 PCGs and two rRNAs showed that Mysmenidae is a sister clade to the family Tetragnathidae. Our data and findings will contribute to the better understanding of the origin and evolution of mysmenid spiders.