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

SIMPLE SUMMARY: The complete mitochondrial genome has been widely applied in the phylogenetics, population genetics and ecological research of animals due to its characteristics such as strict maternal inheritance and comparatively conserved genomic structure. Currently, the complete mitochondrial genomes of the Mysmenidae are not available. In this study, we reported the first four complete mitochondrial genomes of the Mysmenidae, including one aboveground species (Trogloneta yuensis) and three cave-dwelling species (T. yunnanense, Yamaneta kehen and Y. paquini). T. yunnanense was more similar to Yamaneta in mitogenome size than T. yuensis, possibly showing the convergent evolution of cave spiders. High variability was detected between the genera Trogloneta and Yamaneta. The phylogenetic analysis supports that Mysmenidae is a sister clade to the family Tetragnathidae. Our data and findings could enrich the gene database and contribute to the better understanding of the molecular characteristics of the family Mysmenidae, which will provide help for further studies related to the population genetics, molecular biology and phylogenetics of these spiders. ABSTRACT: 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: Trogloneta yuensis, and three cave-dwelling species: T. yunnanense, Yamaneta kehen and Y. paquini). Comparative analyses revealed that their lengths ranged from 13,771 bp (T. yuensis) to 14,223 bp (Y. kehen), 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 T. yunnanense was more similar to that of Yamaneta mitogenomes than that of T. yuensis, which might indicate the convergent evolution of cave spiders. High variability was detected between the genera Trogloneta and Yamaneta. The A + T content, the amino acid frequency of protein-coding genes (PCGs) and the secondary structures of tRNAs showed large differences. Yamaneta kehen and Y. paquini contained almost identical truncated tRNAs, and their intergenic spacers and overlaps exhibited high uniformity. The two Yamaneta species also possessed a higher similarity of start/stop codons for PCGs than the two Trogloneta species. In selective pressure analysis, compared to Yamaneta, Trogloneta 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.