Mitogenomic Insights Into the Ancient Split and Cryptic Diversity of the Poecilochirus carabi Complex (Acari: Mesostigmata).

Phoretic associations with insects are widespread in the Mesostigmata. These temporary symbiotic associations have been important in driving the evolutionary development of this taxon. The phoretic mite genus Poecilochirus, which is considered obligately associated with burying beetles, is of great value in forensic entomology. However, due to the morphological conservatism and the difficulty of obtaining adult specimens, the taxonomy of this genus has long been based on deutonymphal characters, and the true species diversity is far from resolved. Here we used next-generation sequencing to determine, for the first time, the complete mitochondrial genomes of P. austroasiaticus, P. subterraneus, and two distinct lineages of P. carabi (BJ1 and BJ2). All the sequenced mitogenomes contain 37 genes with a gene order identical to that of the ancestral arthropod arrangement and they all show pronounced AT bias and negative GC skew. All 13 protein-coding genes are under strong purifying selection (Ka/Ks < 1). Sliding window analysis identifies nad2 and nad5 as nucleotide diversity hotspots, which could serve as high-resolution markers for future species delimitation. Phylogenetic reconstruction tentatively recovered Poecilochirus as paraphyletic with Parasitus nested within it (ML = 44%, BI = 0.982), while Parasitinae and Pergamasinae are not reciprocally monophyletic. The cox1 genetic distance between P. carabi BJ1 and BJ2 reached 0.409, with both mPTP and ASAP analyses of the mitogenomic dataset partitioning them into separate candidate molecular operational taxonomic units. Bayesian divergence time estimation revealed that the two lineages diverged in the Eocene about 52.4 million years ago (Ma), before the split between the morphologically recognized species P. subterraneus and P. davydovae (39.0 Ma). Our results give strong molecular evidence for cryptic speciation within the P. carabi complex and provide a solid genomic foundation for the future systematic and taxonomic research in the Parasitidae.