The complete mitochondrial genome of Histiostoma blomquisti (Acari: Histiostomatidae)

The mite Histiostoma blomquisti is a microorganism feeder that uses the red imported fire ant (Solenopsis invicta) as a phoretic carrier for dispersal. We sequenced the H. blomquisti mitogenome using next-generation sequencing methods. The circular mitogenome of H. blomquisti is 15,892 bp and is composed of 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNAs, and 6 non-coding regions >100 bp. Most tRNAs are highly reduced, like those found in other Acariformes. Phylogenetic analysis based on the concatenated nucleotide sequence of the 13 protein-coding genes supports Histiostomatid mites forming the basal-most lineage in Astigmata.

Gonads in Histiostoma mites (Acariformes: Astigmata): structure and development

The development of male and female gonads in arrhenotokous and thelytokous species of Histiostoma was studied using transmission electron microscopy (TEM). All instars were examined: larvae, protonymphs, facultative heteromorphic deutonymphs (=hypopi), tritonymphs, and adults. In testis primordium, spermatogonia surrounding a testicular central cell (TCC) with a gradually enlarging, branched nucleus are present already at the larval stage. Spermatogonia and the TCC are connected via narrow, tubular intercellular bridges revealing that the TCC is a germline cell. Spermatocytes appear at the protonymphal stage. At the heteromorphic deutonymph stage, the testis primordium is similar to that of the protonymph, but in the tritonymph it is much larger and composed as in the adult: spermatids as well as sperm cells are present. The latter are congregated ventrally in the testis at the entrance of the deferent duct. In the larval ovary, an eccentrically located ovarian nutritive cell (ONC) is surrounded by oogonia which are connected with the ONC via tubular intercellular bridges. In later stages, the ovary grows and oocytes appear in the protonymph. Meiotic synaptonemal complexes in oocytes occur from the tritonymph stage. At about the time of the final molting, tubular intercellular bridges transform into peculiar diaphragm-crossed bridges known only in Histiostoma mites. In the adult female, growing oocytes at the end of previtellogenesis lose intercellular bridges and move ventro-laterally to the ovarian periphery towards the oviduct entrance. Vitellogenesis occurs in oviducts. Germinal cells in both the testis and ovary are embedded in a few somatic stroma cells which may be well discernible already in the larval ovary; in the testis, somatic stroma cells are evident not earlier than the end of the tritonymphal stage. The ovary has a thin wall of flat somatic cells, whereas the testis is covered by a basal lamina only. The obtained results suggest that gonads in Histiostoma and other Astigmata originate from two primordial cells only.