Integrative taxonomy of Leptotrombidium species collected from murine hosts in Thailand with emphasis on L. deliense

Abstract

Leptotrombidium chiggers (larval trombiculid mites) are medically important vectors of scrub typhus caused by Orientia tsutsugamushi. We performed comparative genetic and morphometric analyses of four Leptotrombidium taxa (L. deliense, L. imphalum, L. turdicola, and Leptotrombidium cf. guzhangense), isolated from murine rodents from various localities across Thailand. Phylogenetic analysis was performed using mitochondrial and nuclear gene sequences using specimens of L. deliense from different geographical localities. Quantitative morphological variation was examined using linear measurements of the larval body (20 measurements), together with two geometric morphometric approaches applied to the scutum: an outline-based analysis of scutal contour and a landmark-based analysis of the seven scutal sensillar positions. Remarkably, geometric morphometric analyses of the scutum provided taxonomic resolution equivalent to traditional morphometry based on 20 linear measurements of the entire body. Morphometric analyses showed some concordance with genetic data but could not help definitively identify an unexpected genetic variation within L. deliense, as revealed by phylogenetic reconstruction. By integrating molecular phylogenetics with traditional and geometric morphometrics, we raised the working hypothesis of a cryptic species within the L. deliense taxon. Our approach offers an effective new framework for identification of Leptotrombidium spp. Our findings improve the identification of medically important chigger species and clarify genetic structuring within Leptotrombidium, and also enhance scrub typhus vector surveillance and control programmes.