Meconial peritrophic matrix and meconial degradation in the biting midge, Culicoides variipennis (Coquillett) (Diptera: Ceratopogonidae)

Luis Renato León Villalba¹, William S Romoser², Lisa Patrican^3
1Medical Entomology Laboratory, Institute of Microbiology, School of Biological and Environmental Sciences, Universidad San Francisco de Quito, Cumbaya, Quito, Ecuador; ²Tropical Disease Institute, Department of Biomedical Sciences, College of Osteopathic Medicine, Ohio University, Athens, OH, USA; ³National Center for Medical Intelligence (NCMI), Infectious Disease Division, Fort Detrick, MD, USA

Background: The peritrophic matrix (PM), a noncellular layer surrounding the food bolus in the gut, has been described in several invertebrate phyla, including Arthropoda. In arthropod vectors, the PM may be a barrier to pathogens in a meal. In mosquito pupae, a PM forms around the sloughed larval midgut epithelial cells (meconium), and evidence suggests this meconial PM (MPM) protects the developing adult midgut epithelium from microbes ingested during the larval stage. Given the probable protective function of MPM and the possibility that it exists in taxa beyond the Culicidae, we looked for MPM in a representative of a related dipteran family, Certatopogonidae (biting midges).
Methods: One hundred thirteen mature Culicoides variipennis (Diptera: Ceratopogonidae) larvae, pupae over the entire pupal stadium, and several adults were paraffin-sectioned, stained, and examined using light microscopy.
Results: Near the end of the larval stage, the midgut epithelium sloughs into the lumen forming the meconium and a new epithelium forms from regenerative cell proliferation. The meconium gradually histolyses as indicated by shrinkage, a staining reaction change from red to blue, and loss of cellular structure. A distinct MPM often forms and persists into the adult stage. Shortly after sloughing, blue staining material accumulates around the meconium and then disappears in correlation with MPM formation.
Conclusion: Our study supports the hypothesis that MPM occurs in taxa beyond the Culicidae, in this study the Ceratopogonidae, specifically in C. variipennis. As in mosquitoes, MPM occurrence is variable, and its formation may be induced in response to a chemotactic stimulus, possibly of microbial origin. As in mosquitoes, MPM in C. variipennis appears to protect the developing adult midgut by sequestering microorganisms remaining from the larval stage. Consistent with MPM induction and microbial sequestration, we saw in a few older pupae and new adults, brownish material, possibly microbial, within the confines of a distinct MPM.

Keywords: pupa, protection of developing midgut, microbial sequestration, diptera/microbe interaction