Egg shells in mites: vitelline envelope and chorion in Acaridida (Acari)

Abstract

This study deals with the formation of vitelline envelope (VE) and chorion compartments in several free living and parasitic acaridid mites. In all investigated mites, the VE is of primary origin (produced by oocyte itself), whereas exochorion material is of tertiary origin (oviduct or chorion gland secretion). In acarid mites Acarus siro and Tyrophagus perniciosus, VE formation starts with the oviductal oocytes in which vitellogenesis already proceeds. It is characterized by stratification (Acarus) or coarse fibrillar texture (Tyrophagus). Oocyte microvilli penetrating VE material were not observed. When the vitellogenesis terminates, VE becomes homogeneous and is transformed into chorion. This is the only layer protecting the deposited egg in A. siro, whereas in T. perniciosus the chorion-coated eggs passing through the distal portion of the oviduct are additionally covered by exochorion material deposited in three distinct forms: dense patches, granules, and most conspicuous locular chambers. In Tyrophagus longior, the egg surface closely resembles that of T. perniciosus, but the locular chambers are smaller. In Aleuroglyphus ovatus the exochorion material forms tiny spherical patches instead of locular chambers. In Sarcoptes scabiei, Notoedres cati and Falculifer rostratus, flocculent VE appears on vitellogenic oocytes in the oviduct. VE development is characterized by formation of numerous lenticular perivitelline spaces, which initially grow to disappear later. Then VE material transforms into fully homogeneous chorion. Chorion glands in Sarcoptes and Notoedres produce multivesicular secretory bodies; their content is released onto the egg surface to form a vesicular monolayer (exochorion) during the egg passage. The chorion gland in Falculifer is composed of two secretory cell types. Its secretion possibly glues the eggs to the host feather barb during highly ordered deposition, and forms the appendage ending with a ribbed plate, here considered to be a print of female undulate lamina acting as an ovipositor. The hatching suture is present. Neither distinct micropyle nor aeropyles have been found in eggs of species under study. The exochorion is proposed to be an adhesive layer which fixes the eggs to the substratum. The same role plays the chorion gland secretion in F. rostratus. It can be argued, however, that locular chambers of Tyrophagus exochorion may participate in reduction of water loss rather than in egg adherence or plastron respiration, as previously suggested in the literature.