First Bisexually Dimorphic Phytoptid Taxon (Eriophyoidea, Phytoptidae) from Gondwanian Angiosperm Host

Acariform mites of the superfamily Eriophyoidea are permanent parasites of higher vascular plants. Seasonal morphological dimorphism in females has been documented across various eriophyoid taxa, while male dimorphism remains poorly understood. In this study, we analyzed morphological, molecular, and biological data from the genus <i>Austracus</i> Keifer 1944, with a particular focus on the type species, <i>A. havrylenkonis</i> Keifer 1944, associated with <i>Nothofagus</i>. Using new material collected from Chile and Argentina, we demonstrated that this species exhibits two distinct forms of both males and females, making it the first known bisexually dimorphic taxon within the family Phytoptidae. The summer form of <i>A. havrylenkonis</i> displays the unstable annulation of the dorsal opisthosoma, characterized by a significant variation in the number of thin, microtuberculated dorsal annuli interspersed among the broader, plate-like annuli typical of the winter form. This finding aligns with the previous observations of atypical deuterogyny in Eriophyoidea and leads us to hypothesize that gall mites employ diverse adaptive strategies-manifesting as either gradual or discrete morphological changes-to cope with seasonal environmental fluctuations. Investigating the genetic mechanisms underlying these adaptive strategies, along with further studies of eriophyoids associated with <i>Nothofagus</i> in the Southern Hemisphere, represents a promising direction for future research.