Review of sexual dimorphism in brachypyline oribatid mites
Citation
Behan-Pelletier, V.M. 2015. Review of sexual dimorphism in brachypyline oribatid mites. Acarologia, 55, 127-146.
Plain language summary
Oribatid mites are major components of Canadian soils, acting as nutrient recyclers and soil health indicators. Understanding their evolution in relation to habitat preferences can improve their use as bioindicators. Sexual dimorphism (i.e. differences in the appearance of males vs females in addition to differences in the sexual organs) was studied in a group of oribatid mites (cohort Brachypylina). It appears that sexual dimorphism has evolved independently many times within the group. The evolution of strongly sexually dimorphism seems to be in response to intermittent dryness (such as that of littoral and arboreal habitats), or aquatic habitats, or spatially discrete microhabitats (such as decaying wood or fungal fruiting bodies).
Abstract
Expressions of strong sexual dimorphism have been found in 77 species of Brachypylina, representing 36 genera, in the superfamilies Gustavioidea, Ameroidea, Oppioidea, Limnozetoidea, Ameronothroidea, Licneremaeoidea, Oripodoidea, Oribatelloidea, Ceratozetoidea and Galumnoidea. There are many examples of convergences, e.g., modifications of tarsus I setae in Cosmogneta (Autognetidae), Hydrozetes (Hydrozetidae) and Erogalumna (Galumnidae), and of possible behavioural constraints, e.g., the paraxial position of modified setae in sexually dimorphic species in these genera. Similarly, there is strong convergence in position and modification of presumed secretory porose organs in species of Autogneta (Autognetidae), Mochloribatula (Mochlozetidae), Symbioribates (Symbioribatidae), Oribatella (Oribatellidae), Zachvatkinibates, Nuhivabates (Punctoribatidae), Xiphobates (Chamobatidae) and Psammogalumna (Galumnidae). The number of superfamilies with sexually dimorphic species and the range of expression of sexual dimorphism suggest multiple independent origins in Brachypylina, as congeneric species in 20 of these 36 genera do not show such modifications. Despite 1% of brachypyline species being strongly sexually dimorphic, the evidence for courtship behaviour is limited to the Galumnidae and an undescribed species of Mochloribatula (Mochlozetidae). Evolution of strongly sexually dimorphic species in Oribatida seems to be in response to intermittent dryness, or aquatic habitats, or spatially discrete microhabitats. The littoral habitat is represented by 11 species showing strong sexual dimorphism, coastal vegetation by 6, the semiaquatic by 5, dry soil by 4 species and crustose lichens by 3 species. Arguably, these 29 species and some of the 19 species reported from arboreal habitats (including lichens and moss) live in microhabitats that can be intermittently dry, with wet-dry periods of varying lengths and intensity. Seven sexually dimorphic species of Hydrozetes are found in aquatic habitats; males of these all show modifications of one or more paraxial seta on tarsus I which may be used to orient the female. The 5 sexually dimorphic species of Autogneta, and Unguizetes mauritius (Jacot) are associated with decaying wood, bark and fungal sporophores, suggesting evolution of sexual dimorphism in this spatially discrete habitat. Undoubtedly, there are many other undiscovered cases of sexual dimorphism in Brachypylina, as microhabitats where they predominantly occur are rarely studied.