Microgeographic Morphophysiological Divergence in an Amazonian Soil Mite

摘要

Geographic isolation plays a major role in biological diversification. Yet, adaptive divergence also can occur with ongoing gene flow, but the minimal spatial scale required for this is unclear. Here, we hypothesized that local gradients in soil clay and water contents respectively select for anti-adherent and hypoxia-tolerant phenotypes in soil invertebrates, thereby driving intraspecific phenotypic divergence despite unlimited, passive dispersal. We tested this idea using the parthenogenetic oribatid mite Rostrozetes ovulum, an abundant species in tropical forest soils. We obtained 40 individuals from valleys and uplands within 4 km(2) of rainforest in central Amazonia, and estimated soil clay and water contents for each site. Then, we experimentally assessed submersion tolerance of each individual, measured its body size, shape and structural traits, and inferred anti-adherence from the extent of debris attached to its body. We found that morphological distance was greater between than within habitats while being independent of geographic distance, which itself was unrelated to habitat. Further, using structural equation modelling, we found that clayish soils harboured mites with fewer, larger dorsal pits that were less likely to have attached debris, consistent with an anti-adherent morphology. To a lower degree, individuals from moister soils tended to survive submersion longer, likely through anaerobiosis. These patterns could reflect phenotypic plasticity, local adaptation or some combination thereof. Altogether, they suggest that environmental gradients may trigger local-scale animal diversification in soils, contributing to the exceptional biodiversity of this substrate.