The tropics as an ancient cradle of oribatid mite diversity

Diversity of most animal and plant taxa increases towards the equator. The reasons for this pattern are manifold, but their relative importance is controversial. Understanding of the radiation of animal taxa is needed to uncover the mechanisms underlying latitudinal gradients in biodiversity. Species may have evolved more quickly in tropical regions, suggesting that the tropics function as cradle of diversity, or they may have survived in the tropics for longer periods of time as compared to higher latitude biomes, suggesting that the tropics function as museum of diversity. We used oribatid mites as a model group to investigate if the high species number of this mainly soil-living taxon can be explained by the cradle or museum hypothesis. We analyzed oribatid mite communities in tropical and temperate forest ecosystems, sequenced 18S and part of the 28S rDNA of common species, and constructed phylogenetic trees using Bayesian Inference and Maximum Likelihood algorithms. Then, we mapped the distribution of species (tropical, temperate, cosmopolitan) onto the phylogenetic tree. Most tropical oribatid mite taxa formed terminal branches indicating that the high diversity of oribatid mites in tropical regions is due to recent radiation, supporting the hypothesis that the tropics function as cradle of oribatid mite diversity. Further, the results suggest that most early-derivative oribatid mite taxa are cosmopolitan indicating that they evolved on a large ancient continent. Overall, our results support the view that oribatid mites are a very old taxon which radiated intensively in tropical regions, but their origin predates the existence of the tropical regions of today.