Population history of a social spider reveals connection between South American tropical forests

Abstract

High levels of sociality in spiders interact with morphological traits that enhance dispersal and dispersal propensity. The social spider Anelosimus eximius occurs in tropical rainforests in South America, including in the north and mid-west Amazon, and we have recently found records of its presence in Atlantic Forest, even though the arid features of Cerrado apparently restrict contact between these two areas. In this study we investigated the population history of A. eximius using a spatiotemporal framework. Focusing on biogeographic history, we used sequences of the mitochondrial gene cytochrome oxidase subunit I (COI), to investigate population structure, diversity and time to the most recent common ancestor of the sampled individuals. We aimed to generate ecological niche models to estimate the distribution of the species from the last interglacial to the present. Our results revealed three major clusters in A. eximius (Trans-Andean, Cis-Andean, and French Guiana–Brazil) sharing common ancestors in the Late Pleistocene, c.a. 360 thousand years ago. The last interglacial period revealed a connection between A. eximius populations in Amazonia and the Atlantic forests, through the area currently occupied by the Cerrado. Dispersal over longer distances could be improved by higher rates of propagule production, growth and proliferation of new colonies. This increased dispersal capacity could be provided by wetter environments that promote prey with greater biomass, thus positively impacting the expansion of larger webs.