Cenário macroevolutivo das nuances do parasitismo em Ciliophora

Abstract

This study contributed to the understanding about the evolutionary relationships associated to parasitism and its nuances for the Ciliophora phylum. We investigated the time and mode of evolution of the symbiont ciliates(mutualists and parasites) and established hypotheses associated with the mode of evolution of the different categories (free-living, mutualism and parasitism). Using a wide taxon sampling of the 18S-rDNA gene, were performed phylogenetic and divergence time inference based on primary fossil data aiming to investigate the macroevolutionary scenario and map ancestralstates and transitions of the ecological associations across the Ciliophora phylum. The results corroborated the previously proposed hypothesis that parasitic ciliates may have arisen from free-living ciliates. However, our results add a second possible evolutionary route, which suggests that parasitic ciliates evolved through ciliates that have lower metabolic dependence. In addition, the present study suggests that symbiotic relationships may have led to an increase of the diversification rate within the phylum. When the symbiotic relationship is benefical, practiced by mutualist ciliate, the increase of the diversification rate is even greater than its cogenerics, the parasite ciliates. Indicating a unique hypothesis that explains the increase in the diversification rate for the symbiotic ciliates was not possible, due the diversity of ecological relationships found in this phylum, and the reduced number of the morphological and ecological data available. However, it is believed that changes in the macroevolutionary rate may be related to the ocupation of new environments, and consequently better conditions, to the acquisition of new characteristics, and also the type of life cycle. The present study indicates the need to apply efforts in studies of morphological nature, especially those related to the life cycle of symbiotic ciliates.