O boto vermelho (Inia spp.) nos Rios Madeira, Mamoré e Guaporé: distribuição, evolução e estrutura populacional

Abstract

This study aimed to evaluate the population structure and historical distribution of the boto of the Madeira, Mamoré and Guaporé Rivers. In the first chapter, I investigated if the geographic features of the upper Madeira River act as potential barriers to the distribution of botos (Inia boliviensis and I. geoffrensis). I observed that I. boliviensis occupies not only the Bolivian rivers above the rapids, but also a large part of the Madeira River until the Borba locality, 870 km below the rapids. The gene flow between populations is unidirectional downriver. The divergence time between upstream and downstream populations was estimated as approximately 122,000 years. The second chapter presented the genetic characterization of the population located between the rapids, evaluating the possibility of connectivity with the dolphins found upstream and downstream. Most individuals between the rapids were identified as I. boliviensis based on mitochondrial DNA analysis; however, the nuclear alleles of individuals below the highest waterfall (Teotônio) were characteristic of the Amazon river dolphin species (I. geoffrensis), therefore proving the occurrence of hybrids in this area. Because of the unique alleles and low migration rate I believe that the group found between the rapids evolved independently and became an evolutionary significant unit (ESU) of the Bolivian pink river dolphin. In the third chapter I investigated the occurrence of hybridization between the two species and identified which biogeographic events have led to the observed result. I found a hybrid zone of about 870 km, where individuals have mitochondrial genome of the Bolivian species and most of the nuclear genome belonging to the Amazon species. Although we can not estimate for how long the species are in contact, I fell confident to affirm that the species remained separated not due to the formation of the rapids, as has been suggested by other authors, but because of the river discontinuity during it`s formation. It is likely that the Bolivian pink river dolphin has occupied the Madeira River up to it`s confluence with Manicoré River during the Pliocene. While the formation of the rest of the river untill it s confluence with the Amazon River is recent, dated from the end of the Pleistocene. Thus, the contact between the two species is likely related to a more recent time, revealing that the geographical barrier that hided this secondary contact were not the rapids, but the formation of the Madeira River itself.