| dc.description.abstract | The network of South American rivers network has experienced deep changes during
Late Miocene through Pleistocene, including major marine transgressions, Andean uplift
driving drainage changes, and changes in eustatic level. After a marine transgression during
Late Miocene in northwest South America, the ancestor of freshwater stingrays
(Potamotrygonidae) adapted to the freshwater environment. Since then, the group has
diversified while colonizing new drainage networks, given rise to new phenotypes and diet
preferences. There are currently 24 species described, a number that will certainly rise once
new species already known are described and new geographical regions are sampled.
In the present work, I present a phylogeny for the Potamotrygonidae family based on
molecular markers, with estimates of age and statistical tests of changes in rates of
diversification while also testing for the occurrence of hybridization among some species of
the family. The pattern of diversification in time and space was interpreted in light of major
paleogeographical events that shaped drainage networks in South America during the
Neogene,
Mitochondrial and nuclear data corroborate the hypothesis of family origin in
northwest South America around 25 Million years ago (MYA), after a major marine
transgression, in a time when the Andes was not a topographic barrier between the
Caribbean Sea and coastal regions. After those hydrographic basins were differentially
colonized, whereas lineages that given rise to the genera Heliotrygon and Plesiotrygon
possibly originated in the Pebas System, the lineage that gave rise to Potamotrygon probably
was restricted to the region that is now the upper Negro river/Orinoco/Essequibo. After the
inversion of the proto-Amazonas direction from East-West to West-East and reorganization of
drainages, the Potamotrygon stingrays colonized both West portions of the Amazon Basin,
previously occupied by Pebas megawetlands, and an Eastern portion, previously isolated by
the Purus Arch. Contrary to the more accepted hypothesis, the estimated speciation ages
suggest that this reorganization occurred around 3 MYA. Alternatively, the inversion of proto-
Amazonas may have occurred earlier but the Negro river basin was kept isolated from the Amazon Basin at least as long to prevent stingrays dispersion.
Following the reorganization of the drainages, a group of Potamotrygon named spotocellated
underwent an increase in speciation rate – a radiation – as new regions were
colonized. Upstream colonization of Crystalline Shields probably occurred in periods of higher
eustatic level at the end of Pliocene, followed by vicariance after reduction of water levels.The
Paraguai-lower Paraná basin was probably colonized at this same time, after headwater
capture between Paraguai and Amazon Basins driven by foredeep formation. During the
radiation, extensive hybridization took place among species of the spot-ocellated group. | |
