Riverscape properties contribute to the origin and structure of a hybrid zone in a Neotropical freshwater fish

Understanding the structure of hybrid zones provides valuable insights
about species boundaries and speciation, such as the evolution of barriers
to gene flow and the strength of selection. In river networks, studying
evolutionary processes in hybrid zones can be especially challenging,
given the influence of past and current river properties along with
biological species-specific traits. Here, we suggest that a natural hybrid
zone between two divergent lineages of the sexually dimorphic Neotropical
fish Nematocharax venustus was probably established by secondary contact
as a result of a river capture event between the Contas and Pardo river
basins. This putative river capture is supported by hydrogeological
evidence of elbows of capture, wind gaps, and geological faults. The
morphological (color pattern) and genetic (mtDNA and RADseq) variation
reveal a clinal transition between parental lineages along the main river,
with predominance of F2 hybrids at the centre of the hybrid zone, absence
of early generation backcrosses, and different levels of hybridization in
the tributaries. We highlight that different sources of information are
crucial for understanding how the riverscape spatial history influences
the connectivity between and within rivers systems and, consequently, the
dynamics of gene flow between freshwater lineages/species.