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.解析杂交带（hybrid zone）的结构，可为物种边界与物种形成（speciation）研究提供重要洞见，例如基因流（gene flow）障碍的演化历程以及选择作用的强度。在河网（river networks）系统中，由于受古今河流属性以及物种特异性生物学性状的共同影响，对杂交带内演化过程的研究往往极具挑战性。本研究提出，两性异形新热带鱼类Nematocharax venustus的两个分化支系间形成的自然杂交带，极有可能是孔特拉斯河（Contas）与帕尔多河（Pardo）流域发生河流袭夺（river capture）事件后，两支系通过次生接触（secondary contact）所形成。该推测的河流袭夺事件得到了水文地质证据（hydrogeological evidence）的支持，包括袭夺弯（elbows of capture）、风口（wind gaps）以及地质断层（geological faults）。形态学（体色模式，color pattern）与遗传学（线粒体DNA（mtDNA）和限制性位点相关DNA测序（RADseq））数据显示，干流沿岸的亲本支系间存在渐变式过渡；杂交带核心区域以F2代杂交个体占优，未检测到早期回交（backcrosses）个体，且各支流间的杂交程度存在差异。本研究强调，多源信息对于解析河网空间历史如何影响河流系统间及内部的连通性，进而影响淡水支系/物种间的基因流动态至关重要。