Assinaturas genéticas associadas à estrutura populacional em Salminus brasiliensis (Cuvier 1816) do alto e médio rio Uruguai

Abstract

The Neotropical region is recognized by its high biodiversity, from which Ostariophysian fish represent a group of wide diversifications. Among these fish, those that carry out long-distance migratory movements, that occur naturally during annual reproduction seasons in tropical basins, stand out. There is still much to investigate about how this behavior is reflected in the genetic structure of migratory freshwater fish, such as Salminus brasiliensis, a fish of economic and ecological importance popularly known as Dourado and whose natural populations have been reduced in the last years. Uruguay River represents an area subjected to intense environmental pressure caused by hydroelectrical dams, which constrain both the upstream movement to spawning areas and the passive downstream movement of egg and larvae to nursery and recruitment areas. Although the presence of dams could suggest the existence of a spatial population structure, previous genetic evidence of isolation by time suggests the hypothesis of the presence of reproductive shoals temporally different that could present genetic signatures of this isolation. Using a Genotyping-by-Sequencing (GBS) approach, Single Nucleotide Polymorphisms markers for 60 individuals of S. brasiliensis were assessed. Patterns of genetic structure were inferred using three datasets: (i) Total SNPs, (ii) putative neutral SNPs, and (iii) putative adaptive SNPs, and three sets of individuals: (i) Total individuals, (ii) Itá e (iii) Turvo, in order to test the pattern of Isolation by time described for S. brasiliensis. Clustering analysis using the total and putative neutral SNPs datasets rejected the hypothesis of isolation by time and suggest that dourados from both regions represent a panmictic population. In contrast, 121 putative adaptive SNPs evidenced four genetic clusters. These findings indicated that selection would drive a genetic differentiation between dourados from the upper and middle Uruguay river, resulting in a “Temporal divergent selection” with the formation of two potential ecotypes of S. brasiliensis: “Early” and “Late ” spawners, at the beginning (October-December) and at the end (January-February) of the reproductive season, respectively. By mapping our adaptive SNPs to an annotated genome assemble, those were identified in genes involved in functions at cellular level. Thus, it was suggested that our SNP markers exhibit an effect of “genetic hitchhiking” with genomic regions that are truly under selection, with the probable existence of “genomic islands” shaping the genomic structure of S. brasiliensis during initial stages of ecological divergence under high levels of gene flow. Overall, this study demonstrates the utility of genomic approaches by applying analysis of putative adaptive genetic variation when neutral data fail in revealing patterns of population structure in freshwater migratory fish species. Nevertheless, further studies including both phenotypic and ecological data are necessary to aid to identify management units that guarantee the conservation status of S. brasiliensis from the upper and middle Uruguay River.