Search

Phylogenetic comparative studies suggest that the direction of deviation from bilateral symmetry (sidedness) might evolve through genetic assimilation; however, the changes in sidedness inheritance remain largely unknown. We investigated the evolution of genital asymmetry in fish of the family Anablepidae, in which males' intromittent organ (the gonopodium, a modified anal fin) bends asymmetrically to the left or the right. In most species, males show a 1 : 1 ratio of left-to-right-sided gonopodia. However, we found that in three species left-sided males are significantly more abundant than right-sided ones. We mapped sidedness onto a new molecular phylogeny, finding that this left-sided bias likely evolved independently three times. Our breeding experiment in a species with an excess of left-sided males showed that sires produced more left-sided offspring independently of their own sidedness. We propose that sidedness might be inherited as a threshold trait, with different thresholds across species. This resolves the apparent paradox that, while there is evidence for the evolution of sidedness, commonly there is a lack of support for its heritability and no response to artificial selection. Focusing on the heritability of the left : right ratio of offspring, rather than on individual sidedness, is key for understanding how the direction of asymmetry becomes genetically assimilated.

Knowledge on the hydrological evolution of the Neotropical region was used along with the current distribution of the anablepid species to investigate the historical biogeography of this family. Areas of endemism were delimited by endemicity analysis resulting in seven individual areas of endemism and three consensus areas located in northwestern Argentina, southern Brazil, and northern South America. These areas were discussed in the context of anablepid species diversification, especially for the genus Anableps and the subgenera Jenynsia and Plesiojenynsia. The existence of areas of endemism for the family analyzed reveals an intimate association with historical events that occurred in the geological evolution of South America, which can be associated with the main diversification patterns and historical hypotheses in the context of Neotropical freshwater fishes biogeography.