Flying fish as useful model for evolutionary investigations in epipelagic zones. A cytogenomic and cytotaxonomic...

Flying fishes (Exocoetidae) constitute a group of fish with adaptations for life in the epipelagic zone, notably featuring one or two pairs of expanded fins that enable gliding flight. Most of their 80 species have an interoceanic distribution, where they stand out as economic resources, as well as a key component of the trophic base of large pelagic predators. However, despite being a charismatic evolutionary model with considerable knowledge about its biology, that group remains neglected regarding its cytogenetic characterization. Here, we performed cytogenetic analyses for the first time in Hirundichthys affinis (Günther, 1866) (two Atlantic populations) and in Cheilopogon exsiliens (Linnaeus, 1771) and C. furcatus (Mitchill, 1815) (Saint Peter and Saint Paul Archipelago, mid-Atlantic region). These analyses included C-banding, silver nitrate staining (Ag-NOR), and fluorescent in situ hybridization (FISH) with 18S rDNA, 5S rDNA, and (CA)₁₅ microsatellite probes. The three species share 2n = 48 but exhibit discernible intergeneric karyotypic divergences. Thus, while the karyotype of H. affinis displays exclusively acrocentric chromosomes (48a, FN = 48), C. exsiliens (2st+46a, FN = 50) and C. furcatus (2m+46a, FN = 50) both differ due to one pair of bi-armed chromosomes. Heterochromatin shows an occasional accumulation of (CA)₁₅ clusters predominantly in pericentromeric regions of the chromosomes. The rDNA sites display variations in number and location, proving to be effective cytotaxonomic and population markers for the group. Our data indicate that the distribution of the flying fishes was accompanied by their chromosomal reorganizations. Thus, cytogenetic data stand out as promising tools for unraveling environmental adaptations, reproductive isolation, and speciation in the vast epipelagic areas they occupy.