BAC recombineering of the agouti loci from spotted gar and zebrafish reveals the evolutionary ancestry of dorsal-ventral pigment asymmetry in fish

Dorsoventral pigment patterning, characterized by a light ventrum and a dark dorsum, is one of the most widespread chromatic adaptations in vertebrate body coloration. In mammals, this countershading depends on differential expression of agouti‐signaling protein (ASIP), which drives a switch of synthesis of one type of melanin to another within melanocytes. Teleost fish share countershading, but the pattern results from a differential distribution of multiple types of chromatophores, with black–brown melanophores most abundant in the dorsal body and reflective iridophores most abundant in the ventral body. We previously showed that Asip1 (a fish ortholog of mammalian ASIP) plays a role in patterning melanophores. This observation leads to the surprising hypothesis that agouti may control an evolutionarily conserved pigment pattern by regulating different mechanisms in mammals and fish. To test this hypothesis, we compared two ray‐finned fishes: the teleost zebrafish and the nonteleost spotted gar (Lepisosteus oculatus). By examining the endogenous pattern of asip1 expression in gar, we demonstrate a dorsoventral‐graded distribution of asip1 expression that is highest ventrally, similar to teleosts. Additionally, in the first reported experiments to generate zebrafish transgenic lines carrying a bacterial artificial chromosome (BAC) from spotted gar, we show that both transgenic zebrafish lines embryos replicate the endogenous asip1 expression pattern in adult zebrafish, showing that BAC transgenes from both species contain all of the regulatory elements required for regular asip1 expression within adult ray‐finned fishes. These experiments provide evidence that the mechanism leading to an environmentally important pigment pattern was likely in place before the origin of teleostsGrant sponsor: Spanish Economy and Competitiveness Ministry; Grant numbers: AGL2011-23581 and AGL2014-52473R; Grant sponsor: NIH; Grant num-bers: R01 OD011116 (alias R01 RR020833) and R24 OD01119004; Grant sponsor: Alexander von Humboldt Foundation; Grant sponsor: Volkswagen Foundation Initiative Evolutionary Biology; Grant number: I/84 815.Peer reviewe