BAC Recombineering of the Agouti Loci from Spotted Gar and Zebrafish Reveals the Evolutionary Ancestry of Dorsal–Ventral Pigment Asymmetry in Fish. Issue 7 (24th May 2017) - Cal, Laura
ABSTRACT: 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 teleosts. GRAPHICAL ABSTRACT: Countershading, 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 pattern results from spatially regulated expression of Agouti‐signaling protein (Asip). This dorso‐ventral graded distribution of asip1 gene expression is also conserved in adults of two distantly related ray‐finned fish (zebrafish and spotted gar), which shows that the dorso‐ventral patterning mechanism previously found in teleost is an ancient feature shared by other groups of ray‐finned fish and not a specific feature of teleost arising as a outcome of the teleost genome duplication (TGD). Therefore, the present study demonstrates that the molecular mechanisms leading to the dorso‐ventral pigment pattern were likely in place before the origin of teleosts.