Poeciliid
Research

The guppy (Poecilia reticulata) represents a particularly informative model for understanding the molecular, physiological, and ecological foundations of vertebrate color vision. This mini-review synthesizes current evidence on the expanded visual opsin repertoire of guppies, retinal specialization, phototransduction, developmental and environmentally induced plasticity, and the ecological consequences of variation in visual sensitivity. Guppies possess an exceptionally diverse complement of cone opsins, including ultraviolet-, short-, middle-, and long-wavelength-sensitive classes, with pronounced molecular diversity among LWS paralogs. This diversity provides the molecular substrate for fine spectral discrimination, particularly within the red-orange region that is relevant to sexual signaling. However, the functional consequences of opsin diversity depend on their spatial expression within the retina, photoreceptor physiology, neural integration, chromophore availability, and adaptive regulation of visual sensitivity. Opsin expression changes with age, sex, illumination, and dietary carotenoid availability, demonstrating that the visual phenotype is both developmentally dynamic and environmentally responsive. The review further emphasizes that visual performance is constrained by the high metabolic demands of photoreceptors, oxidative stress, photoprotection, chromophore recycling, and retinal cellular maintenance. At the ecological level, variation in visual sensitivity is associated with mate preference, foraging behavior, predator detection, and population-level differences in sexually selected traits. Overall, guppy color vision emerges as an integrated phenotype in which molecular diversification, retinal organization, physiological homeostasis, environmental plasticity, and behavioral selection interact to shape visual information processing and fitness.

Female guppies (Poecilia reticulata) generally lack the conspicuous ornamental pigmentation characteristic of males, although the underlying genetic determinants of coloration are present in both sexes. This mini-review examines the genetic, chromosomal, hormonal, developmental, and evolutionary mechanisms underlying sex-limited pigmentation in guppies, with particular emphasis on the relationship between sex linkage, recombination, and phenotypic expression. The guppy possesses an XY sex-determination system on chromosome pair 12, and current genomic, cytogenetic, and linkage evidence indicates that its sex chromosomes remain largely homomorphic and retain substantial regions capable of X–Y recombination. Rather than forming a simple nonrecombining block, chromosome 12 exhibits a recombination gradient, with strongly reduced exchange around the sex-determining region and progressively higher recombination toward the distal pseudoautosomal region. Classical genetic studies have demonstrated extensive Y-linked inheritance of male coloration, together with X- and Y-linked loci whose expression is restricted to males and a smaller contribution from autosomal loci. The absence of ornamental coloration in females therefore reflects predominantly sex-limited and hormonally regulated gene expression rather than the absence of pigmentation genes. Experimental androgen treatment can induce male-like coloration in females, further supporting a regulatory basis for sexual dimorphism. In domesticated ornamental strains, this regulatory restriction can be altered, allowing certain dominant sex-linked color patterns to be expressed in females. The developmental formation of pigmentation involves temporally distinct pigment-cell populations and signaling pathways associated with melanophore, xanthophore, and iridophore differentiation. Finally, female mate choice and predation jointly shape the evolution and maintenance of male coloration, while variation in visual sensitivity may contribute to coevolution between female perception and male ornamental traits. Overall, guppy pigmentation provides a particularly informative model for understanding how chromosome structure, recombination, hormonal regulation, and sexual selection interact to produce sexually dimorphic phenotypes.