The mechanisms underlying nuptial colouration in the white Threespine Stickleback (Gasterosteus aculeatus)

Abstract

Often the most striking displays of colour in animals arise from interactions during mating. Nuptial colouration is exclusively associated with mating and typically functions in either female mate choice by signalling male quality, or male competition by asserting dominance and possession of a territory. Animal colouration is the result of various chromatophores found within the integument. Changes in the number and distribution of chromatophores and pigments over time causes seasonal colour change associated with nuptial colouration. The Threespine Stickleback (Gasterosteus aculeatus) is a teleost fish found throughout the northern hemisphere that displays large variation in nuptial colouration among populations. The white Threespine Stickleback, endemic to Nova Scotia, displays a pearlescent white colour during breeding season in contrast to the common Threespine Stickleback which is a dark green/brown colour. Although Threespine Sticklebacks are a model system for many evolutionary studies, the mechanisms involved in the evolution of the contrasting nuptial colouration of the white and common males are not well understood. The purpose of this study was to determine the cellular mechanisms underlying the differences in nuptial colouration of these ecotypes. In particular, I looked at both melanophore and iridophore coverage as well as melanophore number and percent of isolated iridophores in the two dermal chromatophore layers of stickleback. My results indicate that common males have a higher melanophore coverage and fewer isolated iridophores in the deep dermal layer than the white males. However, the number of melanophores and the iridophore coverage in each layer, as well as the surface melanophore coverage, did not significantly differ between the two groups. These finding indicate that the integumental chromatophores in these males are quite morphologically similar in number and distribution. Future studies should investigate the thickness of chromatophores and the orientation and spacing of platelet crystals within iridophores among white and common stickleback.