Colourful guppy males have the highest number of offspring
Guppies have been popular among aquarium hobbyists for a long time due to their beautiful colours. These diversely coloured fish have also attracted the attention of biologists from the Max Planck Institute for Developmental Biology in Tübingen and the Bioscience and Biotechnology Centre of Nagoya University in Japan. In their recent scientific work, the team around the Ph.D. student Verena Kottler, Professor Christine Dreyer and Professor Detlef Weigel focussed on the structure of the colourful pattern of male guppies.
While guppy females are camouflaged by an inconspicuous reticulate pattern, guppy males are highly colourful, displaying iridescent, orange, and black spots and stripes. Many studies have investigated the animals’ behaviour and interactions with their environment. Previous studies found that guppy females prefer males with high amounts of orange and iridescent pigments.
The guppy (Poecilia reticulata) is a small live-bearing freshwater fish native to northeastern South America. “Nowadays, they can be found worldwide because they were released for mosquito control in many countries,” says Verena Kottler. Guppy populations have been studied most extensively on the Caribbean island of Trinidad, where guppy populations have adapted to different river habitats and vary tremendously in behaviour, life history, and male colouration. While the zebrafish (Danio rerio) is particularly suitable to investigate early development, guppies are considered an excellent example for natural variation since the 1920s, because every wild guppy male has an individual ornamental pattern.
In Trinidad, guppies live in habitats that differ in the amount and kind of predatory fish. If there are only a few predators, guppies become sexually mature later and give birth to fewer offspring. Moreover, guppy males are much more colourful because females are attracted by colourful males, which then have more offspring. If large predators are present, however, the guppy males are less colourful, as predators can spot conspicuously coloured males more easily, which hence have a lower survival rate. The guppy is therefore a species whose evolution can be directly observed in the wild.
Verena Kottler and her team asked how different pigment cell types are organised within the male ornaments, which makes it possible to further understand the genetics that underlies these colours. “We know a lot about the behaviour and ecology of the guppy, but the underlying genetic factors haven’t been investigated much so far,” states Kottler. The research team investigated the pigment cell distribution within the spots and fins of male wild-type guppies from three genetically divergent strains called Cumaná, Quare6, and Maculatus. Cumaná guppies are derived from a wild population in Venezuela. Quare6 guppies are descendants of fish from the Quare River on Trinidad, and Maculatus guppies have been bred in captivity by researchers and hobby breeders. The scientists focused on the central orange and central black spots near the gonopodium, as these two spots are present in all three strains despite their considerably different male ornaments.
The research team identified three different pigment cell types in the skin of male guppies and their experiments revealed that at least two of the three types of pigment cells contribute to each of the investigated ornamental traits. This suggests that complex interactions between different pigment cell types are necessary to form the ornamental pattern of guppy males. Also, the study demonstrated that two layers of pigment cell types, one in the dermis and the other in the hypodermis, are responsible for the colour pattern of the fish. Notably, the researchers found that the pigment cell distribution within the central orange and black spots of Cumaná, Quare6, and Maculatus males are very similar, despite Cumaná and Quare6 guppies being derived from geographically distant populations that may have been separated for almost a million years. This suggests that the pigment cell distribution within these spots is indeed conserved within the guppy and might be controlled by conserved genetic factors.