Competition among female cichlids has led to colour polymorphism among Neochromis omnicaeruleus from Lake Victoria, according to a recent study published in the Proceedings of the Royal Society B: Biological Sciences.
Peter Dijkstra, Ole Seehausen and Ton Groothuis studied aggression bias in a polymorphic population of Neochromis omnicaeruleus from Makobe Island in the western Speke Gulf (Tanzania), in which three distinct female colour morphs (one plain brown and two blotched) coexist.
Females of algae-scraping cichlids such as Neochromis omnicaeruleus typically occupy feeding territories, which they defend against other females.
The authors kept wild-caught test females in aquaria, which were then presented with combinations of pairs of stimulus females of different colour morphs to test for own-type aggression bias.
The stimulus females were individually kept in transparent watertight Perspex tubes before being introduced to the tank containing the test females.
They then recorded the number of attacks of the test female to each of the stimulus females for 5 minutes starting directly after introduction of the stimulus females, with an attack defined as biting or butting at the walls of the tube containing the stimulus female.
The authors then conducted a similar experiment using the female offspring of laboratory-generated crosses between plain males and blotched females (the blotch gene is linked on the x chromosome, and both plain and blotched daughters are produced from such a cross).
The authors found that wild-caught females preferentially attacked females of their own colour morph, whereas no such aggression bias was displayed by the offspring of the laboratory-bred fish.
According to the authors, Ecological niche partitioning between the three morphs of N. omnicaeruleus is absent or at least not known"the morphs have fully overlapping microdistributions and trophic morphologies...Hence, ecological resource competition is unlikely to have led to the evolution of female intrasexual aggression bias towards own-coloured individuals.
However, there is preferential mate choice among the different colour morphs, with males and females both exhibiting a strong preference for mating with members of their own colour morphs.
Mutual mate choice leaves females of the same morph stronger competitors for males than females of different morphs. Intrasexual competition of this nature might have led to the evolution of the observed aggression biases.
Over time, such selective mating would lead to symaptric speciation via the development of reproductive barriers and phenotypic divergence between colour morphs.
The experiments with the laboratory-generated crosses demonstrated no genetic linkage between ccolour and aggression bias.
This data also suggest that ...morph hybrids experience reduced fitness, because they lack an aggression bias towards their most direct competitors for mates.
For more information, see the paper: Dijkstra, PD, O Seehausen and TGG Groothuis (2008) Intrasexual competition among females and the stabilization of a conspicuous colour polymorphism in a Lake Victoria cichlid fish. Proceedings of the Royal Society B: Biological Sciences 275, pp. 519"526.