Abstract
BackgroundWith about 120 colour morphs currently assigned to six nominal species, the genus Tropheus is an ideal model to study evolutionary divergence of populations in allopatry. The morphology of Tropheus has been described as relatively static, but reproductive constraints are sexually dimorphic due to mouthbrooding in females. We analysed phenotypic variation in six populations of T. moorii and one population of T. polli using geometric morphometrics to assess morphological differences among sexes in relation to the differentiation of populations and species.ResultsThe mean shapes differed significantly between sexes, populations, and species even though within-sex variation exceeded the divergence among populations. The first principal component of Procrustes shape coordinates revealed differences between populations and species in mouth position and ventral head shape. The second principal component reflected sex-specific shape differences, mainly comprising a relatively larger female viscerocranium and, in particular, a larger buccal area. While shape variation between populations and between sexes was primarily located in the cranial region, within-sex variation was relatively uniform across all landmarks.ConclusionsDeviations of the between-population and between-sex pattern of shape variation from that within sex indicate that the differences in head shape likely result from both adaptations to female mouthbrooding and population-specific foraging strategies.
Highlights
With about 120 colour morphs currently assigned to six nominal species, the genus Tropheus is an ideal model to study evolutionary divergence of populations in allopatry
The distribution of sex-specific mean shapes in shape space was evaluated with Principal Component Analysis (PCA), called Relative Warp Analysis when applied to Procrustes shape coordinates [26]
The shape deformations depicted by the principal components are visualized by thin-plate spline (TPS) deformation grids [26], superimposed on images of a fish deformed by the same TPS functions
Summary
With about 120 colour morphs currently assigned to six nominal species, the genus Tropheus is an ideal model to study evolutionary divergence of populations in allopatry. The morphology of Tropheus has been described as relatively static, but reproductive constraints are sexually dimorphic due to mouthbrooding in females. Sexual selection typically acts on males, e.g., when females show assortative mating or when mate competition enforces selection on certain male traits. The dimorphic niche hypothesis suggests selection acting mainly on females, due to reproductive constraints [1]. Sexual dimorphism can evolve by ecological selection acting differentially on both sexes and favouring both dimorphic niches and, as a consequence, dimorphic trophic structures [1]. Bolnick and Doebeli [2] found that sexual dimorphism usually evolves more rapidly than new species by disruptive ecological selection [3,4]. In contrast, is likely to reduce sexual dimorphism because of a restricted
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