Sexual dimorphism and population divergence in the Lake Tanganyika cichlid fish genus Tropheus

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.

To feed or to breed: morphological constraints of mouthbrooding in coral reef cardinalfishes

“…Sexual dimorphism of the buccal cavity appears to be widespread among mouthbrooding fishes. Males of the paternally mouthbrooding cardinalfishes [24,34] and jawfishes [37], and females of the maternally mouthbrooding cichlids [38,39], have been shown to have larger buccal cavities than the respective non-brooding sexes. This expanded buccal volume has been attributed to larger osteological elements and/or increased flexibility of soft tissues in the brooding sex [24,34,39].…”

“…Males of the paternally mouthbrooding cardinalfishes [24,34] and jawfishes [37], and females of the maternally mouthbrooding cichlids [38,39], have been shown to have larger buccal cavities than the respective non-brooding sexes. This expanded buccal volume has been attributed to larger osteological elements and/or increased flexibility of soft tissues in the brooding sex [24,34,39]. The significance of the larger buccal volume has been related to increased reproductive potential [40], and increased respiratory efficiency of both the eggs [37] and the brooding parent [41].…”

The Evolution of Sexual Dimorphism: Understanding Mechanisms of Sexual Shape Differences

“…When adults however, sexual size dimorphism was present and differed under conditions such that there is increased plasticity of the larger sex as compared to the smaller. Interestingly however, sexual shape dimorphism still did not vary with differing conditions [57]. It has been suggested for over a century that environment is a major driver of morphological differences [78,79], and new evidence such as this presents an opportunity to further understand the variables at play in the evolution of shape dimorphism.…”

“…These are then projected orthogonally onto a linear tangent space yielding Kendall's tangent space coordinates [47,51,52], which can then be treated as a set of shape variables for further analyses of shape variation and covariation with other variables [e.g., 53,54,55]. In terms of sexual shape dimorphism, dimorphism, sets of both linear measurements and geometric morphometric methods have been utilized to identify patterns of shape dimorphism in numerous taxa, including fish [56], turtles [57], birds [58][59][60][61] and lizards [62,63]. In addition to quantifying sexual shape dimorphism, identifying the potential mechanisms that generate these patterns is a current focus of many evolutionary biologists.…”

Sexual dimorphism in black jaw tiliapia (Sarotherodon melanotheron) and banded jewelfish (Hemichromis fasciatus) from the Great Kwa River, Calabar, Nigeria

“…Phenotypic diversification between sexes, also referred to as sexual morphometric dimorphism, is an ubiquitous phenomenon in animals and has been documented for a number of species both terrestrial and aquatic (Sullivan and Best, 1997;Herler et al, 2010;Nacua et al, 2010;Bhatta et al, 2012). These differences when present often manifest as structural specializations and adaptive mechanisms favouring sexual selection, dimorphic niches, food competition, locomotion, reproduction and other critical life processes of the species (Hedrick and Temeles, 1989;Bolinick and Doebeli, 2003).…”