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Adaptive Phenotypic Plasticity in the Midas Cichlid Fish Pharyngeal Jaw and Its Relevance in Adaptive Radiation


Adaptive Phenotypic Plasticity in the Midas Cichlid Fish Pharyngeal Jaw and Its Relevance in Adaptive Radiation

Moritz Muschick et al. BMC Evol Biol.


Background: Phenotypic evolution and its role in the diversification of organisms is a central topic in evolutionary biology. A neglected factor during the modern evolutionary synthesis, adaptive phenotypic plasticity, more recently attracted the attention of many evolutionary biologists and is now recognized as an important ingredient in both population persistence and diversification. The traits and directions in which an ancestral source population displays phenotypic plasticity might partly determine the trajectories in morphospace, which are accessible for an adaptive radiation, starting from the colonization of a novel environment. In the case of repeated colonizations of similar environments from the same source population this "flexible stem" hypothesis predicts similar phenotypes to arise in repeated subsequent radiations. The Midas Cichlid (Amphilophus spp.) in Nicaragua has radiated in parallel in several crater-lakes seeded by populations originating from the Nicaraguan Great Lakes. Here, we tested phenotypic plasticity in the pharyngeal jaw of Midas Cichlids. The pharyngeal jaw apparatus of cichlids, a second set of jaws functionally decoupled from the oral ones, is known to mediate ecological specialization and often differs strongly between sister-species.

Results: We performed a common garden experiment raising three groups of Midas cichlids on food differing in hardness and calcium content. Analyzing the lower pharyngeal jaw-bones we find significant differences between diet groups qualitatively resembling the differences found between specialized species. Observed differences in pharyngeal jaw expression between groups were attributable to the diet's mechanical resistance, whereas surplus calcium in the diet was not found to be of importance.

Conclusions: The pharyngeal jaw apparatus of Midas Cichlids can be expressed plastically if stimulated mechanically during feeding. Since this trait is commonly differentiated--among other traits--between Midas Cichlid species, its plasticity might be an important factor in Midas Cichlid speciation. The prevalence of pharyngeal jaw differentiation across the Cichlidae further suggests that adaptive phenotypic plasticity in this trait could play an important role in cichlid speciation in general. We discuss several possibilities how the adaptive radiation of Midas Cichlids might have been influenced in this respect.


Figure 1
Figure 1
Induced shape differences. LPJ shape differences between the diet groups along pairwise discriminant functions depicted as interpolated outlines based on analysis of landmark coordinates. Landmark positions are shown in the upper left. Differences are exaggerated five times for illustration purposes.
Figure 2
Figure 2
Morphological separation of treatment groups. Scatterplot for the first two axes derived from a principal component analysis (PCA) of LPJ landmark data. Percentage of variance explained by the axes is given in parentheses. Note that the large overlap of convex hulls of 'ground' and 'with shell' groups is mainly brought about by two extreme individuals in the 'ground' group.
Figure 3
Figure 3
Character divergence between treatment groups. Diet group differentiation for regressed morphometric data from LPJs. Regression was either against body weight or LPJ weight. Significance levels are given in Table 2. Boxes range from the lower to the upper quartile and a bar indicates the median. The whiskers exceed the boxes by 1.5 times the inter-quartile-range of the lower or upper quartile, respectively. Notches are a rough proxy for confidence intervals of the median; if they do not overlap between two plots, the medians are most likely significantly different. They extend to +/- 1.58 inter-quartile-range divided by the square root of the number of observations from the median.

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