Abstract
The effect of the environment may result in different developmental outcomes. Extrinsic signals can modify developmental pathways and result in alternative phenotypes (phenotypic plasticity). The environment can also be interpreted as a stressor and increase developmental instability (developmental noise). Directional and fluctuating asymmetry provide a conceptual background to discriminate between these results. This study aims at assessing whether variation in dentition and shape of pharyngeal arches of the clonal fish Chrosomus eos-neogaeus results from developmental instability or environmentally induced changes. A total of 262 specimens of the Chrosomus eos-neogaeus complex from 12 natural sites were analysed. X-ray microcomputed tomography (X-ray micro-CT) was used to visualize the pharyngeal arches in situ with high resolution. Variation in the number of pharyngeal teeth is high in hybrids in contrast to the relative stability observed in both parental species. The basal dental formula is symmetric while the most frequent alternative dental formula is asymmetric. Within one lineage, large variation in the proportion of individuals bearing basal or alternative dental formulae was observed among sites in the absence of genetic difference. Both dentition and arch shape of this hybrid lineage were explained significantly by environmental differences. Only individuals bearing asymmetric dental formula displayed fluctuating asymmetry as well as directional left-right asymmetry for the arches. The hybrids appeared sensitive to environmental signals and intraspecific variation on pharyngeal teeth was not random but reflects phenotypic plasticity. Altogether, these results support the influence of the environment as a trigger for an alternative developmental pathway resulting in left-right asymmetry in dentition and shape of pharyngeal arches.
Highlights
The phenotype of an organism is the product of developmental processes and depends on the interactions among genetic, epigenetic and environmental factors
This study aims at assessing whether variation of the dentition and the shape of pharyngeal arches result from developmental instability or phenotypic plasticity
A total of seven distinct dental formulae were detected amongst the 262 individuals (Fig 2)
Summary
The phenotype of an organism is the product of developmental processes and depends on the interactions among genetic, epigenetic and environmental factors. The survival of an individual is intimately dependent on the production of a consistent phenotype according to a specified environmental condition, a capacity defined as developmental stability [1, 2]. Organisms are not impervious to random perturbations and developmental instability refers to the deviation from the expected phenotype within a given environment [3, 4]. Measuring developmental instability represents a proximate way to assess the capacity of organisms to deal with different environments. In organisms with bilateral symmetry, the use of bilateral homologous structures across the leftright axis of symmetry offers an accurate way to estimate developmental instability as the development of such structures is expected to be influenced by the same environment and genotype
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