Abstract
BackgroundInsights into the micro-evolutionary patterns of morphological traits require an assessment of the natural variation of the trait within and between populations and closely related species. The mouse mandible is a particularly suitable morphological trait for such an analysis, since it has long been used as a model to study the quantitative genetics of shape. In addition, many distinct populations, sub-species and closely related species are known for the house mouse. However, morphological comparisons among wild caught animals require an assessment in how far environmental and technical factors could interfere with the shape change measurements.ResultsUsing geometric morphometrics, we have surveyed mandible shapes in 15 natural populations of the genus Mus, with a focus on the subspecies Mus musculus domesticus. In parallel we have carefully assessed possibly confounding technical and biological factors. We find that there are distinct differences on average between populations, subspecies and species, but these differences are smaller than differences between individuals within populations. Populations from summer-dry regions, although more ancestral, are less distinct from each other than are populations from the more recently colonized northern areas. Populations with especially distinct shapes occur in an area of sympatry of M. m. domesticus and M. spretus and on recently colonized sub-antarctic islands. We have also studied a number of inbred strains to assess in how far their mandible shapes resemble those from the wild. We find that they fall indeed into the shape space of natural variation between individuals in populations.ConclusionsAlthough mandible shapes in natural populations can be influenced by environmental variables, these influences are insufficient to explain the average extent of shape differences between populations, such that evolutionary processes must be invoked to explain this level of diversity. We discuss that adaptive evolution may contribute to shape changes between populations, in particular in newly colonized areas. A comparison between inbred strains and wild mice suggests that the laboratory environment has no major systematic effect on the mandible shape and that such strains can be used as representatives of the natural shape differences between individuals.
Highlights
Insights into the micro-evolutionary patterns of morphological traits require an assessment of the natural variation of the trait within and between populations and closely related species
The problem is nowadays usually addressed within the framework of quantitative trait genetics and geometric morphometrics [1,2]
There have been a number of comparative morphometric studies in wild house mice in different contexts [15,16,17,18,19,20,21,22,23,24,25,26,27], but except for [25], all of them were limited to specific instances of shape differences between small numbers of populations or species and most of them had a taxonomic focus
Summary
Insights into the micro-evolutionary patterns of morphological traits require an assessment of the natural variation of the trait within and between populations and closely related species. While there is little doubt that purifying selection is required to maintain an optimal function of the mandible, very little is known in how far positive selection to new environmental conditions or neutral divergence processes influence mandible shape between populations and species. It is so far unknown how variation among inbred strains relates to natural variation. To lay the foundations for studying microevolutionary divergence processes of the mandible and to eventually elucidate the genetic basis of shape differences, it is necessary to carefully record wild type variation in different populations and closely related species
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