Abstract
Morphological symmetry and asymmetry of three different sheep geographical populations (n=39), managed under semi-extensive conditions, were decomposed using geometric morphometric methods, on dorsal aspect of skulls. Fluctuating asymmetry (FA) was used as an indicator of environmental stress, and directional asymmetry (DA) as biomechanical constraints. The two-dimensional coordinates of 21 landmarks were digitized and analyzed using geometric morphometrics. Multivariate analyses show ed the presence of subtle but significant directional asymmetry and fluctuating asymmetry in the entire sample, but no distinctive differences were detected between populations. These results are on the whole indicative that environmental stress, if present was not sufficient to infer on symmetry of the skull, and no alterations could be noted on functional loadings. On the whole morphometric studies should open up promising areas of research in this almost unexplored field, in domestic ungulates.
Highlights
Bilateral symmetry, a key feature of vertebrate body plans, is rarely perfect, and mild asymmetries can be found in normal growth and development as a typical adaptation of the organisms to their environments
The use of Fluctuating asymmetry (FA) and directional asymmetry (DA) is not prevented for comparison, because shape variability relative to proportions of asymmetry is preserved
The effect of different sizes does not play as important a role as in traditional morphometry, because asymmetry is considered to be a property of normalized objects with respect to a sample
Summary
A key feature of vertebrate body plans, is rarely perfect, and mild asymmetries can be found in normal growth and development as a typical adaptation of the organisms to their environments. Deviations from expected perfect symmetry can occur, and organisms can develop several kinds of asymmetries, including fluctuating asymmetry (FA) and directional asymmetry (DA). The former represents small random differences between corresponding parts on the left and right side of an individual, and because of its characteristics, it is usually considered to be a measure of developmental noise (Graham et al, 1993; Palmer 1994, 2004). DA occurs whenever one side on the plane of symmetry develops more than the other side, and has a proportion of genetic component (Van Valen, 1962; Palmer and Strobeck, 1986).
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