Abstract
Approximately two decades after the first pioneering analyses, the study of shape asymmetry with the methods of geometric morphometrics has matured and is a burgeoning field. New technology for data collection and new methods and software for analysis are widely available and have led to numerous applications in plants and animals, including humans. This review summarizes the concepts and morphometric methods for studying asymmetry of shape and size. After a summary of mathematical and biological concepts of symmetry and asymmetry, a section follows that explains the methods of geometric morphometrics and how they can be used to analyze asymmetry of biological structures. Geometric morphometric analyses not only tell how much asymmetry there is, but also provide information about the patterns of covariation in the structure under study. Such patterns of covariation in fluctuating asymmetry can provide valuable insight about the developmental basis of morphological integration, and have become important tools for evolutionary developmental biology. The genetic basis of fluctuating asymmetry has been studied from empirical and theoretical viewpoints, but serious challenges remain in this area. There are many promising areas for further research that are only little explored at present.
Highlights
Studies of fluctuating asymmetry have long primarily used measurements of lengths or perhaps angles on the left and right sides of organisms [1,2,3,4,5,6]
Studies of size measurements have found directional asymmetry only sporadically, whereas directional asymmetry for shape appears to be nearly ubiquitous in all animals that have been examined in sufficiently large studies [15]
Morphometric analyses of the shape of mature leaves in tomato and Arabidopsis showed differences in leaf asymmetry between shoots with clockwise and counter-clockwise phyllotaxis, and experiments showed that the differences in auxin concentration were responsible for such asymmetries [191]. These results provide an elegant demonstration of a mechanism that can generate antisymmetry in plant structures, provided populations contain a mix of plants with meristems spiraling in a clockwise and counter-clockwise direction
Summary
Studies of fluctuating asymmetry have long primarily used measurements of lengths or perhaps angles on the left and right sides of organisms [1,2,3,4,5,6]. Many new tools have been developed in the field of geometric morphometrics [7,8,9,10], including methods for studying asymmetry of shape [11,12,13,14]. These methods have been used increasingly for studies of fluctuating asymmetry in a wide range of organisms. It reviews the concepts and methods that underlie these studies and compiles some of the results that have emerged from them
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