Comments on biological asymmetry

Abstract

Gross forms of asymmetry of biological structure, hence of development, are generally considered abnormalities of conformation with "perfect" symmetry, whether bilateral or radial, being regarded as the "ideal" form. This notion, primarily a cultural deceit of neo-Platonic origin, can easily be shown to be wrong or at best only skin-deep by any student of anatomy or surgery who finds the heart not in the midline but, most of the time on the left, liver on the right, gut coiled and disposed in a certain direction with appendix in the right lower quadrant, and so forth. Indeed, since the beginning of Amphioxus, a major effect of evolutionary developmental modification has been the abolition of the visceral symmetry which characterized this cephalochordate with introduction of a specific pattern of asymmetry called laterality determination. This embryonic process, which is beginning to yield its universal molecular basis, is probably not responsible for another type of biological phenomenon designated fluctuating asymmetry well known to anthropologists (on the basis of quantitative studies of morphometric traits of teeth, appendicular skeleton, dermatoglyphics) and well-known to the ancients who in their most beautiful Hellenistic sculptures introduced deliberate asymmetries into facial structure and expression. Photographic images constructed of 2 right or 2 left facial halves may differ to a starling degree from the authentic face (Fig. 1). The relatively random nature of fluctuating asymmetry makes it less likely to be under strong natural selection. 1 Middle panel: Frontal view of face of a normal man. Left panel: "Artificial" face constructed out of two right halves of the same face. Right panel: Face constructed out of two left halves. A careful study of the right and left panels makes it easier to appreciate the actual degree of asymmetry present in the unaltered middle image/face. However, in addition to laterality determination and fluctuating asymmetry, there are additional forms of biological asymmetry which have other biological bases such as Lyonization, somatic/clonal mosaicism, mosaic aneuploidy/polyploidy, chimaerism, and developmental "resistance" seen with especial clarity in virtually every hereditary limb malformation. In this paper we will attempt to enumerate the causal forms and bases of biological asymmetry.