Embryonic motility: environmental influences and evolutionary innovation.

Abstract

Embryos do not passively await hatching from their eggs or amnionic containments but begin active movement very early on in their development. The first muscle contractions in the chick embryo start on the third day of incubation and subsequently assume a characteristic pattern of increasing and decreasing motor activity (Fig. 1) (Hamburger 1963; Bekoff 1981, 1992). It is long known that embryonic motility represents an important epigenetic component of development. In vertebrates, active movement of the embryo is required for the correct development of cartilage, bone, and joints; of muscles, tendons and ligaments; and of connectivities in the central nervous system. The intrauterine disturbance of embryonic movements leads to severe malformations and functional disorders, such as pathological motor patterns and neurological deficits that are retained throughout adult life (Bos et al. 2001). It is equally known that embryonic activity depends on environmental conditions, both chemical and physical, and that changes of environmental parameters can strongly affect the motility patterns. However, the effect of such environmental influences on development, via the alteration of embryonic activity, is little explored. Even less is known about the possible evolutionary consequences of this kind of environment–development interaction. This article provides an outline of the relevance of environment dependent embryonic activity for evolutionary developmental biology.