MOLECULAR HETEROCHRONY IN THE PATTERN OF FIBRONECTIN EXPRESSION DURING GASTRULATION IN AMPHIBIANS.

Abstract

Heterochrony is an evolutionary concept referring to changes in the relative timing of developmental events. Studies of heterochrony have concentrated mostly on morphology (e.g., Alberch and Alberch 1981; Emerson 1986). Because these studies examine heterochrony in the morphogenesis of adult structures, they deal with later aspects of development. A review of heterochrony by Raff and Wray (1989) stressed the lack of heterochronic studies of early (those stages in vertebrates from fertilization through neurulation), and especially molecular, aspects of development. One of the few exceptions involves differences in the timing of expression of three different epitopes (the antigenic determinants msp 130, Meso 1, and SP12) among different species of sea urchins (Wray and McClay 1989). While studies of molecular heterochrony are not new, they are infrequent (Raffet al. 1984; Parks et al. 1988; Wray and McClay 1989). Much as comparative studies on morphological aspects of early development reveal interspecific variability (Ballard 1981; Elinson 1987; del Pino 1989; Raff et al. 1991), molecular heterochronic studies demonstrate that early development is also varied at the level of molecular expression (Raff et al. 1991). A valuable system in which to study molecular heterochrony during early development is the expression of extracellular-matrix molecules on the blastocoel roof during gastrulation. This extracellular matrix is complex and consists of many different molecules, only some of which have