Process heterochronies in endochondral ossification

Heterochrony, evolutionary changes in developmental rates and timing, is a key concept in the construction of a synthesis of development and evolution . Heterochronic changes in vertebrate evolution have traditionally been ide ntified through plesiomorphic-apomorphic comparisons of bone growth. This m ethodological framework assumes that observed heterochronies are the outcom e of dissociations of developmental processes in time. Recent findings of n on-heterochronic developmental changes underlying morphological heterochron y invalidate this assumption. In this paper, a function for bone growth (at the organ level) has been mathematically deduced from the underlying devel opmental mechanisms. The temporal domain of the model spans from the time a t maximum growth rate, after the formation of growth plates, to the time at atrophy of the proliferating stratum of cells. Three organizational levels were considered: (a) cell kinetics of endochondral ossification, (b) varia tion of bone growth rates and (c) variation of accumulated bone growth with increasing age. This quantitative model provides an excellent tool to deal with the problem of the developmental basis of morphological change. I hav e modelled potential evolutionary changes on the system at different levels of biological organization. This new framework involves an epistemological shift in heterochronic analysis from a pattern-oriented inductive way to a process-oriented deductive way. The analysis of the relationships between the evolutionary alterations of endochondral ossification and the morpholog ical expression of these changes reveals that observed pattern heterochroni es can be the outcome of different process heterochronies. Moreover, I disc uss at length the heteroposic hypothesis, that evolutionary changes in the tight regulation of the amount of protein synthesized by a cell population during development would underlie acceleration or deceleration in cases of evolutionary changes in the initial number of proliferating cells at growth plates. Future research on the genetic basis of process heterochronies and heteroposies will complete our understanding of these evolutionary phenome na. (C) 2000 Academic Press.