Permanent distortion of positional system of Xenopus embryo by brief early perturbation in gravity.

The formation of a body plan from an initially radially symmetrical egg during animal development is presumed to involve a 'positional system'and a subsequent mechanism of local response to position values provided by this system. Early gene products intimately connected with the latter response mechanism have been identified in Drosophila, and because these share conserved sequences with possible counterparts in vertebrates , there is renewed interest in understanding the physiological nature of the positional system itself in a vertebrate embryo. In the frog Xenopus, body position value appears to be specified in outline across much of the egg material by stages comprising a few cells, after only approximately 2h of development. This is already suggestive of a structural or mechanical recording system rather than a diffusion-controlled gradient. I describe here an experiment aimed at perturbing the positional system by causing gravity-driven rearrangements within eggs which conflict with their own, self-organizing rearrangements near the time of the first cleavage. The system appears to retain its full regulatory properties during only a brief time interval, so that records of positional profiles disturbed at the close of that interval are permanent, and give rise to systematically abnormal body patterns in otherwise healthy larvae. The results are inconsistent with the notion that Xenopus primary pattern results from a set of determinant 'plasms' in the egg, or from a mechanism dominated by long-range diffusion of molecules.