Morphogenesis and Evolution of the Blastopore

Resumo

The blastopore evolution is considered as a model example of the evolution of morphogenesis mechanism based on dipole interactions between the sources and sinks of epithelial sheet surface energy. The blastopore arises as a singular point of the vector field movement of a spherical surface having a zero velocity of the planar flow being surrounded by a toroidal surface (“border torus,” BT). The BT domain is the region of maximum difference in the planar surface flow velocity and, as a consequence, of maximum variability of the gastrulation movements. The evolution of gastrulation begins with the blastopore closure and continues through the formation of intercalary developmental stages that precede this closure leading to an increase in the blastopore diameter (retrograde evolution). Only three types of robust sink/source distributions at the BT surface are feasible irrespective to their phylogenetic origin. First, it is the “unilateral” gastrulation of Lophotrochozoa with the source and sinks at two opposite BT poles and two bilaterally symmetrical flows along BT. Then, it is the “bilateral” gastrulation of Ecdysozoa with two bilaterally symmetrical surface sources and two sinks at the opposite blastopore poles. Thirdly, it is the “radial” gastrulation of Deuterostomia including Chordates: one of the BT poles is a sink of the surface coming from two sources: from the outer BT surface adjacent to the sink and from the opposite pole of the BT circumference. In the evolution of the blastopore of chordates from the lancelet to amniotes, it is possible to trace the gradual replacement of gastrulation movements with pre-gastrulation cellular flows due to fixation of heterochronies, set out in the normal variability of morphogenesis. Since the variability of structures is reduced as they are formed, the evolution uses variability of earlier developmental stages.