Role of the zebrafish trilobite locus in gastrulation movements of convergence and extension

Convergence and extension are gastrulation movements that participate in th e establishment of the vertebrate body plan. Using new methods for quantify ing convergence and extension movements of cell groups, we demonstrate that in wild-type embryos, dorsal convergence of lateral cells is initially slo w, but speeds up between the end of the gastrula period and early segmentat ion. Convergence and extension movements of lateral cells in trilobite muta nts are normal during the gastrula period but reduced by early segmentation . Morphometric studies revealed that during epiboly wild-type gastrulae bec ome ovoid, whereas trilobite embryos remain rounder. By segmentation, trilo bite embryos exhibit shorter, broader embryonic axes. The timing of these m orphological defects correlates well with impaired cell movements, suggesti ng reduced convergence and extension are the main defects underlying the tr ilobite phenotype. Our gene expression, genetic, and fate mapping analyses show the trilobite mutation affects movements without altering dorsoventral patterning or cell fates. We propose that trilobite function is required f or cell properties that promote increased speed of converging cells and ext ension movements in the dorsal regions of the zebrafish gastrula. genesis 2 7: 159-173, 2000. (C) 2000 Wiley-Liss, Inc.