Role of the zebrafishtrilobite locus in gastrulation movements of convergence and extension

Abstract

Convergence and extension are gastrulation movements that participate in the establishment of the vertebrate body plan. Using new methods for quantifying convergence and extension movements of cell groups, we demonstrate that in wild-type embryos, dorsal convergence of lateral cells is initially slow, but speeds up between the end of the gastrula period and early segmentation. Convergence and extension movements of lateral cells in trilobite mutants are normal during the gastrula period but reduced by early segmentation. Morphometric studies revealed that during epiboly wild-type gastrulae become ovoid, whereas trilobite embryos remain rounder. By segmentation, trilobite embryos exhibit shorter, broader embryonic axes. The timing of these morphological defects correlates well with impaired cell movements, suggesting reduced convergence and extension are the main defects underlying the trilobite 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 for cell properties that promote increased speed of converging cells and extension movements in the dorsal regions of the zebrafish gastrula.

Initiation of convergence and extension movements of lateral mesoderm during zebrafish gastrulation

“…We previously observed that lateral mesodermal cells (80% epiboly, 8.4 hpf) migrated primarily dorsally, frequently with substantial animal or vegetal bias and showed quite variable net dorsal speed (Jessen et al, 2002). Finally, our previous studies in which lateral mesodermal cells labeled at early gastrulation were visualized at early segmentation stages provided another hint of difference in path orientation along the AnVg axis of the mesoderm (Sepich et al, 2000;Topczewski et al, 2001). The labeled cell group moved toward the dorsal midline, forming a broken line along the animal-vegetal (or anterior-posterior) axis.…”

“…1AB) showed that between shield stage and 80% epiboly (6.0 -8.4 hpf), labeled lateral cell populations converge slowly, about 15°in 2.5 h, while undergoing moderate extension (Sepich et al, 2000;Myers et al, 2002). In these experiments, one-cell embryos were injected with caged fluorescein dextran.…”

“…Afterward, the paths of lateral cells become biased toward dorsal. These cell behaviors provide a mechanism for the slow convergence of lateral mesoderm observed at the start of gastrulation (Sepich et al, 2000). In addition, cells closer to the animal pole bias their movement animally and cells closer to the blastoderm margin and vegetal pole bias their paths dorsally or dorsovegetally.…”

“…To identify genes regulating the initiation of convergence movements of lateral mesodermal cells, we analyzed convergence movements in embryos with defects in two pathways involved in C&E. Embryos in which function of Stat3 was downregulated using antisense morpholino oligonucleotides translation interference (Stat3 MO) had populations of labeled lateral cells show reduced convergence by midgastrulation (Yamashita et al, 2002), while embryos homozygous for trilobite m209 (tri) or knypek m119 (kny) null mutations show reduced convergence at late gastrulation (Sepich et al, 2000;Topczewski et al, 2001;Jessen et al, 2002). We analyzed kny or tri mutants and Stat3-MO-injected embryos to learn when lateral convergence becomes apparent.…”

“…Between the onset of gastrulation (6 h post fertilization, hpf) and midgastrulation (80% epiboly, 8.4 hpf), lateral mesodermal cell groups converge slowly toward dorsal, while undergoing extension in the animal to vegetal (AnVg) direction. Later, the rate of convergence increases (Sepich et al, 2000;Myers et al, 2002). These observations raised several questions.…”