Gastrulation defective, a complement factor C2/B-like protease, interprets a ventral prepattern in Drosophila

gastrulation defective (gd) encodes a serine protease required for specification of dorsal-ventral cell fates during Drosophila embryogenesis. Using RNA microinjection, I show that wild-type gd RNA can restore ventrolateral pattern elements with correct polarity with respect to egg shape in embryos lacking gd function. While low RNA concentrations restore ventrolateral pattern elements, higher concentrations ventralize the embryo. Gastrulation defective concentration has a rate-limiting effect on the domain of high Dorsal concentration but little effect upon the slope of the gradient. In embryos from pipe-null females, much higher RNA concentrations generate an ectopic axis oriented with respect to the site of injection. The data suggest that the Dorsal gradient is not directly determined by asymmetric cues in the eggshell but arises de novo within the perivitelline space as a consequence of self-regulatory properties of the protease cascade. A homology to the mammalian complement factors C2 and B is also described.     

Maternal-Zygotic Gene Interactions during Formation of the Dorsoventral Pattern in Drosophila Embryos

Maternal-zygotic interactions involving the three genes dorsal (dl), twist (twi) and snail (sna) are described. The results suggest that all three are involved in the process by which the dorsoventral pattern of the Drosophila embryo is established. First, the lethal embryonic mutant phenotypes are rather similar. In homozygous twi or sna embryos invagination of the ventral presumptive mesodermal cells fails to occur, and the resulting embryos are devoid of internal organs. This is very similar to the dominant phenotype described for dl; in the case of dl, however, the effect is a maternal one dependent on the mutant genotype of the female. Second, a synergistic interaction has been found whereby dominant lethality of twi- or sna-bearing zygotes is observed in embryos derived from heterozygous dl females at high temperature. The temperature sensitivity of this interaction permitted definition of a temperature-sensitive period which is probably that of dl. This was found to extend from approximately 12 hr prior to oviposition to 2–3 hr of embryogenesis. A zygotic action for the dl gene in addition to the maternal effect was revealed by the finding that extra doses of dl⁺ in the zygotes can partially rescue the dominant lethality of heterozygous twi embryos derived from heterozygous dl females. Two possible interpretations of the synergism are considered: (1) twi and sna are activated in the embryos as a result of positional signals placed in the egg as a consequence of the functioning of the dl gene during oogenesis and, thus, play a role in embryonic determination. (2) The gene products of dl⁺ and twi ⁺ (or sna⁺) combine to produce a functional molecule that is involved in the specification of dorsoventral pattern in the early embryo.     

The maternal NF-kappaB/dorsal gradient of Tribolium castaneum: dynamics of early dorsoventral patterning in a short-germ beetle

In the long-germ insect Drosophila melanogaster dorsoventral polarity is induced by localized Toll-receptor activation which leads to the formation of a nuclear gradient of the rel/ NF-kappaB protein Dorsal. Peak levels of nuclear Dorsal are found in a ventral stripe spanning the entire length of the blastoderm embryo allowing all segments and their dorsoventral subdivisions to be synchronously specified before gastrulation. We show that a nuclear Dorsal protein gradient of similar anteroposterior extension exists in the short-germ beetle, Tribolium castaneum, which forms most segments from a posterior growth zone after gastrulation. In contrast to Drosophila, (i) nuclear accumulation is first uniform and then becomes progressively restricted to a narrow ventral stripe, (ii) gradient refinement is accompanied by changes in the zygotic expression of the Tribolium Toll-receptor suggesting feedback regulation and, (iii) the gradient only transiently overlaps with the expression of a potential target, the Tribolium twist homolog, and does not repress Tribolium decapentaplegic. No nuclear Dorsal is seen in the cells of the growth zone of Tribolium embryos, indicating that here dorsoventral patterning occurs by a different mechanism. However, Dorsal is up-regulated and transiently forms a nuclear gradient in the serosa, a protective extraembryonic cell layer ultimately covering the whole embryo.     

sog and dpp exert opposing maternal functions to modify toll signaling and pattern the dorsoventral axis of the Drosophila embryo

The short gastrulation (sog) and decapentaplegic (dpp) genes function antagonistically in the early Drosophila zygote to pattern the dorsoventral (DV) axis of the embryo. This interplay between sog and dpp determines the extent of the neuroectoderm and subdivides the dorsal ectoderm into two territories. Here, we present evidence that sog and dpp also play opposing roles during oogenesis in patterning the DV axis of the embryo. We show that maternally produced Dpp increases levels of the I(kappa)B-related protein Cactus and reduces the magnitude of the nuclear concentration gradient of the NF(kappa)B-related Dorsal protein, and that Sog limits this effect. We present evidence suggesting that Dpp signaling increases Cactus levels by reducing a signal-independent component of Cactus degradation. Epistasis experiments reveal that sog and dpp act downstream of, or in parallel to, the Toll receptor to reduce translocation of Dorsal protein into the nucleus. These results broaden the role previously defined for sog and dpp in establishing the embryonic DV axis and reveal a novel form of crossregulation between the NF(kappa)B and TGF(beta) signaling pathways in pattern formation.