Giant, Krüppel, and caudal act as gap genes with extensive roles in patterning the honeybee embryo

In Drosophila, gap genes translate positional information from gradients of maternal coordinate activity and act to position the periodic patterns of pair-rule gene stripes across broad domains of the embryo. In holometabolous insects, maternal coordinate genes are fast-evolving, the domains that gap genes specify often differ from their orthologues in Drosophila while the expression of pair-rule genes is more conserved. This implies that gap genes may buffer the fast-evolving maternal coordinate genes to give a more conserved pair-rule output. To test this idea, we have examined the function and expression of three honeybee orthologues of gap genes, Krüppel, caudal, and giant. In honeybees, where many Drosophila maternal coordinate genes are missing, these three gap genes have more extensive domains of expression and activity than in other insects. Unusually, honeybee caudal mRNA is initially localized to the anterior of the oocyte and embryo, yet it has no discernible function in that domain. We have also examined the influence of these three genes on the expression of honeybee even-skipped and a honeybee orthologue of engrailed and show that the way that these genes influence segmental patterning differs from Drosophila. We conclude that while the fundamental function of these gap genes is conserved in the honeybee, shifts in their expression and function have occurred, perhaps due to the apparently different maternal patterning systems in this insect.     

A p38 MAPK-CREB pathway functions to pattern mesoderm in Xenopus

Dorsal-ventral patterning is specified by signaling centers secreting antagonizing morphogens that form a signaling gradient. Yet, how morphogen gradient is translated intracellularly into fate decisions remains largely unknown. Here, we report that p38 MAPK and CREB function along the dorsal-ventral axis in mesoderm patterning. We find that the phosphorylated form of CREB (S133) is distributed in a gradient along the dorsal-ventral mesoderm axis and that the p38 MAPK pathway mediates the phosphorylation of CREB. Knockdown of CREB prevents chordin expression and mesoderm dorsalization by the Spemann organizer, whereas ectopic expression of activated CREB-VP16 chimera induces chordin expression and dorsalizes mesoderm. Expression of high levels of p38 activator, MKK6E or CREB-VP16 in embryos converts ventral mesoderm into a dorsal organizing center. p38 MAPK and CREB function downstream of maternal Wnt/β-catenin and the organizer-specific genes siamois and goosecoid. At low expression levels, MKK6E induces expression of lateral genes without inducing the expression of dorsal genes. Loss of CREB or p38 MAPK activity enables the expansion of the ventral homeobox gene vent1 into the dorsal marginal region, preventing the lateral expression of Xmyf5. Overall, these data indicate that dorsal-ventral mesoderm patterning is regulated by differential p38/CREB activities along the axis.     

Maternal effects and temperature-sensitive period of mutations affecting embryogenesis in Caenorhabditis elegans

We have used standard tests to investigate the nature of gene expression of a new set of temperature-sensitive mutants defining 30 emb genes (essential for embryogenesis) in the nematode Caenorhabditis elegans. The mode of gene expression as determined by progeny tests for parental effects divides the genes into four classes. For 18 genes maternal gene expression is necessary and sufficient for normal embryogenesis; for 2 genes zygotic expression is necessary and sufficient; for 7 genes either maternal or zygotic expression is sufficient; for 3 genes both maternal and zygotic expression are necessary. One mutant displayed partial paternal sufficiency. The results of temperature-shift experiments define two “execution stages,” corresponding to the limits of the temperature-sensitive period (TSP), and indicate the nature and the time of action or synthesis of the gene products. Most of the maternally expressed genes have very early execution stages indicating translation before fertilization, but some are temperature sensitive late in embryogenesis. Early execution stages for 2 zygotically necessary genes demonstrate that the zygotic genome can be active in the earliest stages of embryogenesis. All taken together, the mode of gene expression, TSP, and arrest stage (terminal phenotype) allow us to classify functionally and begin to order the genes essential for embryogenesis. The results indicate a preeminent role for maternal genes and gene products in embryogenesis, in agreement with the results of others.