Archiv für Entwicklungsmechanik der Organismen

The primary sex determination signal in Drosophila melanogaster, the ratio of X chromosomes to autosomes, sets the activity state of the switch gene, Sex-lethal (Sxl), by regulating the establishment promoter, m-Sxl-Pe. We have identified and characterized the establishment promoter, v-Sxl-Pe, of the distantly related species Drosophila virilis. Like melanogaster, the virilis Sxl-Pe is organized into four sub-domains: the Sxl-Pe mRNA leader and exon E1 of Sxl protein, the core promoter, the sex-specific element and the augmentation element. The core promoter and sex-specific element of v-Sxl-Pe show considerable sequence similarity to m-Sxl-Pe and contain target sites for components of the X/A signaling system. While the augmentation element of v-Sxl-Pe also has sequence motifs that could function as target sites for the X/A signaling system, it shows little similarity to the melanogaster augmentation element. Functional studies reveal that v-Sxl-Pe drives sex-specific expression in D. melanogaster embryos and that the activity of the virilis promoter is controlled by known components of the melanogaster X/A counting system. Although v-Sxl-Pe responds appropriately to the melanogaster sex determination signal, it is less active than Sxl-Pe from melanogaster. Unexpectedly, the reduced activity is due to differences in the activity of the conserved core promoter, while the non-conserved augmentation element functions effectively. These findings suggest that low-affinity target sites for the X/A counting system are critical for the functioning of Sxl-Pe.

Drosophila Ftz-F1 is an orphan nuclear receptor required for segmentation and metamorphosis. Its mammalian orthologs, SF-1 and LRH-1, function in sexual development and homeostasis, and have been implicated in stem cell pluripotency maintenance and tumorigenesis. These NR5A family members bind DNA as monomers and strongly activate transcription. However, controversy exists as to whether their activity is regulated by ligand-binding. Structural evidence suggested that SF-1 and human LRH-1 bind regulatory ligands, but mouse LRH-1 and Drosophila FTZ-F1 are active in the absence of ligand. We found that Dm-Ftz-F1 and mLRH-1, thought not to bind ligand, or mSF-1 and hLRH-1, predicted to bind ligand, each efficiently rescued the defects of Drosophila ftz-f1 mutants. Further, each correctly activated expression of a Dm-Ftz-F1 target gene in Drosophila embryos. The functional equivalence of ftz-f1 orthologs in these sensitive in vivo assays argues against specific activating ligands for NR5A family members.

We have isolated cDNA clones from a Xenopus laevis embryo library that encode a predicted translation product of 342 amino acids containing a signal sequence for secretion. The predicted protein has 62–70% amino acid identity with the Xenopus oocyte cortical granule lectin (XCGL), the mouse intelectin, the human HL-1/intelectin and HL-2. Onset of gene expression occurs by gastrulation, and the transcripts localize in non-ciliated epidermal cells all over the tailbud embryos. The results suggest that the molecule, designated XEEL (Xenopus embryonic epidermal lectin), is a novel XCGL family molecule secreted from the embryonic epidermis.

The RYK subfamily of receptor tyrosine kinases is characterised by unusual, but highly conserved, amino acid substitutions in the kinase domain. The linotte/derailed gene encodes a Drosophila RYK subfamily member involved in embryonic and adult central nervous system development. Previous studies have shown that the kinase activity of this receptor is not required in vivo for its embryonic function. In this study, we have investigated the role of the cytoplasmic domain and the kinase activity of the linotte/derailed receptor tyrosine kinase in adult brain development. Our results indicate that these domains are not essential for adult brain development but they are required for the proper regulation of the activity of this receptor. This sheds light on a regulatory role for the kinase activity of a RYK subfamily member.