We propose a model in which the interaction of the Crumbs carboxyl terminus with Discs Lost organises a membrane-associated protein complex in the apical cytocortex of epithelial cells. This scaffold mediates the localisation and stabilisation of the zonula adherens component DE-cadherin, a crucial component for the maintenance of epithelial cell polarity and tissue integrity.

The correct assembly of junction components, such as E-cadherin and beta-catenin, into the zonula adherens is fundamental for the function of epithelia, both in flies and in vertebrates. In C. elegans, however, the cadherin-catenin system is not essential for general adhesion, raising the question as to the genetic basis controlling junction morphogenesis in nematodes. Here we show that dlg-1, the C. elegans homologue of the Drosophila tumour-suppressor gene discs-large, plays a crucial role in epithelial development. DLG-1 is restricted to adherens junctions of all embryonic epithelia, which contrasts with the localisation of the Drosophila and vertebrate homologues in septate and tight junctions, respectively. Proper localisation of DLG-1 requires the basolateral LET-413 protein, but is independent of the cadherin-catenin system. Embryos in which dlg-1 activity was eliminated by RNA-mediated interference fail to form a continuous belt of junction-associated antigens and arrest development. Loss of dlg-1 activity differentially affects localisation of proteins normally enriched apically to the zonula adherens. While the distribution of an atypical protein kinase C (PKC-3) and other cytoplasmic proteins (PAR-3, PAR-6) is not affected in dlg-1 (RNAi) embryos, the transmembrane protein encoded by crb-1, the C. elegans homologue of Drosophila crumbs, is no longer concentrated in this domain. In contrast to Drosophila, however, crb-1 and a second crb-like gene are not essential for epithelial development in C. elegans. Together the data indicate that several aspects of the spatial organisation of epithelial cells and its genetic control differ between flies, worms, and vertebrates, while others are conserved. The molecular nature of DLG-1 makes it a likely candidate to participate in the organisation of a protein scaffold that controls the assembly of junction components into the zonula adherens.

Drosophila imaginal disc cells can switch fates by transdetermining from one determined state to another. We analyzed the expression profiles of cells induced by ectopic Wingless expression to transdetermine from leg to wing by dissecting transdetermined cells and hybridizing probes generated by linear RNA amplification to DNA microarrays. Changes in expression levels implicated a number of genes: lamina ancestor, CG12534 (a gene orthologous to mouse augmenter of liver regeneration), Notch pathway members, and the Polycomb and trithorax groups of chromatin regulators. Functional tests revealed that transdetermination was significantly affected in mutants for lama and seven different PcG and trxG genes. These results validate our methods for expression profiling as a way to analyze developmental programs, and show that modifications to chromatin structure are key to changes in cell fate. Our findings are likely to be relevant to the mechanisms that lead to disease when homologs of Wingless are expressed at abnormal levels and to the manifestation of pluripotency of stem cells.

Defects in human germ cell development are common and yet little is known of genes required for germ cell development in men and women. The pathways that develop germ cells appear to be conserved broadly, at least in outline, in organisms as diverse as flies and humans beginning with allocation of cells to the germ cell lineage, migration of these cells to the fetal gonad, mitotic proliferation and meiosis of the germ cells, and maturation into sperm and eggs. In model organisms, a few thousand genes may be required for germ cell development including meiosis. To date, however, no genes that regulate critical steps of reproduction have been shown to be functionally conserved from flies to humans. This may be due in part to strong selective pressures that are thought to drive reproductive genes to high degrees of divergence. Here, we investigated the micro- and macro-evolution of the BOULE gene, a member of the human DAZ (deleted in azoospermia) gene family, within primates, within mammals and within metazoans. We report that sequence divergence of BOULE is unexpectedly low and that rapid evolution is not detectable. We extend the evolutionary analysis of BOULE to the level of phyla and show that a human BOULE transgene can advance meiosis in infertile boule mutant flies. This is the first demonstration that a human reproductive gene can rescue reproductive defects in a fly. These studies lend strong support to the idea that BOULE may encode a key conserved switch that regulates progression of germ cells through meiosis in men.

The Drosophila melanogaster gene Dscam (Down syndrome cell adhesion molecule) can generate thousands of different ectodomains via mutual exclusive splicing of three large exon clusters. The isoform diversity plays a profound role in both neuronal wiring and pathogen recognition. However, the isoform expression pattern at the global level remained unexplored. Here, we developed a novel method that allows for direct quantification of the alternatively spliced exon combinations from over hundreds of millions of Dscam transcripts in one sequencing run. With unprecedented sequencing depth, we detected a total of 18 496 isoforms, out of 19 008 theoretically possible combinations. Importantly, we demonstrated that alternative splicing between different clusters is independent. Moreover, the isoforms were expressed across a broad dynamic range, with significant bias in cell/tissue and developmental stage-specific patterns. Hitherto underappreciated, such bias can dramatically reduce the ability of neurons to display unique surface receptor codes. Therefore, the seemingly excessive diversity encoded in the Dscam locus might nevertheless be essential for a robust self and non-self discrimination in neurons.