Two-step process for photoreceptor formation in Drosophila

The formation of photoreceptor cells (PRCs) in Drosophila serves as a paradigm for understanding neuronal determination and differentiation. During larval stages, a precise series of sequential inductive processes leads to the recruitment of eight distinct PRCs (R1-R8). But, final photoreceptor differentiation, including rhabdomere morphogenesis and opsin expression, is completed four days later, during pupal development. It is thought that photoreceptor cell fate is irreversibly established during larval development, when each photoreceptor expresses a particular set of transcriptional regulators and sends its projection to different layers of the optic lobes. Here, we show that the spalt (sal) gene complex encodes two transcription factors that are required late in pupation for photoreceptor differentiation. In the absence of the sal complex, rhabdomere morphology and expression of opsin genes in the inner PRCs R7 and R8 are changed to become identical to those of outer R1-R6 PRCs. However, these cells maintain their normal projections to the medulla part of the optic lobe, and not to the lamina where outer PRCs project. These data indicate that photoreceptor differentiation occurs as a two-step process. First, during larval development, the photoreceptor neurons become committed and send their axonal projections to their targets in the brain. Second, terminal differentiation is executed during pupal development and the photoreceptors adopt their final cellular properties.B.M. was supported by the Human Frontier Science Program Organization (HFSPO). This work was supported by grants from the National Eye Institute (NEI) to C.D., from HHMI, Research to Prevent Blindness (RPB) and the Retina Research Foundation to N.J.C., and the Dirección General de Investigación from Minesterior de Ciencia y Tecnología (MCYT) to M.D.Peer reviewe

Nosy Neighbors

Scholars argue that third parties make rational calculations and intervene to influence interstate dispute outcomes in favor of their own objectives. Third parties affect not only conflict outcomes but also escalation and duration. Theories of third-party involvement are applied to understand the dynamics of intrastate war. An analysis of event data for three Central American conflicts (El Salvador, Guatemala, and Nicaragua) from 1984 to 2001 is used to examine transnational actors’ influence on the dynamics of civil war. Findings show that transnational third parties often alter levels of cooperation among domestic adversaries, and that consistency affects the strength and direction of third-party influence. </jats:p