Gonad coalescence involves the concerted movements of germ cells and SGPs as they transition from a broad association of cells into a condensed and organized gonad. Coalescence

During development, complex organs are formed from individual cells and simple tissues. Organogenesis is characterized by many cellular behaviors, including cell migration, modification of cell contacts, changes in cell shape, cell proliferation and cell death. Extensive communication between cells ensures that different cell types act together in a coordinated manner to form a properly patterned, functional organ. Gonad formation provides an excellent model for studying organogenesis. The germ cells and specialized somatic cells that make up the gonad are initially located far apart from one another, and must undergo a series of morphogenic movements to come together and create the proper gonad architecture. This process is crucial for continued germ cell development, gametogenesis and the propagation of a species. In Drosophila, germ cells are formed at the posterior pole during the syncytial blastoderm stage of embryogenesis. Gastrulation brings the germ cells to the interior of the embryo, after which they actively migrate through the midgut epithelium and into the mesoderm (reviewed by Starz-Gaiano and Lehmann, 2001). They then make contact with specialized mesodermal cells with which they form the gonad (Sonnenblick, 1950), known as somatic gonadal precursors (SGPs) (Boyle et al., 1997). SGPs are specified in bilateral clusters within parasegments (PS) 10, 11 and 12 (Brookman et al., 1992), and can be identified by their expression of the nuclear proteins Eyes Absent (EYA) (Boyle et al., 1997) and ZFH-1 (Broihier et al., 1998). They arise within the eve domain of the dorsolateral mesoderm, where groups of cells are selected to become either SGPs or fat body. The repressive effects of serpent and the positive regulatory effects of abdA limit the SGPs to PS10, 11 and 12 (Riechmann et al., 1998; Moore et al., 1998a; Hayes et al., 2001). During gonad formation, the three clusters of SGPs come together to form a band of cells on each side of the embryo as the germ cells end their migration, and the two cell types associate along PS1012. Germ cells and SGPs then undergo gonad coalescence to form a rounded structure in PS10. In addition to SGPs that are specified in PS10, 11 and 12, there is an additional cluster of somatic mesoderm cells, called msSGPs (for male-specific SGPs), that arises in PS13 (DeFalco et al., 2003). msSGPs can be distinguished from the SGPs by co-expression of EYA and the nuclear protein Sox100B (DeFalco et al., 2003). As the germ cells and SGPs coalesce to form the gonad, the msSGPs move anteriorly and join the posterior of the gonad specifically in males (DeFalco et al., 2003). Gonad coalescence involves the concerted movements of germ cells and SGPs as they transition from a broad association of cells into a condensed and organized gonad. Coalescence is 4417 Development 130, 4417-4426 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00639