[
WormBook,
2006]
Receptor Tyrosine Kinase (RTK)/Ras GTPase/MAP kinase (MAPK) signaling pathways are used repeatedly during metazoan development to control many different biological processes. In the nematode Caenorhabditis elegans , two different RTKs ( LET-23 /EGFR and EGL-15 /FGFR) are known to stimulate LET-60 /Ras and a MAPK cascade consisting of the kinases LIN-45 /Raf, MEK-2 /MEK and MPK-1 /ERK. This Ras/MAPK cascade is required for multiple developmental events, including induction of vulval, uterine, spicule, P12 and excretory duct cell fates, control of sex myoblast migration and axon guidance, and promotion of germline meiosis. Studies in C. elegans have provided much insight into the basic framework of this RTK/Ras/MAPK signaling pathway, its regulation, how it elicits cell-type specific responses, and how it interacts with other signaling pathways such as the Wnt and Notch pathways.
[
WormBook,
2005]
The features that differentiate the C. elegans male from the hermaphrodite arise during postembryonic development. The major male mating structures, consisting of the blunt tail with fan and rays, the hook, the spicules and proctodeum, and the thin body, form just before the last larval molt. Male and hermaphrodite embryogenesis are similar but some essential male cell fates are already established at hatching. The male mating structures arise from three important sets of male-specific blast cells. These cells generate a total of 205 male-specific somatic cells, including 89 neurons, 36 neuronal support cells, 41 muscles, 23 cells involved in differentiating the hindgut, and 16 hypodermal cells associated with mating structures. Genetic and molecular studies have identified many genes required for male development, most of which also function in the hermaphrodite. Cell-cell interactions play a role in patterning all three of the generative tissues. Male-specific neurons, including sensory neurons of the rays, hook, post-cloacal sensilla, and spicules, differentiate at the end of the last larval stage and send out axons to make connections into the existing neuropil, greatly enlarging the posterior ganglia. The hindgut is highly differentiated to accommodate the spicules and the joining of the reproductive tract to the cloaca. A complex male-specific program generates many new muscles for copulation. The cell lineage and genetic program that gives rise to the one-armed male gonad appears to be a variation on that of the hermaphrodite.