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Vertebrates and invertebrates are thought to employ common mechanisms to guide outgrowing cells to their final destinations. We have chosen the nematode Caenorhabditis elegans as a model system to study guidance processes because of its powerful genetics and transparency, which allows for individual cells to be viewed by microscopy. The excretory cell is an excellent growth cone to use in this type of analysis because the cell body is located in the anterior end of the animal and extends two short processes anteriorly and two long canals posteriorly along the length of the animal. This bidirectional and short vs long trajectory allows for the selection of mutations that specifically affect only certain aspects of longitudinal extension. We screened for mutations in the longitudinal outgrowth of the posterior excretory canals and identified two mutations (pm23 and pm24) which map to LG V. Both of these alleles have similar phenotypes; they are short with epidermal defects and possess very short canals that terminate at the anterior gonad suggesting that pm23 and pm24 may define a locus which is important in the general elongation of growth cones. One intriguing candidate gene which resides on LG V and has a similar phenotype to pm23 and pm24 is the sma-1 gene which encodes a unique [Beta]-spectrin, an important component of the apical cytoskeleton (C. McKeown, V. Praitis, and J. Austin, 1998, Development 125, 2087-2098). We have constructed a transgenic reporter strain that expresses GFP in the excretory canals to facilitate high-density screening for additional mutations in longitudinal path finding of this growth cone.

The Caenorhabditis elegans ubq-2 gene encodes a fusion of ubiquitin and a 52-amino-acid ribosomal protein. This single copy gene is both cis- and trans-spliced. It is expressed in all life stages of the worm and its transcript abundance is unaffected by heat stress. Transgenic analysis shows that expression of ubq-2 is regulated by an upstream promoter and a downstream element. The downstream element is required for ubq-2 promoter activity in embryos and in cells of the somatic gonad, including the distal tip cells, sheath cells, spermathecal cells, and cells of the uterus. The gonad-specific activity of the downstream regulator is transferable to a stress gene promoter such that heat-inducible expression of the transgene occurs in the somatic gonad. Stress-inducible beta-galactosidase expression in the gonad does not occur in all life stages, but is initiated late in the second or early in the third larval stage, when differentiation of gonadal tissues begins. Expression of a beta-galactosidase fusion protein from constructs containing the downstream ubq-2 regulator causes abnormalities of the gonad and embryonic lethality. Gonad abnormalities include arrested development and general disorganization. These abnormalities may be related to the overexpression of ubiquitin in the gonad.