We have continued genetic studies on exc mutants, in which the long hollow processes (canals) of the excretory cell form large fluid-filled cysts. We previously classified these mutants into five groups based on cyst morphology: exc-2, exc-4, let-4, and let-653 mutants form large septate cysts near the excretory cell body; exc-1 and exc-5 mutants form extremely large septate cysts (often visible via dissecting microscope) predominantly at the shortened canal termini; exc-3, exc-6, and exc-8 mutants show regional canal widenings and meanderings, and occasional inappropriate extra branchings to form multiple canal lumena; alleles of sma-1 exhibit a short, very wide, unseptate canal. Finally, the lone exc-7 allele rh252 exhibits small cysts along the length of the canal, terminating in a large group of tiny cysts. We have now made double mutants of several of the genes from various groups. Double mutants made of genes from within any one group showed no increase in severity in canal phenotype. For example, exc-2 (rh90) exc-4 (rh133) animals showed no changes in cyst size or morphology compared to rh90 or rh133 alone, and no increase in larval lethality. Similarly, exc-6 (rh103) exc-8 (rh210) canals appeared no shorter or more meandering than those of the single mutants, and exc-1 (rh26) exc-5 (rh232) mutant canals had similar cyst size and placement as the single mutants alone. Doubles constructed with exc-7 (rh252) showed interesting effects, however. exc-7 is epistatic to exc-6; the double mutants showed no abnormal branchings, only a series of small cysts along the entire canal length. Perhaps not surprisingly, the large cysts of exc-2 and exc-4 are epistatic to exc-7. Most strikingly, the doubles exc-7 (rh252) sma-1 (e30) and especially exc-7 (rh252) exc-3 (rh207) show canal phenotypes different from that of the single mutants; instead, these animals show large cysts near the excretory cell body, like those of exc-4 mutants. Finally, we have been unable so far to isolate doubles of exc-7 (rh252) and either exc-1 (rh26) or exc-5 (rh232); this could indicate that these mutants are lethal in combination. Our previous ultrastructural data, along with sequencing results from the labs of David Baillie (Jones & Baillie, Mol. Gen. Genet. 248, 719-726, !95) and Judith Austin (see McKeown, Patel, Praitis, and Austin, WBG 14 (2), p. 83) suggest that the exc genes encode proteins necessary for the proper placement of structural elements, both extracellular and cytoskeletal, at the apical epithelial surface. We believe that these new genetic results indicate that the differing morphologies of the various groupings of exc genes reflect different elements that interact with each other to shape this surface; Exc-7 may play a central position in these interactions.

Affiliation:
- Dept. of Microbiology, University of Kansas, Lawrence, KS, 66045 Dept. of Biology, Johns Hopkins University, Baltimore, MD, 21218