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.