Worm Breeder's Gazette 11(3): 76
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
The dramatic morphogenesis of the fan and rays at the end of the L4 larval stage of the C. elegans male raises several questions. How is the reproducible shape of the fan determined? Why does the outer layer of the adult cuticle that makes up the fan fold at such precise boundaries? What causes the tips of the rays to attach at specific sites either in the dorsal or ventral surfaces, or at the margin of the fan? Mutations that cause fusions of rays and alteration of ray attachment sites have identified genes required for ray formation (see Baird & Emmons, Newsletter Vol 11 #2, p116, 1990). What is the function of such genes? Studies of the topography of hypodermal cells in the tail during the L3 and L4 have provided some answers to these questions. Hypodermal cell boundaries in males of various ages have been visualized by means of monoclonal antibody MH27, kindly provided by R. Waterston and coworkers. This antibody is presumed to react with belt desmosomes that ring each hypodermal cell and join it to its neighbors. A central role in fan formation appears to be played by the tail seam (SET), a syncytium formed by fusion of the posterior daughters of the five ray precursor cells R1-R5 (Figure panels a and b). In the adult male the SET is a narrow extension of the body seam, to which it is attached by a desmosome (Sulston et al., Dev. Biol 78, 542-576, 1980). We find that earlier in the L4 the SET is much enlarged and covers the surface that will become the dorsal surface of the fan ( Figure panels c and d). The boundary between the SET and the ventral hypodermis (hyp7, or possibly R6.p or a cell descended from T) runs where the margin of the fan will form, and could define this fold- point. The future position of the margin can be determined from the arrangement of the ray tips, as discussed below. The positions of the ray tips in the adult fan appear to be defined by distinct affinities of (probably) the ray structural cells for the two hypodermal domains defined by the SET and the ventral hypodermis. The cells of the rays are born in the early L4 in the hypodermis ( Figure panel b). By mid L4 a single cell remains on the surface for each ray, presumably the structural cell, the two neurons having sunk into the body. The structural cells arrange themselves in a specific way with respect to the boundary between the SET and the ventral hypodermis (Figure panel d). Structural cells of rays that attach to the dorsal surface of the fan (rays 1, 5, and 7) move up into the SET. Those of rays that attach to the ventral surface of the fan (rays 2, and 4; rays 8 and 9 have not been visualized) move downwards into the ventral hypodermis. The structural cell of ray 3, which extends right to the margin of the fan in the adult, remains at the boundary between SET and ventral hypodermis. The arrangement seen with the antibody stain is congruent with the pattern of ray tips visible with Nomarski optics late in the L4 at the beginning of morphogenesis (Figure panel e). The arrangement shown in Figure panel d is in fact still uncertain for rays 5 through 9. It is presented as best we can make it out from our data, and by analogy with rays 1-4. The above observations make it possible to interpret mutations that cause fused rays as resulting in adjacent ray structural cells that lie in the same hypodermal domain. This might be because structural cells have lost or changed identities, or because they cannot express specific affinities for the SET or ventral hypodermis. If structural cells are not segregated into separate hypodermal domains, they may come together and become surrounded by a single sheet of hypodermis. Our evidence for this interpretation is preliminary. In mab-20, which causes fusions of ray 1 to ray 2, and ray 3 to ray 4, compound rays run right to the margin of the fan in the adult, and ray tips have altered positions consistent with the structural cells lying between the SET and the ventral hypodermis (Figure panel f). In mab-18, which causes transformation of ray 6 to a ray 4-like ray, usually fused to ray 4, the tip of ray 6 is found in the position of and adjacent to the tip of ray 4. We have not yet studied mutants with the MH27 antibody. [See Figure 1]