The mutation
mab-22(
bx59ts)III results in missing or truncated rays and "thin" rays in the male tail, this differs from other mab phenotypes, esp. other missing ray phenotypes, since
mab-22 causes a change in morphogenesis rather than a loss of ray cells [see Int. C. elegans Meeting,
l991, p.309].We have used EM serial sections to characterize the Mab phenotype in animals shifted to 25C during the L4 stage. The temperature sensitive period for the Mab phenotype corresponds to the period of ray extension. Cell lineages wae normal (lineage analysis performed at 20C, n=10), and no defects in arrangement of early ray cell groups (RCGs) were noted, as assessed by MH27 staining of
mab-22 males from L3 until the onset of ray extension. By EM, each mutant "ray" is not really missing, as all ray cells are present and fully differentiated into normal fates despite the failure of ray extension. They usually form a papilla (75% of RCGs, N=
l7 tails) with two sensory dendrites and a structural cell (str) surrounded by
hyp7 tissue, but fail to extend normally into the cuticular fan, which is present but reduced at restrictive temperatures in the adult. These papillae resemble intermediate stages of normal RCGs just before ray extension, except that in
mab-22 the str cell is not attached to the outer layer of the adult cuticle (compared to serial thin sections of wild type animal "JSG"; cf. Sulston et al., Dev. Biol 78: 542-76, 1980). In JSG (an advanced L4 male), all RnB neurons are already fully differentiated, with the extreme ciliary tip piercing the developing adult cuticle. In
mab-22 tails, the str cell still forms a distal pocket surrounding ciliated endings of RnA and RnB, and some papillae extend a short way towards the fan, to form a truncated ray. Most papillae (
l5/17) open to the inner cuticle, with the RnB dendrite extending a short distance out from a str pocket (see Figure). RnB sometimes projects from the papilla to the exterior of the fan (7/17). In occasional "closed" papillae, the RnB dendrite fails to eit and may be retracted. In thin rays ( 10-15% of RCGs), the tip of the RnB dendrite runs further through the inner layer of the cuticle, into the fan. It is still uncertain if these thin rays can achieve a distal exit through the outer cuticle. Thin rays and truncated rays can be scored by DIC optics. About 10% of rays extend normally in animals shifted to 25C, with all cellular components displaying normal morphogenesis. Whcn grown at 16C, mab- 22
(bx59ts)III animals have no Mab phenotype, while at 20C they are fully viable with a moderate number of missing rays. At 25C,
bx59ts is an embryonic lethal, but is viable when shifted from low temperature to 25C at late juvenile stages. Ray extension may involve outgrowth of
hyp7 and/or the str cell, and can be blocked by laser ablation of the str cell. Perhaps a signal passes between hyp and str to induce outgrowth;
mab-22 mutations could interfere with this signaling. Alternately,
mab-22 may encode an adhesive or cytoskeletal element by which individual papillae become anchored to the cuticle at the distal tips of the str cells. This anchorage is marked by an electron dense matrix in the JSG series. During tail retraction to form the cuticular fan, each papilla may be transformed into a ray by passive stretching due to this anchorage. The
mab-22 thin ray phenotype consists of an abnormal lengthening of the distal tip of the RnB cilium to run within a narrow channel through the cuticular fan. Thus RnB outgrowth into the ray could be an active process separate from ray extension. Alternately, RnB may be anchored to the outer cuticle independently from the str cell anchorage, and could extend by passive stretching during tail retraction. A specialized zone of cuticle is evident surrounding the dendritic tip of RnB both in L4 and adult stages. We suggest that the "ring and dot" seen by DIC optics of RCGs in wild type and
mab-22 tails may represent the separate anchorages of str and RnB cell tips respectively, and that these can occasionally fail independently in a
mab-22 background to give a closed ray (with the RnB dendrite lacking or foreshortened) or a thin ray. [We thank John Sulston for access to the JSG series.]