We have begun to characterize cell interactions that specify cell fates in the male-specific blast cell, B. The B cell generates the spicules in the male tail. As shown by Sulston and White [Dev Biol 78, 577 (1980)], the fates of two pairs of bipotent cells, alpha/ (B.a( l/r)aa) and gamma/delta (B.a(l/r)pp), depend on their relative positions. Thus they must respond to spatial cues. Our cell ablation experiments indicate that other male-specific blast cells, F and U, provide at least part of such spatial cues. In a wild type male, F daughters are positioned immediately anterior to the gamma/delta pair ( next to r), and U daughters are anterior to alpha/ (next to a). Ablation of both F and U results in three B lineage defects: (l) transformation of alpha to , (2) abnormal truncated gamma lineages, and (3) abnormalities in B.a(l/r)ap. In single ablations of either F or U, the B lineage is either wild type or occasionally abnormal, with truncated gamma lineages (for F-), or alpha to transformation (for U- ). Variation in the lineages of F-U- ablated animals suggests that we have not completely eliminated the inductive signal(s). However, we have clearly identified candidates for inducing cells in the alpha/ and gamma/delta equivalence groups. Genes identified for their role in vulval induction are also involved in induction of the alpha and gamma fates. The lineage defects in mutant males suggest that, as in the vulva, there are two classes of mutations which give two opposite phenotypes. Vulvaless class mutations
lin-3,
let-23,
lin-45, and let- 60(dn) result in transformation of alpha to , truncated gamma lineages, and abnormalities in B.a(l/r)ap. Thus, the Vulvaless mutations mimic F-U- ablations in the male B lineage, in the same way that they mimic anchor cell ablation in the hermaphrodite vulva. In contrast, Multivulva class mutations lin-l and
let-60(gf) result in rare to alpha transformations, and extra divisions in delta.