The Caenorhabditis vulva is formed from a row of precursor cells in the ventral epidermis: P6.p normally adopts a central vulval fate (1), P(5-7).p a lateral vulval fate (2) and P(3,4,8).p a non-vulval fate (3). This spatial pattern ( 332123 ) is organized around the uterine anchor cell (AC). The AC emits a LIN-3/EGF signal that induces the 1 fate at high doses and the 2 fate at low doses. Activation of EGF-Ras signaling in P6.p has two consequences: it induces the 1 fate in P6.p and activates a lateral Notch pathway that induces the 2 and inhibits the 1 fate in P(5,7).p. In C. elegans, Pn.p fates are irreversibly determined by the time they divide: if the AC is ablated at this time, P6.p adopts a 1 fate and P(5,7).p a 2 fate. I found that in two close relatives of C. elegans in the same conditions, P6.p adopts a 2 fate (C. briggsae) and a 3 fate (C. remanei). More precisely, by ablating the AC at successive timepoints in the L3 stage, one uncovers a temporal series of P(5-7).p fate patterns (from 333 to 212), thus revealing relative activities of different vulva signaling pathways. In C. elegans N2, the transition is rapid (Wang & Sternberg, unpubl.), suggesting that P6.p, as soon as it is induced to a 1 fate, activates the 2 fate in P(5,7).p. In C. briggsae AF16 & HK104, P(5-7).p display an intermediate 222 pattern: P6.p adopts an inner 2 fate (vulCDDC), confirmed by L4-stage expression of
Cb-egl-17::GFP and
zmp-1::GFP. The 1 fate requires a late induction of P6.p progeny. In C. remanei PB4641 & PB228, the intermediate AC ablation results in a surprising 232 pattern: P6.p adopts a 3 fate yet induces the 2 fate in P(5,7).p, as they adopt a 3 fate upon simultaneous AC+P6.p ablations. This likely corresponds to changes in the relative weights of pathways downstream of Ras, with a high threshold for 1 fate induction and a low threshold for lateral signaling. In basal Caenorhabditis species and many other nematode genera, the intermediate 222 pattern is observed. When these changes are mapped onto the Caenorhabditis phylogeny (Kiontke et al.), the early specification of the 1 fate appears between the C. spp. DF5070 and PS1010 branches, and a further dramatic evolution occurs in the C. remanei + briggsae branch. Intra-specific variations are also detected (for C. elegans, cf Milloz et al.). Key changes in vulval patterning mechanism thus occurred in Caenorhabditis evolution, which are being further investigated using reporter genes for the different pathways.