Worm Breeder's Gazette 16(5): 32 (February 1, 2001)

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.

Microevolution of vulval cell lineages within two nematode genera : Caenorhabditis and Oscheius.

Marie Delattre, Marie-Anne Félix, Etc.

Institut J.Monod. 2 place Jussieu. 75251 PARIS. France.

The cell lineage of Caenorhabditis elegans and some other nematodes is mostly invariant. Given this invariance, one can wonder how a cell lineage can evolve. We have started a microevolutionary approach by observing lineage variations of vulva precursor cells (VPCs) in different natural nematode populations : 13 strains of the C. elegans species and 5 different species in the Caenorhabditis genus, as well as 32 strains belonging to 3 different species of the Oscheius/Dolichorhabditis genus.

Our results show that, within both genera, lineage variations between species and even between strains of the same species occur mostly for VPCs that do not participate to the vulval invagination (VPCs with a 3 non-vulval fate). These cells are probably not under strong selection pressure and this could explain the large variations observed at a small evolutionary timescale.

In the reference strain C. elegans N2, the lineage of the P3.p cell is not invariant (Sulston and Horvitz, 1977). In 50% of N2 animals, P3.p is a true VPC : it is competent to form the vulva and divides once before fusing with the surrounding hypodermis. In the other 50%, P3.p is not competent, does not divide and fuses with the hypodermis. The percentage of occurrence of P3.p division varies between strains of the C. elegans species (from 10% to 60%) and between species of the Caenorhabditis genus (from 0% to 100%). In the C. elegans N2 strain, P3.p division is correlated with the competence of the cell. However, the correlation does not hold in the C. elegans strain CB4857, where P3.p divides in only 15% of the animals whereas it is competent in about 60% (as determined by ablation of P(4-8).p in the L1 stage). P3.p competence varies between Caenorhabditis species, suggesting that the size of the competence group (controlled by lin-39 /bar-1 / lin-22 in C. elegans N2) has evolved between closely related species, but that the program of cell division can be affected even between strains of the same species.

Species of the Oscheius genus can be easily found in soil samples from around the world (much more easily than Caenorhabditis species). Thus, we have been able to compare a large variety of strains in this group. Variations mostly concern P4.p and P8.p lineages : these cells divide twice, once, or not at all, but are always competent to replace the vulval cells P(5-7).p. Variations observed between strains of the same species are larger in Oscheius sp.1 than in C. elegans. Therefore, we have undertaken a genetic analysis between two pairs of strains of Oscheius sp. 1 that show distinct P4.p and P8.p lineages. For each pair, at least three loci are involved in the phenotypic differences. We also find that variation at one locus has a relatively strong effect on the phenotype (instead of small additive effects of many genes).

Within both Caenorhabditis and Oscheius, a similar range of variations is found between strains of the same species and between closely related species of the same genus. Thus, cell lineage variations observed between species could be due to fixation of variants that segregate within species.