Worm Breeder's Gazette 14(5): 36 (February 1, 1997)
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.
Department of Biochemistry, University of Kentucky College of Medicine, Lexington, KY 40536-0084
We are investigating mechanisms that orient asymmetric vulval cells. The vulva is formed from a symmetric primary lineage flanked by asymmetric secondary lineages. The secondary lineages have mirror-image orientation with respect to each other. Progeny cells of the secondary lineages migrate toward the center of vulval symmetry, and each extends a process either posteriorly or anteriorly toward the center of vulval symmetry.
We previously reported that a signal from the gonad promotes the secondary lineage formed by P7.p to be anteriorly-orientated, and that LIN-18 appears to act in production of or response to the orienting signal (WBG 11(4):98). We also found that high levels of a fragment of inductive signal or elevated activity of its receptor bypasses the requirement for orienting signal from the gonad (WBG 14(2):65). This suggests four possibilities. The orientation and induction pathways might share one or more effectors; relative activities of the two pathways might be modulated by regulatory factors; orientation signaling might use different but similar effectors, for which inductive pathway effectors can substitute when excessively activated; or excess inductive pathway activity might artifactually alter the activity or localization of molecules that regulate orientation signaling.
To exclude the possibility that the apparent effect of inductive signaling on orientation might be an artifact of excess inductive activity, we examined the effect on orientation of reduced vulval inductive activity. We used a reduction-of-function allele of lin-45raf, together with a lin-18 mutation. The lin-18 alleles tested cause the P7.p lineage to be misoriented in about 45% of mutant animals. Their phenotype in trans to a deficiency indicates that each lin-18 allele likely causes only partial loss of function. If a RAF-mediated pathway is important for correct vulval orientation, reduction of RAF activity should enhance the orientation defect of a lin-18 mutation, causing a greater fraction of P7.p lineages to be misoriented in double mutants. We scored vulval phenotypes under Nomarski optics, in early L4 stage larvae.
lin-18 mutations alone did not affect the extent of vulval induction or larval viability. Therefore we were surprised to find that the double mutants displayed twofold less vulval induction and lower larval viability than raf single mutants (see Table). Because of reduced vulval differentiation, P7.p adopted a tertiary or incompletely induced fate in most double mutants. In cases where a secondary lineage was formed, however, a higher fraction were misoriented in the double mutants (17/22) compared to lin-18 single mutants (63/138) or raf single mutants (0/53). While we have not ruled out the possibility of a genetic background effect, these observations support the possibility that the vulval orientation and inductive signaling pathways have one or more effectors in common, and converge upstream of RAF or diverge downstream of RAF. Alternatively, the pathways might be distinct, with relative activities modulated by regulatory factors.