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Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Cover of C. elegans II

C. elegans II. 2nd edition.

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Section VExecution

Laser microsurgery and temperature-shift experiments suggest that the VPCs become specified prior to their division (Greenwald et al. 1983; Sternberg and Horvitz 1986; Ferguson et al. 1987; Euling and Ambros 1996a). After the VPCs are specified, the different fates must be correctly executed to generate the correct numbers and types of descendants. For the purposes of this chapter, I consider “execution” to be the phase of vulval development from the division of the VPCs until the termination of the vulval lineages at the end of the L3 stage.

A. Intrinsic Programming or Extrinsic Influences?

Once the VPC fates have been specified, interactions among VPC descendants may not be important for the execution of vulval fates: For example, if is ablated, P5.pp expresses its normal lineage (TN), and if P5.pp is ablated, expresses its normal lineage (LL) (Sternberg and Horvitz 1986). However, there may be other extrinsic influences on the execution of vulval fates. Indeed, signaling from the somatic gonad appears to influence polarity of the 2o lineages (W. Katz and P. Sternberg, pers. comm.). Furthermore, many of the mutants in VPC specification display abnormal execution of vulval lineages (see, e.g., Ferguson et al. 1987; Sternberg and Horvitz 1989). Although these abnormal lineages may reflect abnormal specification, it is also possible that the same signaling systems are involved in cell-cell interactions during execution. Finally, lin-12 is expressed in cells during the 2o lineages, raising the possibility that lin-12 -mediated signaling is also involved in execution of the 2o fate (Wilkinson and Greenwald 1995).

B. Genes Involved in Execution

Genes involved in execution may be operationally defined by mutations that specifically alter either the 1o or the 2o lineages. Two criteria have been most useful: the plane of the final division and adherence properties of the penultimate and ultimate cells (Sternberg and Horvitz 1986; Ferguson et al. 1987). Thus far, one gene that may be specifically required for the execution of the 1o lineage has been identified. In vex-1 mutants, P6.p expresses an abnormal 1o lineage, but P5.p and P7.p express normal 2o lineages (P. Kayne and P. Sternberg, pers. comm.).

Three genes that appear to be involved in the execution of the 2o lineage have been identified. In lin-11 mutants, the 2o lineages expressed by both P5.p and P7.p are affected and may be represented as LLLL instead of LLTN (P5.p) or NTLL (P7.p) as in wild type (Ferguson et al. 1987). lin-11 encodes a transcription factor containing a homeodomain and a LIM metal-binding domain (Freyd et al. 1990). lin-11 is expressed in the T and N cells and not in the other VPCs (and often not in the VPC daughters), suggesting that lin-11 functions in the cells that are abnormal in lin-11 mutants (Freyd 1991; G. Freyd and H.R. Horvitz, pers. comm.). The lin-11::lacZ transgene that was used in lin-11 expression studies is an excellent marker for expression of the 2o fate by a VPC.

Mutations in lin-17 and lin-18 have virtually no effect on the 2o lineage of P5.p, but they alter the 2o lineage expressed by P7.p, resulting in LLLL (symmetric) or LLTN (reversed polarity) lineages (Ferguson et al. 1987; Sternberg and Horvitz 1988). lin-17 is predicted to encode a multiple transmembrane domain protein that is similar to the Drosophila frizzled gene (Vinson et al. 1989), which is also involved in cell polarity (H. Sawa and H.R. Horvitz, pers. comm.). Expression of lin-11 is altered in lin-17 and lin-18 mutants (Freyd 1991), suggesting that it may be a target of intercellular signaling mediated by lin-17 and lin-18 .

Copyright © 1997, Cold Spring Harbor Laboratory Press.
Bookshelf ID: NBK19977


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