Homeobox-containing genes have been identified in a large number of eukaryotic organisms, where they are thought to be involved in a wide range of developmental processes. In C. elegans, the imminent completion of the genome sequencing project will offer us the unique opportunity to identify and characterize the entire NK-2 class homeobox gene family in a multi-cellular organism. Presently, we have identified four NK-2 homeobox genes in C. elegans:
ceh-22,
ceh-24,
ceh-27, and
ceh-28 (>85% of the genome has now been sequenced). The molecular and genetic characterization of two of these genes,
ceh-22 and
ceh-24, has been described previously (Okkema and Fire, 1994; Harfe and Fire, 1998). The characterization of the remaining two NK-2 homeobox genes,
ceh-27 and
ceh-28 are discussed below. 1.
ceh-28 -
ceh-28 appears to be transcribed at extremely low levels; we were unable to obtain
ceh-28 cDNAs from three different libraries. In addition, no EST hits have been reported for this gene. As a first step in understanding the role ofceh-28 in development, we created transgenic animals carrying a
ceh-28::gfp transcriptional fusion. Strong GFP expression in transgenic animals was reproducibly observed in a single pharyngeal neuron, M4. 2.
ceh-27 - We obtained a putative full length
ceh-27 cDNA from an embryonic library. This cDNA was used to produce dsRNA for RNA inhibition (RNAi; Fire et al., 1998) and as a probe for RNA in situ hybridization analysis.
ceh-27 RNAi injections resulted in 100% F1 embryonic lethality. Anaylis of
ceh-27 RNAi animals using 4-D time-lapse imaging revealed defects in the ventral hyperdermis.
ceh-27 RNAi animals appeared wild-type until they began moving inside the egg shell. The onset of embryonic muscle contractions appeared to cause the embryo to burst at a breach in the ventral hyperdermis. Localization of
ceh-27 RNA by in situ hybridization and examination of transgenic animals containing
ceh-27::gfp and lacZ expression constructs demonstrated that
ceh-27 was expressed in a very dynamic pattern during development. Expression was first seen in four (possibly MS-derived) cells in ~100 cell embryos. Later, numerous cells in the anterior head region were
ceh-27 positive. Presently, we are attempting to better characterize the
ceh-27 expression pattern and identify the cause of the
ceh-27 loss of function hyperdermal defect. Meeting fees and transportation costs for B.D.H were generously provided by MJ Research Fire A., Xu S., Montgomery M.K., Kostas S.A., Driver S.E., Mello C.C. (1998) Nature 391, 806-811 Harfe, B.D. and Fire, A. (1998) Development 125, 421-429. Okkema, P.G. and Fire, A. (1994) Development 120, 2175-2186.