The caudal homolog
pal-1 is expressed both maternally and embryonically. Maternally it functions to specify muscle fate in the C and D lineages (1). Embryonic loss of function results in the severely posteriorly malformed Nob phenotype (2). We are using reporter constructs, 4D lineage analysis, and mosaic analysis of the null allele
ct224 to continue characterization of
pal-1 embryonic function. Lineage analysis shows defects in the C hypodermal lineages (which normally express
pal-1): cells are misplaced from the 4-C-cell stage onward, and the normal contralateral nuclear migration does not occur. Adjacent AB posterior hypodermal cells (which normally do not express
pal-1 at these stages), are also mispositioned, and expression of the seam cell-specific reporter wIS1 is reduced in the posterior. Although C and D muscle lineages appear normal, preliminary results indicate that the number of cells expressing a
myo-3::GFP reporter is reduced, suggesting a function for
pal-1 in the C muscle lineage, even though wild-type embryonic expression of
pal-1 is detectable in this lineage only briefly if at all with antibodies and a GFP construct. This hypothesis is supported by preliminary mosaic analysis: losses in C muscle lineages cause an intermediate lumpy phenotype localized in the area of loss. Other functional foci appear to lie in ABp (
pal-1-expressing progeny contribute to the rectal tissues) and the C hypodermal lineage. Thus there may be more than one way to make a Nob. Based on the finding that a transgenic construct containing
pal-1 exons 1 and 2 (excluding the homeodomain) causes a dominant-negative Nob phenotype, we are beginning a yeast two-hybrid screen, using Pal-1 as bait, to identify possible interacting proteins, one of which could be MyoD (3). Pal-1 does not appear to interact with itself. Initially positive clones from the Okkema embryonic library are currently being tested further. (1) C. Hunter and C. Kenyon, Cell 87: 217-226, 1996. (2) L. Edgar, S. Carr and B. Wood, 1996 International Worm Meeting Abstr. 198. (3) M. Park, R. Littlejohn and M. Krause, WBG 14(5) 1997.