The C. elegans Hox cluster contains only six genes, which are orthologous to only four of the Drosophila Hox transcription factors, possibly because the worm lacks a segmented body plan. Unlike genes in other animal Hox clusters, these Hox genes are not expressed in the same order, anterior to posterior, as their order on the chromosome, suggesting that their regulatory mechanisms may have diverged as well. Furthermore, expression of the anterior Hox ortholog,
ceh-13, extends to parts of the extreme posterior of the animal, raising the question of whether it regulates posterior fates. We investigated the regulation and function of three C. elegans Hox genes:
ceh-13, an anterior Hox1/labial homolog, and
nob-1 and
php-3, two posterior Hox13/Abdominal-B homologs with redundant functions. In most species, expression of these Hox genes does not overlap but we identified high overlap between rescuing
nob-1 and
ceh-13 transgenes, with
ceh-13 expressed one cell cycle earlier than
nob-1 in several posterior daughter lineages within the ABp and E lineages. We identified enhancers sufficient to drive part or all of the spatio-temporal expression pattern of both
ceh-13 and
nob-1, including two putative shadow enhancers. We tested whether
ceh-13 might be an upstream regulator of
nob-1 by assaying
nob-1 reporter expression in
ceh-13 mutants. We found that
ceh-13 is required for
nob-1 reporter expression in the ABplpp and ABprpp sublineages. Arrested
ceh-13 mutant embryos and L1 larvae have major defects in head morphology as well as occasional defects in tail morphology similar to those observed in
nob-1/php-3 mutants. We tested for similarities in single-cell developmental phenotypes of these two mutants by automated lineage tracing. We found that
nob-1/php-3 mutant embryos have widespread defects in cell division and migration in the posterior of the embryo, including non-cell autonomous migration defects, consistent with previous reports.
ceh-13 mutants have a fewer defects, most of which are in CEH-13 expressing cells, including in posterior cells where
nob-1 expression requires
ceh-13. Taken together, these results have several interesting implications. First, it suggests that along with input from the Wnt pathway through POP-1/TCF and SYS-1/ beta -catenin,
nob-1 is regulated by different combinations of transcription factors in different sublineages, with only some dependent on
ceh-13. Second, we observed that an anterior Hox gene positively regulates expression of a posterior Hox gene and plays an important role in the development of extreme posterior structures. This is consistent with the regulation and function of the Hox genes in C. elegans having diverged from the strict positional roles observed in other animals, suggesting that the evolutionary "rules" for animal body plans may be more flexible than previously appreciated.