The four previously described C. elegans Hox genes appear to control only anterior embryonic or postembryonic patterning, raising the question of how posterior patterning in the embryo is dictated. The
nob-1 locus, mapping about 15 cM right of the known Hox cluster, is defined by three mutations that disrupt this patterning: a viable mutation,
ct230 , causing variable tail abnormalities, and two L1-lethal mutations,
ct223 and
ct351 , causing severe posterior morphological abnormalities (the Nob phenotype).
nob-1 is predicted to encode a Hox protein with strong similarity in the homeodomain to products of the posterior Hox paralog (PHP) group genes, including
egl-5 , Drosophila Abd-B (72% similarity) and the vertebrate Hox 9-13 genes. Further sequencing in this region revealed a third PHP group homolog
php-3 , (89% similar to Abd-B ), beginning just ~200 bp from the 3' end of
nob-1 .
nob-1 transcripts are present in pregastrulation embryos, all larval stages, and the adult soma, but appear absent from the germ line.
php-3 transcripts appear to be rarer but can be detected by RT-PCR in embryos and later stages. Both transcripts are trans-spliced to SL1, suggesting that
nob-1 and
php-3 do not constitute an operon. Analysis of the mutant alleles revealed that all are deletions.
ct230 removes only small parts of exon 2 and the adjacent intron in
nob-1 ;
ct351 removes the homeobox of
nob-1 and all of
php-3 ; and
ct223 removes at least 10 kb of flanking sequence 5' of
nob-1 , but no coding sequence. To understand the respective functions of the two genes, we have performed dosage and RNAi experiments. Whereas most
ct230 embryos develop into fertile adults, few
ct230/eDf2 embryos do so, suggesting that
ct230 is a hypomorph. Only about 45% of
nob-1(RNAi) embryos develop into fertile adults, which have variably abnormal tails like those of
ct230 . In contrast, 95% of
php-3(RNAi) embryos develop into fertile adults. The double
nob-1(RNAi);
php-3(RNAi) phenotype is no more severe than that of
nob-1(RNAi) alone. We conclude that C. elegans has a split Hox cluster (like flies) including three PHP group genes (unlike flies, more like vertebrates). The role of
php-3 is still uncertain, but
nob-1 appears to be required for normal posterior patterning in the embryo.