We have previously described the expression patterns of all but one of the 102 homeobox genes present in the C. elegans genome, using either fosmid-based reporter transgenes or CRISPR/Cas9-engineered gfp reporter alleles (Reilly et al., 2020). The last remaining homeobox gene for which we were initially not able to generate a reporter reagent for is
ceh-84 (Reilly et al., 2020). As previously recognized by Thomas Brglin (Hench et al., 2015),
ceh-84 codes for an unusual homeodomain protein with two divergent homeodomains (Fig.1D). It likely is a tandem duplicate of the
ceh-85 gene, which has been classified as a pseudogene (Fig.1B).
ceh-84 has no recognizable homologs in other Caenorhabditis species (Hench et al., 2015). Together with its duplicate,
ceh-85, localizes to a large cluster of extensively duplicated genes on chromosome II, described by James Thomas (Thomas, 2006). Most of the duplicated genes code for MATH and BTB type proteins (Fig.1A). The entire cluster seems to be C. elegans-specific (Thomas, 2006) and, apart from a number of pseudogenes, it also contains the C. elegans-specific
ceh-81,
ceh-82 and
ceh-83 homeobox genes (Fig.1A).