Description

The HMX/NKS homeodomain transcription factor MLS-2 is required to initiate the expression of the AWC terminal selector ceh-36, and as such the AWCs of mls-2 loss-of-function mutant animals fail to express downstream AWC-specific terminal differentiation genes (Kim et al., 2010).  Interestingly, loss of mls-2 function was also shown to result in ectopic expression of ASH markers (sra-6p::gfp and osm-10p::gfp) in at least one neuron in a large percentage of mls-2 mutant animals (Kim et al., 2010).  The cells that ectopically express the ASH markers are adjacent to the native ASHs and, like native ASH neurons, dye-fill with lipophilic dyes such as DiD (Perkins et al., 1986; Kim et al., 2010).

Since ASH expression of both sra-6p::gfp (Baran et al., 1999; Wood and Ferkey, 2019) and osm-10p::gfp (Wood and Ferkey, 2019), as well as ceh-23p::gfp (Wood and Ferkey, 2019), depends upon the paired-like homeodomain transcription factor UNC-42, we assessed whether the ectopic expression of these ASH markers requires UNC-42 function as well.  We first examined the expression pattern of stably integrated reporters for sra-6p::gfp, osm-10p::gfp and ceh-23p::gfp in mls-2(cc615) loss-of-function mutant animals.  In addition to being expressed in the native ASH neurons, for each transgene we confirmed ectopic marker expression unilaterally or bilaterally in the ectopic ASH-like cells of mls-2(cc615) mutant animals, which were identified by dye-filling (Figure 1).  We note that there was no obvious directional bias as to which side of the bilateral ASH-like pair the unilateral ectopic expression arose (Figure 1B).  We found that both native and ectopic expression of all three ASH markers was lost in the unc-42(gk598);mls-2(cc615) double mutants, although the native and ectopic cells retained dye-filling capacity (Figure 1A, C). Thus, the ectopic expression of these ASH markers in the absence of MLS-2 function depends upon UNC-42, as native ASH expression does (Baran et al., 1999; Wood and Ferkey, 2019).