Description

sma-4 encodes the co-Smad of the BMP pathway in C. elegans, which is also known as the Sma/Mab pathway (Savage et al. 1996). Null mutations in core components of this pathway, including the BMP ligand DBL-1, the receptors SMA-6 and DAF-4, and the R-Smads SMA-2 and SMA-3, all result in a small body size phenotype without significantly compromising viability (Gumienny and Savage-Dunn 2013). However, we found that even after multiple rounds of out-crossing, two deletion alleles of sma-4, ok3140 and tm4731, caused late larval lethality and embryonic lethality, respectively. This observation suggests that either SMA-4 has DBL-1/BMP-independent functions that are required for viability, or ok3140 and tm4731 have closely linked lethal mutations. To distinguish between these two possibilities, we generated a deletion allele of sma-4 using CRISPR/Cas9-mediated non-homologous end joining. This allele, jj278, contains a 3,556bp deletion (position: Chromosome III: 5,816,203….5,819,759) that deletes almost the entire coding region of sma-4 (Figure 1A) and represents a true molecular null. jj278 animals are viable and fertile, but are smaller than wild-type animals (Figure 1B), like loss-of-function mutants in other core BMP pathway members (Savage et al. 1996; Krishna et al. 1999). This result suggests that ok3140 and tm4731 likely have closely linked lethal mutations not caused by their respective sma-4 deletions.