Maternal rescue of post-embryonic neuroblast migration in C. elegans C-mannosyltransferase mutants

Abstract

The maternal effect is a phenomenon in which the phenotype of an offspring is determined by the mother’s genotype instead of its own genotype or the environment. Maternal effect genes are essential for the normal development of offspring since they encode proteins important for early embryonic development before zygotic transcription takes place (Wolf & Wade, 2009). This is also why the maternal effect is rarely found to regulate post-embryonic development.

I found one exception in Caenorhabditis elegans when investigating a mutation that affects the post-embryonic differentiation of the Q neuroblasts. This mutation (allele name: unk88) causes the anterior displacement of the PVM neuron, one of the six touch receptor neurons (TRN) in C. elegans, which normally resides in the posterior half of the worm body. Through genetic mapping, unk88 is found to be a Tc1 transposon insertion in the dpy-19 gene encoding a C-mannosyltransferase (Buettner et al., 2013). The loss of dpy-19 leads to defects in the initial polarisation of Q cells, leading to subsequent anterior displacement of QL descendants (PQR, PVM, SDQL) and posterior displacement of QR descendants (AQR, AVM, SDQR). Homozygous unk88 mutants derived from a heterozygous mother did not display any defects, suggesting a maternal rescue of the defects in post-embryonic development.

By examining the phenotypes of unk88 and two other dpy-19 alleles, I found that the maternal effect of dpy-19 regulates the migration of both QL and QR neuroblasts and thus all six Q descendants are affected. Interestingly, heterozygous progeny born from a homozygous dpy-19(-) mother did not show the migration defects, suggesting that the phenotype is also zygotically rescued. Moreover, among the genes involved in Q polarisation and migration, only dpy-19 shows such maternal effect, indicating a unique genetic property (Middelkoop et al., 2012; Buettner et al., 2013; Sundararajan et al., 2012, 2014). By tagging DPY-19 proteins with GFP and visualising its mRNA through smFISH, I found that the maternal influence is mediated by the deposition of maternal mRNA, and not protein, in early embryos.

I also investigated the potential mechanisms underlying the exceptional stability of dpy-19 mRNA from maternal deposition to larval development. I first observed a disruption in the maternal rescue when swapping the dpy-19 3’UTR with that of a non-maternal gene, suggesting the 3’UTR is important for maintaining mRNA stability. Second, deleting the pab-2 gene, a poly(A) binding protein that is involved with both the 3’UTR and mRNA modifications, had a slight effect in removing the maternal rescue in dpy-19(-) animals. I ruled out the involvement of mRNA modifications such as m6A-mediated RNA stabilisation in mediating the maternal effect, since deleting other mRNA modification genes had little effect on the maternal effect. Thus, the 3’UTR and potentially other factors exist to ensure the long-term stabilisation of dpy-19 mRNA. I outlined further experiments that study the half-life of dpy-19 mRNA, the precise 3’UTR location that enables mRNA stability, and potential mechanisms through which this is mediated.