The Annual Killifish, Austrofundulus limnaeus, survives the seasonal drying of their pond habitat in the form of embryos entering diapause midway through development. The diapause trajectory is one of two developmental phenotypes. Alternatively, individuals can "escape" entry into diapause and develop continuously until hatching. The alternative phenotypes of A. limnaeus are a form of developmental plasticity that provides this species with a physiological adaption for surviving stressful environments. The developmental trajectory of an embryo is not distinguishable morphologically upon fertilization and phenotype is believed to be influenced by maternal provisioning within the egg based on observations of offspring phenotype production. However, incubation temperature may override any such maternal pattern suggesting an environmental influence on the regulation of developmental trajectory.

We hypothesize that maternally packaged gene products coordinate the cellular events prior to the maternal-to-zygotic transition (MTZ) that determine developmental trajectory in embryos of A. limnaeus. In addition, we propose that environmentally responsive gene expression after the MTZ can sustain or override any such maternal provisioning. Using high-throughput RNA-sequencing, we have generated transcriptomic profiles of protein-coding messenger RNA and noncoding RNA during development in A. limnaeus. Embryos destined for either the diapause or escape phenotypes display unique expression profiles immediately upon fertilization that support hormone synthesis, well before the stage when phenotypes are morphologically distinct. At stages when the trajectories diverge from one another, differential expression of the vitamin D receptor signaling pathway suggests that vitamin D signaling may be a key regulator of developmental phenotype in this species. These data provide a critical link between maternal and environmental influences on the genetic regulation of phenotypic plasticity.

These results will not only impact our understanding of the genetic mechanisms that regulate entrance into diapause, but also provide insight into the epigenetic regulation of gene expression and development. Uncovering genetic mechanisms in a system exhibiting alternative developmental trajectories will elucidate the role of maternal packaging in regulating developmental decisions, and in sustaining metabolic depression during diapause.