Somatic gonad morphogenesis in C. elegans requires the heterochronic pathway acting through HBL-1

Heterochronic genes are known for acting in succession to control the timing of stage-specific events of the developing Caenorhabditis elegans larva. While most heterochronic mutants have normal gonad development, in a few cases, defective timing regulators lead to variations in gonad development, although with little to no effect on fertility. We found that a double mutant of 2 heterochronic genes, a lin-28 null allele and hbl-1 hypomorphic allele, results in a catastrophic failure of gonad morphogenesis resulting in sterility. This defect includes a high-penetrance disruption of normal gonad arm migration as well as frequent absence of 1 or both spermathecae. We demonstrate that the abnormal gonad morphology and novel sterility phenotype is ultimately due to loss of hbl-1 activity in larval development. To clarify the mechanism of how lin-28 positively influences hbl-1 activity, we demonstrate a direct interaction between the RNA-binding protein LIN-28 and the 5'UTR of lin-46, and in turn, a direct interaction between LIN-46 and 2 zinc fingers of HBL-1. Nevertheless, our genetic analysis indicates that lin-46 accounts for only part of lin-28's regulation of hbl-1 and that some portion of lin-28's effect is independent of lin-46.