Developmental and behavioral plasticity allow animals to prioritize alternative genetic programs during fluctuating environments. In nematodes that interact with host organisms, reproductive adults and the developmentally arrested, host-seeking larvae are likely to have different responses to odorants. To understand the genes that coordinate development and behavior, we used Pristionchus pacificus to characterize two dauer-constitutive mutants that form dauer larvae in the presence of food and at ambient temperatures (Daf-c). Both Daf-c alleles are recessive, temperature-sensitive, and incompletely penetrant, with a subset of the emerging DL exhibiting a transient, dauer-specific exsheathment defect.
csu60 mutant larvae committed to dauer entry spend a prolonged time as pre-dauer J2 larvae (J2d). Like dauers, the mutant J2d rarely ingest fluorescent latex beads, but they lack resistance to SDS. Both P. pacificus Daf-c alleles disrupt steroid synthesis required for proper regulation of the conserved canonical steroid hormone receptor DAF-12, whose dauer-constitutive and cuticle exsheathment phenotypes can be rescued by the feeding of 7-dafachronic acid (7DA), a ligand for DAF-12. The
csu60 allele has a deletion of the sole HydroxySteroid Dehydrogenase (HSD) in P. pacificus, and a transgenic
Ppa-hsd-2 reporter shows expression in CAN neurons, intestine, and excretory gland cells in all stages, and excretory canal expression in dauers only.
csu60 mutants exhibit ectopic expression of
Ppa-odr-3p::rfp in an extra pair of amphid neurons. Unlike C. elegans, both the wild-type and mutant DL show repulsion to CO2. Both
hsd-2(
csu60) adults and dauers show enhanced attraction to a beetle pheromone, possibly due to the heterochronic activation of dauer-specific neuronal development in the adults. While this enhanced odor attraction can be rectified by a
Ppa-hsd-2 rescue transgene, the enhancement acts independently of
Ppa-daf-12, revealing unexpected decoupling of dauer formation and dauer-specific host-seeking behavior upstream of the conserved 7DA/DAF-12 module. Given that mutations in
Ppa-daf-22,
Ppa-daf-19,
Ppa-daf-21, and several TGF-beta homologs do not exhibit dauer formation phenotypes, we speculate that the genetic regulation for dauer entry has significantly diverged between C. elegans and P. pacificus.