The endocrine regulation of male exploratory behavior in the nematode Caenorhabditis elegans
Kleemann, Gunnar Alan
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Sexual dimorphism in behavior is partially due to differences in the motivation to perform sex specific behaviors. However, the biological basis of motivated behaviors is poorly understood. I used C. elegans as a model to study the molecular basis of a motivated sexually dimorphic behavior. In the absence of a mating partner, C. elegans males leave food at a constant rate and search the surrounding area. Since this exploratory behavior ceases when the male encounters a mate on a food source, it effectively allows a male to locate a mating partner. Hence, it can be considered a mate-searching behavior. A quantitative assay for mate-searching behavior was used to investigate its regulation.;I found that a steroid regulated or produced by the gonad, Delta 4-dafachronic acid (DA) appears to modulate male mate-searching behavior. Worms with loss of function mutations in the enzyme necessary for DA synthesis (daf-9), or a null mutation in the DA receptor daf-92 , both have reduced mate searching. DA supplementation restores mate searching of daf-9(-) males. DAF-12 is necessary for the gonad to control mate searching since gonad ablation has no effect on the mate searching in daf-12 null worms while it inhibits mate searching of wild type males. daf-12 appears to activate mate searching downstream of a gonad signal since daf-12(m20), which lacks a ligand binding domain (LBD), promotes mate searching even in the absence of the gonad.;Adult administration of the DA precursor (which was more available than DA) rescued mate searching in animals with only the somatic gonad intact (germ line ablated), but not with the somatic gonad removed (whole gonad ablated). This suggests that the germ line controls production of the DA precursor and the somatic gonad is necessary for the transformation of the DA precursor to mature DA.;Tissue specific expression of daf-12(m20) in the male specific neurons and in the gut rescued -37% to 57% of the daf-12(m20) mate-searching rate, suggesting that both tissues activate mate searching. Additionally, a candidate heterodimeric partner of daf-12, nhr-8, is expressed only in the gut and controls mate searching.