A bHLH Code for Sexually Dimorphic Form and Function of the C. elegans Somatic Gonad

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genetics

Background:

• Sexual dimorphism in gonad form and function is a fundamental biological question.
• In C. elegans, gonad sexual dimorphism arises from distinct roles of somatic gonad primordium regulatory cells.
• The transcription factor HLH-2 (E protein ortholog) is crucial for regulatory cell development in both sexes, but its role in fate determination was unclear.

Purpose of the Study:

• To elucidate the mechanism by which C. elegans regulatory cells adopt different fates for sexual dimorphism.
• To test the hypothesis that a combinatorial 'bHLH code' of transcription factors specifies regulatory cell identity.

Main Methods:

• Analysis of bHLH gene expression patterns in C. elegans regulatory cells.
• Functional assays involving loss-of-function and ectopic expression of bHLH genes.
• Investigating the reprogramming of regulatory cell fates by manipulating bHLH gene complements.

Main Results:

• Each C. elegans regulatory cell type expresses a unique set of bHLH-encoding genes, forming a 'bHLH code'.
• This bHLH gene complement is both necessary and sufficient to determine specific regulatory cell fates.
• Direct reprogramming of cell fates and male-to-female/female-to-male transformations demonstrate the instructive role of the bHLH code in sexual dimorphism.

Conclusions:

• A 'bHLH code' governs the identity and function of somatic gonad regulatory cells in C. elegans.
• This code provides a mechanism for generating sexual dimorphism in gonad development.
• The bHLH code, embedded in a bow-tie regulatory architecture, offers insights into the evolutionary plasticity of nematode gonad form.