Introduction to Sex Determination in C. elegans

Sex determination in Caenorhabditis elegans (C. elegans) is fundamentally based on the ratio of X chromosomes to sets of autosomes (X:A ratio) rather than the presence or absence of a Y chromosome. This system influences whether the organism develops as a hermaphrodite (XX) or as a male (XO).

The Experimental Observation

• When a specific segment of the X chromosome is transferred into an XO male (which normally develops as a male), the animal gains female characteristics.

• This result indicates that the dosage of genes on the X chromosome relative to autosomal genes determines the sexual phenotype.

• It rules out models based on XX-XY systems (where presence of Y determines sex), hormonal signaling as primary signals, or ploidy differences alone.

X:A Ratio and the Master Regulator

• The primary signal is the dose of X chromosome-linked genes compared to autosomal genes, integrated as the X:A ratio.

• The gene xol-1 acts as a master switch, activated in XO males (leading to male development) and repressed in XX hermaphrodites.

• This pathway regulates downstream genes controlling sexual differentiation.

Why Other Modes Are Less Applicable

• XX-XY type (option 2): C. elegans lacks a Y chromosome and does not determine sex by presence of specific sex chromosomes but by gene dosage.

• Hormonal type (option 3): Although some hormones play roles in sexual development, primary sex determination is not hormone-based.

• Ploidy difference (option 4): Ploidy variations are irrelevant since hermaphrodites and males are diploid with different sex chromosome compositions.

Summary

The observation that transferring an X chromosome segment into an XO male restores female characters strongly supports that sex determination in C. elegans follows the X/A ratio mechanism, making it fundamentally dosage-dependent.

Final Answer:
(1) X/A ratio