In the haplodiploid insect order Hymenoptera, males are derived from haploid unfertilized eggs and females from fertilized diploid eggs. In the August 22 Cell, Martin Beye and colleagues from Biozentrum at Martin-Luther-Universitat report the identification of the primary sex-determining signal in honeybees. This signal encodes a product that triggers the female developmental pathway if it results from two different alleles in females, but the product is inactive if it results from two identical alleles or a single allele in males, allowing the default male developmental pathway. This phenomenon is known as complementary sex determination—hence, the csd gene (Cell, 114:419-429, August 22, 2003).

Beye et al. identified genetic markers that flanked the sex-determining locus and by chromosome walking and fine mapping located a single gene that was always heterozygous in females. The ~5-kb region containing it was cloned, sequenced, and analyzed for exon-coding regions. These were assembled into the...

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