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Sex and the honeybee
Genome Biology volume 4, Article number: spotlight-20030827-01 (2003)
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 2003, 114:419-429).
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 1453-bp csd sequence consisting of nine exons that contained an open reading frame of 385 amino acids. Sequence comparisons revealed the protein to be an arginine-serine (RS) -rich protein that was similar to the tra genes of Dipteran insects, representing a distinct member of a gene family that has RS domains, which is involved in pre-mRNA splicing and metabolism. Repression of csdtranslation by RNA interference demonstrated its key role in sex determination in the honeybee.
"As CSD has no RNA binding domain, we propose that a factor with RNA binding function exists in the honeybee that cooperatively binds with CSD and mediates RNA splicing... The Apis system appears to be the most ancient and the simplest system studied so far," the authors conclude.