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Abstract
Haplodiploid organisms comprise about 20% of animals. Males develop from unfertilized
eggs while females are derived from fertilized eggs. The underlying mechanisms of
sex determination, however, appear to be diverse and are poorly understood. We have
dissected the complementary sex determiner (csd) locus in the honeybee to understand
its molecular basis. In this species, csd acts as the primary sex-determining signal
with several alleles segregating in populations. Males are hemizygous and females
are heterozygous at this locus; nonreproducing diploid males occur when the locus
is homozygous. We have characterized csd by positional cloning and repression analysis.
csd alleles are highly variable and no transcription differences were found between
sexes. These results establish csd as a primary signal that governs sexual development
by its allelic composition. Structural similarity of csd with tra genes of Dipteran
insects suggests some functional relation of what would otherwise appear to be unrelated
sex-determination mechanisms.