In the course of this work, the primary signal of haplo-diploid sex determination of Apis mellifera was isolated and molecularly characterized. This is the first contribution for understanding the complementary sex determination at molecular level. The sex-locus was narrowed down at genomic DNA level by a stepwise approach. By positional cloning, 70 kb of the highly unstable sex-locus region were isolated and fine-mapping was performed with four newly developed sex-locus linked markers. Functional analysis of csd with for the honey bee developed RNA-technique establishes that csd acts as the primary signal in the sex determining pathway. In genetic female eggs csd-RNAi resulted in male gonads, whereas in genetic males no effects in gonad differentiation were found. For the first time, sex determining alleles of Apis mellifera were isolated and the DNA-sequences were obtained. Analysis of synonymous and nonsynonymous substitutions indicate diversifying selection. Moreover, the influence of selection on csd-alleles was also detectable in the adjanced genomic region. Selective neutral loci linked to csd show an increased nucleotide diversity that rapidly decline with increasing distance to csd ('hitchhike'-effect). This decline can be explained by a high recombination rate in the sex-locus region, which was measured by fine scale-mapping.