Haplodiploid sex determination occurs in about 20% of all animal species, mainly in the insect order Hymenoptera, but also in a few other lineages within phylum Arthropoda. Males are haploid and develop from unfertilised eggs, while females are diploid and develop from fertilised eggs. The whole genome therefore in many ways acts like and resembles the X0 chromosome system in diploids. Sex determination in itself is believed to be caused through complementary sex determination at the CSD locus. Heterozygotes at this locus become female, while homozygous diploids and hemizygotes become male. A fitness benefit of this system should be resilience to inbreeding depression as recessive deleterious alleles may be purged through expression in haploid males. However, reduced allelic diversity at the CSD locus could result in involuntary production of diploid homozygous males that do not contribute resources to the colony, therefore reducing its fitness. Previous modelling of this system has suggested that reduced allelic richness at the CSD locus can lead to an extinction vortex with an increased diploid male production, decreased population growth rate, reduced population size, and then further reduced allelic richness. While diploid male production has been suggested as a measure of population health, the allelic diversity at the CSD locus could potentially be a more direct and useful indicator. Bumblebees are recognised as some of the most important pollinators of both wild and agricultural plants. However, they have recently suffered great declines across the globe through habitat deterioration caused by agricultural intensification and climate change. The importance of bumblebees for ecosystem services therefore motivates the use of them as a study system. Questions we will approach include: 1. What are the expected fitness consequences of genetic bottlenecks in breeding systems with complementary sex determination? 2. How does population fragmentation affect the genetic variation of the (assumed and known) complementary sex determination in natural populations? 3. How is selection acting on the CSD locus as inferred from genetic analyses? 4. How is colony sex ratio affected by CSD in fragmented populations (i.e., during inbreeding)? 5. How is sexual dimorphism and within species colour morphs related to complementary sex determination?