Hypothesis
The honeybee’s roughly 20% higher mutation rate follows from its exceptionally high recombination rate, so the closely related bumblebee, with far less recombination, should mutate measurably less.
What failed
The bumblebee’s crossover rate was confirmed to be much lower (8.7 against 37 cM/Mb), yet its directly measured mutation rate was 3.6 x 10^-9 per haploid genome per generation (95% CI 2.38-5.37 x 10^-9), about 5% above the honeybee’s rather than lower, with no significant difference between species.
Broken assumption
A species’ recombination rate sets its genome-wide mutation rate.
Why
The authors do find a direct coupling between recombination and mutation within the genome, but so weak that it leaves almost no footprint on between-species differences.
From the published paper: Liu et al. Molecular Biology and Evolution, 2016. ↗CC BY