Variants and QTL associated with variation in genome-wide crossover number

Recombination diversifies the genomes of offspring, influences the
evolutionary dynamics of populations, and ensures that chromosomes
segregate properly during meiosis. Individuals recombine at different
rates but observed levels of variation in recombination rate remain mostly
unexplained. Genetic dissection of differences in recombination rate
within and between species provides a powerful framework for understanding
how this trait evolves. In this Perspective, I amalgamate published
findings from genetic studies of variation in the genome-wide number of
crossovers  within and between species, and I use exploratory
analyses to identify preliminary patterns. The narrow-sense heritability
of crossover count is consistently low, indicating limited resemblance
among relatives and predicting a weak response to short-term selection.
Variants associated with crossover number within populations span the
range of minor allele frequencies. The size of the additive effect of
recombination-associated variants, along with a negative correlation
between this effect and minor allele frequency, raises the prospect that
mutations inducing phenotypic shifts larger than a few crossovers are
deleterious, though the contributions of methodological bias to these
patterns deserve investigation. Quantitative trait loci that contribute to
differences between populations or species alter crossover number in both
directions, a pattern inconsistent with selection toward a constant
optimum for this trait. Building on this characterization of genetic
variation in crossover number within and between species, I describe
fruitful avenues for future research. Better integrating recombination
rate into quantitative genetics will reveal the balance of evolutionary
forces responsible for genetic variation in this trait that shapes
inheritance.