Meiotic effects of MSH4 copy number variation support an adaptive role for post-polyploidy gene loss

Many eukaryotes descend from polyploid ancestors that experienced massive duplicate gene loss. This genomic erosion is particularly strong for duplicated (meiotic) recombination genes that return to a single copy more rapidly than genome average following polyploidy. To better understand the evolutionary forces underlying duplicate loss, we analysed how varying copy numbers of MSH4, an essential meiotic recombination gene, influences crossover formation in allotetraploid Brassica napus. We show that faithful chromosome segregation and crossover frequencies between homologous chromosomes are unchanged with MSH4 duplicate loss; by contrast, crossovers between homoeologous chromosomes (which result in genomic rearrangements) decrease with reductions in MSH4 copy number. We also found that inter-homoeologue crossovers originate almost exclusively from the MSH4-dependent crossover pathway. Limiting the efficiency of this pathway by decreasing the copy number of key meiotic recombination genes could therefore contribute to adaptation to polyploidy, by promoting regular chromosome segregation and genomic stability.