Chromosomal rearrangements in holocentric organisms lead to reproductive isolation by hybrid dysfunction: The correlation between karyotype rearrangements and germination rates in sedges

PREMISE OF THE STUDY: Understanding the drivers of speciation is a central task of evolutionary biology. Chromosomal rearrangements are known to play an important role in species diversification, but the role of rearrangements of holocentric chromosomes-chromosomes without localized centromeresis poorly understood. METHODS: We made numerous artificial crosses between Carex scoparia individuals of different diploid chromosome numbers and, for comparison, between individuals of the same chromosome number. We studied chromosome pairing and chromosomal rearrangements in the F-1 individuals using light microscopy. We then estimated germination rates as a function of geographic distance, genetic distance, chromosome number differences in parents, and pairing irregularities in F-1 individuals, using generalized least squares to fit alternative regression models. KEY RESULTS: The most informative predictors of germination rates in the F-1 generation are chromosome number differences and minimum number of chromosome pairing irregularities in the F-1 individuals. Genetic and geographic distances between parents are not significant predictors. CONCLUSIONS: Holocentric chromosomal rearrangements play an important role in postzygotic reproductive isolation in Carex through F-1 hybrid inviability and sterility. Hybrid dysfunction seems to be a suitable model for chromosomal speciation when there are several chromosomal rearrangements between parents. However, we have not tested the hypothesis that genome rearrangements may also play an important role in suppressing recombination between cytogenetically divergent populations.