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 centromeres-is 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 F1 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 F1 individuals, using generalized least squares to fit alternative regression models.

KEY RESULTS

The most informative predictors of germination rates in the F1 generation are chromosome number differences and minimum number of chromosome pairing irregularities in the F1 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 F1 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.