HoloChromEvol · HOLOCENTRIC CHROMOSOME EVOLUTION AND THE ORIGINS OF BIODIVERSITY IN A HYPER-DIVERSE PLANT LINEAGE

Holocentric chromosomes are commonly referred to as having a diffuse centromere. Holocentric chromosome organization has been described for three of the six supergroups in the domain Eukarya (the Eukaryotes): plants (angiosperms, algae and mosses), animals (numerous arthropod clades, velvet worms, and nematodes), and Rhizaria.It has long been recognized that chromosome fragments that would be lost in monocentric chromosomes may be propagated and become fixed in organisms with holocentric chromosomes. In addition to fission and fusion, which may be only weakly underdominant or nearly neutral in holocentric chromosomes, holocentric chromosome structure facilitates translocations and inversions.In the sedge genus Carex (Cyperaceae), 2n = 12 – 124, chromosome rearrangements contribute to genetic diversity within species. These findings suggest that holocentry is an important determinant of biodiversity patterns in the wide range of lineages in which it occurs. Holocentry undoubtedly also plays an important role in the evolution of recombination rates which may be selected by the environment. There is another dimension to chromosome evolution in organisms with holocentric chromosomes: Chromosome rearrangements may suppress recombination (islands of speciation theory).The goals of this study are to answer four fundamental questions: 1) Do chromosome rearrangements protect ecologically significant genome regions from recombination? 2) To what extent do chromosome rearrangements decrease the fitness of first- and second-generation interpopulation crosses? 3) What are the relative contributions of hybrid dysfunction (decreased fitness of hybrids between individuals with differing chromosome numbers) and recombination suppression to chromosomal speciation? and 4) Do the population dynamics of sedges allow for rapid establishment of chromosome variants, even in the face of underdominance of those mutations?