Chromosomes tell half of the story: the correlation between karyotype rearrangements and genetic diversity in sedges, a group with holocentric chromosomes

Abstract

Chromosome rearrangements may affect the rate and patterns of gene flow within species, through reduced fitness of structural heterozygotes or by reducing recombination rates in rearranged areas of the genome. While the effects of chromosome rearrangements on gene flow have been studied in a wide range of organisms with monocentric chromosomes, the effects of rearrangements in holocentric chromosomes--chromosomes in which centromeric activity is distributed along the length of the chromosome--have not. We collected chromosome number and molecular genetic data in Carex scoparia, an eastern North American plant species with holocentric chromosomes and highly variable karyotype (2n = 56-70). There are no deep genetic breaks within C. scoparia that would suggest cryptic species differentiation. However, genetic distance between individuals is positively correlated with chromosome number difference and geographic distance. A positive correlation is also found between chromosome number and genetic distance in the western North American C. pachystachya (2n = 74-81). These findings suggest that geographic distance and the number of karyotype rearrangements separating populations affect the rate of gene flow between those populations. This is the first study to quantify the effects of holocentric chromosome rearrangements on the partitioning of intraspecific genetic variance.

Comparative cytogenetics of Auchenorrhyncha (Hemiptera, Homoptera): a review

“…As a result, chromosome fragments that would be acentric (lacking a centromere) and hence lost in organisms with monocentric chromosomes may be inherited in holokinetic organisms. The gametes harboring chromosome fragments are consequently expected to be viable (Hipp et al 2010). Fusion/fission rearrangements are therefore conventionally accepted as the commonest mechanisms of chromosome evolution in holokinetic groups.…”

The Glanville fritillary genome retains an ancient karyotype and reveals selective chromosomal fusions in Lepidoptera

“…Due to the holocentric chromosome structure with dispersed kinetochore activity, such events have been expected to be less deleterious than in monocentric chromosomes1112. Conversely, holocentricity may restrict gene flow13 through meiotic14 and recombination suppression mechanisms15.…”

Cytological Studies on Seven Species of Korean <i>Carex</i> (Cyperaceae)

“…The cytogenetic variance is hypothesized to be due to the non-localized centromeres (holocentric chromosomes), which facilitate chromosome number increases (agmatoploidy) and/or decreases (symploidy) by fission and fusion, respectively (Hoshino 1981, Nishikawa et al 1984, Luceño and Guerra 1996, Chung et al 2011, Hipp et al 2013. Cytological studies have been very informative on taxonomy and phylogeny of Carex , Yano et al 2010 and high chromosome number variations have been hypothesized to play an important role on rapid speciation in the genus (Hipp 2007, Hipp et al 2010, Chung et al 2012, Escudero et al 2012.…”