NONUNIFORM PROCESSES OF CHROMOSOME EVOLUTION IN SEDGES (CAREX: CYPERACEAE)

Holocentric chromosomes—chromosomes that lack localized centromeres—occur in numerous unrelated clades of insects, flatworms, and angiosperms. Chromosome number changes in such organisms often result from fission and fusion events rather than polyploidy. In this study, I test the hypothesis that chr...

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Bibliographic Details
Published in:Evolution 2007-09, Vol.61 (9), p.2175-2194
Main Author: Hipp, Andrew L.
Format: Article
Language:English
Subjects:
Biological Evolution
Biological taxonomies
Brownian motion
Brownian motion model
Carex
Carex Plant - genetics
chromosome evolution
Chromosomes
Chromosomes, Plant
Cyperaceae
Evolution
Evolutionary biology
Grasses
holocentric chromosomes
karyotypic orthoselection
Models, Biological
Original s
Ornstein-Uhlenbeck models
Parametric models
phylogenetic dependence
phylogenetic generalized least squares (GLS)
Phylogenetics
Phylogeny
Ploidies
Sedges
Taxa
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Summary:Holocentric chromosomes—chromosomes that lack localized centromeres—occur in numerous unrelated clades of insects, flatworms, and angiosperms. Chromosome number changes in such organisms often result from fission and fusion events rather than polyploidy. In this study, I test the hypothesis that chromosome number evolves according to a uniform process in Carex section Ovales (Cyperaceae), the largest New World section of an angiosperm genus renowned for its chromosomal variability and species richness. I evaluate alternative models of chromosome evolution that allow for shifts in both stochastic and deterministic evolutionary processes and that quantify the rate of evolution and heritability/phylogenetic dependence of chromosome number. Estimates of Ornstein–Uhlenbeck model parameters and tree-scaling parameters in a generalized least squares framework demonstrate that (1) chromosome numbers evolve rapidly toward clade-specific stationary distributions that cannot be explained by constant variance (Brownian motion) evolutionary models, (2) chromosome evolution in the section is rapid and exhibits little phylogenetic inertia, and (3) explaining the phylogenetic pattern of chromosome numbers in the section entails inferring a shift in evolutionary dynamics at the root of a derived clade. The finding that chromosome evolution is not a uniform process in sedges provides a novel example of karyotypic orthoselection in an organism with holocentric chromosomes.
ISSN:0014-3820
1558-5646
DOI:10.1111/j.1558-5646.2007.00183.x