Sedges (Carex: Cyperaceae) exhibit remarkable agmatoploid chromosome series between and within species. This chromosomal diversity is due in large part to the structure of the holocentric chromosomes: fragments that would not be heritable in organisms with monocentric chromosomes have the potential to produce viable gametes in organisms with holocentric chromosomes. The rapid rate of chromosome evolution in the genus and high species diversification rate in the order (Cyperales Hutch., sensu Dahlgren) together suggest that chromosome evolution may play an important role in the evolution of species diversity in Carex. Yet the other genera of the Cyperaceae and their sister group, the Juncaceae, do not show the degree of chromosomal variation found in Carex, despite the fact that diffuse centromeres are a synapomorphy for the entire clade. Moreover, fission and fusion apparently account for the majority of chromosome number changes in Carex, with relatively little duplication of whole chromosomes, whereas polyploidy is relatively important in the other sedge genera. In this paper, we review the cytologic and taxonomic literature on chromosome evolution in Carex and identify unanswered questions and directions for future research. In the end, an integration of biosystematic, cytogenetic, and genomic studies across the Cyperaceae will be needed to address the question of what role chromosome evolution plays in species diversification within Carex and the Cyperaceae as a whole.
Section Ovales is the most species-rich section of the sedge genus Carex in the New World. Phylogenetic analyses of molecular data recover a predominantly New World clade as sister to a solitary east Asian species, C. maackii. Nuclear ribosomal DNA are congruent in the placement of all taxa within the section, with a solitary exception: incongruence between ITS and ETS data in the placement of C. bonplandii and C. roraimensis suggests a hybrid origin for this lineage. Biogeography correlates strongly with phylogeny in the section, but there have been at least two instances of long-range dispersal, one from an eastern North American clade to western North America and one from the New World to Eurasia. Morphological characters studied are all homoplastic. Developing a comprehensive infrasectional classification with a phylogenetic basis would be complicated by the fact that most of the novel morphological characters in the section have evolved within relatively small, independent clades.
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.
Phylogenetic analysis of amplified fragment length polymorphisms (AFLP) was used to infer patterns of morphologic and chromosomal evolution in an eastern North American group of sedges (ENA clade I of Carex sect. Ovales). Distance analyses of AFLP data recover a tree that is topologically congruent with previous phylogenetic estimates based on nuclear ribosomal DNA (nrDNA) sequences and provide support for four species groups within ENA clade I. A maximum likelihood method designed for analysis of restriction site data is used to evaluate the strength of support for alternative topologies. While there is little support for the precise placement of the root, the likelihood of topologies in which any of the four clades identified within the ENA clade I is forced to be paraphyletic is much lower than the likelihood of the optimal tree. Chromosome counts for a sampling of species from throughout sect. Ovales are mapped onto the tree, as well as counts for all species in ENA clade I. Parsimony reconstruction of ancestral character states suggest that: (1) Heilborn's hypothesis that more highly derived species in Carex have higher chromosome counts does not apply within sect. Ovales, (2) the migration to eastern North America involved a decrease in average chromosome count within sect. Ovales, and (3) intermediate chromosome counts are ancestral within ENA clade I. A more precise understanding of chromosomal evolution in Carex should be possible using likelihood analyses that take into account the intraspecific polymorphism and wide range of chromosome counts that characterize the genus.
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