The ability to form interspecific hybrids is often considered to indicate a relatively close relationship between species--the closer the parental species, the more likely they are to form hybrids that are at least partially fertile. Among the 226 interspecific hybrids listed by Cayouette and Catling in their 1992 review of Carex hybridization in North America, about 25% are intersectional hybrids, in some cases between sections thought to be only distantly related. A new phylogenetic hypothesis for Carex based on parsimony analysis of nuclear ribosomal spacer regions and three non-coding chloroplast DNA regions for 220 species from North America, northern Europe, and Japan, reveals clades that are often inconsistent with traditional sectional classification of Carex. Reviewing the pattern of hybridization in light of this new phylogeny indicates that nearly all documented hybridizations in Carex occur between species belonging to the same clade. Furthermore, hybridization is more common in some clades than in others. Molecular markers are also useful in more detailed studies of hybridization and introgression in natural populations. Three species of Carex section Limosae that are known to hybridize differ in trnL intron sequence (cpDNA) and in nuclear ITS sequences, thus giving markers for the parental species that can be easily assessed using specific restriction enzymes. Restriction site analysis of trnL introns amplified from genomic DNA of natural hybrids (confirmed by allozyme analysis) suggests strong bias toward one species as the usual maternal parent. Similar analyses of the ITS region in these hybrids often show ITS sequences identical to the maternal parent, suggesting rapid homogenization of the tandemly repeated ribosomal DNA.

Key words: Carex, Cyperaceae, hybridization, ITS, molecular phylogeny