Dicerandra, an endemic mint of the southeastern United States, comprises nine species, all of which are threatened or endangered and restricted to sandhill vegetation and a mosaic of scrub habitats. Molecular phylogenetic analyses of Dicerandra based on data from the nuclear and plastid genomes for all 13 taxa of the genus, identified two strongly supported clades, corresponding to the four annual and to the five perennial species of Dicerandra. However, the nuclear and plastid trees were incongruent in their placement of two perennial taxa, D. cornutissima and D. immaculata var. savannarum, perhaps due to ancient hybridization or to lineage sorting. Based on these analyses, the widespread D. linearifolia is not monophyletic, with populations of D. linearifolia var. linearifolia falling into either western or eastern clades. The western clade, comprising populations of D. linearifolia var. linearifolia and var. robustior, occurs in an area drained by rivers flowing toward the Gulf of Mexico, whereas the eastern clade, comprising populations of D. linearifolia var. linearifolia, D. densiflora, D. odoratissima, and D. radfordiana (i.e., all the annual species), occupies a region drained by rivers flowing to the Atlantic Ocean. Although this pattern of genetic differentiation between populations from these two river drainages has been documented in several animal species, it has not previously been reported for plants. A revised subgeneric classification is presented to reflect the annual and perennial clades.
Chromosome counts are reported for the first time in Dicerandra Benth. (Labiatae), a genus endemic to the south-eastern United States. Representative taxa are tetraploid (« = 16) and hexaploid (n = 24). Diploids were not found. The systematic, phytogeographic and conservation implications of polyploidy in this genus are discussed. The monophyletic nature of the genus is upheld by the discovery that both sections have a presumed base number of x = 8. Hexaploidy has apparently evolved independently in all branches of sect. Dicerandra, but not in sect. Lecontea.
A new subspecies, Dicerandra frutesceris Slii nners subsp. modesta (Labiatae), is de¬ scribed for Florida. Occurring only in Polk County, tliis new taxon is distinguished from D. frutescens subsp .frutescens in Highlands County by the num¬ ber of flowers per cyme, corolla coloration, shape of the upper lip of the corolla, and chromosome number. A new variety, D. immaculata Lakela var. savannarum, is described from east coastal Florida and is separated from D. immaculata var. imma¬ culata by its wider leaves and lax habit.
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