Abstract
BackgroundThe North American Agalinis are representatives of a taxonomically difficult group that has been subject to extensive taxonomic revision from species level through higher sub-generic designations (e.g., subsections and sections). Previous presentations of relationships have been ambiguous and have not conformed to modern phylogenetic standards (e.g., were not presented as phylogenetic trees). Agalinis contains a large number of putatively rare taxa that have some degree of taxonomic uncertainty. We used DNA sequence data from three chloroplast genes to examine phylogenetic relationships among sections within the genus Agalinis Raf. (=Gerardia), and between Agalinis and closely related genera within Orobanchaceae.ResultsMaximum likelihood analysis of sequences data from rbcL, ndhF, and matK gene regions (total aligned length 7323 bp) yielded a phylogenetic tree with high bootstrap values for most branches. Likelihood ratio tests showed that all but a few branch lengths were significantly greater than zero, and an additional likelihood ratio test rejected the molecular clock hypothesis. Comparisons of substitution rates between gene regions based on linear models of pairwise distance estimates between taxa show both ndhF and matK evolve more rapidly than rbcL, although the there is substantial rate heterogeneity within gene regions due in part to rate differences among codon positions.ConclusionsPhylogenetic analysis supports the monophyly of Agalinis, including species formerly in Tomanthera, and this group is sister to a group formed by the genera Aureolaria, Brachystigma, Dasistoma, and Seymeria. Many of the previously described sections within Agalinis are polyphyletic, although many of the subsections appear to form natural groups. The analysis reveals a single evolutionary event leading to a reduction in chromosome number from n = 14 to n = 13 based on the sister group relationship of section Erectae and section Purpureae subsection Pedunculares. Our results establish the evolutionary distinctiveness of A. tenella from the more widespread and common A. obtusifolia. However, further data are required to clearly resolve the relationship between A. acuta and A. tenella.
Highlights
The North American Agalinis are representatives of a taxonomically difficult group that has been subject to extensive taxonomic revision from species level through higher subgeneric designations
We show that Agalinis is sister to a group composed of the gerardioid genera with yellow or red corollas: Aureolaria, Brachystigma, Dasistoma, and Seymeria with a bootstrap value of 0.99 (Figure 2)
One of our objectives was to evaluate the evolutionary distinctiveness of A. tenella from A. obtusifolia, and A. acuta from A. tenella
Summary
The North American Agalinis are representatives of a taxonomically difficult group that has been subject to extensive taxonomic revision from species level through higher subgeneric designations (e.g., subsections and sections). Agalinis (including Tomanthera Raf.) is a genus of from 40 to 70 species (depending on the taxonomy used) distributed in the eastern part of the United States, Mexico, Central America and South America. The North American Agalinis species are less variable and all have pink-purple, membranaceous, ephemeral corollas, typically with red spots on the anterior lobes. Many species are hemiparasitic; Agalinis represents the largest genus of hemiparasitic plants in the eastern United States. Most North American species are restricted to the coastal plain of the southern and eastern United States where they occupy a range of habitats including dry, sandy pine barrens, grasslands, and edges of wetlands including bogs, ponds, and salt marshes [3,4,14]. Off the coastal plain Agalinis species are found in prairie habitats, other grasslands, and open habitats within shrublands or woodlands
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