Eupatorieae

Index to Genera

Eupatorieae is a group of Asteraceae comprising over 2000 species that is primarily New World in geographic distribution (Robinson et al. 2009). It is characterized by having discoid heads that are white or anthocyanic but never yellow in color, as well as typically long and blunt-tipped style appendages. Although traditionally it was compared to Vernonieae or Astereae, molecular phylogenetic data have revealed it to be an ingroup to the traditional Heliantheae s.l., and its sister group is the newly recognized tribe Perityleae. (See Tree of Life web page .)

Morphological Characterization

Eupatorieae have discoid heads in which all of the florets are bisexual and fertile. A few genera, such as Microspermum, have differentiation between outer and inner florets. The corolla color is white, pink, purple, or blue, and never bright yellow. The style branches, and in particular the sterile appendages, are often the colorful part of the inflorescences. The styles are characterized by having paired stigmatic lines at the bases of the style branches and long, sterile appendages which are blunt at the apex. In some species, such as Eupatorium album or Liatris elegans, the phyllaries may be the most conspicuously colored part of the floral display. Anther appendages are present in most genera although unlike in many helianthoid genera they lack glandular trichomes. The cypselae (achenes) become blackish in color at maturity, a trait shared with the helianthoid groups. The pappus is often of numerous capillary bristles, although some genera characteristically have a pappus of blunt-tipped scales, of scales alternating with bristles, or even lack a pappus. The plants are annual or perennial herbs, or may be woody, ranging in size from subshrubs to shrubs, lianas, or rarely small trees. Many genera exhibit vegetative features common among helianthoid genera including basically opposite leaves, the leaf blades with the main vein and lowest pair of lateral veins conspicuously larger than other veins, and the presence of subsessile glandular trichomes on the leaf abaxial surfaces, phyllaries, and corollas.

Classification

The traditional taxonomic conformation of the Eupatorieae is a planetary system with a large genus Eupatorium containing 1200 species central to a satellite-like series of apparently natural genera each characterized by a distinct morphological feature; but their connection to Eupatorium is not specified. King and Robinson (1987) shocked the sunflower community, and by extension those routinely identifying these taxa, by reducing the Eupatorium to approximately 45 species. Although the realignment was not immediately accepted, there has been increasing recognition that, at least at the generic level, the recommended adjustments are justified if not completely accurate. A continued source of discussion is the status of the large number of unispecific genera (65 of 180), a number of which will are probably phenetically but not phylogenetically distinctive and will be lumped with their sister genus. The subtribal system presented by King and Robinson (1987) will require considerable adjustment, and suggests unlikely biogeographical relationships (e.g. their so-called “Eastern Complex” which places taxa of eastern North America and eastern South America together).

Chromosome Numbers

The reported base chromosome numbers for Eupatorieae at the diploid level vary, from x=4 (Fleischmannia microstemon) to x=25 (Neomirandea). The most commonly encountered number (in terms of occurring in different genera) is x=10. The other commonly encountered base numbers include x=17, x=16, and x=11 or 12. A clade of two genera but multiple species has x=9 (Brickellia and Pleurocoronis) and this number is also found (apparently of independent phylogenetic derivation) in Acritopappus. The number of x=15 characterizes a small group of 2-3 aquatic genera, Trichocoronis/Shinnersia (3 species) and Sclerolepis (1 species). Counts of x=13 or 14 are rare or absent. Counts lower than x=9 occur within genera such as Fleischmannia and Stevia where they are clearly derived from a higher base number. Polyploidy also occurs, although where it has been studied it is usually, perhaps always, associated with agamospermy, the most well known examples being in Eupatorium s.s., although it also occurs in Ageratina, Chromolaena/Praxelis, and Stevia.

It must be noted that information on chromosome numbers of Eupatorieae is extremely incomplete. Chromosome numbers have been reported for only a fraction of the species and even of the genera of the tribe (the summary provided in the linked table shows counts for 100 of the 182 listed genera).

Link to a provisional list of reported chromosome counts by genus

Phylogenetic interpretation of variation in chromosome numbers is still problematic. The possibility that x=10 is the original diploid base number for the tribe was challenged early by information from isozyme studies showing that, at least in Eupatorium s.s., species with this number are functionally polyploid. Thus the information from early molecular phylogenetic results that placed genera with high base chromosome numbers such as Ageratina at the base of phylogenetic trees for the family did not come as a complete surprise. It is tempting to look toward a stepwise dysploid reduction series progressing from x=17 or 18 or 19 to x=10, but the available results do not support this. There appears to be a fundamental split at the base of the tribe between genera characterized by the base numbers of x=9 or x=10 from those that have higher chromosome base numbers. Within the first clade, there is an immediate split between genera with x=9 (Brickellia, Pleurocoronis) and those with x=10. Intriguingly, there is a series of genera that form a partly unresolved polytomy or paraphyletic grade at the base of the x=10 that exhibit one or more morphological features that are otherwise considered to be characteristic for x=9 Brickellia, and many of these have been at one time included within the latter genus. At least one other appearance of x=9 appears to have occured independently within the x=10 clade in the South American Acritopappus. In the second clade characterized by base chromosome numbers above x=10, there is again no evidence for stepwise changes, but rather basically a large unresolved polytomy, within which there are individual clades that do appear to be characterized by one or two base chromosome numbers. The highest apparently diploid base chromosome number is Hofmeisteria (x=19) which forms a distinct clade but is not clearly basal. There is some molecular support for grouping Mikania (x=17, 18, 19, 20, 21) with the aquatics of subtribe Trichocoroninae (Sclerolepis, Shinnersia, Trichocoronis, all x=15). The anomalous Macvaughiella has been reported to be x=12, and may also group with the Mikania clade. The large Ageratina, x=17, clade also includes the x=16 Oxylobus, apparently derived through a dysploid change. Another x=16 genus, Bartlettina, appears to be sister to Ageratina. Still other x=16 genera appear to form another clade, including Decachaeta, for which Erythradenia and Mexianthus appear to be clear ingroups, and Eupatoriastrum. Another clade is formed by Pachythamnus (x=17), Neomirandea (x=17, 25) and Kaunia (reported counts are 20-26). Most or all of the genera with x=11 or x=12 (and in several cases both numbers occur within a single genus) may fall into one or perhaps two clades: Stevia (x=11, 12); Carphochaete (x=11, 12); and Microspermum (x=12); also Piqueria (x=11, 12) and its sister group Piqueriopsis (no chromosome count available) may be part of this group, or they may form an independent clade.

Apomixis

The phenomenon of apomixis, which involves asexual reproduction by seed, may play a significant role in generating diversity in some genera of Eupatorieae. Apomixis is always associated with polyploidy, and often with hybridization - it is a way in which hybrid genotypes can be perpetuated indefinitely, even in odd polyploids which would be sterile through the sexual cycle of meiosis and fertilization. An extensive series of apomictic polyploids has been documented for Eupatorium in eastern North America (Sullivan 1972) which is being further elucidated using molecular data. Apomixis in Eupatorieae is reviewed by Noyes (2007), who indicates that it has been proposed or documented in: Ageratina, Campovassouria, Chromolaena, Eupatorium, Gyptis, and Praxelis.

Chemistry

Eupatorieae present collectively a rich and varied assortment of secondary phytochemicals and has been fertile ground for phytochemists seeking to report novel substances. Flavonoid aglycones (including relatively uncommon poly-methylated types) and sesquiterpene lactones are frequently sequestered in the almost ubiquitous subsessile glandular trichomes. The presence of assorted alkaloids, triterpenoids, and other compounds has also been reported. Unfortunately to date the secondary chemistry has offered little insight into the major issues surrounding classification or phylogenetic relationships within the tribe.

References

Ito, M., T. Yahara, R. M. King, K. Watanabe, S. Oshita, J. Yokoyama, and D. J. Crawford. 2000. Molecular phylogeny of Eupatorieae (Asteraceae) estimated from cpDNA RFLP and its implication for the polyploid origin hypothesis of the tribe. Journal of Plant Research 113: 91-96.

King, R. M. and H. Robinson. 1987. The genera of the Eupatorieae (Asteraceae). Monographs in Systematic Botany, Missouri Botanical Garden 22: 1-581.

Noyes, R. 2007. Apomixis in the Asteraceae: Diamonds in the rough. Functional Plant Science and Biotechnology 1: 207-222.

Robinson, H., E. Schilling, and J. L. Panero. 2009. Eupatorieae. Pp. 731-744 in Systematics, Evolution, and Biogeography of Compositae, eds. V. A. Funk, A. Susanna, T. F. Stuessy, and R. J. Bayer. Vienna: I.A.P.T. pdf

Schilling, E. E., P. B. Cox, and J. L. Panero. 1999. Chloroplast DNA restriction site data support a narrowed interpretation of Eupatorium (Asteraceae). Plant Systematics and Evolution 219: 209-223.

Schmidt, G.J. and E.E. Schilling. 2000. Phylogeny and biogeography of Eupatorium (Asteraceae: Eupatorieae) based on nuclear ITS sequence data. American Journal of Botany 87: 716-726.

Sullivan, V. I. 1972. Investigations of the breeding systems, formation of auto- and alloploids and the reticulate pattern of hybridization in North American Eupatorium (Compositae). Ph.D. Dissertation, Florida State University, Tallahassee.

Turner, B. L. 1997. The Comps of Mexico. Volume 1. Eupatorieae. Phytologia Memoirs 11: 1-272.

Programming help provided by Daniel Schilling