MonocotyledonsWilliam J. Hahn
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The Monocotyledons are one of the most distinctive major lineages of angiosperms and traditionally have been paired with the Dicotyledons as the two main groups of flowering plants. Among the monocots are some of the largest families of angiosperms (such as the orchids with ca. 20,000 species and the grasses with ca. 15,000 species) as well as some of the most economically important species of plants. Numerous vegetation types are dominated by monocots including grasslands, palm savannas, sedge meadows, and cattail marshes.
The shared, derived character that unites all monocots is the single cotyledon, a feature first noted by John Ray in 1703. Other features that are common in monocots but not necessarily unique to or universal among them are: vascular bundles irregularly distributed in cross section of the stem, parallel veins in the leaves (mostly in the more derived groups), and flower parts in multiples of three. Despite the lack of true secondary growth in monocots, most growth habits are found in the group including floating and submerged aquatics, lianas, trees, epiphytes, and forbs of all sizes.
Relationships of Monocots to other Angiosperms
A singular origin for the monocots is generally accepted with numerous morphological synapomorphies proposed for the group (Donoghue and Doyle, 1989; Doyle and Donoghue, 1992; Tucker and Douglas, 1996). Additionally, most molecular analyses of monocots have provided strong support for their monophyly (Chase, et al., 1993, 1995; Duvall, et al., 1993; Hamby and Zimmer, 1992).
Relative to other angiosperms, the monocots are most frequently placed among a group of predominantly herbaceous magnoliid taxa (Lactoridaceae, Aristolochiaceae, Piperaceae, Sauraraceae, Nymphaeaceae) collectively referred to as the "paleoherbs" (Donoghue and Doyle, 1989). The specific relationships among these taxa are still unclear but a close relationship of the monocots with either the woody magnoliid taxa or the more derived, tricolpate, "eudicots" is not supported in any analysis.
The fossil record of the Monocotyledons is old with the first probable monocot remains dating to the Early Cretaceous. The presence of morphologically diverse fossils by the Campanian suggests a relatively rapid radiation into most of the extant major groups early in the course of monocot evolution (Herendeen and Crane, 1995).
Discussion of Phylogenetic Relationships
The terminal taxa in this phylogeny are based on the major groups recognized and delimited by Dahlgren, et al. (1985) and Thorne (1992a, b) with several modifications. Although earlier work by Huber (1969, 1977) provided the context for a more explicit analysis of monocot relationships, Dahlgren and co-workers were the first to utilize cladistic approaches to the study of monocot phylogeny and their taxonomy is still the most complete. As such, it has served as the basis for most recent studies of the monocots (e.g., Rudall, et al, 1995). Alternate taxonomic systems of Cronquist (1981), Thorne (1992a, b) and Takhtajan (1980, 1987) are largely congruent with that of Dahlgren, et al. (1985) but differ in treatment of lilioid and basal monocots. The systems of Dahlgren and Thorne maintain numerous smaller families in the Lilianae as opposed to a few relatively large ones and place lilioid elements at the base of the monocots rather than the Alismatanae and Aranae.
The phylogenetic hypothesis represented here is based largely on the rbcL reconstructions of Duvall, et al. (1993a, b) and Chase, et al. (1995a) with consideration of the morphological studies of Dalhgren, et al. (1983, 1985) and Stevenson and Loconte (1995; see also Chase, et al., 1995b) as well as the nrDNA 18S SSU studies of Hamby and Zimmer (1992) and Bharathan and Zimmer (1995). Resolution within and among certain subgroups is predominantly based on various works presented in Rudall, et al. (1995).
The general relationships depicted in this tree are recovered in most analyses although taxon sampling is not completely comparable from study to study. Differences in topology concern the nature of the basal lineages (specifically the position of the alismatid/aroid and the dioscorealean/melanthioid lineages), placement of certain enigmatic taxa such as the Cyclanthaceae, Eriocaulaceae, Hanguanaceae, Pandanaceae, Velloziaceae, and Xyridaceae, and resolution within the Lilianae and Commelinanae.
While some earlier studies had suggested basal positions for groups such as the palms (Arecanae), most recent analyses place either the Alismatanae and Aranae (e.g., Duvall, et al., 1993a, 1993b; Chase, et al., 1993, 1995), or members of the lilioid orders Dioscoreales (e.g., Huber, 1969; Dahlgren et al. 1983, 1985; Stevenson and Loconte, 1995) and Melanthiales (e.g., Thorne, 1992; Takhtajan, 1969) as sister to the remainder of the monocots. Much of the difference seen in these two alternative hypotheses is due to uncertainties over the position of monocots within the angiosperms and attendant ambiguity in outgroup choice. Character state homologies are not obvious in many cases and alternate outgroups provide different character state polarizations. The current weight of evidence, however, seems to support the alismatid/aroid hypothesis. An example of this is the trans-spliced structure of the 4th intron in the mitochondrial gene nad1 shared by all monocots sampled except Araceae and Alismataceae (Qui & Palmer, 1996).
Bharathan, G & EA Zimmer. 1995. pp 81-107 in: PJ Rudall, PJ Crib, DF Cutler & CJ Humphries. (eds.) Monocotyledons: systematics and evolution,. Royal Botanic Garden, Kew.
Chase, M. W. 2004. Monocot relationships: an overview. American Journal of Botany 91: 1645-1655.
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Georgetown University, Washington, D. C., USA
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