Source: http://live.tolweb.org/Angiosperms/20646
Timestamp: 2019-04-25 01:54:16+00:00

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Relationships after Qiu et al. (1999, 2000), P. Soltis et al. (1999), D. E. Soltis et al. (2000), Zanis et al. (2002) and Hilu et al. (2003).
The angiosperms, or flowering plants, are one of the major groups of extant seed plants and arguably the most diverse major extant plant group on the planet, with at least 260,000 living species classified in 453 families (Judd et al., 2002; APG II, 2003; Soltis et al., 2005). They occupy every habitat on Earth except extreme environments such as the highest mountaintops, the regions immediately surrounding the poles, and the deepest oceans. They live as epiphytes (i.e., living on other plants), as floating and rooted aquatics in both freshwater and marine habitats, and as terrestrial plants that vary tremendously in size, longevity, and overall form. They can be small herbs, parasitic plants, shrubs, vines, lianas, or giant trees. There is a huge amount of diversity in chemistry (often as a defense against herbivores), reproductive morphology, and genome size and organization that is unparalleled in other members of the Plant Kingdom. Furthermore, angiosperms are crucial for human existence; the vast majority of the world's crops are angiosperms, as are most natural clothing fibers. Angiosperms are also sources for other important resources such as medicine and timber.
Despite their diversity, angiosperms are clearly united by a suite of synapomorphies (i.e., shared, derived features) including 1) ovules that are enclosed within a carpel, that is, a structure that is made up of an ovary, which encloses the ovules, and the stigma, a structure where pollen germination takes place, 2) double fertilization, which leads to the formation of an endosperm (a nutritive tissue within the seed that feeds the developing plant embryo), 3) stamens with two pairs of pollen sacs, 4) features of gametophyte structure and development, and 5) phloem tissue composed of sieve tubes and companion cells (see Doyle and Donoghue, 1986; Judd et al. 2002; P. Soltis et al., 2004; and D. Soltis et al., 2005, for further discussion). All available evidence strongly rejects hypotheses of more than one evolutionary origin of extant angiosperms.
Most analyses of the past five years concur in placing the monotypic Amborella as the sister to all other extant angiosperms, although some analyses suggest Amborella plus water lilies may occupy this pivotal position (see below). Amborella trichopoda, endemic to cloud forests of New Caledonia, was described in the mid-nineteenth century (Baillon, 1869) and has since been classified with various groups of basal angiosperms, most often with Laurales (e.g., Cronquist, 1981), a group of magnoliids (see below). However, Amborella lacks those features considered to be synapomorphies for Laurales (Doyle and Endress, 2000; see Laurales later) and clearly differs from most Laurales in having spirally arranged floral organs (except perhaps the carpels; M. Buzgo et al., University of Florida, unpublished data), rather than the whorled arrangement typical of most Laurales (see studies of floral morphology and development by Endress and Igersheim, 2000a; Posluszny and Tomlinson, 2003; Buzgo et al., 2004). Amborella has carpels that are closed only by secretion, rather than by fused tissue as in most angiosperms (Endress and Igersheim, 2000b)–a feature that may represent a plesiomorphy (i.e., ancestral feature) for the angiosperms. Features that unify all extant angiosperms except Amborella include vessels (Judd et al., 2002; but see Feild et al., 2000; Doyle and Endress, 2001) and pollen grains with a reticulate tectum (Doyle and Endress, 2001). Synapomorphies for all extant angiosperms except Amborella and Nymphaeaceae (sensu APG II, 2003) include ethereal oil cells–common throughout basal angiosperms–and columellate pollen grains with a perforate tectum (Doyle and Endress, 2001).
Photo of Amborella trichopoda (Amborellaceae; photo © Sangtae Kim).
The placement of Amborella as sister to all other angiosperms is supported by nearly all multigene analyses of basal angiosperms, including evidence from all three plant genomes (e.g., P. Soltis et al., 1999; Qiu et al., 1999; Mathews and Donoghue, 1999, 2000; Parkinson et al., 1999; Graham and Olmstead, 2000; Graham et al., 2000; D. Soltis et al., 2000; Magallon and Sanderson, 2002; Zanis et al., 2002; see also Nickerson and Drouin, 2004). A few studies have found alternative rootings, using either different genes or different methods of analysis. For example, Amborella + Nymphaeaceae (e.g. Barkman et al., 2000; P. Soltis et al., 2000; Kim et al., 2004a) or Nymphaeaceae alone (e.g., Parkinson et al., 1999; Graham and Olmstead, 2000, with partial sampling of Nymphaeaceae; Mathews and Donoghue, 2000) have occasionally been reported as sister to all other angiosperms. However, statistical analyses of these alternative rootings using a data set of up to 11 genes generally favor the tree with Amborella as sister to the rest, although the Amborella + Nymphaeaceae tree could not always be rejected (Zanis et al., 2002).
Another alternative, which conflicts with all molecular analyses of angiosperms with dense taxon sampling, is based on a selection of 61 genes from the totally sequenced plastid genomes of 13 plant species (Goremykin et al., 2003). This data set placed the monocots (represented by only three grasses–rice, maize, and wheat) as the sister to all other extant angiosperms, while Amborella was sister to Calycanthus of Laurales, a position consistent with the original description of Amborella, but clearly at odds with other aspects of morphology (see Laurales section). Goremykin et al. (2003) attributed their results to the increased character sampling (30,017 nucleotides in their aligned matrix) in their study relative to other analyses that included fewer genes but many more taxa. However, further analyses of a data set of three genes and nearly equivalent taxon sampling as used by Goremykin et al. indicated that the "monocots-basal" topology is an artifact of limited taxon sampling (D. Soltis and Soltis, 2004; Soltis et al., 2004). The recent work of Stefanovic et al. (2005) further indicates that the results of Goremykin et al. reflect poor taxon sampling. Stefanovic et al. added the nearly complete plastid sequence of Acorus to the Goremykin et al. data set, and this simple addition resulted in the "Amborella-basal" topology.
The basal angiosperms represent a grade that includes the following groups: Amborellaceae (discussed above), Nymphaeaceae (sensu APG II, 2003), Austrobaileyales, Ceratophyllaceae, Chloranthaceae, magnoliids, and monocots (although not all investigators choose to consider monocots basal angiosperms). The evolutionary relationships of these groups are discussed below.
Photos of Nuphar japonica sp. (Nymphaeaceae; photo © Sangtae Kim), Austrobaileya sp. (Austrobaileyales; photo © Peter Endress) and Illicium floridanum (Schisandraceae sensu APG II (2003); photo © Doug Soltis).
Nymphaeaceae. The phylogenetic position of Nymphaeaceae as one of the two basalmost (or oldest diverging) lineages of extant angiosperms is strongly supported by nearly all molecular analyses. This clade of eight aquatic genera has a worldwide distribution, consistent with the ancient age of this lineage based on the fossil record. However, despite the ancient origins of this group, molecular analyses indicate that most extant genera of Nympheaceae have relatively recent origins (Yoo et al. 2005).
Austrobaileyales. The positions of Amborellaceae and Nymphaeaceae as successive sisters to the rest of the angiosperms are followed, in turn, by Austrobaileyales. Austrobaileyales are a small clade that comprises Austrobaileyaceae (Austrobaileya) and Trimeniaceae (Trimenia) from Australasia plus a broadly defined Schisandraceae (sensu APG II, 2003, i.e., Schisandra, Kadsura, and Illicium; Qiu et al., 1999; Renner, 1999; Savolainen et al., 2000a, b; P. Soltis et al., 1999; D. Soltis et al., 2000). Although the traditional Illiciaceae and Schisandraceae have typically been considered to be closely related, a relationship between these taxa and Austrobaileya and Trimenia had not been suspected. No morphological characteristics have been identified that unify this group, despite the strong molecular support for its monophyly.
Resolution and support for relationships among Ceratophyllaceae, monocots, Chloranthaceae, magnoliids, and eudicots are poor. Individually, each lineage is well supported, and both the fossil record and molecular-based trees identify these lineages as ancient. However, relationships among them remain unclear. It is clear, however, that angiosperms do not fall into two major groups that correspond to monocots (Liliopsida) and dicots (Magnoliopsida) of most traditional classification systems such as Cronquist (1981), Takhtajan (1997), and their predecessors. Although monocots clearly form a strongly supported group, dicots in the traditional sense do not: most are found in the eudicot clade, but the remaining nonmonocot basal branches (i.e., Amborellaceae, Nymphaeaceae, Austrobaileyales, Ceratophyllaceae, Chloranthaceae, magnoliids) were also "traditional" dicots. It was long suspected that the "dicots" as traditionally recognized represents multiple evolutionary lineages, and the nonmonophyly of dicots based on molecular data precludes their recognition in current classifications (e.g., APG II, 2003). Thus, the concept of "dicot" should be abandoned in favor of eudicots.
Monocots. While dicots are not a valid group, the monocots indeed are a distinct group within the angiosperms. Ray (1703) first identified the monocots as a group, based largely on their possession of a single cotyledon. Nonmolecular phylogenetic studies of monocots (Doyle and Donoghue, 1992; Donoghue and Doyle, 1989; Loconte and Stevenson, 1991) support this grouping; these studies have identified 13 putative synapomorphies for the monocots, including, among others, a single cotyledon, parallel-veined leaves, sieve cell plastids with several cuneate protein crystals, scattered vascular bundles in the stem, and an adventitious root system. An often-overlooked synapomorphy for monocots is their sympodial growth; although there are other angiosperms with sympodial growth, monocots are nearly exclusively so. These synapomorphies are covered in detail by Chase (2004; see also Judd et al., 2002; D. Soltis et al., 2005). In some analyses the monocots appear as the sister to Ceratophyllaceae, with the clade of monocots + Ceratophyllaceae sister to Chloranthaceae + magnoliids + eudicots (e.g., Zanis et al., 2002; Davies et al., 2004). However, relationships of monocots are unclear and further work is needed to assess the validity of their relationships.
Ceratophyllaceae. Ceratophyllaceae (Ceratophyllum) had the distinction of appearing as the sister to all other angiosperms in the first large molecular phylogenetic analysis based on rbcL (Chase et al., 1993). The aquatic habit and simple flowers seemed at odds with most hypotheses about the earliest angiosperms, although Ceratophyllum has a long fossil record, going back at least 125 mya (Dilcher, 1989). Subsequent analyses demonstrated that this placement was unique to the rbcL data set. Our current understanding of the relationship of Ceratophyllum to other angiosperms, based on evidence from many other genes, is still not clear although it may be related to the monocots (see Monocots above).
Chloranthaceae. Chloranthaceae, with their small, simple flowers, have an extensive fossil record, dating back 125 my (e.g., Couper, 1958; Walker and Walker, 1984; Friis et al., 2000; Doyle et al., 2004; Eklund et al., 2004). However, although the origins of the family are ancient, most extant genera are relatively young in age (Zhang and Renner, 2003). Chloranthaceae are clearly an isolated lineage separate from the magnoliid clade (Fig. 2), but their phylogenetic position remains uncertain. In some analyses (e.g., Zanis et al., 2002; Davies et al., 2004), they are sister to a clade of magnoliids + eudicots, albeit with weak support.
Magnoliids. The magnoliid clade comprises most of those lineages typically referred to as "primitive angiosperms" in earlier works (e.g., Cronquist, 1981, 1988; Stebbins, 1974; Takhtajan, 1997). Although the component families of the magnoliid clade were loosely associated in previous classifications, for example, as Cronquist's (1981) subclass Magnoliidae, relationships among the families were not clear. In addition, Magnoliidae contained groups that are not part of the magnoliid clade as recognized by phylogenetic analyses. Reconstructing relationships within this clade is challenging, given the age of the group (some putative members, such as Archaeanthus, Dilcher and Crane, 1984, date to the early Cretaceous) and presumably high levels of extinction. Although the major lineages of the magnoliid clade were identified as well-supported clades in earlier studies (e.g., P. Soltis et al., 1999), composition and interrelationships of the magnoliid clade did not become clear until data sets of at least five genes for a broad sample of taxa were assembled to address these problems (e.g., Qiu et al., 1999, 2000; Zanis et al., 2002). Some phylogenetic analyses weakly support a grouping of magnoliids as sister to the eudicots, although more data are necessary to clarify this relationship.
Eudicots. Eudicots, a clade strongly supported by molecular data, comprise the bulk of angiosperm species (approximately 75%; Drinnan et al., 1994). This large clade is supported by a single morphological synapomorphy–triaperturate pollen. This pollen type is unique to the eudicots, and while not all eudicots have triaperturate pollen due to subsequent changes in pollen structure, triaperturate pollen is clearly distinct from the uniaperturate pollen of basal angiosperms, monocots, and all other seed plants, allowing easy assignment of fossil pollen to the eudicots. The fossil pollen record indicates that the eudicots appeared 125 mya, shortly after the origin of the angiosperms themselves. The extensive fossil pollen collections worldwide, coupled with solid dates, make it unlikely that the eudicots arose much before this time point.
The oldest unambiguous angiosperm fossil extends back at least to the early Cretaceous, conservatively around 132 million years ago (mya) (see Crane et al., 2004). Floral size, structure, and organization in early angiosperms varied tremendously, ranging from small (i.e., <1 cm in diameter) flowers of fossil Chloranthaceae and many other lineages (reviewed in Friis et al., 2000), to the large, Magnolia-like flowers of Archaeanthus (Dilcher and Crane, 1984). However, some early fossils, such as Archaefructus (Sun et al. 2002), appear to have no close extant relatives. The floral diversity in the fossil record is consistent with an early radiation of angiosperms and a diversification in floral form (e.g., Friis et al., 2000).
Initial attempts to estimate the age of the angiosperms and the timing of important divergences based on molecular data span a wide time range (ranging from ~125 to > 400 mya) and many disagree with dates determined from the fossil record (see Sanderson and Doyle, 2001; P. Soltis et al. 2002; Sanderson et al., 2004; Bell et al., 2005). However, more recent efforts to date the origin of the angiosperms using molecular data and improved dating methods have converged on estimates between 180-140 mya, predating the dates inferred from the fossil record by between 45 to 5 million years (Sanderson et al., 2004; Bell et al., 2005). Estimated ages for specific angiosperm clades using molecular estimates are also generally older than inferences from the fossil record (e.g., Wikström et al., 2001, compared with Magallon et al., 1999), but these discrepancies are generally small. Work in the future will likely focus on further reconciliation of age estimates inferred from fossils and molecular data. For example, given the numerous diverse fossils reported from as early as 115-125 mya, perhaps the earliest angiosperms were older than the estimate from the fossil record of 132 mya. Conversely, molecular methods tend to overestimate ages (Rodríguez-Trelles et al., 2002), so refinement of dating approaches is needed to compensate for this bias.
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The authors thank M. Chase, J. Palmer, and K. Schulz. This research was supported in part by NSF grants DEB-0090283 and PGR-0115684.
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