About Australian Pea-flowered Legumes
by Mike Crisp
The Faboideae is found throughout the world except the oceans and polar regions. Its diversity is probably greatest in the tropics, but is also apparent in the subtropics and temperate zone. Within Australia, the greatest concentration of tribes and genera is in the tropics, whereas the centres of species diversity are in the south-west and south-east.
Worldwide: c. 13,800 species: 478 genera
The plants may be trees, shrubs, herbs, or climbers. Their leaves are mostly alternate and in the tropics are usually compound with one or more pairs of leaflets and a single terminal one; three leaflets are especially common. However, the leaflets may be even in number, or reduced to one per leaf. In temperateAustralia the leaves are typically simple or modified to phyllodes, spines, or even tiny scales. There are usually stipules and often in compound leaves, stipels occur at the leaflet bases. The basic inflorescence is a terminal raceme or panicle, but is usually modified to axillary racemes, pseudo-umbels, spikes, pseudo-heads or clusters. The flowers are almost always bisexual and papilionaceous (pea-shaped) in form: i.e. with a large, round, upstanding, upper petal - the standard; two smaller lateral petals - the wings; and two similar lowermost petals which are more or less joined together to form a keel. In bud, the standard encloses the wings, and the wings enclose the keel. The five, rarely four, calyx lobes are partly joined into a short or long tube, and are usually dissimilar in size, shape, and degree of fusion, frequently forming an upper and lower lip. The 10 stamens are enclosed by the keel. They are all unfused or variously joined by their filaments. The anthers may be all alike, or alternately dissimilar. The ovary is superior, of one carpel, single-celled or rarely partly two-celled and terminated by the style, which is incurved and often bearded. The fruit, called a legume or pod, has a great variety of shapes. It is most often dry and bean-like with many seeds, opening by two valves, but may be short and one- or two seeded and / or indehiscent. The pods can be woody, papery, or winged and are often ornamented with veins, hairs, bristles, or warts. In some species the pods are formed underground (e.g. peanuts). Many genera have pods which are transversely jointed (loments), breaking up into one-seeded bits. The seeds are usually round or kidney-shaped, and often have a fleshy appendage or aril which attracts seed-collecting ants. The seed coat is tough and must often be scarified before germination will occur. The seeds can have great longevity.
Classification and phylogeny
The Faboideae is, together with the Mimosoideae and Caesalpinioideae, a subfamily of the family Fabaceae, which is characterised by its pods. The family is alternatively known as Leguminosae and the subfamily as Papilionoideae. Sometimes the pea-flowered subfamily is recognised as a family with the name Fabaceae or Papilionaceae. In 1865 Bentham and Hooker grouped the genera of Faboideae into 11 tribes, but these were increased to 32 by Polhill (1981). Australia's tribes (below) almost represent the full evolutionary range of the subfamily. A comprehensive, well-illustrated and up to date world-wide classification of legumes down to genus level is ‘Legumes of the World' by Lewis et al. (2005). Many studies in the systematics of the legumes are published in the series Advances in Legume Systematics, Parts 1 to 11 (1981 to 2003).
Recent years have seen much progress in understanding the phylogenetic relationships of legumes through the analysis of DNA sequences. Higher level analyses published so far use sequences from both the chloroplast (rbcL, trnL intron and trnK-matK) and nuclear (ITS and 18S) genomes. Figure 1 shows the major groups upon which these studies agree (Crisp et al., 2000; Doyle et al., 2000; Kajita et al., 2001; Pennington et al., 2001; Wojciechowski, 2002; Wojciechowski et al., 2004). The legume family and subfamilies Faboideae and Mimosoideae (most genera) are supported as monophyletic but Caesalpinioideae is paraphyletic, containing the rest of the family (Bruneau et al., 2008). Basal nodes within the Faboideae are not well resolved although a major group appears to be supported by the shared presence of a 50-kb chloroplast DNA inversion (lacking in Dipterygeae and parts of Sophoreae and Swartzieae) (Doyle et al., 1996). Most tribes and some groups of tribes are supported as monophyletic. These are described below.
Swartzieae and Sophoreae
These are mostly pinnate-leaved trees or woody climbers found in tropical regions. The stamens are unfused and the pods are often indehiscent. Both tribes were originally defined by primitive (plesiomorphic) morphological characters and therefore their delimitation has been problematic. A core group of Swartzieae genera, characterized by unique flowers with a single petal and a ring meristem, has been found monophyletic by molecular data and is not found in Australia. Genera previously placed in Sophoreae are scattered among several clades, e.g. the Baphia, Cladastris and Vatairioid clades. Some genera, including the type (Sophora), belong in the Genistoids and are found in Australia.
Trees and shrubs, mostly restricted to the New World tropics and not represented in Australia. Fruits either more or less woody and/or indehiscent including samaras.
Crotalarieae, Euchresteae, Genisteae, Podalyrieae (incl. Liparieae), Thermopsidae and much of the Sophora group (Sophoreae). With Brongniartieae, this is a strongly supported and well understood group (Crisp et al., 2001; Doyle et al., 2001; Wojciechowski et al., 2004; Boatwright et al., 2007, 2008) of largely southern-Hemisphere scleromorphic shrubs characterised by the accumulation of a diversity of quinolizidine alkaloids. In Australia they are mainly represented by Crotalaria, bird flower or rattlepod, which has many species in the tropics and arid zone. These are herbs or shrubs with few leaflets or simple leaves and conspicuous yellow flowers. The stamens are joined in an open sheath, and the pod is often inflated. Genisteae is entirely introduced in Australia, mostly as garden ornamentals (brooms), many of which have escaped. Scotch broom (Cytisus scoparius) and gorse (Ulex europaeus) are serious environmental weeds in temperate SE Austraia, originating from Europe.
Brongniartieae. This is the sister group of the rest of the Genistoids and occurs disjunctly in the New World tropics (Brongniartia, Cyclolobium, Harpalyce and Poecilanthe) and Australia Cristonia, Hovea, Lamprolobium, Plagiocarpus, Templetonia and Thinicola). The Australian genera comprise a minor radiation (52 species) of sclerophyll shrubs that are convergent with Bossiaeeae in their growth habit, reduced leaves and staminal fusion, and were once classified in that tribe.
Aeschynomeneae, Dalbergieae (most), Adesmieae, Bryinae (Desmodieae). These tribes comprise a strongly supported clade that is widespread and diverse in the tropics but with relatively few species in Australia. The group is known collectively as the 'Aeschynomenoids' or 'Dalbergioids' (Kajita et al., 2001; Wojciechowski, 2002; Lavin at al., 2001). Molecular support for monophyly of the group is strong and members share a distinctive 'aeschynomenoid' type of root nodule (Sprent, 2000). Most are herbs or soft shrubs with compound leaves but generally few leaflets. Many have pods that are transversely jointed and break up into one-seeded pieces, but this feature has evolved convergently in the Desmodiaeae sens. strict. and Hedysareae (below). The stipules are often enlarged, appendaged, striate, or joined. The flowers are usually small and inconspicuous. Many species are economic plants e.g. Arachis hypogaea, peanut. Dalbergieae differ from the rest of the group in being tropical trees and lianas with pinnate leaves and woody or winged fruits (Inocarpus fagiferus is the sole Australian species).
Amorpheae. This tribe is the sister group of the Dalbergioids s.s., and together these clades comprise the Dalbergioids s.l. (Wojciechowski et al., 2004). The Amorpheae is well supported as a clade with eight genera and 240 species in the Americas (McMahon & Hufford, 2004). The flowers are variable in the number, placement and fusion of the parts and, in one clade of five genera, non-papilionaceous (Mcmahon and Hufford (2004).
THE CAVANANINE-ACCUMULATING CLADE
A clade that includes the largest radiations of papilionoid legumes world-wide is weakly supported by matK and is united by the capacity of many species to synthesize the non-protein amino acid canavanine (Wojciechowski et al., 2004). This group, known as the ‘Old-World Clade' in Lewis et al. (2005), contains the majority of Australian peas, including the only endemic tribes.
A monophyletic group endemic in Australia, except that one or two species extend to southern New Guinea. This group accounts for 60 per cent of Australia's endemic Faboideae, with about 30 genera and 530 species. Most are scleromorphic shrubs with yellow and red ('egg and bacon') petals. Leaves are often simple or reduced to phyllodes, spines or scales. Traditionally these tribes have been distinguished by their stamens, which are free or coherent in Mirbelieae and monadelphous in Bossiaeeae. Bossiaeeae is monophyletic but Mirbelieae is unresolved at the base and is probably paraphyletic with respect to Bossiaeeae. These tribes occur throughout Australia but have major centres of diversity in the south-west and south-east.
Mirbelieae and Bossiaeeae have unusual embryology in comparison with most other legumes (Cameron and Prakash, 1990, 1994). Typical embryology in the family is Polygonum-type which produces an 8-nucleate embryo sac. Some taxa in the Mirbelieae and Bossiaeeae have Polygonum-type development but produce greatly enlarged antipodal cells, possibly serving a role in nutrition for the embryo (Cameron and Prakash, 1990). Taxa producing giant antipodals comprise the Bossiaeeae and the Daviesia group (Daviesia, Erichsenia, Gompholobium, Sphaerolobium and Viminaria) of Mirbelieae (Crisp and Weston, 1995). The remaining taxa within Mirbelieae (Isotropis and the Mirbelia group) have a more unusual type of development (Cameron and Prakash, 1994). No antipodals are produced and polarity is reversed during megasporogenesis, so that the micropylar (not chalazal) megaspore is functional and any to all megaspores (not just the chalazal) undergo mitosis to produce partially or fully developed embryo sacs. Cameron and Prakash (1994) called this development 'Mirbelia' type. A variant of this type, called 'Jacksonia' type (Cameron and Prakash, 1994), produces multiple archesporial cells and embryo sacs, some of which may be aposporous although apomixis has not been confirmed.
There is molecular support for monophyly of the group with no antipodals (Isotropis and the Mirbelia group of Mirbelieae) and inconsistent evidence for monophyly of the group with giant antipodals (Bossiaeeae and the Daviesia group of Mirbelieae) (Crisp & Cook, 2002). If monophyly of these embryological groups is confirmed, then the two tribes could be redefined by transfer of the Daviesia group to Bossiaeeae. More than 20 genera have been recognized in the Mirbelia group, including three with 100 or more species (Gastrolobium, Jacksonia and Pultenaea), but most genera in this group are not well supported as monophyletic (Crisp & Cook, 2003; Orthia et al., 2005a,b). Therefore, it has been proposed to treat the Mirbelia group as a single genus, which would be called Pultenaea (Orthia et al., 2005a). Recent taxonomic revisions of large genera (40 or more species) include Bossiaea (Ross, 2006), Daviesia (Crisp, 1995), Gastrolobium (Chandler et al., 2002), Gompholobium (Chappill et al., 2008), Jacksonia (Chappill et al., 2007) and Pultenaea (de Kok and West, 2002, 2003, 2004; Orthia et al., 2005c).
A single genus with three species in South Africa. DNA sequences of matK place it with weak support as sister to Mirbelieae and Bossiaeeae.
A largely tropical group recognised by the combination of stipels and desmodioid root nodules which export ureides (Sprent, 2000). Current classification within the Phaseoloids is inadequate because three large tribes (Desmodieae, Millettieae and Phaseoleae) are not monophyletic but are mixed together in grades and polytomies (Doyle et al., 2000; Wojciechowski et al., 2004; Lewis et al., 2005)).
Phaseoleae. This tribe is mainly tropical, with many introduced genera with few species each in Australia. The plants are mostly trifoliolate herbs or soft-wooded shrubs, often with twining stems. This tribe includes a multitude of pulse crops: edible peas and beans, e.g. Phaseolus vulgaris, French or green bean, and Vigna, Mung bean, Lima bean. Hardenbergia and Kennedia are endemic Australian genera well known as cultivated ornamentals. Erythrina (Coral trees) are cultivated as flowering trees.
Millettieae. These are mainly tropical subshrubs or vines with pinnate leaves and a standard which is usually silky-hairy outside. The main genus inAustralia is Tephrosia with about 60 native species of subshrubs in the arid zone and monsoonal tropics. By contrast Callerya is a rainforest vine with spectacular wisteria-like flowers.
Psoraleeae. These are mostly temperate-zone shrubs or herbs, mainly with trifoliolate leaves which are often gland-dotted. The pod is indehiscent and one-seeded. Represented in Australia mainly by Cullen (32 species).
Desmodieae (excl. Bryinae). Mostly shrubs and herbs, commonly with striate stipules and fruits separating into indehiscent 1-seeded articles. Pedley (1999) recognised a number of small genera in Australia.
Abreae. Tropical vines with highly toxic red and black beans inside a follicular fruit. A single species in Australia (Abrus precatorius).
Named after the indigo dye traditionally extracted from some species, this widespread tropical group is sister group to the Millettioiods and is represented in Australia mainly by Indigofera with c. 40 species. The plants are tropical or arid-zone herbs or subshrubs with pinnate leaves, a small raised gland on the anthers, and hairs that are laterally attached.
HOLOGALEGINA (GALEGOIDS )
This group was not recognized until strong molecular support was found for it (Wojciechowski et al., 2000, 2004). Hologalegina comprises two strongly supported sister clades: Robinioids and the 'inverted-repeat-loss clade' (IRLC). It includes over 4800 species, comprising the vast majority of legumes in temperate regions of the world (Wojciechowski et al., 2004).
Robinieae. These are tropical subshrubs or trees with pinnate leaves, long snake-like pods, and often showy flowers. Represented in Australia by Sesbania (10 spp.) and the cultivated tree Robinia pseudoacacia.
Loteae. These are temperate-zone herbs, mostly introduced in Australia. The leaves are pinnate, often with the lowermost pair of leaflets resembling stipules. The inflorescences are condensed into umbels or are head-like. The fruits are often jointed.
Inverted repeat loss clade (IRLC)
The IRLC is very strongly supported by DNA sequence data from the chloroplast and nucleus, as well as by the loss of one copy of the 25 kb inverted repeat in the chloroplast genome (Wojciechowski et al., 2000, 2004). It includes most of the long-recognised (Polhill, 1981) ‘temperate herbaceous' group of tribes as well as the well-known climber Wisteria, formerly assigned to the Millettieae. Members share a combination of mainly herbaceous growth habit, epulvinate compound leaves, stipules adnate to the petiole and base chromosome numbers of n=7 and n=8 (Polhill 1981; Wojciechowski et al., 2004. Species diversity is centred in the Old World temperate areas. It contains economically important legumes such as lucerne, clovers, lentils, peas, vetches, chickpeas.
Galegeae and Carmichaelieae. These are mainly subtropical and arid-zone plants. They are herbs or shrubs (rarely trees) with pinnate leaves and flowers in axillary racemes. The flowers are generally showy shades of white, blue, and red. Spectacular ornamentals include the endemic Swainsona (84 species), especially Sturt's desert pea S. formosus, which is the floral emblem of South Australia.
Trifolieae, Vicieae, Hedysareae, Cicereae. These tribes are entirely introduced in Australia except for the endemic Trigonella suavissima. Trifolieae is a typically Mediterranean group of trifoliolate herbs with toothed leaflets and pods which are variously short, coiled, or indehiscent. Many are important pasture plants e.g. Trifolium, the clovers, and Medicago, the medics. Vicieae are temperate zone herbs which climb by means of tendrils, modified terminal leaflets. All are introduced to Australia. The Garden pea Pisum sativum, Sweet pea Lathyrus, and Broad bean Vicia faba, are members of the tribe. Cicereae includes the Chick pea (Cicer arietinum).
The first definitive legumes appear in the fossil record during the Late Paleocene (about 56 Myr ago) and all three traditionally recognized subfamilies of legumes, the caesalpinioids, mimosoids, and papilionoids appear soon afterward, beginning around 50-55 Ma (Lavin et al., 2005). Pea-flowered legumes have a very poor and recent fossil record in Australia, yet molecular dating (Lavin et al., 2005) indicates that the crown group of the endemic Mirbelieae/Bossieaee commenced their radiation at least 36 Myr ago. This radiation established the lineages of present-day genera (e.g. Daviesia, Gompholobium) or generic groups (Bossiaeeae, Mirbelia group). Later, from about 20 Myr ago, some of these lineages (Bossiaea, Daviesia, Mirbelia group) underwent a second round of explosive radiation, possibly in response to the mid-Cenozoic drying and cooling of the Australian climate (Crisp et al., 2004). These three lineages account for the great majority of present-day species in the two tribes.
I thank Lyn Cook for drafting the figure and for comments on the manuscript.
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