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Hansjörg Eichler Scientific Research Fund – Report

Leaf blades or floral clades - A guide to spinifex phylogeny

Jim Mant, Australian National University

The Australian tribe of grasses, Triodieae or 'spinifex', was well known to the early explorers of the continent's centre. The tough sharpened leaf blades of this 'porcupine grass' were a dreaded obstacle to the adventurers' vain pursuit of fertile lands and inland seas - a real Australian 'thorn in the side'. For most, the endless spinifex grasslands symbolised a harsh and monotonous, if not hostile Australian desert, a place to heroically endure rather than celebrate.

Yet, spinifex grasses are of great importance to what is a dynamic, biologically diverse and culturally rich arid zone. In many areas these grasses form the primary perennial biomass, providing habitat for the world's richest lizard fauna and several endangered mammals, playing a dominant role in the region's fire ecology and supporting a diverse ephemeral flora in inter-hummock areas (Griffin 1984). They form pure hummock grasslands or a dominant understorey on over two million km2 of the continent - the single most extensive vegetation type in Australia (Griffin 1984).

William Dampier, in 1699, was the first European to collect a hummock grass (George 1999), although indigenous Australians have long used and traded the adhesive resin produced by some spinifex species. Dampier collected a specimen of what is now called Triodia danthonioides, from the southwest coast of Western Australia. For many years, the species was included in the genus Plectrachne, along with 15 others with remarkably similar spikelet characters of long linear glumes and three elongated awns on the lemma.

In contrast, Triodia, described by Robert Brown, has traditionally been distinguished from Plectrachne by its relatively short obtuse glumes and variously lobed or emarginate lemmas. In his recent comprehensive revision Mike Lazarides (1997) transferred Plectrachne into Triodia; the original glume and lemma criteria failed to account for the many species newly described in the postwar period. Instead, nine infra generic groups were erected similarly based on features of the spikelet and inflorescence. A couple of smaller genera were retained; Monodia which is monotypic and Symplectrodia with two species from Arnhem Land.

The tribe now comprises 67 described species and includes considerable variation in the floral parts. However, there appear to be two quite different types of leaves that cut across these generic and infrageneric classification schemes. This leaf anatomical variation was first documented by Nancy Burbidge in the 1940's, but is significant enough to have attracted the common names, 'hard' versus 'soft' spinifex. 'Hard' taxa have stomata on both sides of the leaf, whereas 'soft' lack stomata and associated photosynthetic tissues from the outer leaf surface. 'Soft' spinifex species are, with one exception, restricted to the monsoonal region of the arid north. Species with the 'hard' type anatomy are found throughout Australia's north as well as the temperate south (Lazarides, 1997).

A survey of the 67 described species in the tribe identified 40 with the 'hard' type anatomy including T. danthonioides, whilst 27 species are 'soft' (Mant, unpublished). Both types occurred in the former Plectrachne and Triodia, and now six of the infrageneric groups are polymorphic for the leaf character. Monodia is 'soft', while Symplectrodia is 'hard'. Have these leaf forms arisen many times as the classification implies? Or are there, instead, discernible patterns of convergence in spikelet characters?

To resolve these competing claims for character importance, a morphological and molecular phylogeny of the tribe was attempted with some much needed financial assistance from the Hansjörg Eichler Research Fund. Molecular results from the ITS region of nrDNA support the recognition of two lineages corresponding to the hard - soft leaf anatomy, as well as confirming the tribe's monophyly (see Mant et al. in press). Symplectrodia is nested within the 'hard' group, while the relationship of Monodia to the 'soft' clade remains uncertain. Instead of plasticity in leaf anatomical characters, these data point to strong convergence in key spikelet and inflorescence characters. It would seem that both Plectrachne and Triodia were not monophyletic as previously circumscribed.

Outgrouping supports a single derived origin of the 'soft' leaf, with the 'hard' type species forming a basal paraphyletic group in the tribe. Re-examination of the leaf anatomy indicates the two forms are likely to have markedly different physiologies relating to water use and photosynthetic activity (McWilliam and Mison 1974; Craig and Goodchild 1977). The 'soft'-type leaf anatomy may well be an adaptation to the more predictable rainfall of the monsoonal arid north.

In any case, Dampier's T. danthonioides and its other 'hard' allies from S-W Western Australia do not share recent ancestry with the northern monsoonal 'soft' species such as T. schinzii (the type of Plectrachne) and T. pungens (the type of Triodia). Overall, I would argue that features of the spikelet and inflorescence are unsuitable for generic classification in this tribe of grasses. Instead, it turns out that the key to the thorny issue of higher level spinifex relationships is to be found within those harsh spiky desert leaves.

References

Craig, S. and Goodchild, D. J. (1977) Leaf Ultrastructure of Triodia irritans: a C4 grass possessing an unusual arrangement of photosynthetic tissues. Australian Journal of Botany 25: 277-290

George, A. S. (1999) William Dampier in New Holland. Australia's First Natural Historian. Melbourne: Bloomings Books.

Griffin, G. F. (1984) Hummock Grasslands. In G. N. Harrington, A. D. Wilson and D. Young, eds. 'Management of Australia's Rangelands'. pp. 271-284. CSIRO: Melbourne.

Lazarides, M. (1997) A revision of Triodia, including Plectrachne (Poaceae, Eragrostideae, Triodiinae). Australian Systematic Botany 10: 381-489

Mant, J. G., Bayer, R. J., Crisp, M. D. and Trueman, J. W. H. (in press) A phylogeny of Triodieae (Poaceae: Chloridoideae) based on the ITS region of nrDNA: testing conflict between anatomical and inflorescence characters. In: S. W. L. Jacobs and J. Everett, eds. Grasses - Systematics and Evolution - Vol. 2 of Proceedings of the Second International Conference of the Comparative Biology of the Monocots. Sydney: CSIRO: Melbourne.

McWilliam, J. R. and Mison, K. (1974) Significance of the C4 pathway in Triodia irritans (Spinifex), a grass adapted to arid environments. Australian Journal of Plant Physiology 1: 171-175

Jim Mant studied the systematics of Triodieae for his BSc Honours degree at The Australian National University and CSIRO Plant Industry. He is now doing a PhD on the comparative biology of Chiloglottis (Orchidaceae) and their sexually deceived thynnine wasp pollinators at ANU and Royal Botanic Gardens Sydney.

 

Published in Australian Systematic Botany Society Newsletter 102; 5-6 (March 2000)


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