Leptospermum juniperinum, a species from Leptospermum sens. str., showing the persistent, woody capsules with the typical domed summit. [Photo: M. Fagg]
Leptospermum madidum subsp. madidum, a member of the L. brachyandrum subgroup, showing clusters of flowers in leaf axils. [Photo: M. Fagg]
Leptospermum maxwellii, a member of the L. erubescens subgroup showing the small, non-woody developing fruits. [Photo: M. Fagg]
Leptospermum spinescens, showing the distinctive spinescent branchlets and the flowers sessile on the old wood. [Photo: M. Fagg]
Report from an ABRS grantee
Generic position of the non-persistent-fruited species of Leptospermum (Myrtaceae)
Leptospermum is an important genus of shrubby plants. Commonly known as tea-tree, the genus is widespread in eastern Australia, with a smaller number of species in southern and south-western Australia. Outside Australia, a few species occur in New Zealand, New Guinea and South-east Asia. Leptospermum is found in a variety of habitats, from coastal heath (‘wallum’) communities, to shrublands on the slopes and ranges and, particularly in the tropics, on the tops of mountains.
A revision of the genus by Thompson (1989) divided it into two major groups based on a combination of fruit and seed characters. The allocation of currently accepted species, based on the same characters, splits the genus almost exactly in half with 42 species in the persistent-fruited group and 43 in the predominantly non-persistent-fruited group.
Work on the phylogeny of the Leptospermum suballiance by OBrien et al. (2000) demonstrated that the persistent-fruited species of the genus Leptospermum form a well-supported monophyletic group, sister to Homalospermum. However, the non-persistent-fruited species sampled in this study were themselves paraphyletic, appearing to be more closely related to Kunzea and Neofabricia. Since the type of the genus Leptospermum, L. scoparium, has a woody, persistent fruit, just over 50% of the species currently recognised are now of uncertain generic position. So, there is a pressing need for resolution of relationships within Leptospermum sens. lat. In our project the generic position of the non-persistent-fruited species of Leptospermum is being assessed against an estimate of phylogeny derived from sequence data. Two chloroplast regions and one nuclear region are being sequenced across the range of variation of these species plus exemplars from related genera. A number of morphological characters will also be assessed and the molecular estimate of phylogeny will be used to determine directions of evolution in these characters. This will enable the selection of informative states on which to base revised generic concepts.
A complicating factor is that there is a wide range of morphological variation amongst the non-persistent-fruited species. Most species have fruits that have thin walls and are borne singly in leaf axils but there are some major deviations from this pattern. For example, the coastal heath species L. speciosum has its flowers borne in clusters and fruits that are semi-persistent. Another example is L. laevigatum and its near relatives which have slightly fleshy, semi-persistent fruits. A more extreme case is found in the Western Australian species L. spinescens, which has spinescent branchlets, sessile flowers and persistent woody fruits.
At present, all three DNA regions of many of the species have been sequenced. This wider survey has confirmed that the woody-fruited species (Leptospermum sens. str.) are indeed distinct from the remainder of the genus as presently circumscribed. In analyses, the non-persistent-fruited species do come out in a single group, but two problems are apparent. Firstly, the genus Neofabricia is nested amongst species from eastern Australia. Secondly, there is no jackknife support for this entire group, although there is strong jackknife support for Neofabricia and for several subclades.
Thompson (1989) recognised three subgroups amongst the non-persistent-fruited species: the L. brachyandrum, L. erubescens and L. brevipes subgroups, each defined by inflorescence form, with each group including species from both the east and west of Australia. Our study mostly gives only weak support for subdivisions associated with these species and those subgroups have a different species composition. A strongly supported Leptospermum brachyandrum subgroup is recovered in analyses but comprises only taxa from northern and eastern Australia; L. macgillivrayi, L. spinescens and L. exsertum form a separate, moderately well-supported group. The largely unsupported Leptospermum erubescens subgroup includes only Western Australian species, one of which is the type of the generic name Leptospermopsis. The Leptospermum brevipes subgroup is also largely unsupported and includes only species from eastern Australia. The relationships of many species are still unclear. We hope that filling gaps in sequencing and adding morphological data will resolve these and enable clear group delimitation.
Further reading
O’Brien, M.M., Quinn, C.J. & Wilson, P.G. (2000), Systematics of the Leptospermum suballiance, Australian Journal of Botany 48: 621–628.
Thompson, J. (1989), A revision of the genus Leptospermum (Myrtaceae), Telopea 3: 301–449.