Botanic Gardens Trust, Sydney, Australia

Xanthostemoneae

Relationships and generic concepts in Xanthostemoneae (Myrtacae)

Dr Peter Wilson - Principal Research Scientist

The family Myrtaceae is divided into two subfamilies and the largest of these, Myrtoideae, is divided into 15 tribes. Xanthostemoneae is a tribe of three genera. The largest is Xanthostemon with around 40 species that occur in New Guinea, the Solomon islands, Indonesia, the Philippines, Australia and New Caledonia. The other two genera, Pleurocalyptus, with two species, and the monotypic Purpureostemon, are endemic to New Caledonia. Most species of Xanthostemon occur in Australia and New Caledonia with around 16 species each. One of the characteristics of this tribe is the fruit, which is usually a rounded capsule that is strongly exserted, unlike most other capsular Myrtaceae. The species of Xanthostemon can be divided into four separate groups based on placentation (the position of the ovules). The New Caledonian species are members of only one of these groups but Australian species are found in all four. The placenta in Xanthostemon is usually rod-like and projecting into the loculus of the ovary with the ovules arranged in a ring or arc from the apex, depending on the angle. In the most common type, the placenta is at right angles to the central axis of the ovary. In two of the three other types, the placenta is oblique to almost erect; in the fourth group, the placenta appears to be absent. Some other features correlate with some of these placenta variations: leaf arrangement, petal shape and colour and presence of winged seeds.

Peter Wilson has been involved in a cooperative research project on this group of genera with Adrien Wulff (Institut Agronomique néo-Calédonien and University of New Caledonia) which involved a coursework Masters student, Olivier Laporte-Daube, who worked on the project for six months. The project is continuing at the Royal Botanic Gardens and Domain Trust. The aim of the project is to use DNA sequence data to reassess revolutionary relationships.
Partly dissected capsule of the Australian species Xanthostemon umbrosus showing the excerted fruits and the obliquely angled placentas.

Generic concepts and morphological character evolution will be tested via analysis of the molecular data. Questions that will be addressed are:

  • Are the groups of species defined by placenta angle and other features worthy of generic rank?
  • Is generic rank supported for Purpureostemon?
  • Is Pleurocalyptus merely acalyptrate member of a group of bullate-leaved species of Xanthostemon, as suggested by morphology?

Xanthostemon-macrophyllus
Xanthostemon macrophyllus, a bullate-leaved species from New Caledonia.

Xanthostemon-umbrosus
Partly dissected capsule of the Australian species Xanthostemon umbrosus showing the excerted fruits and the obliquely angled placentas.

Photos P. Wilson