Malvaceae Info (Home)
The subfamily Sterculioideae is a division of the angiosperm family Malvaceae. It appears to be a well defined group, characterised by apetalous (lacking petals), exinvolucellate (lacking an epicalyx) flowers with a fleshy, usually petaloid, gamosepalous (fused) calyx, an absence of staminodes, a monadelphous staminal column, an androgynophore (a stalk separating the calyx from the stamens and styles), and apocarpous (i.e. separated) ovaries and fruits. The flowers are typically monoecious, i.e. have separate male and female flowers borne on the same plant.
Of other apetalous taxa in Malvaceae, Fremontodendreae (Bombacoideae) have an epicalyx and oppositifoliate instead of axillary inflorescences, Cullenia (Durioneae) has an epicalyx, Keraudrenia and Seringia (Lasiopetaleae) have oppositifoliate instead of axillary inflorescencs, Guichenotia in part, Lasiopetalum in part and Thomasia in part (Lasiopetaleae) possess an epicalyx (some Guichenotia also have unfused sepals), and Mortoniodendron (in part) (Tilioideae) has unfused sepals. Craigia is sometimes interpreted as apetalous; it has clusters of 4 stamens bracketed by a pair of petaloid structures which are alternatively interpreted as two staminodes, or a petal and a staminode.
Sterculioideae has a pan-tropical distribution, being found in New Caledonia and other Pacific islands, Australasia, South, South East and East Asia, Madagascar, tropical Africa, Central America, the Caribbean and tropical South America. It is most diverse in South East Asia.
The Sterculioideae is composed of 12 to 14 genera, depending on whether Tarrietia and Argyrodendron are separated from Heritiera. It was previously classified as one (Sterculieae) or two tribes (Sterculieae and Tarrietieae) in the family Sterculiaceae, in the later case the distinction between the two tribes being dehiscent versus indehiscent fruits. The genera are
In the past many of the genera (Brachychiton, Firmiana, Hildegardia, Pterocymbium, Pterygota, Scaphium) were treated as sections of Sterculia.
The phylogeny of Sterculioideae is unknown, or at least unpublished (a group at RBGE are working on this topic). I retain an open mind as to whether all genera are monophyletic. Sterculia, as the earliest described genus, with 200 to 300 species, and a pantropical distribution, might be suspected of being polyphyletic or paraphyletic. However it has an unique trait distinguishing it from the other genera; the radicle (seed root) is placed remotely from the hilum (location of attachment of the seed to the ovary), rather than being placed close to it, assuming that this trait has been examined for all species.
It seems likely that the geographically restricted genera (Acropogon, Argyrodendron, Brachychiton and Octolobus) are monophyletic. (Franciscodendron is monotypic.)
Few DNA sequences are published for this group, so molecular evidence is of limited use in analysing the structure of this group. The largest data set svailable in EMBL consists of 29 sequences, representing 26 species, of the chloroplast ndhF gene. This does not appear to give a well resolved cladogram. However, Hildegardia and Heritiera are not close relatives, demonstrating that the separation of the indehiscent fruited genera (Hildegardia and Heritiera) as Tarrietieae was incorrect. Arygrodendron appears to be distinct from Heritiera, falling into a weakly supported Australasian (Acropogon, Argyrodendron, Franciscodendron and Brachychiton) clade.
Malvaceae Info (Home)
© 2005, 2006 Stewart Robert Hinsley