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S. asymmetriphyllum - single bisexual or functionally female flower. Photo: M.Fagg ANBG

S. asymmetriphyllum - congested male flowers (Kakadu NP). Photo: W.R.Barker W.R.Barker.

Group characteristics

Sprawling or erect shrubs, with entire or shallowly or deeply lobed adult leaves. Stem prickles and branchlet prickles straight, widened or not at base. Stellate hairs present in all parts. Inflorescences not branched. Flowers all male apart from one larger and pricklier bisexual flower at the base of a long floral rachis or plants dioecious and male and female flowers of different sizes and borne on separate plants, 5-merous, mauve or purple. Calyx with prickles. Corolla lobes glabrous or stellate-hairy on inner face. Stamens with anthers all of similar size; pollen of male flowers aperturate and fertile, pollen of bisexual flowers inaperturate and sterile. Ovary glabrous, with glandular hairs only or with stellate hairs only. Mature fruits brown, yellow, green or purplish, 10-40 mm diameter, mesocarp dry; calyx exceeding and enclosing mature fruit.

Distribution

18 species endemic in Australia, mostly from northern tropical areas.

Note

All of these species are either functionally dioecious (male flowers are borne on separate plants from what appear to be plants with bisexual or hermaphrodite flowers) or andromonoecious (male flowers are borne on the same plant adjacent to the fewer bisexual or hermaphrodite flowers). While the bisexual or hermaphrodite flowers do produce pollen, it is inaperturate and does not germinate, making the flowers effectively female (Knapp et al., 1998).

Martine et al. (2006) have shown the Dioicum group to consist of 5 clades. 

Functionally dioecious species

  1. One clade consists of all but two of the functionally dioecious species (S. dioicum, S. tudununggae, S. cunninghamii, S. petraeum, S. carduiforme, S. vansittartense, S. cataphractum and S. leopoldense). 
  2. S. asymmetriphyllum and S. sejunctum, both from Kakadu, form a separate clade and have arisen separately from the other functionally dioecious species.

Andromonoecious species

  1. Symon (1981) indicated that the relationships of S heteropodium were obscure, with it appearing to have no close relationship except the sharing of a similar fruit structure to S. oedipus.  DNA studies support this suggestion with these two species forming a distinct clade of the Dioicum group.

  2. DNA studies indicated that S. chippendalei, together with S. diversiflorum, S. beaugleholei, S. phlomoides, and probably S. eburneum (not included in the study), forming another clade.

  3. Symon (1981) indicated that S. clarkiae was closely related to S. melanospermum, an observation supported by the DNA studies where these two species form another clade which has arisen separately from the other andomonoecious clades.

Many of these species remain undercollected and poorly understood. Twelve collections of S. dioicum included in the analysis showed considerable variation, likewise S. carduiforme, and it is likely that some species limits will undergo revision in the future.

References: S.Knapp, V.Persson & S.Blackmore (1998). Pollen morphology and functional dioecy in Solanum (Solanaceae). Pl. Syst. Evol. 210:113-139; Martine, C.T., D. Vanderpool, G.J. Anderson, and D.H. Les (2006). Phylogenetic relationships of andromonoecious and dioecious Australian species of Solanum subgenus Leptostemonum section Melongena: Inferences from ITS sequence data. Systematic Botany 31: 410-420.

Australian members of group

Members of the S. dioicum group of Solanum subg. Leptostemonum (Bean 2004).

S. asymmetriphyllum

S. beaugleholei

S. carduiforme

S. cataphractum

S. chippendalei

S. clarkiae

S. cunninghamii

S. dioicum

S. diversiflorum

S. eburneum

S. heteropodium

S. leopoldense

S. melanospermum

S. oedipus

S. petraeum

S. phlomoides

S. tudununggae

S. vansittartense

See Bean, A.R. (2004). The taxonomy and ecology of Solanum subg. Leptostemonum (Dunal)Bitter (Solanaceae) in Queensland and far north-eastern New South Wales, Australia. Austrobaileya 6: 639-816.