Journal of Applied Sciences & Environmental Management, Vol. 8, No. 2, Dec, 2004, pp. 59-62

Distribution and origins of members of the Family Portulacaceae (Centrospermae)

NYANANYO, B L; *MENSAH,S I

Department of Plant Science and Biotechnology, University of Port Harcourt, Choba, P.M.B. 5323, Port Harcourt, Rivers State, Nigeria.

Code Number: ja04026

ABSTRACT

The present day distribution of members of the family Portulacaceae shows that whilst some genera such as Portulaca L., and to some extent Montia L. and Talinum Adanson are cosmopolitan in distribution, others such as Ceraria Pearson and Stephens, Lyallia Hooker fil., Portulacaria Jacquin, Silvaea Philippi and Talinella Baillon with more than six species at the most are endemic to Africa, Australia, Eurasia and South America respectively. This pattern of distribution shows that members of the family Portulacaceae could have originated in Gondwanaland and migrated to new niches by long range dispersal after the breakup of Gondwanaland by continental drift. @JASEM

Whilst the family Portulacaceae is virtually cosmopolitan in distribution (Nyananyo, 1990; Nyananyo and Olowokudejo, 1986), its component genera show interesting patterns of distribution, which may shed some light on its history and origins. The purpose of this article is two fold:-

1.  To place the genera as now recognized (Nyananyo, 1990) into groups based on their geographical location together with such other systematic lines of evidence as cytology, gross morphology, physiology, palynology and phytochemistry and
2. To try to ascertain the centres of diversity and origin of the different genera and generic groups and to hypothesize about the means of which the family achieved its present day distribution.

The palynology of the family is eurypalynous varying in aperture from tricolpate (three colpi running from the north to the south pole of the pollen grain), pantocolpate, polypantocolpate (with colpi of between eight and over thirty in number distributed all over the surface of the pollen grain) to pantoporate to polypantoporate (with between four and over thirty pores distributed all over the surface of the pollen grain), surface ornamentation from echinate (with spines on the surface of the pollen grain) to almost glabrous (smooth) (Nowicke, 1975; Nyananyo, 1985, 1990). Cytologically, the basic chromosome numbers range from two to twelve (Nyananyo, 1986a, b, 1987). The phytochemistry of the Portulacaceae is interesting with the occurrence of betacyanins in their floral and vegetative parts as opposed to anthocyanins as is found in the majority of dicotyledons (Nyananyo, 1986c). Morphologically, members range from small succulent plants of a few centimeters in height to shrubby plants of about a few meters in height occurring in a variety of habitats from deserts to waste places in urban centres.

MATERIALS AND METHODS

The following account is based on the literature, living plants and herbarium specimens. Herbarium specimens were obtained from the British Museum of Natural History (BM.), London, Royal Botanic Garden, Herbarium, Kew (K.), Forest Herbarium, Ibadan (FHI.), Nigeria.

RESULTS AND DISCUSSION

The results are presented in Tables 1 and 2. Portulaca occurs in all the principal regions of the world. Montia and Talinum are absent from one of the five regions recognized. Calandrinia H.B.K. from two and Anacampseros L (Rowley, 1994) from three (Table 1). The other genera show increasing levels of restricted distribution. Four genera, Calandrinia, Portulaca, Talinum and Montia occur in both the northern and Southern hemispheres, whereas the other genera, Calyptridium Nuttall, Claytonia L., Lewisia Pursh and Talinopsis Gray occur in the Northern hemisphere. The remaining twelve genera as now recognized (Nyananyo, 1990) which include the succulent tree genera, Calyptrotheca Gilg, Ceraria Pearson and Stephens and Portulacaria Jacquin (Applequist and Wallace, 2001; Landrum, 2002) are restricted to the Southern hemisphere particularly Africa and south America with representatives in Australasia (Table 1). The Kerguelen Islands in the Southern Ocean and New Zealand each has an endemic species of the ditypic genus, Lyallia (Nyananyo and Heywood, 1987) (Tables 1 and 2).

The family Portulacaceae could probably have originated in Gondwanaland and attained its present day distribution with taxa occurring in all the continents of the world (Table 1), as a result of continental drift and long distance dispersal. The apparent ability of numerical and structural chromosome changes to increase the capacity of plants to expand into new areas (Stebbins, 1971; Moore, 1972; Nyananyo and Okoli, 1987; Stern, 1997) is of value in some members of the family Portulacaceae supporting the distribution data in for example Portulaca and Talinum. These genera have their centres of diversity in the northern hemisphere where polyploids occur, but diploids alone are found in the Southern hemisphere which is probably their centre of origin.

Also, the successiform theory in pollen evolution (Van Campo, 1976), which  basically says that the ancestral angiosperm pollen was tricolpate and that this evolved sequentially into pantocolpate, pantoporate, polypantocolpate and polypantoporate types. The considerable variation in aperture type and number seen in the family Portulacaceae (Nilsson, 1967; Nowicke, 1975; Nyananyo, 1985, 1989, 1992) suggests that the successiform series in pollen evolution has occurred many times in different lineages. Hence, by implication, the change from tricolpate to other forms is developmentally straight forward, and supports the possible origin of the Portulacaceae in Gondwanaland.

After the breakup of Gondwanaland due to continental drift, members of the Portulacaceae in the component parts, Africa, including Madagascar, Australia, Eurasia, Islands in the Southern Ocean, New Zealand and South America could have evolved independently of each other and migrated to new niches in Laurasia by long distance dispersal leading to the present day distribution of the family (Table 1).

REFERENCES

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