Annals of Botany 89: 67-76, 2002
© 2002 Annals of Botany Company
Functional Heterostyly in Tylosema esculentum (Caesalpinioideae)
1Department of Biological Sciences, Queen Mary, University of London, 2Thusano Lefatsheng Gaborone, Botswana and 3School of Biological Sciences, University of Sussex, Brighton
* For correspondence. Fax +44 (0)20 8983 0973, e-mail m.l.hartley{at}qmul.ac.uk
Received: 11 April 2001; Returned for revision: 16 May 2001; Accepted: 14 September 2001.
Tylosema esculentum is a long-lived perennial species endemic to arid areas of southern Africa. Its potential as a crop species has long been recognized as a result of the high oil and protein content of its seeds. The reproductive biology and breeding systems of the species were investigated in wild and experimental populations growing in Botswana. Field observations confirmed that the species is heterostylous with the pistil and anthers exhibiting reciprocal heights in the two morphs, although pollen size and sculpturing do not vary. The wet, non-papillate stigma characteristic of the species is the first to be reported in the Caesalpinioideae. In vivo and in vitro diallel crossing experiments demonstrated that a diallelic self-incompatability system exists in T. esculentum. The major site of pollen tube inhibition in the intramorph crosses was found to be in the style. This is the first report of functional heterostyly in the Fabaceae and of a confirmed self-incompatibility system in the Caesalpinioideae. Three separate lines of evidence, the monitoring of fruit development in open-pollinated plants, fruit set in diallel crossing experiments, and observations made in wild populations, demonstrated that fruit set and, by implication, seed set, are very low in this species. Floral abscission was a major limitation to the production of mature pods but there were also significant losses at other developmental stages of fruit production. The results suggest that low seed set may be an adaptation of the species to an environment in which rainfall is scarce.
Key words: Tylosema esculentum, marama bean, morama bean, Caesalpinieae, heterostyly, seed set, self-incompatability, ovule, fruit production, pollination.