Trimorphic Populations Research Articles

Floral trimorphism and monomorphism in continental and island populations of Eichhornia paniculata (Spreng.) Solms. (Pontederiaceae)

Eichhornia paniculata (Spreng.) Solms. (Pontederiaceae) is a short-lived perennial or annual of marshes, seasonal pools and ditches of lowland tropical South America, primarily NE Brazil, and the Caribbean islands of Cuba and Jamaica. Comparisons, made under uniform glasshouse conditions, of populations originating from seed collected in the two regions revealed striking differences in their floral biology and breeding systems. The majority of populations sampled in NE Brazil are tristylous. Floral trimorphism is associated with pollen trimorphism and minor differences in anther size and pollen production among the three stamen levels. Unlike the majority of heterostylous plants the floral morphs of E. paniculata are highly self-fertile. Populations sampled on the island of Jamaica are composed exclusively of self-pollinating, semi-homostylous, mid-styled forms. Flowers from these populations are smaller and less showy, with reduced pollen heteromorphism and significantly fewer pollen grains and ovules per flower, in comparison with trimorphic populations from Brazil. Individual genotypes from Jamaican populations display considerable developmental instability in floral expression, particularly with respect to filament elongation of the lower stamen level. It is proposed that self-pollinating populations of E. paniculata are evolutionarily derived from tristylous ancestors and that the shift in breeding system is favoured at low density, following population bottlenecks, where pollinator service is unreliable.

Pollination biology of heteromorphic populations of Oxalis alpina (Rose) (Oxalidaceae) in south-eastern Arizona

In south-eastern Arizona, heterostylous (often trimorphic) populations of Oxalis alpina (Rose) Knuth are visited most commonly by females of the solitary bee Heterosarus bakeri Cockerell. Bees collect pollen from flowers and are presumed to be the major pollinators of O. alpina in this region. Analysis of pollen loads from the corbiculae of H. bakeri suggests that individual bees may specialize temporally on different floral forms. However, the apparent preferential collection of pollen probably results from spatial segregation of morphs and the localized foraging behaviour of bees rather than preference on the part of individual bees for particular stylar forms. As a group, bees appear to visit floral morphs of O. alpina indiscriminately, even though individual bees may have a preponderance of pollen from one morph type. Despite spatial segregation leading to pollinator flights between members of the same incompatibility group, capsule and seed production in populations of O. alpina is high for all forms. Loss of the mid form in some populations cannot be attributed to pollinator preferences for individual style morphs.

BREEDING SYSTEM POLYMORPHISM IN A HETEROSTYLOUS SPECIES.

Heterostylous breeding systems have been the subject of continued interest since the work of Hildebrand (1866, 1871, 1887) and Darwin (1865, 1877) revealed the association of floral heteromorphism with an incompatibility system enforcing outcrossing. However, little is known of the selective factors that favor the development of heterostyly, or the selective forces which act to cause modifications of heteromorphic breeding systems. The evolutionary relationship of distyly and tristyly where they occur together in the same family has remained unclear. In Oxalis section Corniculatae, where both reproductive systems occur together, the derivation of distyly from tristyly has been suggested (Ornduff, 1972). The occurrence of distylous and tristylous populations of Oxalis alpina (section Ioenoxalis), a species of the southwestern United States, Mexico and Guatemala, suggested that studies of this species could provide further information on the evolutionary relationship of distyly and tristyly, and indicate how changes in heterostylous breeding systems have occurred. In trimorphic populations of Oxalis alpina the stigmas of one floral form occupy the lowest position in the flower, and the two sets of anthers occupy the middle and upper positions (Fig. 1). In a second floral