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

Abstract

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.