Abstract A survey of 32 populations of the self-compatible, tristylous, aquatic, Eichhornia paniculata (Pontederiaceae), from N. E. Brazil and Jamaica, was undertaken to examine the role of demographic and reproductive factors on variation in outcrossing rate (t), and to investigate the association between t and population genetic structure. Multi-locus outcrossing rates, estimated using isozyme techniques, varied widely (t̂= 0.002–0.960) among populations and were uniformly distributed across the entire range of t. Sixty percent of variation in t̂ was explained by style morph diversity (E) and the frequency of selfing variants within populations (M). Population size (N) and plant density (d) also accounted for a significant portion of variation in t̂, particularly in Jamaica, where variation in style morph diversity was low.
Outcrossing rates were significantly correlated with the proportion of loci that were polymorphic (P), the average number of alleles per polymorphic loci (Na), the average observed heterozygosity, (Ho), and the inbreeding coefficient (f̂). A strong regional effect was evident in the association of f̂ with P and Na. This largely results from the confounding effect of genetic bottleneck(s) and high levels of self-fertilization associated with the colonization of Jamaica. Comparisons of f̂ with the equilibrium inbreeding coefficient, (Feq), indicated that outcrossing populations had a significant deficiency of heterozygotes, while selfing populations had an excess. This demonstrates that levels of heterozygosity cannot be accounted for by the mating system alone. Outcrossing rate variation in E. paniculata is controlled by a complex interplay of local demographic and genetic factors. These influences operate within contrasting regional contexts as a result of the different evolutionary histories of populations in N. E. Brazil and Jamaica.