ORIGINAL ARTICLE Breeding system of tristylous Eichhornia azurea (Pontederiaceae) in the southern Pantanal, Brazil Nicolay Leme da Cunha Æ Erich Fischer Received: 11 April 2008 / Accepted: 17 February 2009 / Published online: 7 April 2009 Ó Springer-Verlag 2009 Abstract The breeding system of Eichhornia azurea (Pontederiaceae) has been described as being both self- and heteromorphic incompatible based on crossing experiments performed on plants grown in an experimental garden. We studied the breeding system of tristylous E. azurea popu- lation under natural conditions in the Pantanal wetlands, Brazil. Controlled pollinations were conducted using 35 individuals of each floral morph. Legitimate pollinations produced more fruits than self- and illegitimate pollina- tions, except for the mid-styled morph which was highly self- and heteromorphic compatible. The number of seeds per fruit was higher under legitimate pollinations than in the other treatments, but self- and illegitimate pollinations produced more fruits and seeds in the Pantanal than for individuals of E. azurea in other populations. The higher fruit and seed production resulting from legitimate polli- nations corroborate previous studies, but self-compatibility of mid-styled plants was not previously reported. Overall results indicate a partially self- and heteromorphic com- patible system for this species in the Pantanal. Keywords Aquatic macrophyte Á Eichhornia azurea Á Fruit production Á Heterostyly Á Seed set Á Self-compatibility Á Wetlands Introduction Heterostyly is regarded as the most complex floral poly- morphism because it provides both mechanical and genetic barriers to prevent autogamy (Vuilleumier 1967; Ganders 1979; Barrett and Husband 1990). Heterostylous species can exhibit either two (distyly) or three (tristyly) floral morphs that differ reciprocally according to the stigmas and anthers heights. Ancillary polymorphism of pollen and stigmas are generally associated with heterostyly and such differences are expected to be involved in self- and intra- morph incompatibility (Barrett and Cruzan 1994). Tristyly is conspicuous in the Pontederiaceae, occur- ring in three of eight Eichhornia and four of five Pontederia species (Graham et al. 1998). However, het- eromorphy and self-incompatibility appear to be stronger in the Pontederia than Eichhornia species. Most Pont- ederia species present evident pollen trimorphism, and populations are composed of three floral morphs occur- ring in equivalent proportions (Barrett 1977; Price and Barrett 1982; Barrett and Wolfe 1986, but see Barrett et al. 1983). However, the majority of tristylous Eich- hornia species also have populations expressing a partial breakdown of heteromorphy and self-incompatibility resulting in monomorphic and dimorphic self-compatible populations and weak intermorph differentiation of pollen grains (Barrett 1978, 1979, 1985; Barrett and Harder 1992; Alves dos Santos 2002). The breeding system of E. azurea has been described to be heteromorphic self- incompatible in the Amazon, southern Brazil and northern Argentina (Barrett 1978; Alves dos Santos and Wittmann 2000; Bianchi et al. 2000). In addition, there are popu- lations with semi-homostylous floral morphs (breakdown of tristyly) in Costa Rica and southeastern Brazil (Barrett 1978; Alves dos Santos 2002). N. L. d. Cunha (&) Programa de Po ´s-Graduac ¸a ˜o em Ecologia e Conservac ¸a ˜o, Universidade Federal de Mato Grosso do Sul, Campo Grande, Mato Grosso do Sul 79070-900, Brazil e-mail: nicolaycunha@gmail.com E. Fischer Departamento de Biologia, Universidade Federal de Mato Grosso do Sul, Campo Grande, Mato Grosso do Sul 79070-900, Brazil 123 Plant Syst Evol (2009) 280:53–58 DOI 10.1007/s00606-009-0170-z