Pollen-ovule ratios in some Neotropical Araceae and their putative significance M. Chouteau 1 , D. Barabe´ 1, *, and M. Gibernau 2 1 Institut de Recherche en Biologie Ve´ge´tale, Universite´ de Montre´al, Jardin Botanique de Montre´al, Montre´ al, (Que´ bec) Canada 2 Laboratoire d’Evolution & Diversite´ Biologique (UMR 5174), Universite´ Paul Sabatier, Toulouse, France Received August 27, 2004; accepted March 2, 2005 Published online: February 16, 2006 Ó Springer-Verlag 2006 Abstract. The correlation between pollen-ovule (P/O) ratio and breeding system has generally been analysed with respect either to pollination efficiency, or in terms of sex allocation theory. Pollen/ovule ratios were measured in nine species of Araceae belonging to two genera with bisexual flowers (Anaphyllopsis, Monstera) and three genera with unisexual flowers (Dieffenbachia, Philoden- dron, Montrichardia). The family Araceae with its unique inflorescence morphology allows the anal- ysis of variations of the P/O ratio with respect to two basal morpho-functional pollination units: the flower or the inflorescence. We found a relation- ship between the value of the P/O ratio and the breeding system that is partially different from Cruden’s results (1977). Some facultative xenoga- mous species have a higher P/O than the obliga- tory xenogamous species. A link was found between the P/O and the type of inflorescence, the floral cycle, and the mode of growth. Key words: Inflorescence, pollination, pollen-ovule ratios, French Guiana, Anaphyllopsis, Dieffenbachia, Monstera, Montrichardia, Philodendron. Introduction The ratio of pollen grains to number of ovules (P/O) in a flower is a useful tool to study plant breeding systems (autogamy vs. xenogamy) in relation to habitat or successional stage (Cru- den 1977, Ramirez and Seres 1994). Xenogamy occurs by the transfer of pollen between flowers of different individuals, and autogamy by pollen transfer among flowers of a single individual (Endress 1994, p. 199). In the evolution of plants, the shift from xenogamy to autogamy may have induced alterations in floral morphology (Ornduff 1969), particularly a reduction of the energetic cost of flowering (Cruden 1977). It was also reported that the existence of certain factors (habitat, pollinator, flower size…) influences differences in P/O ratios between species (Gallardo et al. 1994, Ramirez and Seres 1994, Ju¨rgens et al. 2002). Cruden (1977), after studying 80 different species, concluded that the P/O ratio was related to the breeding system of the plant. Pl. Syst. Evol. (2006) DOI 10.1007/s00606-005-0328-2