ORIGINAL ARTICLE Self-incompatibility in a distylous species of Rubiaceae: is there a single incompatibility response of the morphs? D. E. Klein Æ L. Freitas Æ M. Da Cunha Received: 18 November 2008 / Accepted: 2 May 2009 Ó Springer-Verlag 2009 Abstract Heterostyly is a genetically controlled floral polymorphism usually associated with an incompatibility system. This set of features is known to occur in several angiosperm families, but some aspects of its biology has not been well studied. The present study investigates cel- lular aspects of the pollen–pistil interaction after compat- ible and incompatible pollinations of Psychotria nuda, to increase our knowledge of heteromorphic self-incompati- bility (HetSI). The use of bright field, fluorescence and transmission electron microscopy methods allowed us to demonstrate that pollen tubes behave differently after incompatible and compatible pollinations. Pollen tubes were particularly distinct after incompatible pollinations of L- and S-morph flowers. Relative to compatible pollen tubes, incompatible L-morph tubes had a drastic reduction in cellular contents, but no cell rupture. Incompatible S-morph tubes exhibited dense cytoplasm in apical regions, as well as in other regions, accompanied by a rupture of the apex. These results support the hypothesis that L- and S-morph flowers have different incompatibility mecha- nisms during HetSI. Keywords Pollen–pistil interactions Á Pollen tube growth Á Pollen ultrastructure Á Heteromorphic self-incompatibility system Á Psychotria nuda Introduction Distylous species commonly possess a heteromorphic self- incompatibility system (HetSI). HetSI is a sporophytically controlled, diallelic incompatibility system, which prevents or reduces self- and intra-morph fertilizations (Endress 1998). In this system, pollen from the same flower or from a flower of the same morph is inhibited in germination, hydration or growth in the pistil, preventing fecundation (e.g., Shivanna et al. 1981; Faivre 2002; Castro and Arau ´jo 2004). Although several studies have revealed differences between L- and S-morphs in several traits related to pre- pollination biology, such as pollen production and removal patterns (McKenna 1992), there are no data regarding the interaction between the pollen and pistil cells during the heteromorphic incompatibility reaction. The pistil, which may be open or closed, includes sev- eral tissues and these include the stigmatic epidermal cells that may or may not be secretory and transmitting tissue. The presence of these tissues indicates interactions not only between the pollen tube and the female gametophyte, but also between the pollen tube and any sporophytic inter- vening tissues. During an incompatibility reaction, pollen tube growth may be interrupted in several regions between the stigma and the ovule (Cheung 1995). The site of interruption may be related to the incompatibility system. In homomorphic gametophytic self-incompatibility (GSI), this reaction frequently occurs when the pollen tube is within in the transmitting cells of the style, although Communicated by Teh-hui Kao. D. E. Klein Á M. Da Cunha (&) Laborato ´rio de Biologia Celular e Tecidual, Centro de Biocie ˆncias e Biotecnologia (CBB), Universidade Estadual do Norte Fluminense (UENF), Av. Alberto Lamego, 2000, Campos dos Goytacazes, RJ CEP 28013-602, Brazil e-mail: maurauenf@gmail.com L. Freitas Instituto de Pesquisa Jardim Bota ˆnico do Rio de Janeiro/ Programa Mata Atla ˆntica, Rio de Janeiro, RJ, Brazil 123 Sex Plant Reprod DOI 10.1007/s00497-009-0097-0