Abstract Petunia axillaris occurs in temperate South America and consists of three allopatric subspecies: axillaris, parodii, and subandina. Previous studies have revealed that subsp. axillaris is self-incompatible (SI), subsp. parodii is self-compatible (SC) in Uruguay, and subsp. subandina is SC in Argentina. The SI/SC status over the entire distribution range is not completely understood, however. The objective of this study was to examine the overall SI/SC status of the respective subspecies in comparison with floral morphology. The results confirmed that subsp. parodii and subsp. su- bandina were SC throughout the distribution range, and that subsp. axillaris was also SC in Brazil and in most of the Argentinean territory. The SI P. axillaris occurs in the natural population only between 34 and 36°S, along the eastern shore of South America. The Brazilian and Uruguayan subsp. axillaris differed in SI/ SC status and floral morphology. We discuss the cause of this difference. Keywords Allopatric subspecies Æ Argentina Æ Brazil Æ Differentiation in self-(in)compatibility Æ Paraguay Æ South America Introduction Self-incompatibility is a widespread mechanism in flowering plants to avoid inbreeding and promote outcrossing (de Nettancourt 2001). Self-incompatibility is controlled by a multi-allelic locus, called the S-locus. Gametophytic self-incompatibility (GSI) is observed for plants of the Solanaceae, Rosaceae, and Scrophu- lariaceae families. In GSI, the haploid pollen is re- jected if it carries an S-allele that matches either S- allele in the diploid pistil sporophytic tissue. The specificity of the GSI response is determined by at least two genes, one controlling pistil specificity (pistil-S) and the other controlling pollen specificity (pollen-S). In the families mentioned above, pistil-S and pollen-S are an S-ribonuclease (S-RNase; Anderson et al. 1986; McClure et al. 1989; Sassa et al. 1992, 1996; Lee et al. 1994; Murfett et al. 1994; Broothaerts et al. 1995; Xue et al. 1996) and an F-box protein (SLF or SFB; H. Kokubun Æ H. Watanabe Center for Environment, Health and Field Sciences, Chiba University, 6-2-1 Kashiwanoha, Kashiwa, Chiba 277-0882, Japan M. Nakano Æ T. Tsukamoto Æ T. Ando (&) Faculty of Horticulture, Chiba University, 648 Matsudo, Matsudo City, Chiba 271-8510, Japan e-mail: andot@faculty.chiba-u.jp G. Hashimoto Centro de Pesquisas de Histo ´ ria Natural, Rua Jaime Ribeiro Wright 618, Itaquera, Sa ˜ o Paulo 08201-970, Brazil E. Marchesi Facultad de Agronomia, Universidad de la Repu ´ blica, Garzo ´ n 780, Montevideo, Uruguay L. Bullrich Instituto de Floricultura, INTA, Las Caban ˜ as y Los Reseros, 1712 Castelar, Buenos Aires, Argentina I. L. Basualdo Seccio ´ n Bota ´nica, Facultad de Ciencias Quı´micas, Universidad Nacional de Asuncio ´ n, San Lorenzo, Paraguay T. Kao Department of Biochemistry and Molecular Biology, Pennsylvania State University, 403 Althouse Lab., University Park, PA 16802, USA J Plant Res (2006) 119:419–430 DOI 10.1007/s10265-006-0002-y 123 REGULAR PAPER Distribution of self-compatible and self-incompatible populations of Petunia axillaris (Solanaceae) outside Uruguay Hisashi Kokubun Æ Makoto Nakano Æ Tatsuya Tsukamoto Æ Hitoshi Watanabe Æ Goro Hashimoto Æ Eduardo Marchesi Æ Laura Bullrich Æ Isabel L. Basualdo Æ Teh-hui Kao Æ Toshio Ando Received: 27 February 2006 / Accepted: 12 April 2006 / Published online: 17 August 2006 Ó The Botanical Society of Japan and Springer-Verlag 2006