ORIGINAL ARTICLE Seed abortion in the sexual counterpart of Brachiaria brizantha apomicts (Poaceae) Ana Claudia Guerra Araujo Æ Rosana Falca ˜o Æ Vera Tavares de Campos Carneiro Received: 22 June 2006 / Accepted: 10 March 2007 / Published online: 3 May 2007 Ó Springer-Verlag 2007 Abstract Gametophytic organization, fertilization and reproductive success are described for the fertile diploid Brachiaria brizantha accession BRA-002747 which is being raised for use in Brachiaria breeding programs, as well as to understand and control of apomixis in this genus. The current paper reports on reproductive biology and analysis of seed set in field experiments during three con- secutive years. Unsuccessful seed production in this plant is believed to correlate with early inbreeding depression, based on the reproductive features analyzed. Caryopsis development was observed using differential interference contrast microscopy with seed set determined by the number of self- and open-pollinated pistils that fully developed into viable seeds. Developing and mature female and male gametophytes were observed in the context of flower phenology, morphology and anthesis patterns. Pol- len viability was determined by acetocarmine staining and by observation of germination in vivo, which was also used to observe pollen tube/pistil interaction. Although normal development was observed in floral structures, anthesis and gametophytes, seed set was low, with 2 and 6% in self- and open-pollination, respectively, producing seed. Variations observed in the female organs, such as the presence of a hermaphrodite flower in 50% of the inferior floscules and the presence of multiple embryo sacs of the Polygonum type within the same ovule in 15% of the pistils, are not related to low fertility. The majority of pollen grains are viable, in spite of the reduced number of pollen tubes within the style and ovary carpel, and a developing caryopsis was observed in 70% of self-pollinated pistils, indicating successful double fertilization from 2 days after anthesis (DAA). Nevertheless, abortion gradually increased from 2 until 7 DAA and remains elevated until 12 DAA, when caryopsis maturity is achieved. These data confirm low seed set in this accession and indicate that low fertility is not a consequence of abnormalities, either in the floral or gamete structures, or pollen tube rejection, but most likely a consequence of inbreeding depression. Keywords Embryogenesis Á Embryo sac Á Fertilization Á Gametophyte Á Seed set Introduction The genus Brachiaria (Trin.) Griseb. belongs to the family Poaceae and comprises around 100 species (Renvoize et al. 1996, http://www.ipni.org/index.html). A number of Bra- chiaria species are cultivated as forage grasses in tropical areas (Keller-Grein et al. 1996) but few reproduce sexually. A Brachiaria collection was established at the International Center for Tropical Agriculture—CIAT, Colombia and duplicated at Embrapa, Brazil (Valle 1990). Brachiaria accessions are mostly polyploids that reproduce through apomixis, whereas the diploid accessions are sexual (Valle 1986, 1990; Valle and Savidan 1996). Apomixis, as an asexual mode of seed-based reproduction produces prog- eny that are genetically identical to the mother plant (Asker and Jerling 1992). Among the 275 B. brizantha accessions of the collec- tion, only one accession has been identified to date as Communicated by Scott Russell. A. C. G. Araujo (&) Á R. Falca ˜o Á Vera Tavares de CamposCarneiro Embrapa Genetic Resources and Biotechnology, Parque Estac ¸a ˜o Biolo ´gica, Av. W/5 Norte (Final), P.O. Box 02372, 70770-900 Brasilia, DF, Brazil e-mail: guerra@cenargen.embrapa.br 123 Sex Plant Reprod (2007) 20:109–121 DOI 10.1007/s00497-007-0048-6