In this article, the available literature characterizing apomixis in Paspalum spp. and its use in breeding is critically reviewed. In particular, a comparison is made across species of the structure and function of the genomic region controlling apomixis in order to identify a common core region shared by all apomictic Paspalum species and where apomixis genes are likely to be localized. Candidate genes are discussed, either as possible genetic determinants (including homologs to signal transduction and RNA methylation genes) or as downstream factors (such as cell-to-cell signalling and auxin response genes) depending, respectively, on their co-segregation with apomixis or less. Strategies to validate the role of candidate genes in apomictic process are also discussed, with special emphasis on plant transformation in natural apomictic species.

Paspalum plicatulum Michx. is a wild forage grass species. The common races are tetraploid and apomictic, while sexual diploid representatives have been reported sporadically. Objectives of this study were to induce sexual 4x individuals from sexual diploids, determine the capacity for hybridization with other apomictic 4x species closely related with P. plicatulum, and thus create a tetraploid sexual material cross compatible with apomictic 4x species of the Plicatula group of Paspalum. Two induced tetraploid plants were recovered from germinating 2x seeds treated for 24 h with colchicine. Bivalent and quadrivalent chromosomes were the most common association at meiosis in these autotetraploids. Embryological analysis and progeny tests using molecular markers revealed that both induced tetraploids reproduced sexually. Single‐seed screening by flow cytometry confirmed full sexual reproduction. These plants retained the high self‐incompatibility system of the diploids, but set seed after reciprocal crosses and when crossed with pollen of apomictic 4x P. guenoarum Arechav. of the Plicatula group. Both sexual 4x plants constitute the foundational material for plant improvement through gene exchange and selection in apomictic 4x P. plicatulum and possibly in several apomictic species of the Plicatula group.

Gametophytic apomixis is an asexual mode of reproduction by seeds. This trait is present in several plant families and is strongly associated with polyploidy. Paspalum rufum is a forage grass with sexual self-incompatible diploids (2n = 2x = 20) and aposporous-apomictic pseudogamous tetraploids (2n = 4x = 40). In previous work embryological observations of the diploid genotype Q3754 showed 8.8-26.8% of the ovaries having one meiotic plus an aposporous-like embryo sac, suggesting some capability for apomictic reproduction. The objective of this work was to characterize progenies derived from Q3754 to determine if aposporous sacs were functional and generated progenies via apomixis at the diploid level. Re-examination of Q3754 ovaries showed that 12.5% of them contained one sexual plus an aposporous sac confirming previous results. Progeny tests were carried out on two experimental families (H 1 and S 1 ) employing heterozygous RAPD marker loci. Family H 1 was obtained crossing Q3754 with a natural diploid genotype (Q3861) and S 1 derived from the induced self-pollination of Q3754. Genetic analysis of H 1 showed that all individuals derived from sexual reproduction. However, 5 out of 95 plants from S 1 showed the same heterozygous state as the mother plant for 14 RAPD loci suggesting a clonal origin. Further experiments, designed to test the functionality of aposporous sacs by flow cytometric analyses, were carried out on a third family (M 1 ) obtained by crossing Q3754 with the tetraploid plant Q3785. Histograms of 20 M 1 plants showed 15 diploids (75%), 4 triploids (20%) and 1 tetraploid (5%). Triploids and the tetraploid may have originated from functional aposporous embryo sacs. Likewise, the reconstruction of the developmental route of 40 individual seeds demonstrated that 11 of them (27.5%) derived from fertilized aposporic sacs. The results presented in this work indicate that gametophytic apomixis is effectively expressed at the diploid level in Paspalum rufum and could be the foundation of a recurrent auto-polyploidization process in the species.

Abbreviations: FCSS, fl ow cytometry seed screen; GR, cultivar Rojas of P. guenoarum; 4PT, sexual induced-tetraploid P. plicatulum; PMC, pollen mother cell; RAPD, random amplifi ed polymorphic DNA.

Traditionally, tetraploid Paspalum notatum was considered an obligate or a facultative apomict according to cytoembryological analyses. The degree of facultativeness was usually determined by the relative amount of mature ovules bearing aposporous or meiotic (sexual) embryo sacs, or both together. We established, through progeny tests conducted with the aid of AFLP markers, the degree of residual sexuality expressed in four selected biotypes. The results showed it to be substantially and significantly lower than predicted by previous embryological analyses for the same biotypes. Moreover, the lowest expression of residual sexuality was coincident with maximum flowering period. Seed development in facultative apomictic P. notatum shows a definite bias against meiotic embryo sacsFil: Rebozzio, Romina Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Nordeste. Instituto de Botánica del Nordeste (i); Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Agrarias; ArgentinaFil: Sartor, Maria Esperanza. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Nordeste. Instituto de Botánica del Nordeste (i); Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Agrarias; ArgentinaFil: Quarin, Camilo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Nordeste. Instituto de Botánica del Nordeste (i); Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Agrarias; ArgentinaFil: Espinoza, Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Nordeste. Instituto de Botánica del Nordeste (i); Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Agrarias; Argentin

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The results presented demonstrate the existence of variation in the functionality of apomixis components in natural diploid genotypes of P. rufum and have identified individuals with contrasting reproductive behaviours. Genotypes identified here can be crossed to generate segregating populations in order to study apomixis determinants at the diploid level. Moreover, analysis of their expression patterns, quantification of their transcript levels and an understanding of their regulation mechanisms could help to design new strategies for recreating apomixis in a diploid genome environment.

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