Genetic and embryological evidences of apomixis at the diploid level in Paspalum rufum support recurrent auto-polyploidization in the species

Abstract

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.

Cytological and reproductive aspects in the Caespitosa group of Paspalum

“…Unreduced egg-cells may be functional and, when fertilized by a reduced male gamete, they give rise to triploid plants (2x+x=3x). These triploids may, in turn, be the origin of new tetraploids (3x+x=4x) (SIENA et al, 2008). Otherwise, the 2n gamete of an additional aposporous sac can be fertilized by a male haploid gamete from a neighbouring apomictic 4x plant (2n) to establish a new apomictic 4x genotype, as was achieved experimentally in P. rufum (NORRMANN et al, 1994).…”

Reproductive System and Molecular Phylogenetic Relationships of Fonio Millets (Digitaria spp., Poaceae) with Some Polyploid Wild Relatives

“…Reproductive systems of fonio species were determined following the method described by Marshall and Brown (1974) and recently applied in various plants including Hypericum perforatum L. (Arnhold-Schmitt 2000) and Paspalum rufum Nees (Siena et al 2008). This method involves progeny testing of known genotypes and assumes that auto-segregation is absent, and thus, all progenies derived from apomixis are identical to the mother plant.…”

“…Apomixis or agamospermy is an asexual mode of reproduction in which the ovule develops into a seed without involving meiosis and fertilization (Nogler 1984). Since apomixis results in a transmission of an exact copy of the maternal genotype, it is then possible to discriminate between the two systems as genotypic deviation from maternal profile within the offspring is expected in sexual mating conditions (Marshall and Brown 1974;Siena et al 2008).…”

Evolution of gametophytic apomixis in flowering plants: an alternative model from Maloid Rosaceae

“…Unlike model 2 below, it does not suggest any reason for the repeated origins of apomixis in different genera of the allopolyploid clade (Rosaceae subtribe Pyrinae) that includes Crataegus. Quarin's model was recently challenged by indications that apomixis occurs in diploid Paspalum, but, for unknown reasons, is rarely expressed (Siena et al 2008). …”