Maria Esperanza Sartor

Harnessing apomictic reproduction in grasses: what we have learned from Paspalum

BACKGROUND Apomixis is an alternative route of plant reproduction that produces individuals genetically identical to the mother plant through seeds. Apomixis is desirable in agriculture, because it guarantees the perpetuation of superior genotypes (i.e. heterotic hybrid seeds) by self-seeding without loss of hybrid vigour. The Paspalum genus, an archetypal model system for mining apomixis gene(s), is composed of about 370 species that have extremely diverse reproductive systems, including self-incompatibility, self-fertility, full sexual reproduction, and facultative or obligate apomixis. Barriers to interspecific hybridization are relaxed in this genus, allowing the production of new hybrids from many different parental combinations. Paspalum is also tolerant to various parental genome contributions to the endosperm, allowing analyses of how sexually reproducing crop species might escape from dosage effects in the endosperm. SCOPE 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.

Ploidy levels and reproductive behaviour in natural populations of five Paspalum species

Knowledge of variation in ploidy levels and reproductive behaviour in natural populations is essential in order to understand the functioning of agamic complexes. The aim of this study was to analyse the ploidy level and mode of reproduction in several wild Paspalum populations. A total of 19 populations representing five different species (P. alcalinum, P. denticulatum, P. lividum, P. nicorae, and P. rufum) were collected. Ploidy level was determined in 1,187 individuals by using flow cytometry. Among these individuals, 2x, 3x, 4x, 5x, 6x, and 7x chromosome constitutions were observed. Diploid sexual cytotypes of P. denticulatum were detected for the first time; this will allow the development of future breeding strategies for this particular species. Flow cytometry seed screen (FCSS) in bulked and single seeds revealed the reproductive diversity of these species, ranging from complete sexuality in diploids and varying levels of facultative apomixis in most tetraploids, to obligate apomixis in pentaploids and hexaploids. A fully sexual tetraploid plant was never detected. Nevertheless, most tetraploid genotypes produced both maternal (by apomixis) and non-maternal (by sexuality) progeny. This residual sexuality is very interesting from an evolutionary point of view, since it would allow the creation of new genotypic combinations in natural populations. In addition, the residual sexuality found in some apomictic tetraploid populations can be used as a source of variability for genetic improvement.

Patterns of genetic diversity in natural populations of Paspalum agamic complexes

Paspalum has many multiploid species displaying a wide range of ploidy levels and reproductive systems including apomixis. However, not much is known about the genetic structure of natural populations of the apomictic species of Paspalum. The aim of this work was to evaluate the genetic diversity of several natural populations belonging to five species of Paspalum. A total of 13 populations were analyzed using amplified fragment length polymorphism (AFLP). The AFLP data revealed maximal genotypic diversity and significant levels of genetic diversity in diploid and mixed diploid–tetraploid populations of P. denticulatum and P. rufum, where all individuals represent different genotypes. This may be mainly due to the reproductive system of diploid members and the gene flow from diploids to polyploids. The pure populations of tetraploids consist of either multiple genotypes (P. nicorae) or of one dominant genotype with a few deviated genotypes (P. denticulatum and P. lividum). Here, the main source of variability may be the residual sexuality, which continues generating new genotypic combinations. The hexaploid populations of P. buckleyanum consist of a single AFLP genotype and each population represents a particular genotype suggesting that populations arose from independent polyploidization events. This study represents one of the first reports of genetic diversity in natural populations of several Paspalum agamic complexes. Apomixis in these five species may be acting as a successful method for the dispersion of better adapted genotypes.

Analysis of variation for apomictic reproduction in diploid Paspalum rufum

BACKGROUND AND AIMS The diploid cytotype of Paspalum rufum (Poaceae) reproduces sexually and is self-sterile; however, recurrent autopolyploidization through 2n + n fertilization and the ability for reproduction via apomixis have been documented in one genotype of the species. The objectives of this work were to analyse the variation in the functionality of apomixis components in diploid genotypes of P. rufum and to identify individuals with contrasting reproductive behaviours. METHODS Samples of five individuals from each of three natural populations of P. rufum (designated R2, R5 and R6) were used. Seeds were obtained after open pollination, selfing, conspecific interploidy crosses and interspecific interploidy self-pollination induction. The reproductive behaviour of each plant was determined by using the flow cytometric seed screen (FCSS) method. Embryo sacs were cleared using a series of ethanol and methyl salicylate solutions and observed microscopically. KEY RESULTS In open pollination, all genotypes formed seeds by sexual means and no evidence of apomeiotic reproduction was detected. However, in conspecific interploidy crosses and interspecific interploidy self-pollination induction, variations in the reproductive pathways were observed. While all plants from populations R2 and R6 formed seeds exclusively by sexual means, three genotypes from the R5 population developed seeds from both meiotic and aposporous embryo sacs, and one of them (R5#49) through the complete apomictic pathway (apospory + parthenogenesis + pseudogamy). Cytoembryological observations revealed the presence of both meiotic and aposporous embryo sacs in all the genotypes analysed, suggesting that parthenogenesis could be uncoupled from apospory in some genotypes. CONCLUSIONS 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.