Carlos Alberto Acuña

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.

Sexual diploid and apomictic tetraploid races in Thrasya petrosa (Gramineae)

Thrasya petrosa (Trin.) Chase is the most widespread species of a grass genus indigenous to the New World. Genetic systems in diploid (2n = 2x = 20) and tetraploid (2n = 4x = 40) races of T. petrosa were investigated. The diploid race exhibited embryological development typical of sexual reproduction, but failed to produce seed because of self-incompatibility, whereas the tetraploid showed embryological pathways characteristic of facultative apomixis. Consequently, some ovules showed a normal meiotic embryo sac, others had one to several aposporous sacs, and, finally, some ovules had one or more aposporous sacs beside the meiotic one. A uniform progeny test assisted by molecular markers confirmed that the main reproductive mode for the tetraploid race was apomixis, despite some sexual reproductive structures observed by cytoembryological analyses. The chromosome pairing patterns at meiosis suggested that autoploidy was the genetic origin of the tetraploid races of T. petrosa. In addition, the close relationship between Thrasya Kunth and Paspalum L. previously supported by phylogenetic analyses is further sustained by the particular genetic system shared by the two genera. The system involves co-specific sexual self-incompatible diploids and apomictic, pseudogamous and self-compatible polyploids.

Apospory followed by sterility in a hypotriploid hybrid (2x X 4x) of Paspalum

Abstract – A self-incompatible diploid plant (2n = 2x = 20) of Paspalum limbatum grown in a field nursery surrounded by tetraploid accessions of several Paspalum species produced a hypotriploid descendant (2n-1 = 3x = 29). Molecular fingerprinting by RAPD markers indicated that an apomictic 4x accession of P. guenoarum was the pollen parent. Meiotic chromosome pairing in the hypotriploid hybrid averaged 16.3 univalents (range 9-29), 5.4 bivalents (0-9), and 0.7 trivalents (0-3). Since P. guenoarum has an autopolyploid or segmental allopolyploid origin, the bivalent associations in the hypotriploid hybrid could be ascribed to an autosyndetic pairing between the two chromosome sets contributed by the male parent. The trivalent chromosome associations suggested some degree of homology between the chromosomes of P. limbatum and P. guenoarum. Embryological analysis of the hypotriploid hybrid showed complete abortion of the megaspore mother cell before or during the first meiotic division. One to several aposporous embryo sacs developed from somatic nucellar cells in over 97% of the ovules analysed. The plant was 100% male sterile and seed sterility reached 99.9%, despite hand-pollination with several pollen sources. Thus, apospory was fully inherited from the apomictic 4x male parent, but the trait did not prevent seed sterility.