C. B. do Valle

Identification of a maize linkage group related to apomixis in Brachiaria

Abstract A bulked segregant analysis using RFLPs and RAPDs was carried out to identify molecular markers co-segregating with apomixis in a Brachiaria F1 population. The test population used was a cross between sexual B. ruziziensis R44 and the aposporous apomictic Brachiaria brizantha cv Marandu. The Brachiaria genome was systematically scanned using 61 cDNA and genomic maize clones detecting 65 loci located at 40 cM, on average, one from each other in the maize genome. The finding of a clone that presented a polymorphic band co-segregating with apomixis (umc147) led to the identification of another marker within the same area (umc72). The clones belong to a duplicated linkage group that maps to the distal part of maize chromosome-1 long arm and chromosome-5 short arm. RAPD analysis using 184 primers from Operon sets yielded one more marker (OPC4) significantly linked to the trait mapping the same locus. OPC4 had been previously reported as a potential marker for apospory in Pennisetum. A map of the region was constructed using additional clones that belong to the same maize linkage group. Since that was the only genomic region that presented an apomixis-linked polymorphism our observations support the existence of a single locus directing apospory in Brachiaria.

Breeding Panicum maximum in Brazil. 1. Genetic resources, modes of reproduction and breeding procedures

SummaryApomixis is widely distributed among tropical forage grasses, and has long been merely regarded as an impediment to breeding. Panicum maximum is presented as the first opportunity for Brazilian geneticists to develop and test original breeding schemes adapted to an apomictic species.A large and representative germplasm of P. maximum has been introduced and is currently being evaluated. Basic knowledges on biology and reproduction are also available, which demonstrate an easy manipulation of apomixis and sexuality. Several limiting traits have already been detected during evaluation, which justify breeding attempts. An ideal scheme is given to transfer new qualities to already selected varieties.

Development of microsatellite markers for Brachiaria humidicola (Rendle) Schweick

We describe the first panel of nuclear simple sequence repeats (SSRs) loci for Brachiaria humidicola (Rendle) Schweick., a warmseason grass with facultative apomixis, variation in ploidy levels (6X–9X), and important forage grass species in the Tropics. Of 38 pairs of primers obtained by using an enriched-library methodology, 27 revealed polymorphism in 58 accessions of the Germplasm Collection of B. humidicola held at Embrapa Beef Cattle, Brazil. Eleven loci amplified in B. dictyoneura, a closely related species with unclear taxonomic boundaries with B. humidicola. Transferability to other three Brachiaria species was also evaluated. The developed microsatellites are potentially useful for genetic studies of B. humidicola, as well as phylogenetic evaluations, conservation and breeding applications.

Isolation and characterization of microsatellite markers for Brachiaria brizantha (Hochst. ex A. Rich.) Stap

The first set of nuclear simple sequence repeat (SSR) loci for Brachiaria brizantha (Hochst. ex A. Rich.) Stap is described. A microsatellite-enriched library was constructed and 19 loci were characterized. About 13 SSR loci were found to be polymorphic and across-taxa amplification tests showed that six of them can be transferred to four other species of Brachiaria. This new SSR resource will be a powerful tool for population genetic studies of B. brizantha, for interspecific genetic studies within the genus Brachiaria, for mapping and for marker assisted selection in breeding.

Embryo rescue of interspecific hybrids of Brachiaria spp

Age of explant and six different media were evaluated with the objective of regenerating higher numbers of interspecific hybrids between sexual and apomictic Brachiaria. Immature embryos of 7–8, 9–10 and 11–12 days after pollination (DAP), from artificial hybridization between Brachiaria ruziziensis (R) as female parent, and B. brizantha (B) or B. decumbens (D) as male parent, were cultured in modified MS media (M4) – supplemented with different combinations of growth regulators and vitamins. Embryos cultured 9–12 DAP showed high percentage (85–100%) of germination for all the crosses examined. Germination and survival rates varied according to accessions within crosses. Six different media (all modified MS with different growth regulators and vitamins) were tested with the objective of inducing multiple shoots from 7 to 10 DAP embryos, from crosses between R × B. The media M1, supplemented with Kinetin (13.94 μM) and NAA (5.37 μM), and media M3, supplemented with BA (4.44 μM and IAA 2.85 μM), regenerated adventitious shoots and calli about 30–40 days after inoculation. The highest multiplication rate observed was 2.85 shoots per explant in media M1, 60–70 days after culturing. Two other media, M6, supplemented with 2,4-D (13.57 μM) and M2, with 2,4-D (9.05 μM) and BA (8.87 μM) exclusively induced the formation of calli. The described protocols proved to be efficient in regenerating healthy seedlings from immature embryos of interspecific hybrids in Brachiaria.