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Association analysis was applied to a panel of accessions of Embrapa Rice Core Collection (ERiCC) with 86 SSR and field data from two experiments. A clear subdivision between lowland and upland accessions was apparent, thereby indicating the presence of population structure. Thirty-two accessions with admixed ancestry were identified through structure analysis, these being discarded from association analysis, thus leaving 210 accessions subdivided into two panels. The association of yield and grain-quality traits with SSR was undertaken with a mixed linear model, with markers and subpopulation as fixed factors, and kinship matrix as a random factor. Eight markers from the two appraised panels showed significant association with four different traits, although only one (RM190) maintained the marker-trait association across years and cultivation. The significant association detected between amylose content and RM190 was in agreement with previous QTL analyses in the literature. Herein, the feasibility of undertaking association analysis in conjunction with germplasm characterization was demonstrated, even when considering low marker density. The high linkage disequilibrium expected in rice lines and cultivars facilitates the detection of marker-trait associations for implementing marker assisted selection, and the mining of alleles related to important traits in germplasm.

The objectives of this study were to determine the genetic structure of 242 accessions from the EMBRAPA Rice Core Collection (ERiCC), to create a mini-core collection and to develop a multiplex panel of fluorescent labeled simple sequence repeats (SSRs). Eighty-six SSRs were used to identify 1,066 alleles, with an average number of 12.4 alleles/locus and average polymorphism information content (PIC)/locus of 0.75. A model-based clustering method recognized the structure of the accessions on two levels, according to their cultivation system and origin. The most divergent subgroup identified was the worldwide lowland accessions, with the highest values for gene diversity (0.75), average Rogers distance modified by Wright (0.80), average number of alleles/locus (11.7) and private alleles (132). A mini-core was assembled with the most divergent 24 lowland and upland accessions. This mini-core displayed an average distance of 0.86, an average number of alleles/locus of 8.4 and an average PIC/locus of 0.8. From the 86 SSRs, 24 were selected to compose six multiplex panels in order to optimize the process of rice genotyping. This set of markers distinguished all 242 accessions, and showed an average PIC of 0.80 and an average number of alleles/locus of 15.4, higher than the entire set of 86 SSRs. Since the heterogeneity found in lines and cultivars of ERiCC was higher than expected, it is necessary to analyze pooled DNA samples to get a better estimate of genetic variability. The SSR characterization of ERiCC clearly indicates that there is high genetic variability in rice accessions stored in genebanks worldwide which can be promptly explored by rice pre-breeding programs.

Knowledge of natural diversity and population structures of wild species, which might be related to cultivated species, is fundamental for conservation and breeding purposes. In this study, a genetic characterization of a large population of Oryza glumaepatula, occurring in a 10 km(2) area located at Tamengo Basin (Paraguay River, Brazil), was performed using SSR markers. This population is annually dragged from the river to permit navigation; one goal of this study was to examine the impact of this removal on genetic variability. From 18 polymorphic SSR markers, a total of 190 alleles were detected in a sample of 126 individuals, with an average of 10.3 alleles/locus, and a H(e) of 0.67. The five QTL-related markers showed an average H(e) value of 0.56, while the remaining 13 markers detected an average estimate of 0.70. An apparent outcrossing rate of 30%, a high proportion of alleles at low frequencies (56%), and the presence of exclusive alleles (9.5%) were found, with strong evidence of the establishment of individuals from different populations upstream in the Paraguay River. For conservation purposes, the river drag has no effect on the population. However, periodical seed collection from the Corumbá population can preserve part of the genetic variability present in upstream populations reducing the need for upriver collecting expeditions.

The genus Oryza comprises 22 species which are potentially useful as a source of genetic variability that can be introgressed into the worldwide cultivated rice, Oryza sativa. Molecular markers are useful tools for monitoring gene introgressions and for detecting polymorphism among species. In this study, cross-amplification was estimated among 28 accessions of 16 Oryza species, representing the genomes AA, BB, CC, BBCC and CCDD, using 59 microsatellite (OG, OS and RM series) and 15 STS (Sequence Tagged Sites) markers. All markers amplified at least one Oryza species, indicating different levels of transferability across species. Markers based on microsatellite sequences amplified 37 % of the accessions, with an average of 6.58 alleles per locus and an average polymorphism information content (PIC) of 70 %. For STS markers, the amplification level was 53.3 %, and the average number of alleles and PIC values were 1.6 and 10 %, respectively. These Results showed that although the STS markers detected a reduced level of genetic diversity, the transferability was higher, indicating that they can be used for genetic analysis when evaluating less genetically related species of Oryza. Among the microsatellite markers, an analysis of species with an AA genome showed that the OG markers produced the highest level of polymorphic loci (54.6 %), followed by RM markers (48 %). Highly polymorphic and transferable molecular markers in Oryza can be useful for exploiting the genetic resources of this genus, for detecting allelic variants in loci associated with important agronomic traits, and for monitoring alleles introgressed from wild relatives to cultivated rice.