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The world population is expected to be larger and wealthier over the next few decades and will require more animal products, such as milk and beef. Tropical regions have great potential to meet this growing global demand, where pasturelands play a major role in supporting increased animal production. Better forage is required in consonance with improved sustainability as the planted area should not increase and larger areas cultivated with one or a few forage species should be avoided. Although, conventional tropical forage breeding has successfully released well-adapted and high-yielding cultivars over the last few decades, genetic gains from these programs have been low in view of the growing food demand worldwide. To guarantee their future impact on livestock production, breeding programs should leverage genotyping, phenotyping, and envirotyping strategies to increase genetic gains. Genomic selection (GS) and genomewide association studies play a primary role in this process, with the advantage of increasing genetic gain due to greater selection accuracy, reduced cycle time, and increased number of individuals that can be evaluated. This strategy provides solutions to bottlenecks faced by conventional breeding methods, including long breeding cycles and difficulties to evaluate complex traits. Initial results from implementing GS in tropical forage grasses (TFGs) are promising with notable improvements over phenotypic selection alone. However, the practical impact of GS in TFG breeding programs remains unclear. The development of appropriately sized training populations is essential for the evaluation and validation of selection markers based on estimated breeding values. Large panels of single-nucleotide polymorphism markers in different tropical forage species are required for multiple application targets at a reduced cost. In this context, this review highlights the current challenges, achievements, availability, and development of genomic resources and statistical methods for the implementation of GS in TFGs. Additionally, the prediction accuracies from recent experiments and the potential to harness diversity from genebanks are discussed. Although, GS in TFGs is still incipient, the advances in genomic tools and statistical models will speed up its implementation in the foreseeable future. All TFG breeding programs should be prepared for these changes. Made available in DSpace on 2021-08-10T02:06:56Z (GMT). No. of bitstreams: 1 Genomic-selection-tropical.pdf: 2481038 bytes, checksum: c1c166bd9f912ae4d0970aee8c98aa94 (MD5) Previous issue date: 2021

ruzigrass ( Urochloa ruziziensis (r. Germain & Evrard) Crins, syn. Brachiaria ruziziensis Germain & Evrard) is a tropical forage native to Africa, first introduced in Brazil in the 1960s. r uzigrass is diploid, presents sexual reproduction, and is a model species for the generation of genomic resources in the Brachiaria genus. Brachiaria species are the most important livestock forage in the tropics, covering more than 70 million ha in Brazil alone. We have used multiplex panels of microsatellite markers to characterize the genetic diversity of ruzigrass germplasm collected in Africa, and of a local population collected in Bra - zil, to obtain information for its conservation and use in breeding programs. Fifteen SS - ers were used to Genotype 114 ruzigrass sam ples. Summary statistics, as well as estimates of F ST and partitioning of genetic diversity were obtained. Clustering and genetic structure analy - ses were performed. r esults showed high val- ues of heterozygosity in the African and Brazilian populations. Although African and Brazilian sam - ples formed distinct groups in clustering analy - ses, and Bayesian analysis of genetic structure distributed samples into three clusters, estimates of pairwise F ST values showed no differentiation between African and Brazilian groups of sam - ples. results indicated that the introduction of ruzigrass in Brazil did not cause a major decrease in genetic diversity. Brazilian local populations might harbor favorable alleles that will be useful for ongoing and future breeding programs. They offer great opportunities for ruzigrass germplasm collection and conservation of genetic diversity, before attempts to access germplasm in its cen - ter of diversity are undertaken. M. Pessoa-Filho, Embrapa Cerrados, P.O. Box 08223, Brasilia, DF, Brazil 73310-970; A.L.S. Azevedo, F.S. Sobrinho, Embrapa Dairy Cattle, Rua Eugenio do Nascimento, 610, Juiz de Fora, MG, Bra

Background Breeding programs are usually reluctant to evaluate and use germplasm accessions other than the elite materials belonging to their advanced populations. The concept of core collections has been proposed to facilitate the access of potential users to samples of small sizes, representative of the genetic variability contained within the gene pool of a specific crop. The eventual large size of a core collection perpetuates the problem it was originally proposed to solve. The present study suggests that, in addition to the classic core collection concept, thematic core collections should be also developed for a specific crop, composed of a limited number of accessions, with a manageable size. Results The thematic core collection obtained meets the minimum requirements for a core sample - maintenance of at least 80% of the allelic richness of the thematic collection, with, approximately, 15% of its size. The method was compared with other methodologies based on the M strategy, and also with a core collection generated by random sampling. Higher proportions of retained alleles (in a core collection of equal size) or similar proportions of retained alleles (in a core collection of smaller size) were detected in the two methods based on the M strategy compared to the proposed methodology. Core sub-collections constructed by different methods were compared regarding the increase or maintenance of phenotypic diversity. No change on phenotypic diversity was detected by measuring the trait "Weight of 100 Seeds", for the tested sampling methods. Effects on linkage disequilibrium between unlinked microsatellite loci, due to sampling, are discussed. Conclusions Building of a thematic core collection was here defined by prior selection of accessions which are diverse for the trait of interest, and then by pairwise genetic distances, estimated by DNA polymorphism analysis at molecular marker loci. The resulting thematic core collection potentially reflects the maximum allele richness with the smallest sample size from a larger thematic collection. As an example, we used the development of a thematic core collection for drought tolerance in rice. It is expected that such thematic collections increase the use of germplasm by breeding programs and facilitate the study of the traits under consideration. The definition of a core collection to study drought resistance is a valuable contribution towards the understanding of the genetic control and the physiological mechanisms involved in water use efficiency in plants.

Background This study aimed to analyze the efficiency of three new microsatellite multiplex panels, which were designed to evaluate a total of 16 loci of the rice genome, based on single PCR reactions of each panel. A sample of 548 accessions of traditional upland rice landraces collected in Brazil in the last 25 years was genotyped, a database of allelic frequencies was established, estimates of genetic parameters were performed and analysis of genetic structure of the collection was developed. Results The three panels yielded a combined matching probability of 6.4 × 10-21, polymorphism information content (PIC) of 0.637, and a combined power of exclusion greater than 99.99%. A few samples presented a genetic background of indica rice. The 16 SSR loci produced a total of 229 alleles. Gene diversity values averaged 0.667, and PIC values averaged 0.637. Genetic structure analysis of the collection using a Bayesian approach detected three possible major clusters, with an overall F ST value of 0.177. Important inputs on the knowledge about upland rice germplasm differentiations which happened in Brazil in the last few centuries were also achieved and are discussed. Conclusion The three multiplex panels described here represent a powerful tool for rice genetic analysis, offering a rapid and efficient option for rice germplasm characterization. The data gathered demonstrates the feasibility of genotyping extensive germplasm collections using panels of multiplexed microsatellite markers. It contributes to the advancement of research on large scale characterization and management of germplasm banks, as well as identification, protection and assessments of genetic relationship of rice germplasm.