Soybean is one of the most valuable and profitable oil crop species and a thorough knowledge of the genetic structure of this crop is necessary for developing the best breeding strategies. In this study, a representative collection of soybean cultivars recommended for farming in all Brazilian regions was genotyped using 27 simple sequence repeat (SSR) loci. A total of 130 alleles were detected, with an average allelic number of 4.81 per locus. These alleles determined the core set that best represented this soybean germplasm. The Bayesian analysis revealed the presence of two clusters or subgroups within the whole collection (435 soybean cultivars) and the core set (31 entries). Cultivars of similar origin (ancestral) were clustered into the same groups in both analyses. The genetic diversity parameters, based on the SSR loci, revealed high similarity between the whole collection and core set. Differences between the two clusters detected in the core set were attributed more to the frequency of their ancestors than to their genetic base. In terms of ancestry, divergent groups were presented and a panel is shown which may foster efficient breeding programs and aid soybean breeders in planning reliable crossings in the development of new varieties.

Genetic diversity is essential for crop breeding and one way to estimate it is through the concept of genetic base, which can be defined as the number of ancestors and their relative genetic contributions (RGC) to each cultivar. The RGC can be estimated through the coefficient of parentage between the ancestors and cultivars. Previous studies determined that the genetic base of Brazilian soybean was very narrow. The objective of this work was to evaluate the pedigree of 444 Brazilian soybean cultivars to estimate their genetic base. The cultivars were divided according to their release dates and according to their origin (public or private), and the genetic base for each group was also estimated. We found 60 ancestors, of which the top four (CNS, S-100, Roanoke and Tokyo, respectively) contribute 55.3% of the genetic base. Only 14 ancestors have an RGC over 1.0%, and they represent 92.4% of the genetic base. Analysis of the release dates indicated that there has been an increase in the number of ancestors over time, but the four main ancestors were the same over all periods, and their cumulative RGC increased from 46.6% to 57.6%, indicating a narrowing of the genetic base.