Genetic Diversity of Released Rice Varieties Derived From a Global Exchange and Evaluation Network.

The International Network for Genetic Evaluation of Rice (INGER; http://seeds.irri.org/inger/) has been a primary source of germplasm used by national rice breeding programs worldwide since 1975. Over 25,000 unique breeding lines and genebank accessions have been distributed by INGER to 85 countries during the last three decades.  Of these, several hundreds have been directly released as varieties in more than 60 countries and thousands more have been used to breed new varieties by hybridizing them with locally adapted cultivars.  We analyzed the genetic diversity of 176 rice varieties selected from INGER nurseries and commercially released in 56 countries using 32 morphological-agronomic traits and 50 simple sequence repeat (SSR) markers covering the 12 rice chromosomes. A total of 377 SSR alleles were detected and compared with 15 SNP collections as reference. Similarity of alleles was computed using the Dice coefficient and cluster analysis was performed using the unweighted pair-group method of arithmetical means (UPGMA) and the NTSYS-pc version 2.0 software. On the other hand, similarity of rice varieties based on morpho-agronomic traits was determined using Euclidian coefficients. Similarity analysis based on marker data showed that at approximately 0.25, 2 major clusters separated japonica and indica varieties and branches out into 3 sub-clusters at 0.45. No trends were observed on the distribution of released varieties coming from different geographical origins. Varieties from the same geographical origin were distributed in the four sub-clusters. Moreover, similarity of varieties based on morpho-agronomic traits showed ambiguous clustering. At 0.27, 11 clusters covering several overlapping sub-clusters were observed, without any major cluster. Clustering patterns based on SSR data did not correlate well with the clustering based on morpho-agronomic traits.  The INGER mechanism appears to have contributed to the enhancement of the genetic diversity utilized in rice breeding programs worldwide.