Although a large number of genes are expected to correctly solve a phylogenetic relationship, inconsistent gene
tree topologies have been observed. This conflicting evidence in gene tree topologies, known as gene tree
discordance, becomes increasingly important as advanced sequencing technologies produce an enormous
amount of sequence information for phylogenomic studies among closely related species. Here, we aim to
characterize the gene tree discordance of the Asian cultivated rice Oryza sativa and its progenitor, O. rufipogon,
which will be an ideal case study of gene tree discordance. Using genome and cDNA sequences of O. sativa and
O. rufipogon, we have conducted the first in-depth analyses of gene tree discordance in Asian rice. Our
comparison of full-length cDNA sequences of O. rufipogon with the genome sequences of the japonica and
indica cultivars of O. sativa revealed that 60% of the gene trees showed a topology consistent with the expected
one, whereas the remaining genes supported significantly different topologies. Moreover, the proportions of
the topologies deviated significantly from expectation, suggesting at least one hybridization event between the
two subgroups of O. sativa, japonica and indica. In fact, a genome-wide alignment between japonica and indica
indicated that significant portions of the indica genome are derived from japonica. In addition, literature
concerning the pedigree of the indica cultivar strongly supported the hybridization hypothesis. Our molecular
evolutionary analyses deciphered complicated evolutionary processes in closely related species. They also
demonstrated the importance of gene tree discordance in the era of high-speed DNA sequencing.