Evidence suggests japonica rice moved from tropical south to north

Also, in keeping with Solheim's theory is a recent study that suggests the temperate japonica rice of East Asia split off from earlier tropical japonica rice in a movement from south to north

http://www.genetics.org/cgi/content/full/169/3/1631

"In addition, the two japonica groups represent an adaptive spectrum of an ancient subpopulation from tropical origins to temperate latitudes, with the necessary adaptations to environmental signals such as day length and temperature. As the only pairwise comparison that embodies such obvious adaptation to a new environment, the temperate and tropical japonica groups offer a valuable tool for studying the genetic basis of adaptation. The statistical significance of the larger allele size in the temperate relative to the tropical japonica group supports the hypothesis that temperate japonica were derived from the tropical japonica group. One explanation for the differences in average allele lengths is a higher mutation rate in the temperate population. Previous observations of enhanced transposable element activity in temperate compared to tropical japonica groups (JIANG et al. 2002) suggest that this hypothesis may be worthy of further investigation."

Regards,
Paul Kekai Manansala
Sacramento