Morphological and molecular genetics of ancient remains and modern rice (Oryza sativa) confirm diversity in ancient Japan

Abstract

Rice was introduced to northern Japan in the prehistoric era, when the local climate was relatively cool for rice cultivation. Ancient strains of rice could potentially be productive in unfavorable conditions for cultivation; modern strains sharing genetic traits with ancient rice may contribute useful genetic diversity. To understand the variation in Japanese rice, a comparison of seed sizes and chloroplast and nuclear DNA sequences was conducted, on modern rice accessions from diverse geographical areas, and eleven populations with seven reference populations of rice seed remains from 800 BC to 1500 AD. The populations of rice seed remains shared short grain shape and a specific chloroplast genotype with modern Japanese and worldwide rice accessions. This morphology and genotype may have been introduced to Japan along with farming techniques. Variability of seed shape and diversity of the nuclear genome both reduced through time, indicating genetic erosion in Japanese rice. Greater diversity in the populations of rice seed remains, taken together with archaeological and historical evidence, suggests that older rice populations had genetic traits that could adapt to unfavorable conditions of cultivation, such as flooding and low temperatures. Modern landraces share a genome structure with rice seed remains populations, which suggests that the modern landraces may have useful breeding traits such as tolerance of flooding and low temperatures.