Colocalization of QTLs for hull-cracked rice and grain size in elite rice varieties in Japan

Artificial selection in the expansion of rice cultivation

Gene distributions and population genomics suggest artificial selection of ghd7 osprr37, for extremely early heading date of rice, in the Tohoku region of Japan. The ranges of cultivated crops expanded into various environmental conditions around the world after their domestication. Hokkaido, Japan, lies at the northern limit of cultivation of rice, which originated in the tropics. Novel genotypes for extremely early heading date in Hokkaido are controlled by loss-of-function of both Grain number, plant height and heading date 7 (Ghd7) and Oryza sativa Pseudo-Response Regulator 37 (OsPRR37). We traced genotypes for extremely early heading date and analyzed the phylogeny of rice varieties grown historically in Japan. The mutations in Ghd7 and OsPRR37 had distinct local distributions. Population genomics revealed that varieties collected from the Tohoku region of northern Japan formed three clusters. Mutant alleles of Ghd7 and OsPRR37 appear to have allowed rice cultivation to spread into Hokkaido. Our results show that the mutations of two genes might be occurred in the process of artificial selection during early rice cultivation in the Tohoku region.

Translation of continuous artificial selection on phenotype into genotype during rice breeding programs

Understanding genetic diversity among local populations is a primary goal of modern crop breeding programs. Here, we demonstrated the genetic relationships of rice varieties in Hokkaido, Japan, one of the northern limits of rice cultivation around the world. Furthermore, artificial selection during rice breeding programs has been characterized using genome sequences. We utilized 8,565 single nucleotide polymorphisms and insertion/deletion markers distributed across the genome in genotype-by-sequencing for genetic diversity analyses. Phylogenetics, genetic population structure, and principal component analysis showed that a total of 110 varieties were classified into four distinct clusters according to different populations geographically and historically. Furthermore, the genome sequences of 19 rice varieties along with historic representations in Hokkaido, nucleotide diversity and F_ST values in each cluster revealed that artificial selection of elite phenotypes focused on chromosomal regions. These results clearly demonstrated the history of the selections on agronomic traits as genome sequences among current rice varieties from Hokkaido.

A novel genotype DATTO5 developed using the five genes exhibits the fastest heading date designed in rice

The optimization of heading date is a key aspect for maximizing grain productivity in cereal crops including rice. The combinations of genes for heading date, a quantitative trait, are a major driver in the wide adaptability of cultivated rice worldwide. Here, we identified a novel QTL, qDTH3 (quantitative trait locus for days-to-heading on chromosome 3), for early flowering time in the F₂ population derived from a cross between Hoshinoyume (HS) and Daichinohoshi (DH) among local rice populations with extremely early heading date. The DH allele at qDTH3, qDTH3^DH , headed 2.7 days earlier than the HS allele at qDTH3, qDTH3^HS . We sought to design a genotype for earlier heading date by pyramiding of five heading date genes. We designated this aggregate of the five genes as DATTO5. Plants with DATTO5 were selected from the F₂ population derived from a cross between DH and HShd5, which is a near-isogenic line carrying a loss-of-function of days to heading 8 in a genetic background of HS. Plants with DATTO5 exhibited earlier heading date but reduced fitness, including shorter culm and panicle length and fewer seeds compared with HS, as a representative local rice variety with extremely early heading date.

Introgression of the chromosomal region with the Pi-cd locus from Oryza meridionalis into O. sativa L. during rice domestication

The genotype of the Pi-cd locus found in blast-resistant rice variety Kitakurin, which is a cultivated rice from Japan belonging to Oryza sativa japonica, is identical to that of its wild relative O. meridionalis. Crop domestication from wild relatives to cultivated species has encompassed significant phenotypic changes. However, little is known about the genetic changes involved in domestication. Here, we surveyed the origin of the Pi-cd locus across Oryza species with AA genomes by comparison with the genome sequences of Hoshinoyume (HS), which does not carry the Pi-cd blast resistance gene, and Kitakurin (KK), which carries the Pi-cd blast resistance gene. We found that variety-specific transposons were enriched at the Pi-cd locus. The genotype of the Pi-cd locus characterized by transposons in HS and KK was specific to each Oryza species with the AA genome. The Kitaake (KT) genotype at the Pi-cd locus found in KK was identical only to that of O. meridionalis and distributed only in subgroups of japonica in the World Rice Collection and tropical japonica in the Japanese Rice Collection, whereas it was not present in O. rufipogon accessions. The distinct distributions of genotypes of the Pi-cd locus clearly demonstrated that the Pi-cd locus was introgressed from O. meridionalis into O. sativa, specific to tropical japonica.