Reduction in seed shattering was an important phenotypic change during cereal domestication. Here we show that a simple morphological change in rice panicle shape, controlled by the SPR3 locus, has a large impact on seed-shedding and pollinating behaviors. In the wild genetic background of rice, we found that plants with a cultivated-like type of closed panicle had significantly reduced seed shedding through seed retention. In addition, the long awns in closed panicles disturbed the free exposure of anthers and stigmas on the flowering spikelets, resulting in a significant reduction of the outcrossing rate. We localized the SPR3 locus to a 9.3-kb genomic region, and our complementation tests suggest that this region regulates the liguleless gene (OsLG1). Sequencing analysis identified reduced nucleotide diversity and a selective sweep at the SPR3 locus in cultivated rice. Our results suggest that a closed panicle was a selected trait during rice domestication.

It is known that the common cultivated rice (Oryza sativa) was domesticated from Asian wild rice, O. rufipogon. Among the morphological differences between them, loss of seed shattering is one of the striking characters specific for the cultivated forms. In order to understand the genetic control on shattering habit, QTL analysis was carried out using BC 2 F 1 backcross population between O. sativa cv. Nipponbare (a recurrent parent) and O. rufipogon acc. W630 (a donor parent). As a result, two strong QTLs were detected on chromosomes 1 and 4, and they were found to be identical to the two major seed-shattering loci, qSH1 and sh4, respectively. The allelic interaction at these loci was further examined using two sets of backcross populations having reciprocal genetic backgrounds, cultivated and wild. In the genetic background of cultivated rice, the wild qSH1 allele has stronger effect on seed shattering than that of sh4. In addition, the wild alleles at both qSH1 and sh4 loci showed semi-dominant effects. On the other hand, in the genetic background of wild rice, non-shattering effects of Nipponbare alleles at both loci were examined to inspect rice domestication from a viewpoint of seed shattering. It was serendipitous that the backcross plants individually having Nipponbare homozygous alleles at either shattering locus (qSH1 or sh4) shed all the seeds. This fact strongly indicates that the non-shattering behavior was not obtained by a single mutation in the genetic background of wild rice. Probably, some other minor genes are still associated with the formation or activation of abscission layer, which enhance the seed shattering.

In the near future, global average temperature is expected to increase due to the accumulation of greenhouse gases, and increased temperatures will cause severe sterility in many crop species. In rice, since wild species show high genetic variation, they may have the potential to improve the flowering characters of cultivars. In this study, we investigated flowering characters under natural conditions by comparing an Asian wild rice accession of Oryza rufipogon W630 (originated from Myanmar) with a Japanese rice cultivar, O. sativa Japonica cv. Nipponbare. Further, QTL analysis for days to heading (DH) and spikelet opening time (SOT: the time of day when the spikelet opens) was carried out using BC(2)F(8) backcross population derived from the cross between them. Regarding DH, four QTLs were detected, and two of them were found to have wild alleles with strong effects leading to longer days to heading during the Japanese summer. These wild alleles may be used to produce late-heading cultivars that do not flower during the high summer temperatures anticipated in the future. As for SOT, two parameters of SOTb (beginning time when the first spikelet opens) and SOTm (median time when 50% of the spikelets open) were recorded and the time differences from Nipponbare were investigated. Two QTLs on chromosomes 5 and 10 and two QTLs on chromosomes 4 and 5 were detected for SOTb and SOTm, respectively. The wild alleles were responsible for early spikelet opening time at all loci. If the wild alleles detected in this study have the same effects in the genetic background of other cultivars, they will be very useful in producing early-flowering rice cultivars that complete fertilization in the morning before the temperature rises.

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Seed awning is one of the important traits for successful propagation in wild rice. During the domestication of rice by ancient humans, plants with awnless seeds may have been selected because long awns hindered collection and handling activities. To investigate domestication of awnless rice, QTL analysis for seed awning was first carried out using backcross recombinant inbred lines between Oryza sativa Nipponbare (recurrent parent) and O. rufipogon W630 (donor parent). Two strong QTLs were detected in the same regions as known major seed-awning loci, An-1 and RAE2. Subsequent causal mutation surveying and fine mapping confirmed that O. rufipogon W630 has functional alleles at both loci. The gene effects and interactions at these loci were examined using two backcross populations with reciprocal genetic backgrounds of O. sativa Nipponbare and O. rufipogon W630. As awn length in wild rice varied among seeds even in the same plant, awn length was measured based on spikelet position. In the genetic background of cultivated rice, the wild alleles at An-1 and RAE2 had awning effects, and plants having both wild homozygous alleles produced awns whose length was about 70% of those of the wild parent. On the other hand, in the genetic background of wild rice, the substitution of cultivated alleles at An-1 and RAE2 contributed little to awn length reduction. These results indicate that the domestication process of awnless seeds was complicated because many genes are involved in awn formation in wild rice.