Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 52 (2019)

Recent progress in overcoming interspecific hybrid sterility in rice
Y. Koide,∗1 K. Onishi,∗2 Y. Hayashi,∗3 T. Abe,∗3 Y. Fukuta,∗4 Y. Okumoto,∗5 and A. Kanazawa∗1Inter-specific hybridization enables breeders totransfer valuable genes from one species to anotherfor improving crops. However, reproductive barriers,
which are also known as “species barriers,” often prevent gene flow between two species. In two cultivatedrice species (Oryza sativa and O. glaberrima), hybridsterility is considered as the main reproductive barrier.
To date, more than 10 loci for hybrid sterility between
these species have been found.1) Among these loci, theHYBRID STERILITY 1 (S1 ) locus on the short armof chromosome 6 has been frequently detected,2) suggesting that the S1 locus is the major cause of thesterility barrier. Here, we review the recent progressin overcoming the hybrid sterility caused by the S1 locus and highlight the usefulness of a forward genetic
screening for a mutant with a “neutral” allele of hybridsterility loci.
In 1990, Sano3) showed that the hybrid between astrain of O. sativa and near-isogenic lines (NILs) containing a segment of chromosome 6 from O. glaberrimain the genetic background of O. sativa exhibited partial sterility in pollen and seeds. This phenomenon
was explained by the genetic interaction between theS1g allele (formerly the S1 allele of Sano3) ) and S1s allele (formerly the S1a allele of Sano3) ), which were derived from O. glaberrima and O. sativa, respectively.
The S1g allele acts as a “gamete eliminator,” and bothmale and female gametes possessing the S1s allele areaborted only in the heterozygote (S1g /S1s ).4) Althoughthese studies revealed the genetic nature of the S1 locus, it was still unclear how we can overcome the sterility barrier.
Recently, significant progress has been made by artificial mutagenesis using genome editing5) or heavy-ion
beam irradiation.6) Koide et al.6) used Acc108 (a variety of O. sativa) and NIL, which contain the S1s allele
and the S1g allele at the S1 locus, respectively. Theyobtained a total of 2,478 F1 seeds for heavy-ion beamirradiation via artificial pollination. From 1,817 F1 hybrids (M1 generation) irradiated with carbon-ion beam
(LET 30 keV/nucleonm, 150 Gy) at the RIKEN RIbeam factory, Wako, Japan, they obtained one plantthat had a panicle with >50% seed fertility from theM1 population (Fig. 1). Then, they developed the M2
family through self-pollination of the M1 plant and obtained the mutant Acc108S1 M, which does not induce
∗1
∗2
∗3
∗4
∗5ANILBAcc108x
S1s/S1s

S1g/S1gF1

Heavy-ionbeam
irradiation

S1g/S1s

Select fertile plants

C>50%
seed ferlity
Acc108S1M

Fig. 1. Forward genetic screening for a mutant with a “neutral” allele of the hybrid sterility locus. (A) The scheme
for selecting a mutant with a neutral allele at the hybrid
sterility locus, S1 . (B) A sterile rice panicle observed in
the M1 plant. The seed sterility was due to the hybrid
sterility locus, S1 . (C) A fertile panicle observed in theM1 plant. Seed sterility did not occurr because of amutation induced at the S1 locus.

sterility in either the hybrid with the S1g carrier or thatwith the S1s carrier.
The results of the crossing experiments, genetic
mapping, and nucleotide sequencing suggested that thecausative mutation of Acc108S1 M was a 5-bp deletionin the peptidase-coding gene (denoted by SSP ) in theS1 locus. These results indicated that the SSP was
one of the essential genes for inducing hybrid sterility
in heterozygotes (S1g /S1s ) and Acc108S1 M has a neutral allele “S1mut ” at the S1 locus.6)
In summary, recent studies5,6) have focused on theS1 locus and have shown how to obtain a neutral allele
through reverse or forward genetic approaches. In general, the number of hybrid sterility loci that have beenannotated and characterized in crop gene pools is still
limited. In such a case, a forward genetic screening ismore practical than approaches in which gene identification is a prerequisite, e.g., genome editing, for creating neutral alleles. Although these two approaches
have their own specific advantages, recent studies havedemonstrated a technique that allows broader accessto desirable traits in distantly related species duringcrop breeding.
References
1) K. Doi et al., Curr. Opin. Plant Biol. 11, 144–148(2008).
2) A. Garavito et al., Genetics 185, 1425–1440 (2010).
3) Y. Sano, Genetics 125, 183–191 (1990).
4) Y. Koide et al., New Phytol. 179, 888–900 (2008).
5) Y. Xie et al., Mol. Plant 10, 1137–1140 (2017).
6) Y. Koide et al., Proc. Natl. Acad. Sci. USA, 115,E1955–E1962 (2018).

Research Faculty of Agriculture, Hokkaido University
Department of Agro-Environmental Science, Obihiro University of Agriculture and Veterinary Medicine
RIKEN Nishina Center
Japan International Research Center for Agricultural Sciences
Faculty of Agriculture, Kyoto University- 25 -