Wide-compatibility Varieties Research Articles

A minimal genome design to maximally guarantee fertile inter-subspecific hybrid rice

Hybrid rice has made considerable contributions to achieve the ambitious goal of food security for the world’s population. Hybrid rice from indica/xian and japonica/geng subspecies shows much higher heterosis and is thereby an important innovation in promoting rice production in the next decade. However, such inter-subspecific hybrid rice has long suffered from serious hybrid sterility, which is a major challenge that needs to be addressed. In this study, we performed a genome design strategy to produce fertile inter-subspecific hybrid by creation of wide compatibility varieties that are able to overcome hybrid sterility. Based on combined genetic analyses in two indica-japonica crosses, we determined that four hybrid sterility loci, S5, f5, pf12 and Sc, are the major QTLs controlling inter-subspecific hybrid sterility and thus the minimal targets that can be manipulated for breeding sub-specific hybrid rice. We then cloned the pf12 locus, one of the most effective loci for hybrid male sterility, by map-based cloning, and showed that artificial disruption of pf12A gene at this locus could successfully rescue hybrid fertility. We further dissected the genetic basis of wide compatibility using three pairwise crosses from a wide-compatibility variety Dular and representative indica and japonica varieties. On this basis, we constructed and assembled different combinations of naturally compatible alleles of four loci, S5, Sc, pf12, and f5, and found that the improved lines could fully recover pollen and embryo sac fertility in test-crossed F1s, thereby completely fulfilling the demands of inter-subspecific hybrid spikelet fertility in agricultural production. This breeding scheme would facilitate redesign of future inter-subspecific hybrid rice with a higher yield potential.

Identification of a New Wide-Compatibility Locus in Inter-Subspecific Hybrids of Rice (Oryza sativa L.)

As a special class of rice germplasm, wide-compatibility varieties (WCVs) guarantee the fertility of hybrids when there is cross-fertilization between two subspecies. In this study, Chenghui9348 was identified as a new member of the WCV family that improves pollen fertility in an inter-subspecific hybrid. Cytological analysis showed that the abnormal mitosis of microspores resulted in the sterility of pollens at the early bicellular stage in the inter-subspecific hybrid. Furthermore, the new F12 locus, corresponding to improvements in fertility of the indica-japonica hybrid, was found to co-segregate with the RM1047 marker and associated with a region of approximately 630 kb flanked by the D1101 and D1164 markers on chromosome 12. In this region, two putative genes were predicted as the candidates for wide-compatibility genes (WCGs). Sequence analysis revealed that, compared with indica/japonica alleles, deletion/insertion occurred within exons of both putative genes. Together, the present study identified another new WC locus, F12, and offers more opportunities for further exploitation of inter-subspecific hybrids in rice.

Population Dynamics of Wide Compatibility System and Evaluation of Intersubspecific Hybrids by indica-japonica Hybridization in Rice.

The exploitation of heterosis through intersubspecific hybridisation between indica and japonica has been a major breeding target in rice, but is marred by the cross incompatibility between the genomes. Wide compatibility (WC) is a triallelic system at the S5 locus on chromosome 6 that ensures the specificity of hybridisation within and between indica and japonica. The S5n allele that favours intercrossing is sparsely distributed in the rice gene pool and therefore warrants identification of diverse WC sources to develop superior intersubspecific hybrids. In this study, we have identified several novel WC sources through the marker-assisted screening of a large set of 950 rice genotypes. Seventeen percent of the genotypes carried S5n, which fell into two subpopulations. The WC genotypes showed wide phenotypic and genotypic variability, including both indica and japonica lines. Based on phenotypic performance, the WC varieties were grouped into three clusters. A subset of 41 WC varieties was used to develop 164 hybrids, of which WC/japonica hybrids showed relative superiority over WC/indica hybrids. The multilocation evaluation of hybrids indicated that hybrids derived from WC varieties, such as IRG137, IRG143, OYR128, and IRGC10658, were higher yielding across all the three different locations. Most of the hybrids showed the stability of performance across locations. The identified diverse set of wide compatible varieties (WCVs) can be used in the development of intersubspecific hybrids and also for parental line development in hybrid rice breeding.

Genetics of anther indehiscence with exerted stigmas and its application in hybrid rice breeding

ABSTRACTComplete indehiscence of anthers accompanied with exerted stigmas outside the hull could be useful in hybrid rice breeding if the genetics and possible environmental effects are well understood. Studies were undertaken to identify the sources, confirm the mode of inheritance, and examine the possible environmental effects and potential application of anther indehiscence with exerted stigmas in hybrid rice breeding. Direct and reciprocal crosses were made between Jasmine 85, selected interspecific and intersubspecific lines and two wide compatibility varieties. The resulting F1 progenies were evaluated for their expression of anther indehiscence and exerted stigmas. Expression of anther indehiscence with exerted stigmas by both direct and reciprocal crosses suggested no effect of cytoplasmic factors on the expression of the trait. Segregation analysis supported the 7:1 (indehiscence:dehiscence) mode of inheritance. Temperature and relative humidity could not modulate the fertility/sterility status of anther indehiscent plants. Utilization of anther indehiscence with exerted stigmas in hybrid rice breeding was hindered by trait fixation and propagation.

Progress of <italic>indica-japonica</italic> hybrid sterility and wide-compatibility in rice

As a main staple food for world population, utilization of heterosis in rice has increased crop productivity tremendously. The Asian cultivated rice ( Oryza sativa L.) consists of two subspecies, indica and japonica . Genetic differentiation between indica and japonica subspecies appears to be a major source of genetic diversity in the cultivated rice gene pool. A strategic plan for utilization of inter-subspecific heterosis was proposed half a century ago, indicating that strong heterosis in indica-japonica hybrids provides an effective approach to increase the yield potential for rice breeding. Hybrid sterility that caused by postzygotic reproductive isolation has been a major barrier for utilization of such heterosis between indica and japonica . Genetic analyses have identified a large number of loci conditioning hybrid sterility in rice, which regulate fertility in heterozygous genetic background by complex allelic and epistasis interactions. Six locus-pairs regulating hybrid embryo-sac or pollen sterility have been cloned recently in rice, providing fresh understandings in mechanism of hybrid sterility. The molecular and evolutionary mechanisms underlying reproductive isolation are summarized with respect to the rice hybrid sterility genes, which reveal three evolutionary genetic models including the parallel divergence model, the sequential divergence model, and the parallel-sequential divergence model. There also exists a special group of rice germplasm, referred to as wide-compatibility varieties that produce fertile hybrids when crossed to both indica and japonica varieties. The wide-compatibility genes in these varieties enable hybridization and provide valuable genetic resources for utilization of inter-subspecific heterosis, which take diverse forms owing to the divergent mechanisms in reproductive barriers. Therefore, understanding in hybrid sterility and reproductive isolation in rice would greatly facilitate the utilization of heterosis in breeding programs. This paper summarized the progress in genetic and molecular mechanisms regulating rice hybrid sterility, and discussed the recent understanding in wide-compatibility genes and their utilizations in breeding of wide-compatibility varieties. These advances may greatly facilitate the exploration of inter-subspecific heterosis in breeding programs.

Prezygotic Reproductive Isolation and Fertility in Crosses between <italic>Indica</italic> and <italic>Japonica</italic> Subspecies

Reproductive isolation is essential to the formation and maintenance of new species. Understanding the mechanism of reproductive isolation is fundamental in biological science. Reproductive isolation occurred between the indica and japonica subspecies in rice. However, mechanisms of prezygotic reproductive isolation in rice are poorly understood, which require further investigation. In this study, we have investigated the adherence and germination of the pollens using 17 crosses from a total of 12 varieties representative of indica , japonica , and wide-compatibility varieties. Prezygotic reproductive isolation was observed between the indica and japonica subspecies, which showed that the average pollen adhered was less than 5 per stigma in 2/3 inter-subspecific crosses. However, the average number of adherent pollen was more than 5 per stigma in 3/4 intra-subspecific crosses and all the crosses with the wide-compatibility varieties. Various abnormalities of pollen tube growth were observed in inter-subspecific crosses, but not in intra-subspecific crosses and crosses with the wide-compatibility varieties. The amount of the pollen adhered was irrelevant to the fertility in F1 hybrids, indicating that prezygotic and postzygotic reproductive isolation occurred independently. The reduced affinity between the pollens and stigma in indica and japonica subspecies prevents the formation of hybrid zygotes, which contributes to prezygotic reproductive isolation in rice. Our findings provide valuable insight in understanding the mechanism of reproductive isolation and have implications for utilization of heterosis.

IDENTIFICATION OF WIDE COMPATIBILITY VARIETIES IN SOME TROPICAL JAPONICA RICE

The F1 hybrid sterility in indica/japonica crosses is the major barrier in developing hybrid rice varieties between these two diverse germplasm. The sterility problem in japonica/indica hybrids can be overcome by using wide compatibility genes. The objective of this study was to identify wide compatibility varieties (WCVs) in some tropical japonica rice. Twenty five tropical japonica varieties as male parents were crossed with indica (IR64) and japonica (Akitakomachi) testers as female parents. The crosses were planted following a randomized complete block design with three replications. Varieties having average spikelet fertility of more than 70% with both the indica and japonica testers were rated as WCVs. Result from this study showed that six tropical japonica varieties were classified as WCVs, i.e., Cabacu, Grogol, Kencana Bali, Klemas, Lampung Lawer, and Napa. Hybrid sterility is caused by partial sterility of male and female gametes. The WCVs from the present study can be used in hybrid rice breeding program to solve hybrid sterility in indica/japonica hybrids.

IDENTIFICATION OF WIDE COMPATIBILITY VARIETIES IN SOME TROPICAL JAPONICA RICE

The F1 hybrid sterility in indica/japonica crosses is the major barrier in developing hybrid rice varieties between these two diverse germplasm. The sterility problem in japonica/indica hybrids can be overcome by using wide compatibility genes. The objective of this study was to identify wide compatibility varieties (WCVs) in some tropical japonica rice. Twenty five tropical japonica varieties as male parents were crossed with indica (IR64) and japonica (Akitakomachi) testers as female parents. The crosses were planted following a randomized complete block design with three replications. Varieties having average spikelet fertility of more than 70% with both the indica and japonica testers were rated as WCVs. Result from this study showed that six tropical japonica varieties were classified as WCVs, i.e., Cabacu, Grogol, Kencana Bali, Klemas, Lampung Lawer, and Napa. Hybrid sterility is caused by partial sterility of male and female gametes. The WCVs from the present study can be used in hybrid rice breeding program to solve hybrid sterility in indica/japonica hybrids.

A new CMS line with a wide compatibility gene

In the three-line system (A/B/R) of heterosis breeding in rice, genetic diversity of the A line is very important. The F1 hybrids of indica/japonica crosses show semisterility. Some wide compatibility varieties do produce fertile F1 hybrids when crossed with indica as well as with japonica varieties. A cytoplasmic male sterile (CMS) line with a wide compatibility gene will be useful in developing indica/japonica hybrids that can overcome sterility problems.

A new gene controlling hybrid sterility in rice (Oryza sativa L.)

Hybrids between rice subspecies indica and japonica display strong heterosis. However, semi-sterility of inter-subspecific hybrids between indica and japonica varieties is a major obstacle for application of hybrid vigor in rice production. Semi-sterility was previously ascribed to allelic interaction at a number of different loci, whereas, wide-compatibility varieties can overcome hybrid sterility. Variety Nekken 2, which is a source of wide compatibility genes, showed sterility when crossed to the Korean variety Yeong Pung. Genetic and cytological analyses revealed that the semi-sterility was caused by partial abortion of the embryo sac. Genome-wide analysis of the backcross population, Nekken 2/Yeong Pung//Nekken 2 identified two independent loci for hybrid sterility on chromosomes 1 and 12, explaining 18.99 and 18.03% of the phenotypic variance, respectively. To confirm this result, another population of the same backcross containing 216 individuals was tested at a different site in a different year. The locus on chromosome 12 was detected again. Based on the study, the stable QTL on chromosome 12 appeared to be different from previously reported genes for this trait, and was designated as S35(t).

Identification of an S 5 n allele in Oryza rufipogon Griff. and its effect on embryo sac fertility

Oryza rufipogon exhibits abundant genetic diversity and contains many elite genes. In this work, a total of 441 accessions of O. rufipogon were genetically examined using the functional marker S5n, revealing that 18 accessions carried the S5n allele with a heterozygous genotype (S5nS5i/j). Of these accessions, 14 were from Guangdong Province, 2 from Guangxi Zhuang Autonomous Region, 1 from Hainan Province and 1 from Jiangxi Province. Further sequence analysis spanning both sides of the deletion in S5n allele of O. rufipogon confirmed that this allele is identical with that of 02428, a wide-compatibility variety of domesticated rice. The segregation ratio of the 3 genotypes (S5iS5i/S5jS5j:S5nS5i/j:S5nS5n) among 15 self-bred progenies of O. rufipogon significantly deviated from the Mendelian expectation (1:2:1) in that the proportion of plants exhibiting S5n heterozygous and homozygous genotypes was much lower in the progeny. Sequence analysis of full-length S5n alleles indicated that a few base pairs were different between O. rufipogon and O. sativa, and it was presumed that S5n may be an ancient allele. The fertility of the embryo sac in plants with an S5n heterozygous genotype was low, suggesting the existence of other genes controlling embryo sac fertility in the wild rice.

Two sequence alterations, a 136 bp InDel and an A/C polymorphic site, in the S5 locus are associated with spikelet fertility of indica-japonica hybrid in rice

The rice indica/japonica hybrid shows strong heterosis. However, such inter-subspecific hybrid can't be directly used in rice production due to its low spikelet fertility. The S5 locus was proved to be associated with fertility of indica/japonica hybrid and its S5n allele from wide-compatibility variety (WCV) is capable to overcome fertility barrier. In the present study, we reported the causal sites in the S5 locus responsible for compatibility of indica/japonica hybrid. Fine-mapping of the S5 locus using the 11 test-cross families pinpoints a candidate S5 locus encoding aspartic protease (Asp). Intragenic recombination within the Asp gene happened in a number of recombinants, resulting in chimeric S5j-S5n alleles. Just like S5n, the chimeric S5j-S5n allele displayed higher spikelet fertility when combined with the S5i allele. In the complementary test, however, the S5n allele from WCVs failed to enhance fertilities of the indica/japonica hybrids. Compared to both indica and japonica varieties, all nine WCVs from different resources are characterized with a 136 bp deletion in the Asp N-terminus, which probably renders the S5n allele non-functional. Furthermore, an A/C polymorphic site is detected 1,233 bp downstream of the Asp start codon. The heterozygous A/C site of the Asp gene in indica/japonica hybrid is believed to be the casual factor to cause partial sterility. The functional makers based on the two polymorphic sites will be broadly used in developing wide-compatibility rice varieties.

Mapping of a wide compatibility locus in indica rice using SSR markers

Hybrid sterility between indica and japonica subspecies in rice is basically caused by partial abortion of gametes and hybrid fertility could be recovered by a single wide compatibility (WC) allele. In this study, a typical indica germplasm source of rice, UPRI 95-162, with strong wide compatibility in cross with japonica rice was studied for location of its WC locus. Bulked segregant analysis was performed and SSRs (simple sequence repeats) were conducted on a F1 population derived from a three-way cross (UPRI 95-162/T8//Akihikari). The locus was located on chromosome 1 approximately 0.2 cM to SSR markers RM581 on one side and 1.5 cM to RM292 on another. This WC locus, tentatively designated as S-20 n (t), and its tight linkage markers, RM581 and RM292, would be very useful for efficient marker-assisted selection for breeding new WC varieties and for map-based cloning of the gene.

A triallelic system of S5 is a major regulator of the reproductive barrier and compatibility of indica–japonica hybrids in rice

Hybrid sterility is a major form of postzygotic reproductive isolation. Although reproductive isolation has been a key issue in evolutionary biology for many decades in a wide range of organisms, only very recently a few genes for reproductive isolation were identified. The Asian cultivated rice (Oryza sativa L.) is divided into two subspecies, indica and japonica. Hybrids between indica and japonica varieties are usually highly sterile. A special group of rice germplasm, referred to as wide-compatibility varieties, is able to produce highly fertile hybrids when crossed to both indica and japonica. In this study, we cloned S5, a major locus for indica-japonica hybrid sterility and wide compatibility, using a map-based cloning approach. We show that S5 encodes an aspartic protease conditioning embryo-sac fertility. The indica (S5-i) and japonica (S5-j) alleles differ by two nucleotides. The wide compatibility gene (S5-n) has a large deletion in the N terminus of the predicted S5 protein, causing subcellular mislocalization of the protein, and thus is presumably nonfunctional. This triallelic system has a profound implication in the evolution and artificial breeding of cultivated rice. Genetic differentiation between indica and japonica would have been enforced because of the reproductive barrier caused by S5-i and S5-j, and species coherence would have been maintained by gene flow enabled by the wide compatibility gene.

Fine mapping of S31, a gene responsible for hybrid embryo-sac abortion in rice (Oryza sativa L.)

Partial abortion of female gametes and the resulting semi-sterility of indica x japonica inter-subspecific rice hybrids have been ascribed to an allelic interaction, which can be avoided by the use of wide compatibility varieties. To further understand the genetic mechanism of hybrid sterility, we have constructed two sets of hybrids, using as male parent either the typical japonica variety Asominori, or the wide compatibility variety 02428; and as female, a set of 66 chromosome segment substitution lines in which various chromosomal segments from the indica variety IR24 have been introduced into a common genetic background of Asominori. Spikelet semi-sterility was observed in hybrid between CSSL34 and Asominori, which is known to carry the sterility gene S31 (Zhao et al. in Euphytica 151:331-337, 2006). Cytological analysis revealed that the semi-sterility of the CSSL34 x Asominori hybrid was caused primarily by partial abortion of the embryo sac at the stage of the mitosis of the functional megaspore. A population of 1,630 progeny of the three-way cross (CSSL34 x 02428) x Asominori was developed to map S31. Based on the physical location of linked molecular markers, S31 was thereby delimited to a 54-kb region on rice chromsome 5. This fragment contains eight predicted open reading frames, four of which encode known proteins and four putative proteins. These results are relevant to the map-based cloning of S31, and the development of marker-assisted transfer of non-sterility allele inducing alleles to breeding germplasm, to allow for a more efficient exploitation of heterosis in hybrid rice.

Fine location of theS-5locus responsible for wide compatibility in rice using SSR markers

Partial abortion of gametes possessing S-5j in S-5i/S-5j genotype at locus S-5 is responsible for hybrid sterility between indica and japonica subspecies in rice (Oryza sativa L.), while a single wide compatibility (WC) allele S-5n can restore normal hybrid fertility between the two groups. In this study, Pei’ai 64S, one of the most popular WC line widely used for subspecific hybrid rice breeding program in South China was studied for location of its S-5 locus. Twenty SSR (Simple Sequence Repeat) markers derived from Cornell SSR linkage map and 9 developed using sequences from GenBank database were employed to perform bulked segregant analysis of the mapping population derived from a three-way cross (Pei’ai 64S/T8//Akihikari) to tag fine location of the hybrid sterility locus, S-5. This S-5 locus was mapped on chromosome 6 approximately 0.2 cM from GXR6 and RM276 SSR markers. This tight linkage of the markers and the S-5 locus would be very useful for efficient marker-assisted selection for WC varieties and for map-based cloning of the gene.

Fine mapping of S32(t), a new gene causing hybrid embryo sac sterility in a Chinese landrace rice (Oryza sativa L.)

Ketan Nangka, the donor of wide compatibility genes, showed sterility when crossed to Tuanguzao, a landrace rice from Yunnan province, China. Genetic and cytological analyses revealed that the semi-sterility was primarily caused by partial abortion of the embryo sac. Genome-wide analysis of the linkage map constructed from the backcross population of Tuanguzao/Ketan Nangka//Ketan Nangka identified two independent loci responsible for the hybrid sterility located on chromosomes 2 and 5, which explained 18.6 and 20.1% of phenotypic variance, respectively. The gene on chromosome 5 mapped to the previously reported sterility gene S31(t), while the gene on chromosome 2, a new hybrid sterility gene, was tentatively designated as S32(t). The BC1F2 was developed for further confirmation and fine mapping of S32(t). The gene S32(t) was precisely mapped to the same region as that detected in the BC1F1 but its position was narrowed down to an interval of about 1.9 cM between markers RM236 and RM12475. By assaying the recombinant events in the BC1F2, S32(t) was further narrowed down to a 64 kb region on the same PAC clone. Sequence analysis of this fragment revealed seven predicted open reading frames, four of which encoded known proteins and three encoded putative proteins. Further analyses showed that wide-compatibility variety Dular had neutral alleles at loci S31(t) and S32(t) that can overcome the sterilities caused by these two genes. These results are useful for map-based cloning of S32(t) and for marker-assisted transferring of the neutral allele in hybrid rice breeding.

Fine mapping of f5-Du, a gene conferring wide-compatibility for pollen fertility in inter-subspecific hybrids of rice (Oryza sativa L.)

Wide-compatibility varieties (WCVs), comprising a special class of rice germplasm, are able to produce fertile hybrids when crossed with both indica and japonica varieties. Dular, a landrace variety from India, has both a wide spectrum and high level of wide-compatibility when crossed with a range of indica and japonica varieties. In previous studies, an allele at the f5 locus from Dular (f5-Du) was identified as a neutral allele conferring wide-compatibility with a large effect on both pollen and spikelet fertility. Using a population of 1993 hybrid plants derived from a cross between ZS(f5-Du/f5-ZS) (F1 of near isogenic line of Zhenshan 97 containing f5-Du) and Balilla (a japonica tester), f5-Du was genetically mapped to an interval of about 1.6 cM, with 0.8 cM from both SSR markers WFPM3 and WFPR1. Combined with bioinformatic analysis, a contig map was constructed for the f5 region, consisting of five bacterial artificial chromosome (BAC) or P1 artificial chromosome (PAC) clones and spanning approximately 437 kb in length. By assaying the recombinant events in the region with markers developed using the sequence information, the f5 locus was further narrowed down to a 70 kb fragment containing nine predicted genes. The result will be very useful for cloning this gene and marker-assisted transferring of the neutral allele in rice breeding programs.

Delimitation of the rice wide compatibility gene S5 n to a 40-kb DNA fragment

Wide compatibility varieties (WCVs) are a special class of rice (Oryza sativa L.) germplasm that produces hybrids with normal pollen and spikelet fertility when crossed with both indica and japonica subspecies. The wide compatibility gene S5 ( n ) has been used extensively in inter-subspecific hybrid breeding programs. We previously mapped the S5 locus to a 2.2-cM genomic region between RM253 and R2349 on chromosome 6, using a population of 356 F(1) plants derived from the three-way cross 02428/Nanjing11//Balilla. In this study, a chromosome walking strategy was employed to construct a physical map covering this genomic region using these two closest markers as the starting points. A physical map consisting of six overlapping BAC clones was formed, spanning a genomic region of 540-kb in length. By analyzing recombination events from a population of 8,000 F(1) plants derived from a three-way cross based on near isogenic lines of the S5 locus, the S5 locus was localized to a DNA fragment of 40-kb in length, flanked by two shotgun subclones, 7B1 and 15D2. Sequence analysis of this fragment predicted five open reading frames, encoding xyloglucan fucosyltransferases, dnak-type molecular chaperone BiP, a putative eukaryotic aspartyl protease, and a hypothetical protein. This result will be very useful in molecular cloning of the S5 ( n ) allele and marker-assisted transferring of the wide compatibility gene in rice breeding programs.

Identification and confirmation of three neutral alleles conferring wide compatibility in inter-subspecific hybrids of rice (Oryza sativa L.) using near-isogenic lines

Wide-compatibility varieties (WCVs) are a special class of rice germplasm that is able to produce fertile hybrids when crossed to both indica and japonica subspecies. Previous studies determined 'Dular' and 02428 as two WCVs and identified a number of QTLs as having large effects on fertility of inter-subspecific hybrids. In this study, we developed five near-isogenic lines (NILs) for three of the QTLs, f5, f6 and S5, by backcrossing and marker-assisted selection, using "Dular" and 02428 as the donors and "Zhenshan 97" as the recipient. Three of the NILs each carried one introgressed allele, and two NILs each carried two introgressed alleles in combinations. The NILs were testcrossed to an indica tester "Nanjing 11" and a japonica tester "Balilla". The results showed that the f5 allele from "Dular"(f5-Du) is a neutral allele conferring wide compatibility, with a large effect on both pollen and spikelet fertility, and the f6 allele from "Dular" (f6-Du) is a neutral allele for spikelet fertility with smaller effect. The S5 allele from 02428 (S5-08) was confirmed to be a neutral allele for spikelet fertility. It is likely that f6 and S5 are the same locus as deduced by their genomic locations and effects. The results also showed that even in combination, two neutral alleles of different loci were not able to produce normal fertility hybrids in typical indica-japonica crosses. The implications of the findings in rice breeding programs are discussed.