Intersubspecific Heterosis Research Articles

Identification of neutral genes at pollen sterility loci Sd and Se of cultivated rice (Oryza sativa) with wild rice (O. rufipogon) origin

Pollen sterility is one of the main hindrances against the utilization of strong intersubspecific (indica-japonica) heterosis in rice. We looked for neutral alleles at known pollen sterility loci Sd and Se that could overcome this pollen sterility characteristic. Taichung 65, a typical japonica cultivar, and its near isogenic lines E7 and E8 for pollen sterility loci Sd and Se were employed as tester lines for crossing with 13 accessions of wild rice (O. rufipogon). Pollen fertility and genotypic segregations of the molecular markers tightly linked with Sd and Se loci were analyzed in the paired F(1)s and F(2) populations. One accession of wild rice (GZW054) had high pollen fertility in the paired F(1)s between Taichung 65 and E7 or E8. Genotypic segregations of the molecular markers tightly linked with Sd and Se loci fit the expected Mendelian ratio (1:2:1), and non-significances were shown among the mean pollen fertilities with the maternal, parental, and heterozygous genotypes of each molecular markers tightly linked with Sd and Se loci. Evidentially, it indicated that the alleles of Sd and Se loci for GZW054 did not interact with those of Taichung 65 and its near isogenic lines, and, thus were identified as neutral alleles Sd(n) and Se(n). These neutral genes could become important germplasm resources for overcoming pollen sterility in indica-japonica hybrids, making utilization of strong heterosis in such hybrids viable.

Yield potential and radiation use efficiency of “super” hybrid rice grown under subtropical conditions

China's “super” hybrid rice breeding project has developed many new varieties using a combination of the ideotype approach and intersubspecific heterosis. It is controversial whether these “super” hybrid varieties have increased the yield potential of irrigated rice. This study was conducted to compare grain yield and yield attributes among “super” hybrid, ordinary hybrid, and inbred varieties. Field experiments were done in Liuyang (moderate-yielding site) and Guidong (high-yielding site) counties, Hunan Province, China, in 2007 and 2008. Two varieties from each varietal group were grown in each field experiment under moderate and high N rates. Grain yield, yield components, aboveground total dry weight, harvest index, total N uptake, and crop radiation use efficiency (RUE) were measured for each variety. A significant difference in grain yield was observed among the varieties and varietal groups but not between the two N rates. “Super” hybrid varieties have increased rice yield potential by 12% compared with ordinary hybrid and inbred varieties. The higher grain yield of “super” hybrid varieties was attributed to improvement in both source and sink. “Super” hybrid varieties produced more biomass than ordinary hybrid and inbred varieties. Long growth duration and high accumulated incident radiation were partially responsible for high biomass production for the “super” hybrid varieties. “Super” hybrid varieties had significantly larger panicle size (spikelets per panicle) than ordinary hybrid and inbred varieties, which resulted in larger sink size (spikelets per m 2). Crop RUE did not explain the yield superiority of “super” hybrid rice. Our study suggests that “super” hybrid rice varieties do not necessarily require more N fertilizer to produce high grain yield.

Prospects of Developing Hybrid Rice with Super High Yield

Approaches to develop hybrid rice (Oryza sativa L.) with super high yield involve the improvement of plant type, the exploitation of intersubspecific heterosis, the pyramiding of heterosis genes in different rice ecotypes, and the utilization of favorable genes from distant relatives. Three main plant types have been suggested in the literature for improving hybrid rice yield: (i) heavy panicle type, (ii) super high yielding plant type, and (iii) super high yielding ideotype. Recently, intersubspecific heterosis has been partially exploited with three strategies: (i) developing parental lines of indica‐japonica intermediate type with favorable genes from both parents, (ii) introgressing intersubspecific gene with the help of wide compatibility and thermosensitive genic male sterility (TGMS) genes, and (iii) breeding pro‐indica japonica lines. It is also possible to pyramid different heterosis genes from various rice ecotypes through molecular marker aided reciprocal recurrent selection. The identification of yield enhancing genes from wild rice, the construction of autoregulated senescence delaying gene, and the cloning of key enzymes related to C4 pathway have been accomplished using molecular biotechnology. By introducing these genes into the parents of the hybrid rice, the heterosis of the hybrid rice can be dramatically improved.

Identification and fine mapping of S-d, a new locus conferring the partial pollen sterility of intersubspecific F1 hybrids in rice (Oryza sativa L.)

The partial pollen abortion of hybrids between the indica and japonica subspecies of Asian cultivated rice is one of the major barriers in utilizing intersubspecific heterosis in hybrid rice breeding. Although a single hybrid pollen sterility locus may have little impact on spikelet fertility, the cumulative effect of several loci usually leads to a serious decrease in spikelet fertility. Isolating of the genes conferring hybrid pollen sterility is necessary to understand this phenomenon and to overcome the resulting genetic barrier. In this study, a new locus for F1 pollen sterility, S-d, was identified on the short arm of chromosome 1 by analyzing the genetic effect of substituted segments of the near-isogenic line E11-5 derived from the japonica variety Taichung 65 (recurrent parent) and the indica variety Dee-geo-woo-gen (donor parent). The S-d locus was first mapped to a 0.8 cM interval between SSR markers PSM46 and PSM80 using a F2 population of 125 individuals. The flanking markers were then used to identify recombinants from a population of 2,160 plants derived from heterozygotes of the primary F2 population. Simultaneously, additional markers were developed from genomic sequence divergence in this region. Analysis of the recombinants in the region resulted in the successful mapping of the S-d locus to a 67-kb fragment, containing 17 predicted genes. Positional cloning of this gene will contribute to our understanding of the molecular basis for partial pollen sterility of intersubspecific F1 hybrids in rice.

Fixation of hybrid vigor in rice: opportunities and challenges

Rice, one of most important crops, is emerging not only as a model plant for molecular biological studies in cereal crops, but also as a model system for hybrid breeding methodologies, which include three-line system, two-line system and one-line system, corresponding to inter-varietal heterosis, inter-subspecific heterosis and heterosis of distant hybridization based on enhancing heterosis level, respectively. The developmental goal and direction of rice breeding are to simplify procedure of hybrid breeding and to enhance heterosis level. One-line breeding is not only to use distance cross for enhancing heterosis but also to lock hybrid vigor and to produce permanent hybrid. Rice of early generation stability, sporophytic apomictic rice or gametophytic apomictic rice might be used as a genetic tool for one-line breeding, and also represent three main methods and three development stages in one-line breeding.

Combining ability and heterosis over environments for yield and yield components in two‐line hybrids involving thermosensitive genic male sterile lines in rice (Oryza sativa L.)

AbstractPresence of substantial heterosis and economic hybrid seed production are two most desirable components for success of any commercial hybrid breeding programme. Thermosensitive genic male sterile (TGMS) lines of rice, in this regard, have tremendous potential in realizing further quantum jump in yield and economical hybrid seed cost. Analyses for combining ability and heterosis over optimum (120N : 60P2O5 : 40K2O kg/ha) and high (200N : 90P2O5 : 60K2O kg/ha) fertility environments for six traits were made in 2 years (2001 and 2002) using 120 hybrids of inter‐ and intra‐subspecific nature derived from hybridization of 30 elite indica TGMS lines and four cultivars, viz., ‘Pant Dhan 4’ and ‘Ajaya’ (I = indica), ‘Taichung 65’ (J = japonica) and ‘IR 65598‐112‐2’ (TJ = tropical japonica) in line × tester mating design. Predominance of non‐additive genetic variance suggested good prospects of hybrid breeding. Pooled analysis revealed highly significant variances for lines, general combining ability (GCA), specific combining ability (SCA) and line x tester. TGMS line 365‐8S was the best general combiner for all the six traits including grain yield. Trend of relative mid‐parent heterosis for grain yield, panicle length, grain number per panicle and earliness in flowering was I/TJ > I/J > I/I. For panicle number per plant and 1000‐grain weight, trends were I/TJ > I/I > I/J and I/I > I/TJ > I/J, respectively. Grain yield recorded heterosis of 49.3%, 71.9% and 92.7% for I/I, I/J and I/TJ hybrid groups respectively. Effect of environments on the hybrid performance indicated better response of hybrids at high fertilizer dose. Study suggests greater prospects of combining improved japonica and tropical japonica germplasms having wide compatible gene with indica TGMS lines for exploitation of intersubspecific heterosis.

Yield Potential Trends of Tropical Rice since the Release of IR8 and the Challenge of Increasing Rice Yield Potential

ABSTRACTSince the release of IR8 in 1966, 42 additional indica rice (Oryza sativa L.) cultivars developed by the International Rice Research Institute (IRRI) for the irrigated and favorable rainfed lowlands have been released in the Philippines. The maximum yield of IR8 has been reduced by about 2 Mg ha−1 during the past 30 yr. Empirical breeding for population improvement within the indica germplasm has resulted in the maintenance of rice yield potential in the tropics of about 10 Mg ha−1. To break the yield barrier, several approaches are being explored. These include development of a new plant type (NPT) with low tillering capacity and large panicles from tropical japonica germplasm and exploitation of heterosis through intervarietal and intersubspecific hybrids. Hybrid rice between indicas increased yield potential by about 9% under the tropical conditions. The higher yield potential of indica/indica hybrids compared with indica inbred cultivars was attributed to the greater biomass production rather than harvest index. New plant type breeding has not yet improved yield potential due to poor grain filling and low biomass production. Factors that cause poor grain filling and low biomass production of the NPT lines have been identified. Selecting parents with good grain filling traits, introduction of indica genes into NPT's tropical japonica background, and a refinement of the original NPT design are expected to improve the performance of the NPT lines. Further enhancement in yield potential may be possible from use of intersubspecific heterosis between indica and NPT lines.

The extent of parental genotypic divergence determines maximal heterosis by increasing fertility in inter-subspecific hybrids of rice (Oryza sativa L.)

The magnitude of heterosis in F1 hybrids is related not only to the performance of parents per se but also to the genetic diversity between two parents. The extent of genotypic divergence between hybrid rice parents was investigated at the molecular level, using two subsets of rice materials: a subset of doubled haploid (DH) lines derived from an Indica × Japonica cross (Gui630/02428) and another subset of Indica or Japonica lines representative of a broad spectrum of the Asian cultivated rice gene pool, including landraces, primitive cultivars, historically important cultivars, modern elite cultivars, super rice and parents of superior hybrids. 57 entries deliberately selected from the 81-DH lines (in total) were testcrossed to two widely used rice lines in China, photoperiod-sensitive genic male sterile (PGMS) N422s and thermo-sensitive genic male sterile (TGMS) Peiai64s. Results of the two sets of test-cross F1 populations showed congruently that parental genotypic divergence has a relatively low impact on heterosis for the two yield components, i.e., panicle number and 1000-grain weight, but it has a great bearing on fertility parameters, i.e., filled grains per plant and seedset. Heterosis for grain yield in the two test-cross populations exhibited a sharp maximum when the proportion of Japonica alleles in the male parent was between 50 and 60%, so was the heterosis for fertility parameters correspondingly. Thus fertility parameters were the most sensitive and important factors which were influenced by the extent of parental genotypic divergence. Moreover, our results showed that parents with moderate extent of genotypic divergence played an important role in the use of inter-subspecific rice heterosis.

Molecularmarker diversity and hybrid sterility in indica-japonica rice crosses

The partial sterility of hybrids between the indica and japonica rice subspecies of Asian cultivated rice is a serious constraint for utilizing inter-subspecific heterosis in hybrid rice breeding. In this study, we have investigated the relationship between molecular-marker polymorphism and indica-japonica hybrid fertility using a diallel set involving 20 rice accessions including 9 indica and 11 japonica varieties. Spikelet fertility of the resulting 190 F1s and their parents was examined in a replicated field trial. Intra-subspecific hybrids showed much higher spikelet fertility than inter-subspecific hybrids except in crosses involving wide-compatibility varieties. The parents were surveyed for DNA polymorphism using 96 RFLP and ten SSR markers, which revealed extensive genetic differentiation between indica and japonica varieties. A large number of markers detected highly significant effects on hybrid fertility. The chromosomal locations for many of the positive markers coincided well with previously identified loci for hybrid sterility. The correlation between hybrid fertility and parental distance was low in both intra- and inter-subspecific crosses. The results suggest that the genetic basis of indica-japonica hybrid sterility is complex. It is the qualitative, rather than the quantitative, difference between the parents that determines the fertility of hybrids.

Use of androgenesis in intersubspecific hybrid rice breeding

AbstractSignificant differences of callus induction, green plant regeneration and culture efficiency were observed among restorer lines and their hybrids in rice. Average callus induction percentage of F1, hybrids was about twice that of their parents. Twenty doubled haploid (DH) lines that showed normal fertility with both indica and japonica CMS lines, were selected as widely compatible restorers (WCRs). TG8 derived from CPSLO17/Minghui 63 not only showed normal fertility to both indica and japonica test varieties and CMS lines, but also exhibited superior agronomic traits, in particular short plant, desired plant type, high tillerability, large panicles, good grain quality and resistance to rice blast disease. Strong heterosis for yield was observed in crosses between indica or japonica CMS lines and TG8. TG8 possessed a dominant, widely compatible gene (WCG) that was inherited from the variety CPSLO17. Results confirmed that the anther culture technique is a quick and effective way of developing widely compatible restorers in rice and that it is applicable to the direct use of intersubspecific heterosis with the three‐line method.

RFLP-based phylogenetic analysis of wide compatibility varieties in Oryza sativa L.

Twenty-one wide compatibility varieties (WCVs) of rice together with three indica and three japonica testers were assayed with 160 DNA probes that were selected to represent the entire RFLP map at an average interval of 11 cM. On the basis of four enzyme digestion 125 probes detected polymorphisms among the WCVs and subspecies' testers. Among these polymorphic probes there were 68 that could distinguish the indica from the japonica testers. Two dendrograms were constructed on the basis of 398 polymorphic fragments of 125 probes and 139 polymorphic fragments of 68 subspecies' differentiating probes in combination with single enzymes, respectively. The reliability and representativeness of the testers and the levels of DNA variations among WCVs were estimated. The potential of WCVs in the utilization of intersubspecific heterosis is discussed.