Fine mapping of a gene causing hybrid pollen sterility between Yunnan weedy rice and cultivated rice (Oryza sativa L.) and phylogenetic analysis of Yunnan weedy rice

Construction of three half-sib SSR linkage maps derived from overwintering cultivated rice and segregation distortion loci mapping

Segregation distortion is a common phenomenon that has been observed in genetics and plant breeding; however, the mechanism of segregation distortion is unknown. In the present study, three half-sib F₂ populations derived from three japonica overwinter (perennial) rice varieties (W1, W2, and W3) crossed to the indica rice variety Minghui725 (MH725) were developed to construct three half-sib linkage maps. We established linkage map lengths of 2032.8, 2317.4, and 2108.7 cM with average intervals of 20.1, 20.5, and 19.7 cM using 101, 113, and 107 SSR markers in W1/MH725, W2/MH725, and W3/MH725, respectively. Discrepancies in marker order and genetic linkage distance occurred in the three half-sib linkage maps due to segregation distortion. A total of 88 markers exhibited segregation distortion across the three linkage maps at P < 0.01 level, 42 segregation distortion loci (SDLs) were detected across the three half-sib populations and exhibited variable LOD value that ranged from 3.2 (SDL2f) to 30.1 (SDL5d), and 13 of the 42 SDLs were repeatedly located at the same chromosomal regions of the previously published hybrid sterility quantitative trait loci. Data from this study provide an extensive archive for investigating the genetic characteristic of overwintering cultivated rice and the future exploration and innovation of overwintering rice breeding.

Allele distributions at hybrid incompatibility loci facilitate the potential for gene flow between cultivated and weedy rice in the US

The accumulation of independent mutations over time in two populations often leads to reproductive isolation. Reproductive isolation between diverging populations may be reinforced by barriers that occur either pre- or postzygotically. Hybrid sterility is the most common form of postzygotic isolation in plants. Four postzygotic sterility loci, comprising three hybrid sterility systems (Sa, s5, DPL), have been recently identified in Oryza sativa. These loci explain, in part, the limited hybridization that occurs between the domesticated cultivated rice varieties, O. sativa spp. japonica and O. sativa spp. indica. In the United States, cultivated fields of japonica rice are often invaded by conspecific weeds that have been shown to be of indica origin. Crop-weed hybrids have been identified in crop fields, but at low frequencies. Here we examined the possible role of these hybrid incompatibility loci in the interaction between cultivated and weedy rice. We identified a novel allele at Sa that seemingly prevents loss of fertility in hybrids. Additionally, we found wide-compatibility type alleles at strikingly high frequencies at the Sa and s5 loci in weed groups, and a general lack of incompatible alleles between crops and weeds at the DPL loci. Our results suggest that weedy individuals, particularly those of the SH and BRH groups, should be able to freely hybridize with the local japonica crop, and that prezygotic factors, such as differences in flowering time, have been more important in limiting weed-crop gene flow in the past. As the selective landscape for weedy rice changes due to increased use of herbicide resistant strains of cultivated rice, the genetic barriers that hinder indica-japonica hybridization cannot be counted on to limit the flow of favorable crop genes into weeds.

Responses of Weedy Rice to Drought Stress at Germination and Seedling Stages

Drought is a world-spread problem seriously influencing grain production and quality, the loss of which is the total for other natural disasters, with increasing global climate change making the situation more serious. Rice is the staple food for more than 23% of world population, so rice anti-drought physiology study is of importance to rice production and biological breeding for the sake of coping with abiotic and biotic conditions. Much research is involved in this hot topic, but the pace of progress is not so large because of drought resistance being a multiple-gene-control quantitative character. On the other hand, stress adaptive mechanisms are quite different, with stress degree, time course, materials, soil quality status and experimental plots, thus increasing the complexity of the issue in question. Additionally, a little study is related to weedy rice.In order to investigate the effects of drought stress on germination and early seedling growth of weedy rice (Oryza sativa f. spontanea L.) and cultivated rice (Oryza sativa L.), polyethyleneglycol-6000 (PEG-6000) are used to generate -1.33MPa and 0MPa water stress in a laboratory condition (28±3°C). Complete randomized design with three replications is used in the study. After 10 days of germination, shoot length, the longest root length, root fresh weight, root dry weight, shoot fresh weight, shoot dry weight and root numbers are measured; germination percentage, and root to shoot ratio are calculated. Germination index (GI), shoot length stress index (SLSI), root length stress index (RLSI) and dry matter stress index (DMSI) are used to evaluate the response of different genotypes to PEG-induced water stress. Results of ANOVA analysis show that responses of weedy rice accessions and cultivated rice varieties to water stress are significant different, demonstrating the germplasm of weedy rice and cultivated rice are diverse which enables us to screen the germplasm tolerant to drought stress.

Molecular evolution and genetics of postzygotic reproductive isolation in plants

In just the last few years, plant geneticists have made tremendous progress in identifying the molecular genetic basis of postzygotic reproductive isolation. With more than a dozen genes now cloned, it is clear that plant hybrid incompatibilities usually evolve via two or more mutational steps, as is predicted by the Dobzhansky-Muller model. There is evidence that natural selection or random genetic drift can be responsible for these incompatibilities.

Molecular analysis of an additional case of hybrid sterility in rice (Oryza sativa L.)

Hybrid sterility hinders the exploitation of the heterosis displayed by japonica × indica rice hybrids. The variation in pollen semi-sterility observed among hybrids between the japonica recipient cultivar and each of two sets of chromosome segment substitution lines involving introgression from an indica cultivar was due to a factor on chromosome 5 known to harbor the gene S24. S24 was fine mapped to a 42 kb segment by analyzing a large F(2) population bred from the cross S24-NIL × Asominori, while the semi-sterility shown by the F(1) hybrid was ascribable to mitotic failure at the early bicellular pollen stage. Interestingly, two other pollen sterility genes (f5-Du and Sb) map to the same region (Li et al. in Chin Sci Bull 51:675-680, 2006; Wang et al. in Theor Appl Genet 112:382-387, 2006), allowing a search for candidate genes in the 6.4 kb overlap between the three genes. By sequencing the overlapped fragment in wild rice, indica cultivars and japonica cultivars, a protein ankyrin-3 encoded by the ORF2 was identified as the molecular base for S24. A cultivar Dular was found to have a hybrid-sterility-neutral allele, S24-n, in which an insertion of 30 bp was confirmed. Thus, it was possible to add one more case of molecular bases for the hybrid sterility. No gamete abortion is caused on heterozygous maternal genotype with an impaired sequence from the hybrid-sterility-neutral genotype. This result will be useful in understanding of wide compatibility in rice breeding.