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.

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.

Weedy rice represents an important resource for rice improvement. The F(1) hybrid between the japonica wide compatibility rice cultivar 02428 and a weedy rice accession from Yunnan province (SW China) suffered from pollen sterility. Pollen abortion in the hybrid occurred at the early bicellular pollen stage, as a result of mitotic failure in the microspore, although the tapetum developed normally. Genetic mapping in a BC(1)F(1) population (02428//Yunnan weedy rice (YWR)/02428) showed that a major QTL for hybrid pollen sterility (qPS-1) was present on chromosome 1. qPS-1 was fine-mapped to a 110 kb region known to contain the hybrid pollen sterility gene Sa, making it likely that qPS-1 is either identical to, or allelic with Sa. Interestingly, F(1) hybrid indicated that Dular and IR36 were assumed to carry the sterility-neutral allele, Sa ( n ). Re-sequencing SaM and SaF, the two component genes present at Sa, suggested that variation for IR36 and Dular may be responsible for the loss of male sterility, and the qPS-1 sequence might be derived from wild rice or indica cultivars. A phylogenetic analysis based on microsatellite genotyping suggested that the YWR accession is more closely related to wild rice and indica type cultivars than to japonica types. Thus it is probable that the YWR accession evolved from a spontaneous hybrid between wild rice and an ancient cultivated strain of domesticated rice.

[QTL analysis of low-temperature-sensitive pollen sterility in Indica-japonica hybrid rice (Oryza sativa L.)].

There existed a number of biological constraints in exploiting the heterosis of indica-japonica hybrid rice. The low-temperature-sensitive sterility (LTSS) of indica-japonica hybrid has become one of the major problems in indica-japonica hybrid rice breeding after the solution of poor fertility of the hybrids by the finding of wide-compatibility gene. Previous studies revealed that the LTSS might be caused by low-temperature-sensitive pollen sterility (LTSPS). However, the genetic basis of LTSPS remained unclear. To explore the genetic basis of LTSPS in indica-japonica hybrid rice, an F2 genetic population derived from 3037 (indica) and 02428 (japonica) was developed. At the booting stage, pollen fertility of F2 population together with parents were surveyed after the treatment with low temperature daily average of 21-23 degrees C. The linkage map was constructed containing 108 SSR markers distributed throughout the whole 12 chromosomes with average marker interval of 16.26 cM. Using software MapMaker/QTL, two putative QTLs, namely qLTSPS2 and qLTSPS5 on chromosomes 2 and 5 were detected by interval mapping, which could explain the phenotypic variation 15.6% and 11.9% respectively. The additive effects were 0.021 and 0.045, dominant effects were -0.246 and -0.215, and the degrees of dominance were 11.7 and 4.8, respectively for the two QTLs. Therefore, the mode of gene action in response to low-temperature stress was overdominance and LTSPS was mainly the result of interaction between the indica and japonica alleles within each locus. In addition, two-way ANOVA showed that the two QTLs acted essentially independent of each other in conditioning LTSPS.

Self-incompatibility (S) alleles of the Rosaceae encode members of a distinct class of the T2/S ribonuclease superfamily.

Stylar ribonucleases (RNases) are associated with gametophytic self-incompatibility in two plant families, the Solanaceae and the Rosaceae. The self-incompatibility-associated RNases (S-RNases) of both the Solanaceae and the Rosaceae were recently reported to belong to the T2 RNase gene family, based on the presence of two well-conserved sequence motifs. Here, the cloning and characterization of S-RNase genes from two species of Rosaceae, apple (Malus x domestica) and Japanese pear (Pyrus serotina) is described and these sequences are compared with those of other T2-type RNases. The S-RNases of apple specifically accumulated in styles following maturation of the flower bud. Two cDNA clones for S-RNases from apple, and PCR clones encoding a further two apple S-RNases as well as two Japanese pear S-RNases were isolated and sequenced. The deduced amino acid sequences of the rosaceous S-RNases contained two conserved regions characteristic of the T2/S-type RNases. The sequences showed a high degree of diversity, with similarities ranging from 60.4% to 69.2%. Interestingly, some interspecific sequence similarities were higher than those within a species, possibly indicating that diversification of S-RNase alleles predated speciation in the Rosaceae. A phylogenetic tree of members of the T2/S-RNase superfamily in plants was obtained. The rosaceous S-RNases formed a new lineage in the tree that was distinct from those of the solanaceous S-RNases and the S-like RNases. The findings suggested that self-incompatibility mechanisms in Rosaceae and Solanaceae are similar but arose independently in the course of evolution.