Common oat (Avena sativa) is an important cereal crop serving as a valuable source of forage and human food. Although reference genomes of many important crops have been generated, such work in oat has lagged behind, primarily owing to its large, repeat-rich polyploid genome. Here, using Oxford Nanopore ultralong sequencing and Hi-C technologies, we have generated a reference-quality genome assembly of hulless common oat, comprising 21 pseudomolecules with a total length of 10.76 Gb and contig N50 of 75.27 Mb. We also produced genome assemblies for diploid and tetraploid Avena ancestors, which enabled the identification of oat subgenomes and provided insights into oat chromosomal evolution. The origin of hexaploid oat is inferred from whole-genome sequencing, chloroplast genomes and transcriptome assemblies of different Avena species. These findings and the high-quality reference genomes presented here will facilitate the full use of crop genetic resources to accelerate oat improvement.

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The nucleotide sequence of the class G tetracycline resistance determinant previously isolated from Vibrio anguillarum has been determined. Two open reading frames of divergent polarity were identified. A resistance gene (tet A) encodes a protein of 393 amino acid residues (deduced molecular mass of 40.9 kDa), and a repressor gene (tet R) encodes a protein consisting of 210 amino acids with a calculated molecular mass of 23.6 kDa. Based on the deduced amino acid sequences, the proteins of tet A(G) and tet R(G) are about 60% homologous with those of RP1/ Tn1721 (class A) and pSC101/pBR322 (class C), and about 50% homologous with Tn10 (class B). The relationship of the tet (G) sequence to five known tetracycline resistance determinants (class A to E) is discussed.Tetracycline resistance (TCr) determinants are widespread among bacterial species. Five classes of TCr determinants (A through E) defined by DNA-DNA hybridization have been described among Enterobacteriaceae (16, 18). They are encoded by either plasmids or transposons and share the resistance mechanism involving active efflux of the drug from resistance cells (17,18). Each of them consists of the resistance gene tet A and the repressor gene tet R, which have divergent polarities and overlapping promoters (12, 23). The resistance gene encodes a membrane-associated protein (Tet A) which is responsible for resistance (7, 13). The repressor gene encodes a regulatory protein (Tet R) that has been characterized as interacting with tetracycline and the tet operator (9, 11). The nucleotide sequences of three resistance genes (27) and five repressor genes (23) have been determined. About 40-60% homology was revealed among encoded proteins of the three resistance genes as well as the five repressor genes (23).Vibrio anguillarum is a gram-negative fish pathogenic bacterium. It is a causative agent of vibriosis in marine and freshwater fish throughout the world. Infections with multiple drug-resistant strains of V. anguillarum have been found in cultured ayu (Plecoglossus altivelis) in various areas of Japan since 1973 (1). These strains were usually resistant to ampicillin, chloramphenicol, colistin, nalidixic acid, nitrofurazone, sulfonamides, streptomycin, tetracycline (TC) and/or trimethoprim. In most cases, these resistant strains carried a transferable R plasmid. In a previous study, a tetracycline determinant (designated class G) was cloned from a V. anguillarum strain MZ8122 carrying the R plasmid pJA8122 (2). This determinant does 105 1