The pathogenicity of avian H5N1 influenza viruses to mammals has been evolving since the mid-1980s. Here, we demonstrate that H5N1 influenza viruses, isolated from apparently healthy domestic ducks in mainland China from 1999 through 2002, were becoming progressively more pathogenic for mammals, and we present a hypothesis explaining the mechanism of this evolutionary direction. Twenty-one viruses isolated from apparently healthy ducks in southern China from 1999 through 2002 were confirmed to be H5N1 subtype influenza A viruses. These isolates are antigenically similar to A/Goose/Guangdong/1/96 (H5N1) virus, which was the source of the 1997 Hong Kong “bird flu” hemagglutinin gene, and all are highly pathogenic in chickens. The viruses form four pathotypes on the basis of their replication and lethality in mice. There is a clear temporal pattern in the progressively increasing pathogenicity of these isolates in the mammalian model. Five of six H5N1 isolates tested replicated in inoculated ducks and were shed from trachea or cloaca, but none caused disease signs or death. Phylogenetic analysis of the full genome indicated that most of the viruses are reassortants containing the A/Goose/Guangdong/1/96-like hemagglutinin gene and the other genes from unknown Eurasian avian influenza viruses. This study is a characterization of the H5N1 avian influenza viruses recently circulating in ducks in mainland China. Our findings suggest that immediate action is needed to prevent the transmission of highly pathogenic avian influenza viruses from the apparently healthy ducks into chickens or mammalian hosts.

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A total of 63 strains of rhizobia isolated from Hainan Province, a tropical region of the People's Republic of China, and 27 representative strains belonging to the genera Rhizobium, Bradyrhizobium, and Agrobacterium were compared by performing numerical taxonomy, DNA hybridization, and DNA base composition analysis to determine the relationships among these rhizobia. The results indicated that the strains isolated from Hainan Province fell into two distinct phena, the slowly growing rhizobia and the fast-growing rhizobia. The slowly growing rhizobia, which formed three subphena that seemed to be three subspecies, are Bradyrhizobium japonicum strains. The fast-growing strains belong to the genus Rhizobium and might be further divided into three specific groups. Sometimes both slowly growing rhizobia and fast-growing rhizobia were isolated from host plants belonging to the same genus or species or even from the same nodule. There was no correlation between hosts and the distribution of rhizobia in the subphena. Isolates obtained from members of the same host genus or species fell into different groups or subgroups.The root nodule bacteria of tropical leguminous plants have been investigated by workers in many laboratories (1,(5)(6)(7)(8)10,23,25), but taxonomic studies of these bacteria have been sparse and have been limited to a few leguminous symbionts (16,19,24,27,28). Dreyfus and Dommergues (6) classified 13 Acacia species into three groups on the basis of effective nodulation with fast-growing and slowly growing tropical strains of rhizobia. The first group formed nodules effectively with slow growers; the second group formed effective nodules with fast growers; and the third group formed effective nodules with both fast and slow growers. Zhang et al. (27) obtained 12 clusters by using numerical taxonomy to study 97 strains isolated from root nodules of Acacia senagal, Prospopis chilensis, and other legumes. Thus, it is interesting to study the taxonomy of rhizobia isolated from a wide range of tropical legumes.In this work 63 rhizobial strains were isolated from various legumes, including trees, herbs, and vines, growing in different geographic regions of Hainan Province of the People's Republic of China, a tropical region south of latitude 20" N. These strains were compared with previously described species of rhizobia by performing numerical taxonomy, DNA composition, and DNA-DNA hybridization analysis. MATERIALS AND METHODSBacterial strains. A total of 90 strains were used in this study (Table 1); 27 of these strains were type and representative strains of Rhizobium, Bradyrhizobium, and Agrobacterium species, and 63 strains were isolated from root nodules collected in different geographic regions of Hainan Province. The purity of the cultures was assured by using single-colony isolates and checking colony morphology, bacterial morphology, and the Gram stain reaction. The identity of each strain was checked by performing a plant infection test with the original host plant.

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The oriental fruit fly, Bactrocera dorsalis, is a serious pest of fruits and vegetables in South‐east Asia, and, because of quarantine restrictions, impedes international trade and economic development in the region. Revealing genetic variation in oriental fruit fly populations will provide a better understanding of the colonization process and facilitate the quarantine and management of this species. The genetic structure in 15 populations of oriental fruit fly from southern China, Laos and Myanmar in South‐east Asia was examined with a 640‐bp sequence of the mitochondrial cytochrome oxidase subunit I (COI) gene. The highest levels of genetic diversity were found in Laos and Myanmar. Low to medium levels of genetic differentiation (FST ≤ 0.134) were observed among populations. Pooled populations from mainland China differed from those in Laos and Myanmar (FST = 0.024). Genetic structure across the region did not follow the isolation‐by‐distance model. The high genetic diversity observed in Laos and Myanmar supports the South‐east Asian origin of B. dorsalis. High genetic diversity and significant differentiation between some populations within mainland China indicate B. dorsalis populations have been established in the region for an extended period of time. High levels of genetic diversity observed among the five populations from Hainan Island and similarity between the Island and Chinese mainland populations indicate that B. dorsalis was introduced to Hainan from the mainland and has been on the island for many years. High genetic diversity in the recently established population in Shanghai (Pudong) suggests multiple introductions or a larger number of founders.