[The occurrence of actual F factor transfer during the stationary-phase mutation in Escherichia coli FC40].

The experiment of adaptive mutation was performed by using Escherichia coli GM133 rif(r) as test cells and HB214 str(r) as scavenger cells. Transfer frequency between GM133 and HB214 was estimated,based on the number of revertants appeared on the selective plates when GM133 were killed by addition of M9 selective medium containing 100 microg/mL of streptomycin at different time. After 30 minutes the cells of GM133 and HB214 were mixed,the estimated transfer frequency was about 0.07%, and two days, 7.47%. After selection of 7 days,some HB214 cells with F' factor from GM133 cells and lac+ mutation were observed, but these cells failed to form the colonies which can be seen by the naked-eye. It was demonstrated that actual F' factor transfer events from test cells GM133 to scavenger cells HB214 occurred during the selection.

[Some genetic features of adaptive mutation].

The research based on the Escherichia coli FC40 showed that adaptive mutations required the enzymes of RecBCD recombination pathway and some unknown proteins of SOS response, and the mutation spectrum of lac+ revertants is single-base deletions in the small mononucleotide repeats. Some evidence showed that the revertants with adaptive mutations partly come from one (or some) subset of transient hypermutable subpopulation of cells, in which high frequently losing of transposons and genome-wide mutations were observed. It was suggested that this kind of transient hypermutability may be due to the transient deficient activity of mismatch repair (MMR) system, or a defective epsilon unit of DNA polymerase III generated by mistranslation. Although other systems demonstrated some different mechanisms from FC40, all research works suggested that, adaptive mutations occurred in nondividing or nongrowing cells under environmental stresses, for example, starvation, displayed different genetic features from growth-dependent spontaneous mutation.

Arbuscular mycorrhizal status of plants and the spore density of arbuscular mycorrhizal fungi in the tropical rain forest of Xishuangbanna, southwest China.

The arbuscular mycorrhizal status of 112 plant species and the spore density of arbuscular mycorrhizal fungi (AMF) in the rhizosphere soil of these plants in the tropical rain forest of Xishuangbanna, southwest China, were surveyed. It was found that 56% of the surveyed species were arbuscular mycorrhizal, 31% were possibly arbuscular mycorrhizal and 13% were non-mycorrhizal. The spore density of AMF ranged from 55 to 1,908 per 100 g soil, with an average of 476. The rhizosphere soil from the arbuscular mycorrhizal plants did not always have a higher AMF spore density than that from the possibly mycorrhizal and non-mycorrhizal plants. The clumped distribution of AMF spores and the complex structure of the underground component of the tropical rain forest may be two important factors that affected the spore density of AMF. Fungi belonging to the genera Acaulospora and Glomus are the dominant AMF in the soil of the tropical rain forest of Xishuangbanna.