| A selection of 23 transgenic birch plants of bt,gus,and nptⅡgenes have been maintained under natural environmental conditions over a period of 4-5 years.A detailed scanning of the plants allowed us to detect a variation of transgene expression and integration patterns in the transgenic birch population,through the way of Northern blot,RT-PCR,Southern blot,and PCR.Moreover,by the isoschizomer digestion of genomic DNA and Southern blot,the methylated cytosines at recognition sequences and cutting sites were showed.At last,by the method of bisulphate sequencing,the frequencies of methylated cytosines were studied in the promoter and code regions of foreign genes.Transgene expression,integration and DNA methylation analyses revealed that:(1)All nptⅡgenes were stable expression,and the percentage of stable expression of bt and gus genes respectively were 78.3%,during three years.Along with increasing months rang from May to September,the trend of the expression of foreign genes was descending. Additionally,the expression of transgenes in distinct tissues or organs was different,the order of transgenic expression level from high to low was leaves＞flowers＞barks＞roots.(2) Base on the expression of gus gene,the transcriptional silence plants included TP96, TP74,TP73,TP28,and the post-transcriptional silence ones were TP67 and TP46.(3) No significant correlation was found between copy number and expression of foreign genes. Besides,the majority of twenty-eight transgenic birches involved vector sequences(part or whole).(4) The data of restriction enzyme digestion and bisulphate sequencing analyses indicated that DNA methylation resulted in the expression of transgenic birches,but it was not the only factor which affected expression.Furthermore,the action of DNA methylation was different among transgenic trees.
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