Abstract:   Objective To construct a suppression subtractive hybridization（SSH） cDNA library of half-blood males of Dermacentor silvarum， and analyze the differentially expressed genes. Methods Total RNA was extracted from the half-blood males and unfed males of D.　silvarum. cDNA was synthesized following the protocol of SMARTER cDNA synthesis kit. After RsaⅠdigestion， cDNA was ligated to adaptors. The cDNA from the half-blood males was used as the tester， and unfed males as the driver. The SSH library was constructed using TaKaRa PCR-select cDNA subtraction kit. Differentially expressed cDNAs were amplified by nested PCR， cloned into PMD-18T vector， transformed into E. coli DH5α, and the white-blue plaque selection was used to get the positive clones. The titer of SSH library and the recombination efficiency were calculated. Individual colonies were randomly selected from library. Subtractive efficiency of the subtracted cDNA library was examined by reverse Northern blotting and RT-PCR. Positive clones with differentially expressed genes were sequenced. Homology comparison and function prediction were performed by Blastn and Blastx. Results The bands of double-stranded cDNAs from half-blood males and unfed males of D. silvarum were dispersed and longer than 500 bp. After RsaⅠ digestion， the ds cDNA-fragments were 100-1 000 bp. The ligation reaction efficiency of adaptor was more than 25%. Nested PCR showed that the bands of subtracted ds cDNA were gathered， ranging from 250 to 500 bp. The titer of SSH library was 700 000 pfu/ml， and the recombination efficiency was 88.5% （239/270）. Reverse Northern hybridization revealed that the clones showed stronger signals in half-blood males cDNA probes than in unfed males cDNA probes. RT-PCR showed that among the eight random selected positive clones， 5 clones were up-expressed under half-blood condition. A total of 87 differentially expressed sequence tags（ESTs， 200-800 bp） were obtained from 115 positive clones. Among the 87 ESTs， 53 ESTs showed sequence similarities to genes from other tick species， and 34 were homologous with genes from other insects. The main biological function of obtained ESTs were related to blood sucking and digestion， such as energy metabolism， signal transduction， and transcription regulation. Conclusion The SSH cDNA library of half-blood male Dermacentor silvarum is constructed. The differential expressed genes are related to blood sucking and digestion.

Key words: Dermacentor silvarum, Suppression subtractive hybridization, cDNA library, Differentially expressed gene