A graph G is called to be chromatic choosable if its choice number is equal to its chromatic number. In 2002, Ohba conjectured that every graph G with 2Х（G） ＋ 1 or fewer vertices is chromatic choosable. It is easy...A graph G is called to be chromatic choosable if its choice number is equal to its chromatic number. In 2002, Ohba conjectured that every graph G with 2Х（G） ＋ 1 or fewer vertices is chromatic choosable. It is easy to see that Ohba＇s conjecture is true if and only if it is true for complete multipartite graphs. But at present only for some special cases of complete multipartite graphs, Ohba＇s conjecture have been verified. In this paper we show that graphs K6,3,2＊（k-6）,1＊4 （k ≥ 6） is chromatic choosable and hence Ohba＇s conjecture is true for the graphs K6,3,2＊（k-6）,1＊4 and all complete k-partite subgraphs of them.展开更多
To study the quasispecies diversity of porcine reproductive and respiratory syndrome virus (PRRSV), open reading frame 5 (ORF5) of strain SD0612 was amplified and cloned. Sixty clones of ORF5 were sequenced and analyz...To study the quasispecies diversity of porcine reproductive and respiratory syndrome virus (PRRSV), open reading frame 5 (ORF5) of strain SD0612 was amplified and cloned. Sixty clones of ORF5 were sequenced and analyzed with DNAStar software. Nucleic acid sequence homology was 97.7%-100%, with 78 mutations observed. Among these 60 clones, the sequences of 17 clones were identical and recognized as the dominant quasispecies of strain SD0612. Evolution of SD0612 quasispecies diversity under antibody selective pressure was also studied. SD0612 was passed continuously in the Marc-145 cell line over 40 passages in 6 independent lineages. SD0612 antiserum was not added to lineage A, B, and C cultures; however, antiserum was added to culture medium for lineages D, E, and F. PRRSV ORF5 was then amplified, cloned, and sequenced from each of the 6 lineages, designated as A40-F40. F40 was further passed in Marc-145 cells using 6 independent lineages with or without F40 antiserum for another 40 passages. ORF5 from the 6 newly-derived virus lineages, which we designated as a40-f40, were amplified, cloned and sequenced. The proportion of dominant quasispecies increased with passage number in cell cultures supplemented with antibodies, but decreased when antibodies were lacking. Our work has demonstrated a diversity of quasispecies for ORF5 in PRRSV SD0612. Antibody selective pressure was able to significantly influence quasispecies diversity and promote a dominant quasispecies that was able to evade immune reactions.展开更多
基金the Science Foundation of the Education Department of Hebei Province (2005108).
文摘A graph G is called to be chromatic choosable if its choice number is equal to its chromatic number. In 2002, Ohba conjectured that every graph G with 2Х（G） ＋ 1 or fewer vertices is chromatic choosable. It is easy to see that Ohba＇s conjecture is true if and only if it is true for complete multipartite graphs. But at present only for some special cases of complete multipartite graphs, Ohba＇s conjecture have been verified. In this paper we show that graphs K6,3,2＊（k-6）,1＊4 （k ≥ 6） is chromatic choosable and hence Ohba＇s conjecture is true for the graphs K6,3,2＊（k-6）,1＊4 and all complete k-partite subgraphs of them.
文摘To study the quasispecies diversity of porcine reproductive and respiratory syndrome virus (PRRSV), open reading frame 5 (ORF5) of strain SD0612 was amplified and cloned. Sixty clones of ORF5 were sequenced and analyzed with DNAStar software. Nucleic acid sequence homology was 97.7%-100%, with 78 mutations observed. Among these 60 clones, the sequences of 17 clones were identical and recognized as the dominant quasispecies of strain SD0612. Evolution of SD0612 quasispecies diversity under antibody selective pressure was also studied. SD0612 was passed continuously in the Marc-145 cell line over 40 passages in 6 independent lineages. SD0612 antiserum was not added to lineage A, B, and C cultures; however, antiserum was added to culture medium for lineages D, E, and F. PRRSV ORF5 was then amplified, cloned, and sequenced from each of the 6 lineages, designated as A40-F40. F40 was further passed in Marc-145 cells using 6 independent lineages with or without F40 antiserum for another 40 passages. ORF5 from the 6 newly-derived virus lineages, which we designated as a40-f40, were amplified, cloned and sequenced. The proportion of dominant quasispecies increased with passage number in cell cultures supplemented with antibodies, but decreased when antibodies were lacking. Our work has demonstrated a diversity of quasispecies for ORF5 in PRRSV SD0612. Antibody selective pressure was able to significantly influence quasispecies diversity and promote a dominant quasispecies that was able to evade immune reactions.
