DESeq to identify DEG without biological replicates

DESeq.R

library(DESeq);
data_pnas<-read.table("readCount.res",header=T,row.names=1);

head(data_pnas);

data_pnas_desi<-data.frame(row.names=colnames(data_pnas),condition = c("Sample_g1","Sample_g2","Sample_g3","Sample_g4"),libType  =  c("single-end","single-end","single-end","single-end"));

condition = data_pnas_desi$condition;condition;

cds  =  newCountDataSet(data_pnas,  condition);

cds  =  estimateSizeFactors(  cds  );

sizeFactors(  cds  );

### This step is telling the DESeq you don't have replicates. 
#Note the option sharingMode="fit-only". Remember that the default, sharingMode="maximum", takes care of
#outliers, i.e., genes with dispersion much larger than the fitted values. Without replicates, we cannot catch such
#outliers and so have to disable this functionality.
cds  =  estimateDispersions(  cds ,method="blind",  sharingMode="fit-only" ) ;

#gene_id Sample_g1  Sample_g2  Sample_g3  Sample_g4

Sample_g1_g2 = nbinomTest(cds,  "Sample_g1", "Sample_g2");
write.table(Sample_g1_g2 , "DEG_g1_g2.res", append = FALSE, quote = FALSE, sep = "\t");
pdf("plotMA_Sample_g1_g2.pdf");plotMA(Sample_g1_g2,col=ifelse(Sample_g1_g2$padj >= 0.05, "gray32", "red3"));dev.off()

Sample_g1_g3 = nbinomTest(cds,  "Sample_g1", "Sample_g3");
write.table(Sample_g1_g3 , "DEG_g1_g3.res", append = FALSE, quote = FALSE, sep = "\t");
pdf("plotMA_Sample_g1_g3.pdf");plotMA(Sample_g1_g3,col=ifelse(Sample_g1_g3$padj >= 0.05, "gray32", "red3"));dev.off()

Sample_g1_g4 = nbinomTest(cds,  "Sample_g1", "Sample_g4");
write.table(Sample_g1_g4 , "DEG_g1_g4.res", append = FALSE, quote = FALSE, sep = "\t");
pdf("plotMA_Sample_g1_g4.pdf");plotMA(Sample_g1_g4,col=ifelse(Sample_g1_g4$padj >= 0.05, "gray32", "red3"));dev.off()

readCount.res

gene_id Sample_g1  Sample_g2  Sample_g3  Sample_g4
AT1G01010       38      37      21      20
AT1G01020       39      43      13      19
AT1G01030       8       11      7       1
AT1G01040       205     258     103     146
AT1G01046       2       1       0       2
AT1G01050       109     100     49      63
AT1G01060       243     220     73      166
AT1G01070       19      17      11      27
AT1G01080       73      53      45      65
AT1G01090       268     167     142     160
AT1G01100       588     615     301     334
AT1G01110       10      18      4       8
AT1G01115       0       0       0       1
AT1G01120       80      152     89      77
......