Population bottlenecks can have major effects in the evolution of RNA viruses, 
but their possible influence in the evolution of DNA viruses is largely unknown. 
Genetic and biological variation of herpes simplex virus type 1 (HSV-1) has been 
studied by subjecting 23 biological clones of the virus to 10 plaque-to-plaque 
transfers. In contrast to large population passages, plaque transfers led to a 
decrease in replicative capacity of HSV-1. Two out of a total of 23 clones did 
not survive to the last transfer in 143 TK(-) cells. DNA from three genomic 
regions (DNA polymerase, glycoprotein gD and thymidine kinase) from the initial 
and passaged clones was sequenced. Nucleotide substitutions were detected in the 
TK and gD genes, but not in the DNA polymerase gene. Assuming a uniform 
distribution of mutations along the genome, the average rate of fixation of 
mutations was about five mutations per viral genome and plaque transfer. This 
value is comparable to the range of values calculated for RNA viruses. Four 
plaque-transferred populations lost neurovirulence for mice, as compared with 
the corresponding initial clones. LD(50) values obtained with the populations 
subjected to serial bottlenecks were 4- to 67-fold higher than for their 
parental clones. These results equate HSV-1 with RNA viruses regarding fitness 
decrease as a result of plaque-to-plaque transfers, and show that population 
bottlenecks can modify the pathogenic potential of HSV-1. Implications for the 
evolution of complex DNA viruses are discussed.