Restriction-modification CRM) systems are cognate gene complexes that code
for an endonuclease and a methylase. They are often thought to have develop
ed in bacteria as protection against invading genetic material, e.g., phage
DNA. The high diversity of RM systems, as observed in nature, is often asc
ribed to the coevolution of RM systems (which 'invent' novel types) and pha
ges. However, the extent to which phages sue insensitive to RM systems cast
s doubts on the effectiveness of RM systems as protection against infection
and thereby on the reason for the diversity of RM systems. We present an e
co-evolutionary model in order to study the evolution of the diversity of R
M systems. The model predicts that in general diversity of RM systems is hi
gh. More importantly, the diversity of the RM systems is expressed either a
t the individual level or at the population level. In the first case all in
dividuals carry RM systems of all sequence specificities, whereas in the se
cond case they carry only one RM system or no RM systems at all. Neverthele
ss, in the second case the same number of sequence specificities are presen
t in the population. (C) 2000 Society for Mathematical Biology.