Individual- and population-based diversity in restriction-modification systems

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.