Abstract
Self-transmissible mobile genetic elements drive horizontal gene transfer between prokaryotes. Some of these elements integrate in the chromosome, whereas others replicate autonomously as plasmids. Recent works showed the existence of few differences, and occasional interconversion, between the two types of elements. Here, we enquired on why evolutionary processes have maintained the two types of mobile genetic elements by comparing integrative and conjugative elements (ICE) with extrachromosomal ones (conjugative plasmids) of the highly abundant MPFT conjugative type. Plasmids encode more replicases, partition systems, and antibiotic resistance genes, whereas ICEs encode more integrases and metabolism-associated genes. Plasmids are more variable in size, have more DNA repeats, and exchange genes more frequently. On the other hand, ICEs are more frequently transferred between distant taxa, and this drives the conversion of plasmids into ICEs after transfer to distantly related hosts. Hence, differential plasticity and transmissibility explain the occurrence of both integrative and extra-chromosomal elements in microbial populations.