Why do mobile genetic elements transfer DNA of their hosts?

The prokaryote world is replete with mobile genetic elements (MGEs) - self-replicating entities that can move within and between their hosts. Many MGEs not only transfer their own DNA to new hosts but also transfer host DNA located elsewhere on the chromosome in the process. This could potentially lead to indirect benefits to the host when the resulting increase in chromosomal variation results in more efficient natural selection. We review the diverse ways in which MGEs promote the transfer of host DNA and explore the benefits and costs to MGEs and hosts. In many cases, MGE-mediated transfer of host DNA might not be selected for because of a sex function, but evidence of MGE domestication suggests that there may be host benefits of MGE-mediated sex.