Frequency-dependent selection in human immunodeficiency virus type 1.

Genetic variation is the main evolutionary strategy adopted by RNA viruses and retroviruses. Evolution operates through competition between different individuals in the same environment, resulting in the imposition of the fittest variant. The process of competition could be affected by various factors, including the frequency of the different competing individuals. In order to investigate this aspect, individual virus populations derived from a human immunodeficiency virus type 1 isolate were studied at different competing proportions. The dynamics of variant imposition in each competition experiment permitted the detection of frequency-dependent selection (FDS); i.e. the imposition of variants is related to their biological fitness, which is also affected by the proportions at which they compete. The existence of FDS in different viruses with RNA genomes would indicate a general mechanism favouring genetic heterogeneity.