Frequency-dependent selection in human immunodeficiency virus type 1

This study demonstrates frequency-dependent selection (FDS) in HIV-1, where viral fitness varies depending on the proportion of competing clones. Researchers isolated 10 biological clones from a single HIV-1 strain and conducted competition experiments in cell culture at three different initial proportions (1:9, 1:1, and 9:1) over five passages. Using heteroduplex tracking assay to quantify competing populations, they found that variant frequency significantly influenced selective outcomes. Statistical analysis revealed negative FDS—lower-frequency variants exhibited higher fitness—and identified equilibrium points where competing variants converged. Importantly, FDS occurred in the absence of immune pressure, suggesting it operates through intrinsic viral mechanisms rather than external selective forces. The authors propose that differences in replication efficiency, reverse transcription rates, and viral morphogenesis among quasispecies members drive this phenomenon. These findings parallel earlier observations of FDS in vesicular stomatitis virus, suggesting FDS may be a general evolutionary mechanism in RNA viruses that preserves genetic heterogeneity within viral populations by maintaining minor variants with superior fitness characteristics.

Key findings

• Frequency-dependent selection operates in HIV-1: variant fitness depends on their frequency in the competing population
• Negative FDS identified across five passages: rare variants consistently showed higher fitness than frequent ones
• Equilibrium points detected where competing viral variants stabilized at predictable frequencies
• FDS maintained viral quasispecies complexity by preserving minor variants despite population fluctuations
• This mechanism likely operates through intrinsic differences in viral replication efficiency among clones, independent of immune pressure