Understanding Basic Concepts of Viral Quasispecies: From Evolutionary Dynamics to Clinical Relevance.

This review examines the viral quasispecies concept and its implications for understanding viral evolution, pathogenesis, and the development of effective antiviral therapies. Quasispecies are dynamic populations of closely related but genetically distinct viral genomes, which evolve under mutation and Darwinian selection. Notably, minority variants, often dismissed as "genetic noise", may harbor significant biological differences-such as drug resistance-and become dominant under changing selective pressures. Next-generation sequencing (NGS) has become an indispensable tool for characterizing genetic diversity within quasispecies, enabling detection and quantitative analysis of minority variants often missed by conventional Sanger sequencing, as well as the calculation of diversity indices. We highlight two NGS-based studies of hepatitis B virus quasispecies as illustrative examples of the relevance of minority variants in antiviral resistance and clinical outcomes. These studies also exemplify transcomplementation, whereby defective viral genomes can be replicated and packaged through functional proteins provided by co-infecting variants. Finally, we discuss how the quasispecies concept may extend beyond viruses, with parallels in biological systems such as the adaptive immune system and tumor cell populations. Recognizing quasispecies as dynamic evolving populations rather than static entities is crucial for developing successful strategies to address infectious diseases and other complex biological challenges.