Differential evolution of eastern equine encephalitis virus populations inresponse to host cell type

Arthropod-borne viruses (arboviruses) cycle between hosts in two widely sep arated taxonomic groups, vertebrate amplifying heats and invertebrate vecto rs, both of which may separately or in concert shape the course of arboviru s evolution. To elucidate the selective pressures associated with virus rep lication within each portion of this two-host life cycle, the effects of ho st type on the growth characteristics of the New World alphavirus, eastern equine encephalitis (EEE) virus, were investigated. Multiple lineages of an ancestral EEE virus stock were repeatedly transferred through either mosqu ito or avian cells or in alternating passages between these two cell types. When assayed in both cell types, derived single host lineages exhibited si gnificantly differences in infectivity, growth pattern, plaque morphology, and total virus yield, demonstrating that this virus is capable of host-spe cific evolution. Virus lineages grown in alternation between the two cell t ypes expressed intermediate phenotypes consistent with dual adaptation to b oth cellular environments. Both insect-adapted and alternated lineages grea tly increased in their ability to infect insect cells. These results indica te that different selective pressures exist for virus replication within ea ch portion of the two-host life cycle, and that alternation of hosts select s for virus populations well adapted for replication in both host systems.