Accumulation of point mutations and reassortment of genomic RNA segments are involved in the microevolution of Puumala hantavirus in a bank vole (Myodes glareolus) population.

The genetic diversity of Puumala hantavirus (PUUV) was studied in a local population of its natural host, the bank vole (Myodes glareolus). The trapping area (2.5 x 2.5 km) at Konnevesi, Central Finland, included 14 trapping sites, at least 500 m apart; altogether, 147 voles were captured during May and October 2005. Partial sequences of the S, M and L viral genome segments were recovered from 40 animals. Seven, 12 and 17 variants were detected for the S, M and L sequences, respectively; these represent new wild-type PUUV strains that belong to the Finnish genetic lineage. The genetic diversity of PUUV strains from Konnevesi was 0.2-4.9 % for the S segment, 0.2-4.8 % for the M segment and 0.2-9.7 % for the L segment. Most nucleotide substitutions were synonymous and most deduced amino acid substitutions were conservative, probably due to strong stabilizing selection operating at the protein level. Based on both sequence markers and phylogenetic clustering, the S, M and L sequences could be assigned to two groups, 'A' and 'B'. Notably, not all bank voles carried S, M and L sequences belonging to the same group, i.e. S(A)M(A)L(A) or S(B)M(B)L(B). A substantial proportion (8/40, 20 %) of the newly characterized PUUV strains possessed reassortant genomes such as S(B)M(A)L(A), S(A)M(B)L(B) or S(B)M(A)L(B). These results suggest that at least some of the PUUV reassortants are viable and can survive in the presence of their parental strains.