Knowledge of the mechanisms of
pathogen persistence in relation to fluctuations in host
density is crucial to our understanding of disease dynamics. In the case of insect baculoviruses, which are typically
transmitted horizontally via a lifestage that can persist outside the host, a key issue that remains to be elucidated is whether the
virus can also be
transmitted vertically as a sublethal
infection. We show that
RNA transcripts for the Plodia interpunctella GV
granulin gene are present in a high proportion of P. interpunctella insects that survive
virus challenge.
Granulin is a late-expressed gene that is only transcribed after viral genome replication, its presence thus strongly indicates that viral genome replication has occurred. Almost all insects surviving the
virus challenge tested positive for viral
RNA in the
larval and pupal stage. However, this proportion declined in the emerging adults.
Granulin mRNA was also detected in both the
ovaries and
testes, which may represent a putative mechanism by which reduced fecundity in sublethally affected hosts might be manifested.
RNA transcripts were also detected in 60-80% of second-generation larvae that were
derived from mating surviving adults, but there was no difference between the sexes, with both males and females capable of
transmitting a sublethal
infection to their offspring. The data indicate that low-level persistent
infection, with at least limited
gene expression, can occur in P. interpunctella following survival of a granulovirus challenge. We believe that this is the first demonstration of a persistent, sublethal
infection by a
baculovirus to be initiated by a sublethal
virus dose. We hypothesize that the 'latent'
baculovirus infections frequently referred to in the literature may also be low level persistent, sublethal
infections resulting from survival from initial
baculovirus exposure.
