Transovarial maintenance of San Angelo virus in sequential generations of Aedes albopictus

An Aedes albopictus line originally selected for efficient transovarial transmission (TOT) of San Angelo (SA) virus displayed a progressive decline in filial infection rate (FIR) whenever artificial selection pressure was relaxed. This observation brought into question whether the efficiency of TOT in this vector-virus model was sufficiently high to permit persistent vertical transmission over numerous freely reproducing generations. By tracing descendants of individual transovarially infected females for several generations, it was found in the present study that this decline in FIR resulted not only from the spontaneous appearance of uninfected females, but also from an accumulation of transovarially infected females which were themselves inefficient transovarial transmitters. Some efficient transovarial transmitters continued to be produced even when FIR had declined to low levels, assuring transmission of virus to subsequent generations, irrespective of the overall infection rate. Outcrossing experiments suggested that a "refractory" genetic factor or factors had been selected out of the TOT-efficient line during its long history of inbreeding. New TOT-efficient lines were more readily established from parenterally infected Ae. albopictus of a Taiwan strain than had been the original TOT-efficient line from a Hawaiian mosquito strain. One of the newly selected lines differed from the others in producing progenies with FIRs no higher than 36% in the third and fourth generations of transovarial passage. The familial and strain variation revealed by these studies suggests a genetic influence on both the establishment and maintenance of persistent oogonial infections.