Squash vein yellowing virus from California emerged in the Middle East via intragenic and intergeneric recombination events in the hypervariable potyvirus P1 and ipomovirus P1a genes.

We present the complete sequence of the genomic RNA of an isolate of squash vein yellowing virus () from California (SqVYV-CA) and show it is a recombinant virus with a highly divergent 5' UTR and proximal P1a gene. The evolution of SqVYV-CA involved an intrageneric event between unknown potyviruses, related to isolates of papaya ringspot virus () from the Old World, and an intergeneric event between this recombinant potyvirus (minor parent) and an isolate of SqVYV from Israel (SqVYV-IL) (major parent). These events occurred in mixed infections and in the potyvirus P1 and ipomovirus P1a recombination hotspots and resulted in SqVYV-CA having a potyvirus 5' UTR and chimeric P1-P1a gene/protein and the remainder of the genome from SqVYV-IL. The SqVYV-CA chimeric P1-P1a gene is under positive selection, and the protein is intrinsically disordered and may localize to the nucleus via nuclear localization signals in the P1 part. The C-terminal SqVYV-IL P1a part also diverged but retained the conserved serine protease motif. Furthermore, substantial divergence in SqVYV isolates from the Middle East was associated with genetic drift and a long evolutionary history in this region. The finding that the host range and symptomatology in cucurbits of SqVYV-CA is similar to those of SqVYV from Florida and SqVYV-IL, indicated that the recombinant part of the genome had no obvious effect on the virus-host interaction. A divergent part of the P1 sequence of the SqVYV-CA P1-P1a gene was used to develop a primer pair and RT-PCR test for specific detection of SqVYV-CA. This test was used to detect spread of SqVYV-CA to a new production area of California in 2021 and 2022. Together, these results demonstrate (i) a high level of genetic diversity exists among isolates of SqVYV and involved intra- and intergeneric recombination and genetic drift (mutation), (ii) evidence that SqVYV originated in the Middle East and that there were independent introductions into the New World and (iii) the remarkable genetic flexibility of the 5' proximal genes of these viruses.