Simmons, H. E., Holmes, E. C., Gildow, F. E., Bothe-Goralczyk, M. A., and Stephenson, A. G. 2011. Experimental verification of seed transmission of Zucchini yellow mosaic virus. Plant Dis. 95:751-754.Within two decades of its discovery, Zucchini yellow mosaic virus (ZYMV) achieved a global distribution. However, whether or not seed transmission occurs in this economically significant crop pathogen is controversial, and the relative impact of seed transmission on the epidemiology of ZYMV remains unclear. Using reverse transcriptionpolymerase chain reaction, we observed a seed transmission rate of 1.6% in Cucurbita pepo subsp. texana and show that seed-infected C. pepo plants are capable of initiating horizontal ZYMV infections, both mechanically and via an aphid vector (Myzus persicae). We also provide evidence that ZYMV-infected seeds may act as effective viral reservoirs, partially accounting for the current geographic distribution of ZYMV. Finally, the observation that ZYMV infection of C. pepo seeds results in virtually symptomless infection, coupled with our finding that an antibody test failed to detect vertically transmitted ZYMV in infected seed, highlights the urgent need to standardize current detection methods for seed infection.Since the discovery of Zucchini yellow mosaic virus (ZYMV) in Italy in 1973, and its subsequent description in 1981 (14), this emerging RNA virus has spread rapidly and achieved an effectively global distribution (5). Although a number of explanations have been put forward to account for the widespread geographic distribution and persistence of this virus, including the international trading of infected fruit, plants, or seeds, as well as overwintering in alternative hosts and noncolonizer aphids, the mechanisms underlying the rapid dissemination and persistence of ZYMV remain unclear (13). ZYMV is a single-stranded positive-sense RNA virus of the family Potyviridae that can result in yellowing and stunting of the plant, as well as severe leaf and fruit deformities that can reduce yields up to 94% (1). Given that cucurbit (squash, melon, and cucumber) production in the United States alone is estimated to be worth approximately $1.5 billion per year (2), the economic significance of this crop pathogen is enormous. Understanding the epidemiology and evolution of ZYMV is therefore central to controlling this devastating crop disease.Viral transmission generally occurs in one of two ways: horizontally, which is the transmission of the virus between unrelated hosts, or vertically, which is the transmission of the virus from parent to offspring. ZYMV is horizontally transmitted in a nonpersistent manner by at least 26 aphid species (12). Transmission occurs as a result of an interaction between the stylet of the aphid, the helper component protein (HC-Pro), and the conserved DAG (AspAla-Gly) region of the coat protein (CP) (18). However, the current worldwide distribution of ZYMV is unlikely to have resulted from aphid transmission alone, particularly as the aphid vector remains virul...

The role played by seed transmission in the evolution and epidemiology of viral crop pathogens remains unclear. We determined the seed infection and vertical transmission rates of zucchini yellow mosaic virus (ZYMV), in addition to undertaking Illumina sequencing of nine vertically transmitted ZYMV populations. We previously determined the seed-to-seedling transmission rate of ZYMV in Cucurbita pepo ssp. texana (a wild gourd) to be 1.6%, and herein observed a similar rate (1.8%) in the subsequent generation. We also observed that the seed infection rate is substantially higher (21.9%) than the seed-to-seedling transmission rate, suggesting that a major population bottleneck occurs during seed germination and seedling growth. In contrast, that two thirds of the variants present in the horizontally transmitted inoculant population were also present in the vertically transmitted populations implies that the bottleneck at vertical transmission may not be particularly severe. Strikingly, all of the vertically infected plants were symptomless in contrast to those infected horizontally, suggesting that vertical infection may be cryptic. Although no known virulence determining mutations were observed in the vertically infected samples, the 5’ untranslated region was highly variable, with at least 26 different major haplotypes in this region compared to the two major haplotypes observed in the horizontally transmitted population. That the regions necessary for vector transmission are retained in the vertically infected populations, combined with the cryptic nature of vertical infection, suggests that seed transmission may be a significant contributor to the spread of ZYMV.

Zucchini yellow mosaic virus (ZYMV) is an economically important virus of cucurbit crops. However, little is known about the rate at which this virus has evolved within members of the family Cucurbitaceae, or the timescale of its epidemiological history. Herein, we present the first analysis of the evolutionary dynamics of ZYMV. Using a Bayesian coalescent approach we show that the coat protein of ZYMV has evolved at a mean rate of 5.0¾10"4 nucleotide substitutions per site, per year. Notably, this rate is equivalent to those observed in animal RNA viruses. Using the same approach we show that the lineages of ZYMV sampled here have an ancestry that dates back no more than 800 years, suggesting that human activities have played a central role in the dispersal of ZYMV. Finally, an analysis of phylogeographical structure provides strong evidence for the in situ evolution of ZYMV within individual countries.