Dating the origins of the maize-adapted strain of maize streak virus, MSV-A

Abstract

Maize streak virus (MSV), which causes maize streak disease (MSD), is one of the most serious biotic threats to African food security. Here, we use whole MSV genomes sampled over 30 years to estimate the dates of key evolutionary events in the 500 year association of MSV and maize. The substitution rates implied by our analyses agree closely with those estimated previously in controlled MSV evolution experiments, and we use them to infer the date when the maize-adapted strain, MSV-A, was generated by recombination between two grass-adapted MSV strains. Our results indicate that this recombination event occurred in the mid-1800s, ∼20 years before the first credible reports of MSD in South Africa and centuries after the introduction of maize to the continent in the early 1500s. This suggests a causal link between MSV recombination and the emergence of MSV-A as a serious pathogen of maize.

Reconstructing the History of Maize Streak Virus Strain A Dispersal To Reveal Diversification Hot Spots and Its Origin in Southern Africa

“…Importantly, these estimated dates have broadly overlapping 95% HPDs encompassing the first credible reports of MSV-A-like disease in South Africa during the 1870s (17). These estimates are very similar to the MSV-A MRCA dates determined previously with much smaller data sets using far less parameter-rich models (21).…”

“…The estimated timing of this recombination event, the observation that it involved the exchange of an experimentally demonstrated pathogenicity-determining mp and cp gene module (40), the fact that it is detectable in all known MSV-A variants (69), and the observation that MSV-A is the only MSV strain that is associated with MSD (7,39,47) have prompted speculation that this recombination event was key to the emergence of MSV-A as a viral pathogen of maize (21).…”

“…Maize was first introduced to West Africa by the Portuguese in the early 1500s and to southern Africa by the Dutch in the mid-1600s (24,44), and a recombination event between ancestral MSV-B and MSV-G/F variants is credited with having generated a maize-adapted MSV-A prototype (54), possibly within 20 years of the first credible reports of MSD in southern Africa during the 1870s (17,21). The estimated timing of this recombination event, the observation that it involved the exchange of an experimentally demonstrated pathogenicity-determining mp and cp gene module (40), the fact that it is detectable in all known MSV-A variants (69), and the observation that MSV-A is the only MSV strain that is associated with MSD (7,39,47) have prompted speculation that this recombination event was key to the emergence of MSV-A as a viral pathogen of maize (21).…”

“…We analyzed the FG and MPCP data sets with both the discrete and continuous phylogeographic models implemented in BEAST under a relaxed-clock model assuming the Hasegawa-Kishino and Yano substitution model with four gamma-distributed substitution rate categories (HKYϩG4), and the flexible nonparametric Bayesian skyline plot (BSP) demographic model (selected for MSV as the best-fit clock) (the substitution and demographic models have been described by Harkins et al [21].) Replicate runs of the Markov chain varying in length between 2 ϫ 10 8 and 8 ϫ 10 8 steps were performed with BEAST and inspected for convergence using Tracer v1.6 (53).…”

Recombination hotspots and host susceptibility modulate the adaptive value of recombination during maize streak virus evolution

“…adapted MSVs [3,13]. MSV-A is distributed throughout sub-Saharan Africa where it jeopardizes sustainable maize production in some of the world's poorest countries [19-21].…”