The combinatorial nature of genetic recombination can potentially provide organisms with immediate access to many more positions in sequence space than can be reached by mutation alone. Recombination features particularly prominently in the evolution of a diverse range of viruses. Despite rapid progress having been made in the characterization of discrete recombination events for many species, little is currently known about either gross patterns of recombination across related virus families or the underlying processes that determine genome-wide recombination breakpoint distributions observable in nature. It has been hypothesized that the networks of coevolved molecular interactions that define the epistatic architectures of virus genomes might be damaged by recombination and therefore that selection strongly influences observable recombination patterns. For recombinants to thrive in nature, it is probably important that the portions of their genomes that they have inherited from different parents work well together. Here we describe a comparative analysis of recombination breakpoint distributions within the genomes of diverse single-stranded DNA (ssDNA) virus families. We show that whereas nonrandom breakpoint distributions in ssDNA virus genomes are partially attributable to mechanistic aspects of the recombination process, there is also a significant tendency for recombination breakpoints to fall either outside or on the peripheries of genes. In particular, we found significantly fewer recombination breakpoints within structural protein genes than within other gene types. Collectively, these results imply that natural selection acting against viruses expressing recombinant proteins is a major determinant of nonrandom recombination breakpoint distributions observable in most ssDNA virus families.

The ongoing global spread of Tomato yellow leaf curl virus (TYLCV; Genus Begomovirus, Family Geminiviridae) represents a serious looming threat to tomato production in all temperate parts of the world. Whereas determining where and when TYLCV movements have occurred could help curtail its spread and prevent future movements of related viruses, determining the consequences of past TYLCV movements could reveal the ecological and economic risks associated with similar viral invasions. Towards this end we applied Bayesian phylogeographic inference and recombination analyses to available TYLCV sequences (including those of 15 new Iranian full TYLCV genomes) and reconstructed a plausible history of TYLCV's diversification and movements throughout the world. In agreement with historical accounts, our results suggest that the first TYLCVs most probably arose somewhere in the Middle East between the 1930s and 1950s (with 95% highest probability density intervals 1905–1972) and that the global spread of TYLCV only began in the 1980s after the evolution of the TYLCV-Mld and -IL strains. Despite the global distribution of TYLCV we found no convincing evidence anywhere other than the Middle East and the Western Mediterranean of epidemiologically relevant TYLCV variants arising through recombination. Although the region around Iran is both the center of present day TYLCV diversity and the site of the most intensive ongoing TYLCV evolution, the evidence indicates that the region is epidemiologically isolated, which suggests that novel TYLCV variants found there are probably not direct global threats. We instead identify the Mediterranean basin as the main launch-pad of global TYLCV movements.

Maize streak virus (MSV; family Geminiviridae, genus Mastrevirus), the causal agent of maize streak disease, ranks amongst the most serious biological threats to food security in subSaharan Africa. Although five distinct MSV strains have been currently described, only one of these – MSV-A – causes severe disease in maize. Due primarily to their not being an obvious threat to agriculture, very little is known about the ‘grass-adapted’ MSV strains, MSV-B, -C, -D and -E. Since comparing the genetic diversities, geographical distributions and natural host ranges of MSV-A with the other MSV strains could provide valuable information on the epidemiology, evolution and emergence of MSV-A, we carried out a phylogeographical analysis of MSVs found in uncultivated indigenous African grasses. Amongst the 83 new MSV genomes presented here, we report the discovery of six new MSV strains (MSV-F to -K). The non-random recombination breakpoint distributions detectable with these and other available mastrevirus sequences partially mirror those seen in begomoviruses, implying that the forces shaping these breakpoint patterns have been largely conserved since the earliest geminivirus ancestors. We present evidence that the ancestor of all MSV-A variants was the recombinant progeny of ancestral MSV-B and MSV-G/-F variants. While it remains unknown whether recombination influenced the emergence of MSV-A in maize, our discovery that MSV-A variants may both move between and become established in different regions of Africa with greater ease, and infect more grass species than other MSV strains, goes some way towards explaining why MSV-A is such a successful maize pathogen.