Cotton leaf curl Gezira alphasatellite associated with tomato leaf curl Sudan virus approaches the expected upper size limit in the evolution of alphasatellites

The Role of Extensive Recombination in the Evolution of Geminiviruses

Mutation, recombination and pseudo-recombination are the major forces driving the evolution of viruses by the generation of variants upon which natural selection, genetic drift and gene flow can act to shape the genetic structure of viral populations. Recombination between related virus genomes co-infecting the same cell usually occurs via template swapping during the replication process and produces a chimeric genome. The family Geminiviridae shows the highest evolutionary success among plant virus families, and the common presence of recombination signatures in their genomes reveals a key role in their evolution. This review describes the general characteristics of members of the family Geminiviridae and associated DNA satellites, as well as the extensive occurrence of recombination at all taxonomic levels, from strain to family. The review also presents an overview of the recombination patterns observed in nature that provide some clues regarding the mechanisms involved in the generation and emergence of recombinant genomes. Moreover, the results of experimental evolution studies that support some of the conclusions obtained in descriptive or in silico works are summarized. Finally, the review uses a number of case studies to illustrate those recombination events with evolutionary and pathological implications as well as recombination events in which DNA satellites are involved.

High-Throughput Sequencing Identified Distinct Bipartite and Monopartite Begomovirus Variants Associated with DNA-Satellites from Tomato and Muskmelon Plants in Saudi Arabia

The studies on the prevalence and genetic diversity of begomoviruses in Saudi Arabia are minimal. In this study, field-grown symptomatic tomato and muskmelon plants were collected, and initially, begomovirus infection was confirmed by the core coat protein sequences. Four tomato and two muskmelon plants with viral infections were further evaluated for Illumina MiSeq sequencing, and twelve sequences (2.7-2.8 kb) equivalent to the full-length DNA-A or DNA-B components of begomoviruses were obtained along with eight sequences (~1.3-1.4 kb) equivalent to the begomovirus-associated DNA-satellite components. Four begomovirus sequences obtained from tomato plants were variants of tomato yellow leaf curl virus (TYLCV) with nt sequence identities of 95.3-100%. Additionally, two tomato plants showed a mixed infection of TYLCV and cotton leaf curl Gezira virus (CLCuGeV), okra yellow crinkle Cameroon alphasatellite (OYCrCMA), and okra leaf curl Oman betasatellite (OLCuOMB). Meanwhile, from muskmelon plants, two sequences were closely related (99-99.6%) to the tomato leaf curl Palampur virus (ToLCPalV) DNA-A, whereas two other sequences showed 97.9-100% sequence identities to DNA-B of ToLCPalV, respectively. Complete genome sequences of CLCuGeV and associated DNA-satellites were also obtained from these muskmelon plants. The nt sequence identities of the CLCuGeV, OYCrCMA, and OLCuOMB isolates obtained were 98.3-100%, 99.5-100%, and 95.6-99.7% with their respective available variants. The recombination was only detected in TYLCV and OLCuOMB isolates. To our knowledge, this is the first identification of a mixed infection of bipartite and monopartite begomoviruses associated with DNA-satellites from tomato and muskmelon in Saudi Arabia. The begomovirus variants reported in this study were clustered with Iranian isolates of respective begomovirus components in the phylogenetic dendrogram. Thus, the Iranian agroecological route can be a possible introduction of these begomoviruses and/or their associated DNA-satellites into Saudi Arabia.

Molecular characterization of two previously undescribed begomovirus-associated alphasatellite molecules infecting malvaceous species in Cameroon

Two begomovirus-associated alphasatellites were isolated from okra and a malvastrum plant (Malvaceae) in Cameroon. The complete nucleotide sequences of the okra- and malvastrum-infecting alphasatellites were 1375 and 1416-1418 nucleotides, respectively, and both exhibited features characteristic of other alphasatellites. Based on pairwise sequence comparisons, these previously undescribed alphasatellites are members of distinct species in the genera Colecusatellite and Gosmusatellite and have been tentatively named "pepper yellow vein Mali alphasatellite" and "cotton leaf curl Gezira alphasatellite3", respectively. Taken together with previous studies, alphasatellites endemic to Cameroon appear to be more diverse and infect plants of many more species and families than currently recognized.