Is it possible to be better than wild type TYLCV?

Begomoviruses (Family Geminiviridae) are highly recombinogenic ssDNA viruses. Two begomovirus infectious clones of Tomato yellow leaf curl and Tomato leaf curl Mayotte viruses (Tyx, Tox; 18% nt divergence) were inoculated in 6 tomato plants. Recombinants were isolated from 30 days post inoculation (dpi). At 150 and 330 dpi more than 40% of the isolated genomes were recombinants. Unexpectedly, Tyx the most fit of the two parents tend to be eliminated at 330dpi. The most frequently isolated recombinant (R4) was prepared as an infectious clone and tested for its fitness in coinfection with Tyx and Tox. Its infectivity (% of infected plants) was significantly higher than Tox, the parent with the lowest infectivity, but similar to Tyx. Viral DNA accumulation of R4 was 10 times lower than Tyx, the parent which accumulated the most. It is proposed that complementation between several recombinants may eliminate TYLCV but possibly none of them individually may be better. (Texte intégral

The viral content ratio between abdomen and head is informative of the relative efficiency with which Bemisia tabaci populations transmit begomoviruses. [P.12]

Begomoviruses (family Geminiviridae) are transmitted by the whitefly Bemisia tabaci in a circulative non propagative manner. B. tabaci is a species complex composed of at least 24 morphocryptic species which differ in host range, insecticide resistance, endosymbionts and virus transmission. Begomoviruses are supposed to cross the gut barrier at the midgut level and salivary gland barrier in the principal salivary gland (PSG) cells because of the highest virus concentrations in these organs. Thus, the critical steps of the virus circulation in the insect body are (i) the exit of virion from the midgut, (ii) their preservation in the hemocoel and (iii) their entry in the PSG. Thus, we proposed that the efficiency of viral transfer from midgut to PSG may be assessed by measuring the viral content in both compartments and that the deduced viral content ratio may be correlated to viral transmission efficiency by the vector. Our predicition was tested with two invasive B. tabaci species, Middle East-Minor Asia 1 (MEAM1), and Mediterranean (MED), and three begomoviruses: the invasive species Tomato yellow leaf curl virus-Mild (TYLCV-Mld), Tomato leaf curl Comoros virus (ToLCKMV), indigenous from Mayotte and R4, a recombinant between TYLCV and ToLCKMV. In a first approach, PSG and midgut were separated by a cross section through the prothorax and viral loads were estimated in both sections by measuring viral DNA using real time PCR. As the midgut of B. tabaci was reported to be sometimes pushed through the diaphragm separating the abdomen and the thorax, the estimation of the viral content ratio between PSG and midgut may be biased by thorax sectioning. The simple cross sectioning was however validated because the ratio determined with such sections and the ratio determined after a careful gut dissection was similar. Using the simple cross section, the viral content ratio between head and abdomen was higher for MEAM1 than for Med for the three begomoviruses. As predicted, the transmission efficiency was higher with MEAM1 than Med Q2 for the three viruses. These results indicate that viral content ratio may be a reliable predictor of the relative transmission efficiency between different B. tabaci populations. Measuring transmission efficiency is time consuming, involves technically difficult experiments with acquisition and inoculation steps and needs specialized cage and containment equipment. However measuring viral content ratios needs only a few cages for the acquisition step, a binocular lens and an access to the commonly used qPCR machines. This approach might be extended to estimate the relative transmission efficiency of other circulative non propagative viruses. (Résumé d'auteur

Is the assistance of satellite by TYLCV strictly cell autonomous?

Begomoviruses are circular single stranded DNA (css) plant viruses with bipartite (A and B) or monopartite (A-like component) genomes. They are sometimes associated with satellites, cssDNA molecules, namely alphasatellites and betasatellites. Like the B component of bipartite begomoviruses, satellites depend on the A or A-like component for their replication (betasatellite) and encapsidation (alphasatellites and betasatellites). Although Tomato yellow leaf curl virus (TYLCV) was only rarely reported with satellites, alphasatellites and betasatellites of various geographic origins are readily assisted by TYLCV in experimental conditions. This result was consistent with the observation that satellite DNA contents were mostly higher than that of TYLCV (Conflon et al., 2018). The ease with which satellites can be assisted with TYLCV was supported further by FISH observations, which showed that the frequency of TYLCV-infected cells that were co-infected with a satellite exceeded 85% for an alphasatellite, and 95% for a betasatellite. Interestingly, a substantial number of cell nuclei were positive only for the satellite, suggesting that the assistance seems to be possible, even with a low amount of TYLCV DNA, and possibly no TYLCV DNA. This later possibility that need to be confirmed with further tests, is according to the “multicellular way of life” theory proposed recently for Faba bean necrotic stunt virus, a multipartite nanovirus with eight separately encapsidated components (Sicard et al.,2019)

Plant resistance-driven emergence of recombinant begomoviruses

The analysis of plant virus genomes reveals that many were shaped by recombination. However, the history of the emergence dynamics of these recombinants is mostly unknown as well as the underlying evolutionary forces that drove their frequency increase. The pivotal role of recombination in geminivirus evolution is supported by the detection of numerous recombination events in sequence data, and by their high propensity to recombine. These typical features were observed with Tomato yellow leaf curl virus (TYLCV), a tomato begomovirus that was extensively studied because of its global economic importance. TYLCV-IS76 is a recombinant TYLCV detected initially in Morocco. It inherited a 76-nt region of tomato yellow leaf curl Sardinia virus (TYLCSV) starting from the origin of replication (OR) towards the V2 gene. Based on field surveys carried out in Morocco and laboratory analysis, a real time emergence of TYLCV-IS76 has been reconstructed from its generation to the displacement of its parental viruses (1). Its emergence coincided with the deployment of Ty-1 resistant tomato cultivars, and a causal link was demonstrated with various competition tests in which positive selection of TYLCV-IS76 was observed in Ty-1 resistant plants (2). TYLCV-IS141 is a TYLCV recombinant detected in Italy (1, 3, 4, 5). It inherited a 141-nt region of TYLCSV between OR and the initiation codon of the V2 gene. TYLCV-IS141 and TYLCV-IS76 exhibit similar recombination profiles and fitness phenotypes in Ty-1 resistant plants. It was inferred from competition tests carried out with various natural and artificially generated TYLCV-IS76 and TYLCV-IS141 clones, that the fitness phenotype of these recombinants was determined by new beneficial intra genomic interaction rather than by a direct effect of specific mutations. Gene silencing is suspected to be involved in the positive selection of these recombinants because Ty-1 is a RNA dependent RNA polymerase gene

Assessing the possible maintenance of TYLCV-satellite association. [O.21]

Viruses of the genus Begomovirus (Family Geminiviridae) are frequently detected with half genome size DNA molecules, either defective DNAs or satellite DNA (? or ?). Whereas some begomoviruses, like Tomato yellow leaf curl virus (TYLCV) were never detected with satellite DNAs, other begomoviruses, like Cotton leaf curl virus (CLCuV), depend upon a betasatellite for their infectivity. Besides the CLCuV-type begomoviruses which may be considered as bipartite begomoviruses, most of the begomoviruses detected with satellites were shown to be infective without their satellites. The alphasatellite was rarely proved to have any impact on the helper virus but the betasatellite was often shown to increase the virulence of its helper virus. Although satellites were never detected with TYLCV in natural conditions, TYLCV was reported as a helper virus for both satellites in artificial conditions and its virulence was dramatically increased when co-inoculated with betasatellites. We have confirmed these results with the Cotton leaf curl Gezira betasatellite (CLCuGB) and two alphasatellites, Cotton leaf curl Gezira alphasatellite (CLCuGA) and Okra leaf curl Burkina Faso alphasatellite (OLCA). If the co-infection of TYLCV and a betasatellite would occur in natural conditions, tomato production may be severely affected. As the probability of such a scenario mainly depends on the maintenance of TYLCV-satellite associations over time, we have studied various factors potentially determining this maintenance: (i) the relative intra-plant accumulation of TYLCV and the satellites, (ii) the cellular co-infection level of TYLCV and satellites, and (iv) the transmission efficiency of satellites by the vector Bemisia tabaci. These various factors were analyzed with CLCuGB, CLCuGA and OLCA. Besides the specific question of the possible maintenance of satellites with TYLCV, the results of our study are expected to provide a new insight on begomoviruses detected in co-infection with satellites in natural conditions, but which were proved to be infectious without satellites. (Résumé d'auteur

Identification des plantes hôtes de Bemisia tabaci (Gennadius 1889) au Maroc et évaluation du risque d'infection de la tomate par le TYLCV à partir de quelques espèces adventices

Les virus responsables de la maladie des feuilles jaunes en cuillère de la tomate sont transmis par l'aleurode Bemisia tabaci. Des prospections ont été réalisées dans la région de Berkane, Sais, Gharb et Agadir pour dresser l'inventaire des plantes hôtes de ce vecteur. Cette étude a permis d'identifier 64 espèces de plantes hôtes, appartenant à 21 familles différentes. Par ailleurs, l'évaluation de l'importance des plantes adventices en tant que source d'inoculum viral a été vérifiée sur la base d'un essai de transmission du virus à la tomate à partir des plantes adventices. Un taux de transmission de 100 % a été obtenu dans le cas de Malva parviflora L. et Datura stramonium L. 80 % dans le cas de Sonchus oleraceus L., 60 % dans le cas Conyza canadensis L. et 20 % dans le cas de Chenopodium murale L. Le risque d'infection des tomates à partir de quelques espèces adventices hôtes du virus et de l'insecte a ainsi été démontré expérimentalement. Ce résultat confirme que ces hôtes pourraient jouer un rôle dans le maintien de l'inoculum viral dans les régions où la tomate n'est pas cultivée toute l'année. (Résumé d'auteur

TYLCV-IS76 and TYLCV-IS141: fraternal but not identical tomato yellow leaf curl twin recombinant viruses

Tomato yellow leaf curl virus (TYLCV), tomato yellow leaf curl Sardinia virus (TYLCSV) and TYLCV/TYLCSV recombinants (Begomovirus, Geminiviridae) are responsible of the damaging tomato leaf curl disease (TYLCD) in the Mediterranean. Tomato cultivars having the Ty-1 resistance gene were deployed in the Mediterranean Basin in the 2000s. Plants of those cultivars prevent symptom expression and reduce virus accumulation. The shift from susceptible to Ty-1 resistant cultivars in Morocco coincided with the invasion of a peculiar TYLCV/TYLCSV recombinant virus (TYLCV-IS76) discovered in 2010 in symptomatic resistant plants (1). It replaced the parental-type viruses in the South of Morocco and spread to the whole country. The selective advantage of TYLCV-IS76 assessed experimentally in comparison with parental type viruses is consistent with a selection driven emergence (2). According to phylogenetic analysis, and the recombination profiles of recombinants generated in tomato plants co-inoculated with TYLCV and TYLCSV, TYLCV-IS76 has emerged through a rare process (3). TYLCV-IS141, another peculiar TYLCV/TYLCSV exhibit some but not all the features of TYLCV-IS76. Like TYLCV-IS76 it inherited a very short TYLCSV fragment of about 120nts, it was detected on symptomatic Ty-1 resistant plants and exhibit a selective advantage over parental viruses in experimental conditions. However, unlike TYLCV-IS76, TYLCV-IS141 was detected in Italy (1, 4, 5) and was found to be generated and positively selected in experimental conditions (3). The sampling effort is presently too low to know if TYLCV-IS141 has the same invasion potential as TYLCV-IS76. Both recombinants will be useful to identify the determinants of their similar competitiveness