The impact of parasitism on resource allocation in a fungal host: the case of Cryphonectria parasitica and its mycovirus, Cryphonectria Hypovirus 1

Parasites are known to profoundly affect resource allocation in their host. In order to investigate the effects of Cryphonectria Hypovirus 1 (CHV1) on the life-history traits of its fungal host Cryphonectria parasitica, an infection matrix was completed with the cross-infection of six fungal isolates by six different viruses. Mycelial growth, asexual sporulation, and spore size were measured in the 36 combinations, for which horizontal and vertical transmission of the viruses was also assessed. As expected by life-history theory, a significant negative correlation was found between host somatic growth and asexual reproduction in virus-free isolates. Interestingly this trade-off was found to be positive in infected isolates, illustrating the profound changes in host resource allocation induced by CHV1 infection. A significant and positive relationship was also found in infected isolates between vertical transmission and somatic growth. This last relationship suggests that in this system, high levels of virulence could be detrimental to the vertical transmission of the parasite. Those results underscore the interest of studying host-parasite interaction within the life-history theory framework, which might permit a more accurate understanding of the nature of the modifications triggered by parasite infection on host biology.

Multiple introductions and recombination in Cryphonectria hypovirus 1: perspective for a sustainable biological control of chestnut blight

Cryphonectria hypovirus 1 (CHV1) is a mycovirus which decreases the virulence of its fungal host Cryphonectria parasitica, the causal agent of chestnut blight recently introduced in Europe. The understanding of the evolutionary processes which have shaped CHV1 populations in Europe is required to develop a sustainable biocontrol strategy targeting chestnut blight and effective in European chestnut forests. To retrace the evolutionary history of CHV1, we analyzed sequences from two genomic regions on a collection of 55 CHV1 strains from France and northern Spain, two countries where multiple introductions of C. parasitica occurred. Several recombination events and variable selection pressures contributed to CHV1 evolution, agreeing with a non-clock-like diversification rate. These two mechanisms may be at the origin of CHV1 population diversity observed in western Europe. Considering the actual prevalence of CHV1 and its association with host genotypes, multiple introductions of CHV1 may have occurred in Europe, some of them directly from Asia and some of them through North America. Although some viral strains remained with low frequency in their introduction area, multiple infections might have allowed homologous recombination within parental sequences. Some of these recombinant lineages are associated with the spread of CHV1 in European regions.

Sigma virus and male reproductive success in Drosophila melanogaster

The risk of disease transmission can affect female mating rate, and thus sexual conflict. Furthermore, the interests of a sexually transmitted organism may align or diverge with those of either sex, potentially making the disease agent a third participant in the sexual arms race. In Drosophila melanogaster, where sexual conflict over female mating rate is well established, we investigated how a common, non-lethal virus (sigma virus) might affect this conflict. We gave uninfected females the opportunity to copulate twice in no-choice trials: either with two uninfected males, or with one male infected with sigma virus followed by an uninfected male. We assessed whether females respond behaviorally to male infection, determined whether male infection affects either female or male reproductive success, and measured offspring infection rates. Male infection status did not influence time to copulation, or time to re-mating. However, male infection did affect male reproductive success: first males sired a significantly greater proportion of offspring, as well as more total offspring, when they were infected with sigma virus. Thus viral infection may provide males an advantage in sperm competition, or, possibly, females may preferentially use infected sperm. We found no clear costs of infection in terms of offspring survival. Viral reproductive success (the number of infected offspring) was strongly correlated with male reproductive success. Further studies are needed to demonstrate whether virus-induced changes in reproductive success affect male and female lifetime fitness, and whether virus-induced changes are under male, female, or viral control.

Virulence evolution in a host-parasite system in the absence of viral evolution

QUESTION

How does virulence evolve in the Drosophila melanogaster/sigma virus (DMelSV) system?

ORGANISMS

Drosophila melanogaster (host) and DMelSV (parasite).

EMPIRICAL METHODS

Artificial selection on whole-carcass viral titre of infected flies, including two selection regimes (maternal and biparental transmission) and three treatments within each regime (increased titre, decreased titre, and control). The maternal transmission selection regime lasted for six generations, while the biparental transmission selection regime lasted for twelve generations. We further quantified virulence by estimating the fecundity, viability, and development time of infected flies. Finally, we sequenced virus strains at the end of selection.

PREDICTIONS AND CONCLUSIONS

Titre is defined here as the number of viral genomes inside a single fly, while virulence is defined as harm to host. We predicted that titre would respond to both increased and decreased selection, that virulence would evolve as a positively correlated response, and that sequence evolution in the viruses would be responsible for these changes. Titre did respond to selection in the biparental regime, although both high and control lines both demonstrated increased titre, while the titre of the low lines did not change. One component of virulence, development time, was positively correlated with titre in the biparental transmission lines (maternal transmission lines were not scored for virulence). However, we detected few (and in some cases, no) genomic changes in the virus, making viral evolution unlikely to be responsible for the response to selection and the association between development time and titre.

Mycovirus transmission revisited by in situ pairings of vegetatively incompatible isolates of Cryphonectria parasitica

In disease ecology, parasite transmission is a key parameter important at both epidemiological and evolutionary scales. Mycoviruses can be transmitted both horizontally and vertically. Their horizontal transmission is strongly restricted by the host vegetative compatibility system, which controls the outcome of somatic fusion in fungi, and by the same way, may limit mycovirus transmission. However, most of current knowledge and predictive capabilities regarding these host/pathogen systems are derived from studies pairing fungal mycelia on artificial medium. An original bioassay method, using infected mycelia as well as asexual spores, had been developed to assess in situ transmission of Cryphonectria Hypovirus-1 (CHV1), a mycovirus of Cryphonectria parasitica that causes chestnut blight. For every pair of different vegetative compatibility types tested, rates of CHV1 transmission were always superior in situ than in vitro. This study supports the hypothesis that the natural ability of CHV1 to migrate within a fungal population composed of different vegetative compatible types may have been underestimated by in vitro essays. This result offers opportunities for a biological control of fungal diseases with mycoviruses.

Parasitism and maintenance of diversity in a fungal vegetative incompatibility system: the role of selection by deleterious cytoplasmic elements

In fungi, horizontal transmission of deleterious cytoplasmic elements is reduced by the vegetative incompatibility system. This self/non-self recognition system may select for greater diversity of fungal incompatibility phenotypes in a frequency-dependent manner but the link between the diversity of fungal phenotypes and the virulence of cytoplasmic parasites has been poorly studied. We used an epidemiological model to show that even when transmission between incompatibility types is permitted, parasite pressure can lead to high levels of polymorphism for vegetative incompatibility systems. Moreover, high levels of polymorphism in host populations can select for less virulent cytoplasmic parasites. This feedback mechanism between parasite virulence and vegetative incompatibility system polymorphism of host populations may account for the general avirulence of most known mycoviruses. Furthermore, this mechanism provides a new perspective on the particular ecology and evolution of the host/parasite interactions acting between fungi and their cytoplasmic parasites.