Examining Phenotypic Traits Contributing to the Spread in Northern European Potato Crops of EU_41_A2, a New Clonal Lineage of Phytophthora infestans.

Until recently, genotypes of Phytophthora infestans were regionally distributed in Europe, with populations in western Europe being dominated by clonal lineages and those in northern Europe being genetically diverse because of frequent sexual reproduction. However, since 2013 a new clonal lineage (EU_41_A2) has successfully established itself and expanded in the sexually recombining P. infestans populations of northern Europe. The objective of this study was to study phenotypic traits of the new clonal lineage of P. infestans, which may explain its successful establishment and expansion within sexually recombining populations. Fungicide sensitivity, aggressiveness, and virulence profiles of isolates of EU_41_A2 were analyzed and compared with those of the local sexual populations from Denmark, Norway, and Estonia. None of the phenotypic data obtained from the isolates collected from Denmark, Estonia, and Norway independently explained the invasive success of EU_41_A2 within sexual Nordic populations. Therefore, we hypothesize that the expansion of this new genotype could result from a combination of fitness traits and more favorable environmental conditions that have emerged in response to climate change.

Exploring the causes of small effective population sizes in cyst nematodes using artificial Globodera pallida populations.

The effective size of a population is the size of an ideal population which would undergo genetic drift at the same rate as the real population. The balance between selection and genetic drift depends on the effective population size ( Ne), rather than the real numbers of individuals in the population ( N). The objectives of the present study were to estimate Ne in the potato cyst nematode Globodera pallida and to explore the causes of a low Ne/ N ratio in cyst nematodes using artificial populations. Using a temporal analysis of 24 independent populations, the median Ne was 58 individuals (min Ne = 25 and max Ne = 228). Ne is commonly lower than N but in the case of cyst nematodes, the Ne/ N ratio was extremely low. Using artificial populations showed that this low ratio did not result from migration, selection and overlapping generations, but could be explain by the fact that G. pallida populations deviate in structure from the assumptions of the ideal population by having unequal sex ratios, high levels of inbreeding and a high variance in family sizes. The consequences of a low Ne, resulting in a strong intensity of genetic drift, could be important for their control because G. pallida populations will have a low capacity to adapt to changing environments.

Functional Transcomplementation between Wheat Dwarf Virus Strains in Wheat and Barley.

Wheat dwarf virus, transmitted by the leafhopper Psammotettix alienus in a persistent, non-propagative manner, infects numerous species from the Poaceae family. Data associated with wheat dwarf virus (WDV) suggest that some isolates preferentially infect wheat while other preferentially infect barley. This allowed to define the wheat strain and the barley strain. There are contradictory results in the literature regarding the ability of each of these two strains to infect its non-preferred host. To improve knowledge on the interactions between WDV strains and barley and wheat, transmission experiments were carried out using barcoded P. alienus and an experimental design based on single/sequential acquisitions of WDV strains and on transmissions to wheat and barley. Results showed that (I) WDV strains are transmitted with similar efficiencies by P. alienus males, females and larvae, (II) WDV wheat and barley strains do not infect barley and wheat plants, respectively, and (III) a functional transcomplementation between the wheat and barley strains allows a mixed infection of barley and wheat. The described ability of each WDV strain to infect a non-host plant in the presence of the other viral strain must be considered to analyze data available on WDV host range.

The Effectiveness of Induced Defense Responses in a Susceptible Potato Genotype Depends on the Growth Rate of Phytophthora infestans.

Phytophthora infestans causes the devastating potato late blight disease, which is widely controlled with fungicides. However, the debate about chemical control is fueling a promotion toward alternative methods. In this context, the enhancement of natural plant immunity could be a strategy for more sustainable protection. We previously demonstrated that a concentrated culture filtrate (CCF) of P. infestans primes defense reactions in potato. They are genotype-dependent and metabolites produced decrease pathogen growth in vitro but not in vivo on tubers. Induced potato defenses are assumed to affect P. infestans life history traits depending on strains. This assumption was studied in vivo through induced leaflets on a susceptible genotype inoculated with four P. infestans strains differing for lesion growth rate. This study combines both defenses mechanistic analysis and ecological observations. Defense-gene expressions were thus assessed by quantitative reverse transcription-polymerase chain reaction; pathogen development was simultaneously evaluated by measuring necrosis, quantifying mycelial DNA, and counting sporangia. The results showed that CCF pretreatment reduced the pathogenicity differences between slow- and fast-growing strains. Moreover, after elicitation, PR-1, PR-4, PAL, POX, and THT induction was strain-dependent. These results suggest that P. infestans could develop different strategies to overcome plant defenses and should be considered in biocontrol and epidemic management of late blight.

Mating behavior of Psammotettix alienus (Hemiptera: Cicadellidae).

The Wheat dwarf virus, the causal agent of the wheat dwarf disease, is transmitted by leafhoppers from the genus Psammotettix and currently the main protection strategy is based on the use of insecticide treatments. Sustainable management strategies for insect vectors should include methods that are targeted to disrupt reproductive behavior and here we investigated the mating behavior of Psammotettix alineus (Dahlbom 1850) in order to determine the role of vibrational signals in intra-specific communication and pair formation. Both genders spontaneously emit species- and sex-specific calling songs that consisted of regularly repeated pulse trains and differ primarily in pulse train duration and pulse repetition time. Females preferred the conspecific male calling song. After a coordinated exchange of pulse trains, the male approached the stationary female. During the close range courtship and also immediately prior to copulatory attempts distinct male vibrational signals associated with wing flapping and wing vibrations were recorded from the substrate. In the presence of a receptive female, competing males emitted vibrational signals most likely aimed to interfere with male-female interaction. Mated females regained sexual receptivity after they laid eggs. Although results suggest that the viruliferous status of insects may have an effect on vibrational songs, our current results did not reveal a significant effect of virus on leafhopper performance in mating behavior. However, this study also suggests, that detailed understanding of plant-vector-virus interactions relevant for vector mating behavior is essential for trying new approaches in developing future control practices against plant viruses transmitted by insect vectors.

A Trade-Off Between Sporangia Size and Number Exists in the Potato Late Blight Pathogen Phytophthora infestans, and Is Not Altered by Biotic and Abiotic Factors.

The negative relationship between offspring size and number is a classic example of trade-off between life-history traits, reported many times in animal and plant species. Here, we wanted to ascertain whether such a trade-off occurred in the oomycete Phytophthora infestans, and whether it was impacted by biotic and abiotic factors. We thus conducted three infection experiments under controlled conditions and measured the number and the size of sporangia (asexual propagules) produced on potato by different P. infestans isolates. In all experiments, we observed a negative relationship between sporangia size and number, demonstrating the existence of a trade-off. Moreover, although the potato host cultivar, temperature and host of origin (tomato or potato) all affected sporangia number, sporangia size or both, none of these biotic and abiotic factors did change the trade-off. Therefore, the trade-off between sporangia size and number could maintain the polyphenism for these traits in P. infestans populations, and favors the coexistence of distinct reproductive strategies within this species. Our results emphasize the relevance to focus on the relationship between offspring size and number in other fungal plant pathogens, as well as to study the impact of offspring size on fitness-linked traits (virulence and disease lesion development) in these organisms.

The coexistence of generalist and specialist clonal lineages in natural populations of the Irish Famine pathogen Phytophthora infestans explains local adaptation to potato and tomato.

Phytophthora infestans, causing late blight on Solanaceae, is a serious threat to potato and tomato crops worldwide. P. infestans populations sampled on either potato or tomato differ in genotypes and pathogenicity, suggesting niche exclusion in the field. We hypothesized that such niche separation can reflect differential host exploitation by different P. infestans genotypes. We thus compared genotypes and phenotypes in 21 isolates sampled on potato (n = 11) or tomato (n = 10). Typing at 12 microsatellite loci assigned potato isolates to the 13_A2, 6_A1 and 1_A1 lineages, and tomato isolates to the 23_A1, 2_A1 and unclassified multilocus genotypes. Cross-inoculations on potato and tomato leaflets showed that all isolates were pathogenic on both hosts. However, tomato isolates performed much better on tomato than did potato isolates, which performed better on potato than did tomato isolates, thus revealing a clear pattern of local adaptation. Potato isolates were significantly fitter on potato than on tomato, and are best described as potato specialists; tomato isolates appear to be generalists, with similar pathogenicity on both hosts. Niche separation in the field may thus result mainly from the large fitness gap on tomato between generalists and unadapted potato specialists, while the small, but significant fitness difference on potato between both types of isolates may prevent population invasion by generalists. Extreme specialization to potato seems very costly relative to performance loss on the alternative host. This study therefore shows that local adaptation and niche separation, commonly expected to involve and generate specialists, can occur with generalists.

Local adaptation to temperature in populations and clonal lineages of the Irish potato famine pathogen Phytophthora infestans.

Environmental factors such as temperature strongly impact microbial communities. In the current context of global warming, it is therefore crucial to understand the effects of these factors on human, animal, or plant pathogens. Here, we used a common-garden experiment to analyze the thermal responses of three life-history traits (latent period, lesion growth, spore number) in isolates of the potato late blight pathogen Phytophthora infestans from different climatic zones. We also used a fitness index (FI) aggregating these traits into a single parameter. The experiments revealed patterns of local adaptation to temperature for several traits and for the FI, both between populations and within clonal lineages. Local adaptation to temperature could result from selection for increased survival between epidemics, when isolates are exposed to more extreme climatic conditions than during epidemics. We also showed different thermal responses among two clonal lineages sympatric in western Europe, with lower performances of lineage 13_A2 compared to 6_A1, especially at low temperatures. These data therefore stress the importance of thermal adaptation in a widespread, invasive pathogen, where adaptation is usually considered almost exclusively with respect to host plants. This must now be taken into account to explain, and possibly predict, the global distribution of specific lineages and their epidemic potential.