Maintenance and expansion of genetic and trait variation following domestication in a clonal crop.

Clonal propagation enables favourable crop genotypes to be rapidly selected and multiplied. However, the absence of sexual propagation can lead to low genetic diversity and accumulation of deleterious mutations, which may eventually render crops less resilient to pathogens or environmental change. To better understand this trade-off, we characterize the domestication and contemporary genetic diversity of Enset (Ensete ventricosum), an indigenous African relative of bananas (Musa) and a principal starch staple for 20 million Ethiopians. Wild enset reproduction occurs strictly by sexual outcrossing, but for cultivation, it is propagated clonally and associated with diversification and specialization into hundreds of named landraces. We applied tGBS sequencing to generate genome-wide genotypes for 192 accessions from across enset's cultivated distribution, and surveyed 1340 farmers on enset agronomic traits. Overall, reduced heterozygosity in the domesticated lineage was consistent with a domestication bottleneck that retained 37% of wild diversity. However, an excess of putatively deleterious missense mutations at low frequency present as heterozygotes suggested an accumulation of mutational load in clonal domesticated lineages. Our evidence indicates that the major domesticated lineages initially arose through historic sexual recombination associated with a domestication bottleneck, followed by the amplification of favourable genotypes through an extended period of clonal propagation. Among domesticated lineages, we found a significant phylogenetic signal for multiple farmer-identified food, nutrition and disease resistance traits and little evidence of contemporary recombination. The development of future-climate adapted genotypes may require crop breeding, but outcrossing risks exposing deleterious alleles as homozygotes. This trade-off may partly explain the ubiquity and persistence of clonal propagation over recent centuries of comparative climate stability.

Europe as a bridgehead in the worldwide invasion history of grapevine downy mildew, Plasmopara viticola.

Europe is the historical cradle of viticulture, but grapevines (Vitis vinifera) have been increasingly threatened by pathogens of American origin. The invasive oomycete Plasmopara viticola causes downy mildew, one of the most devastating grapevine diseases worldwide. Despite major economic consequences, its invasion history remains poorly understood. We analyzed a comprehensive dataset of ∼2,000 samples, collected from the most important wine-producing countries, using nuclear and mitochondrial gene sequences and microsatellite markers. Population genetic analyses revealed very low genetic diversity in invasive downy mildew populations worldwide and little evidence of admixture. All the invasive populations originated from only one of the five native North American lineages, the one parasitizing wild summer grape (V. aestivalis). An approximate Bayesian computation-random forest approach allowed inferring the worldwide invasion scenario of P. viticola. After an initial introduction into Europe, invasive European populations served as a secondary source of introduction into vineyards worldwide, including China, South Africa, and twice independently, Australia. Only the invasion of Argentina probably represents a tertiary introduction, from Australia. Our findings provide a striking example of a global pathogen invasion resulting from secondary dispersal of a successful invasive population. Our study will also help designing quarantine regulations and efficient breeding for resistance against grapevine downy mildew.

Identification of the First Oomycete Mating-type Locus Sequence in the Grapevine Downy Mildew Pathogen, Plasmopara viticola.

Mating types are self-incompatibility systems that promote outcrossing in plants, fungi, and oomycetes. Mating-type genes have been widely studied in plants and fungi but have yet to be identified in oomycetes, eukaryotic organisms closely related to brown algae that cause many destructive animal and plant diseases. We identified the mating-type locus of Plasmopara viticola, the oomycete responsible for grapevine downy mildew, one of the most damaging grapevine diseases worldwide. Using a genome-wide association approach, we identified a 570-kb repeat-rich non-recombining region controlling mating types, with two highly divergent alleles. We showed that one mating type was homozygous, whereas the other was heterozygous at this locus. The mating-type locus encompassed 40 genes, including one encoding a putative hormone receptor. Functional studies will, however, be required to validate the function of these genes and find the actual determinants of mating type. Our findings have fundamental implications for our understanding of the evolution of mating types, as they reveal a unique determinism involving an asymmetry of heterozygosity, as in sex chromosomes and unlike other mating-type systems. This identification of the mating-type locus in such an economically important crop pathogen also has applied implications, as outcrossing facilitates rapid evolution and resistance to harsh environmental conditions.

A High-Quality Grapevine Downy Mildew Genome Assembly Reveals Rapidly Evolving and Lineage-Specific Putative Host Adaptation Genes.

Downy mildews are obligate biotrophic oomycete pathogens that cause devastating plant diseases on economically important crops. Plasmopara viticola is the causal agent of grapevine downy mildew, a major disease in vineyards worldwide. We sequenced the genome of Pl. viticola with PacBio long reads and obtained a new 92.94 Mb assembly with high contiguity (359 scaffolds for a N50 of 706.5 kb) due to a better resolution of repeat regions. This assembly presented a high level of gene completeness, recovering 1,592 genes encoding secreted proteins involved in plant-pathogen interactions. Plasmopara viticola had a two-speed genome architecture, with secreted protein-encoding genes preferentially located in gene-sparse, repeat-rich regions and evolving rapidly, as indicated by pairwise dN/dS values. We also used short reads to assemble the genome of Plasmopara muralis, a closely related species infecting grape ivy (Parthenocissus tricuspidata). The lineage-specific proteins identified by comparative genomics analysis included a large proportion of RxLR cytoplasmic effectors and, more generally, genes with high dN/dS values. We identified 270 candidate genes under positive selection, including several genes encoding transporters and components of the RNA machinery potentially involved in host specialization. Finally, the Pl. viticola genome assembly generated here will allow the development of robust population genomics approaches for investigating the mechanisms involved in adaptation to biotic and abiotic selective pressures in this species.

Soft selective sweeps in fungicide resistance evolution: recurrent mutations without fitness costs in grapevine downy mildew.

Adaptation produces hard or soft selective sweeps depending on the supply of adaptive genetic polymorphism. The evolution of pesticide resistance in parasites is a striking example of rapid adaptation that can shed light on selection processes. Plasmopara viticola, which causes grapevine downy mildew, forms large populations, in which resistance has rapidly evolved due to excessive fungicide use. We investigated the pathways by which fungicide resistance has evolved in this plant pathogen, to determine whether hard or soft selective sweeps were involved. An analysis of nucleotide polymorphism in 108 field isolates from the Bordeaux region revealed recurrent mutations of cytb and CesA3 conferring resistance to quinone outside inhibiting (QoI) and carboxylic acid amide (CAA) fungicides, respectively. Higher levels of genetic differentiation were observed for nucleotide positions involved in resistance than for neutral microsatellites, consistent with local adaptation of the pathogen to fungicide treatments. No hitchhiking was found between selected sites and neighbouring polymorphisms in cytb and CesA3, confirming multiple origins of resistance alleles. We assessed resistance costs, by evaluating the fitness of the 108 isolates through measurements of multiple quantitative pathogenicity traits under controlled conditions. No significant differences were found between sensitive and resistant isolates, suggesting that fitness costs may be absent or negligible. Our results indicate that the rapid evolution of fungicide resistance in P. viticola has involved a soft sweep.

Past and present dynamics of sorghum and pearl millet diversity in Mount Kenya region.

Crop populations in smallholder farming systems are shaped by the interaction of biological, ecological, and social processes, occurring on different spatiotemporal scales. Understanding these dynamics is fundamental for the conservation of crop genetic resources. In this study, we investigated the processes involved in sorghum and pearl millet diversity dynamics on Mount Kenya. Surveys were conducted in ten sites distributed along two elevation transects and occupied by six ethnolinguistic groups. Varieties of both species grown in each site were inventoried and characterized using SSR markers. Genetic diversity was analyzed using both individual- and population-based approaches. Surveys of seed lot sources allowed characterizing seed-mediated gene flow. Past sorghum diffusion dynamics were explored by comparing Mount Kenya sorghum diversity with that of the African continent. The absence of structure in pearl millet genetic diversity indicated common ancestry and/or important pollen- and seed-mediated gene flow. On the contrary, sorghum varietal and genetic diversity showed geographic patterns, pointing to different ancestry of varieties, limited pollen-mediated gene flow, and geographic patterns in seed-mediated gene flow. Social and ecological processes involved in shaping seed-mediated gene flow are further discussed.

Draft Genome Sequence of Plasmopara viticola, the Grapevine Downy Mildew Pathogen.

Plasmopara viticola is a biotrophic pathogenic oomycete responsible for grapevine downy mildew. We present here the first draft of the P. viticola genome. Analysis of this sequence will help in understanding plant-pathogen interactions in oomycetes, especially pathogen host specialization and adaptation to host resistance.