From generalist to specialists: Variation in the host range and performance of anther-smut pathogens on Dianthus

Host specificity in the fungal plant parasite Anthracoidea sempervirentis (Anthracoideaceae, Ustilaginales) reveals three new species and indicates a potential split in the host plant Carex sempervirens

The smut fungal genus Anthracoidea contains more than 100 species that parasitize hosts predominantly in the sedge genus Carex. Anthracoidea species are mainly found in the boreal zones of the Northern Hemisphere and many species have an arctic-alpine distribution. Recent re-organization of the taxonomy of the main host genus Carex questions current understanding of host associations in Anthracoidea. Host specificity for many of the species in this genus is considered to be quite broad and a host spectrum of over 10 host species is common. One aim of the study is to understand the potential influence that host taxonomy has on the evolutionary patterns of Anthracoidea. Additionally, by including more specimens, we clarify host specificity and species delimitation in Anthracoidea sempervirentis, a prevalent species occurring on different host species in different Carex subgroups using molecular data. Host colonization patterns within Anthracoidea are complex, and different subclades of Carex have been colonized several times independently, whereas clades of related Anthracoidea species often occur on Carex species from the same host clade. Parasites previously thought to be Anthracoidea sempervirentis occurring on the different Carex host are shown to be at least four distinct species that are restricted to individual host species. Three new species, Anthracoidea ferrugineae on Carex ferruginea from the Alps and the Carpathians, A. firmae on Carex firma from the Alps, and A. kitaibelianae on Carex kitaibeliana from mountains in the Balkan Peninsula, are described and illustrated. An emended description of Anthracoidea sempervirentis is also provided. Anthracoidea sempervirentis in its emended circumscription consists of two clades that correspond to respective clades within Carex sempervirens. The study shows that host colonization in Anthracoidea is more complex than current host taxonomy suggests. Further, including several specimens per host species results in a much higher diversity within Anthracoidea than previously assumed. Citation: Kemler M, Denchev TT, Feige A, Denchev CM, Begerow D (2024). Host specificity in the fungal plant parasite Anthracoidea sempervirentis (Anthracoideaceae, Ustilaginales) reveals three new species and indicates a potential split in the host plant Carex sempervirens. Fungal Systematics and Evolution 13: 91-110. doi: 10.3114/fuse.2024.13.04.

Genomic factors shape carbon and nitrogen metabolic niche breadth across Saccharomycotina yeasts

Organisms exhibit extensive variation in ecological niche breadth, from very narrow (specialists) to very broad (generalists). Two general paradigms have been proposed to explain this variation: (i) trade-offs between performance efficiency and breadth and (ii) the joint influence of extrinsic (environmental) and intrinsic (genomic) factors. We assembled genomic, metabolic, and ecological data from nearly all known species of the ancient fungal subphylum Saccharomycotina (1154 yeast strains from 1051 species), grown in 24 different environmental conditions, to examine niche breadth evolution. We found that large differences in the breadth of carbon utilization traits between yeasts stem from intrinsic differences in genes encoding specific metabolic pathways, but we found limited evidence for trade-offs. These comprehensive data argue that intrinsic factors shape niche breadth variation in microbes.

Alternative host shapes transmission and life-history trait correlations in a multi-host plant pathogen

Most pathogens are generalists capable of infecting multiple host species or strains. Trade-offs in performance among different hosts are expected to limit the evolution of generalism. Despite the commonness of generalism, the variation in infectivity, transmission, and trade-offs in performance among host species have rarely been studied in the wild. To understand the ecological and evolutionary drivers of multi-host pathogen infectivity and transmission potential, I studied disease severity, transmission dynamics, and infectivity variation of downy mildew pathogen Peronospora sparsa on its three host plants Rubus arcticus, R. chamaemorus, and R. saxatilis. In a survey of 20 wild and cultivated sites of the three host species, disease severity varied by host species and by host population size but not among wild and cultivated sites. To understand how alternative host presence and plant diversity affect transmission of the pathogen, I conducted a transmission experiment. In this experiment, alternative host abundance and plant diversity together modified P. sparsa transmission to trap plants. To understand how resistance to P. sparsa varies among host species and genotypes, I conducted an inoculation experiment using 10 P. sparsa strains from different locations and 20 genotypes of the three host species. Significant variation in infectivity was found among host genotypes but not among host species. When trade-offs for infectivity were tested, high infectivity in one host species correlated with high infectivity in another host species. However, when pathogen transmission-related life-history correlations were tested, a positive correlation was found in R. arcticus but not in R. saxatilis. The results suggest that host resistance may shape pathogen life-history evolution with epidemiological consequences in a multi-host pathogen.

Host density shapes the relative contribution of vector-based and aerial transmission of a pathogenic fungus

Pathogen transmission mode is a key determinant of epidemiological outcomes. Theory shows that host density can influence the spread of pathogens differentially depending on their mode of transmission. Host density could therefore play an important role in determining the pathogen transmission mode. We tested theoretical expectations using floral arrays of the alpine carnation Dianthus pavonius in field experiments of spore dispersal of the anther-smut fungus, Microbotryum, by vector (pollinator)-based floral transmission and passive aerial transmission at a range of host densities. Pollinators deposited fewer spores per plant at high host density than at lower density (ranging from a 0.2-2 m spacing between plants), and vector-based spore deposition at higher densities declined more steeply with distance from diseased plant sources. In contrast, while aerial spore deposition declined with distance from the diseased source, the steepness of this decline was independent of host density. Our study indicates that the amount and distance of vector-based transmission are likely to be a nonmonotonic function of host density as a result of vector behavior, which is not readily encapsulated by fixed dispersal functions. We conclude that the spatial spread of pathogens by vectors is likely to be greater at lower and intermediate densities, whereas the spatial spread of aerially transmitted pathogens would be greater at high densities. These contrasting patterns could lead to differential importance of each transmission mode in terms of its contribution to subsequent infections across host densities.

Effective specialist or jack of all trades? Experimental evolution of a crop pest in fluctuating and stable environments

Understanding pest evolution in agricultural systems is crucial for developing effective and innovative pest control strategies. Types of cultivation, such as crop monocultures versus polycultures or crop rotation, may act as a selective pressure on pests' capability to exploit the host's resources. In this study, we examined the herbivorous mite Aceria tosichella (commonly known as wheat curl mite), a widespread wheat pest, to understand how fluctuating versus stable environments influence its niche breadth and ability to utilize different host plant species. We subjected a wheat-bred mite population to replicated experimental evolution in a single-host environment (either wheat or barley), or in an alternation between these two plant species every three mite generations. Next, we tested the fitness of these evolving populations on wheat, barley, and on two other plant species not encountered during experimental evolution, namely rye and smooth brome. Our results revealed that the niche breadth of A. tosichella evolved in response to the level of environmental variability. The fluctuating environment expanded the niche breadth by increasing the mite's ability to utilize different plant species, including novel ones. Such an environment may thus promote flexible host-use generalist phenotypes. However, the niche expansion resulted in some costs expressed as reduced performances on both wheat and barley as compared to specialists. Stable host environments led to specialized phenotypes. The population that evolved in a constant environment consisting of barley increased its fitness on barley without the cost of utilizing wheat. However, the population evolving on wheat did not significantly increase its fitness on wheat, but decreased its performance on barley. Altogether, our results indicated that, depending on the degree of environmental heterogeneity, agricultural systems create different conditions that influence pests' niche breadth evolution, which may in turn affect the ability of pests to persist in such systems.

Can disease resistance evolve independently at different ages? Genetic variation in age-dependent resistance to disease in three wild plant species

Juveniles are typically less resistant (more susceptible) to infectious disease than adults, and this difference in susceptibility can help fuel the spread of pathogens in age-structured populations. However, evolutionary explanations for this variation in resistance across age remain to be tested.One hypothesis is that natural selection has optimized resistance to peak at ages where disease exposure is greatest. A central assumption of this hypothesis is that hosts have the capacity to evolve resistance independently at different ages. This would mean that host populations have (a) standing genetic variation in resistance at both juvenile and adult stages, and (b) that this variation is not strongly correlated between age classes so that selection acting at one age does not produce a correlated response at the other age.Here we evaluated the capacity of three wild plant species (Silene latifolia, S. vulgaris and Dianthus pavonius) to evolve resistance to their anther-smut pathogens (Microbotryum fungi), independently at different ages. The pathogen is pollinator transmitted, and thus exposure risk is considered to be highest at the adult flowering stage.Within each species we grew families to different ages, inoculated individuals with anther smut, and evaluated the effects of age, family and their interaction on infection.In two of the plant species, S. latifolia and D. pavonius, resistance to smut at the juvenile stage was not correlated with resistance to smut at the adult stage. In all three species, we show there are significant age × family interaction effects, indicating that age specificity of resistance varies among the plant families. Synthesis. These results indicate that different mechanisms likely underlie resistance at juvenile and adult stages and support the hypothesis that resistance can evolve independently in response to differing selection pressures as hosts age. Taken together our results provide new insight into the structure of genetic variation in age-dependent resistance in three well-studied wild host-pathogen systems.

Digest: Evolutionary dynamics of specialization of a fungal pathogen

Pathogen specialization may not always arise due to an adaptive trade-off between infecting one or many species. Bruns et al. characterized the potential host range of four lineages of the fungal pathogen Microbotryum on three Dianthus species. They found that both generalists and specialists co-occurred in nature and no clear fitness benefit is found for either. They suggested that specialization may be related to the specific geographic distribution of these plants.