Differences In Host Range Research Articles

Mouse innate resistance to Neospora caninum infection is driven by early production of IFNγ by NK cells in response to parasite ligands.

Pathogen differences in host range are poorly understood at the molecular level even though even closely related pathogen species can have dramatically distinct host ranges. Here, we study two related parasite species that have a dramatic difference in their ability to infect mice. Here, we show that soluble proteins from these species determine one driver of this difference: induction of interferon gamma by cells of the innate immune system.

A novel genus of Pectobacterium bacteriophages display broad host range by targeting several species of Danish soft rot isolates

The bacterial diseases black leg and soft rot in potatoes cause heavy losses of potatoes worldwide. Bacteria within the genus Pectobacteriaceae are the causative agents of black leg and soft rot. The use of antibiotics in agriculture is heavily regulated and no other effective treatment currently exists, but bacteriophages (phages) have shown promise as potential biocontrol agents. In this study we isolated soft rot bacteria from potato tubers and plant tissue displaying soft rot or black leg symptoms collected in Danish fields. We then used the isolated bacterial strains as hosts for phage isolation. Using organic waste, we isolated phages targeting different species within Pectobacterium. Here we focus on seven of these phages representing a new genus primarily targeting P. brasiliense; phage Ymer, Amona, Sabo, Abuela, Koroua, Taid and Pappous. TEM image of phage Ymer showed siphovirus morphotype, and the proposed Ymer genus belongs to the class Caudoviricetes, with double-stranded DNA genomes varying from 39 kb to 43 kb. In silico host range prediction using a CRISPR-Cas spacer database suggested both P. brasiliense, P. polaris and P. versatile as natural hosts for phages within the proposed Ymer genus. A following host range experiment, using 47 bacterial isolates from Danish tubers and plants symptomatic with soft rot or black leg disease verified the in silico host range prediction, as the genus as a group were able to infect all three Pectobacterium species. Phages did, however, primarily target P. brasiliense isolates and displayed differences in host range even within the species level. Two of the phages were able to infect two or more Pectobacterium species. Despite no nucleotide similarity with any phages in the NCBI database, the proposed Ymer genus did share some similarity at the protein level, as well as gene synteny, with currently known phages. None of the phages encoded integrases or other genes typically associated with lysogeny. Similarly, no virulence factors nor antimicrobial resistance genes were found, and combined with their ability to infect several soft rot-causing Pectobacterium species from Danish fields, demonstrates their potential as biocontrol agents against soft rot and black leg diseases in potatoes.

Genomic analysis of Salmonella bacteriophages revealed multiple endolysin ORFs and importance of ligand binding site of receptor binding protein.

Salmonella is a prevalent foodborne pathogen causing millions of global cases annually. Antimicrobial resistance is a growing public health concern, leading to search for alternatives like bacteriophages. 97 bacteriophages, isolated from cattle farms (n= 48), poultry farms (n=37), and wastewater (n=5) samples in Türkiye, were subjected to host range analysis using 36 Salmonella isolates with 18 different serotypes. The broadest host range belonged to an Infantis phage (MET P1-091), lysing 28 hosts. Ten phages with the widest host range underwent further analysis, revealing 7 unique genomes (32 to 243kb), including a jumbophage (>200kb). Except for one with lysogenic properties, none of them harbored virulence or antibiotic resistance genes, making them potential Salmonella reducers in different environments. Examining ORFs of endolysin enzymes revealed surprising findings: 5 of 7 unique genomes contained multiple endolysin ORFs. Despite sharing same endolysin sequences, phages exhibited significant differences in host range. Detailed analysis unveiled diverse receptor binding protein sequences, with similar structures but distinct ligand binding sites. These findings emphasize the importance of ligand binding sites of receptor binding proteins. Additionally, bacterial reduction curve and virulence index revealed that Enteritidis phages inhibit bacterial growth even at low concentrations, unlike Infantis and Kentucky phages.

Emergence of Poultry-Associated Human Salmonella enterica Serovar Abortusovis Infections, New South Wales, Australia.

Salmonella enterica serovar Abortusovis is a ovine-adapted pathogen that causes spontaneous abortion. Salmonella Abortusovis was reported in poultry in 2009 and has since been reported in human infections in New South Wales, Australia. Phylogenomic analysis revealed a clade of 51 closely related isolates from Australia originating in 2004. That clade was genetically distinct from ovine-associated isolates. The clade was widespread in New South Wales poultry production facilities but was only responsible for sporadic human infections. Some known virulence factors associated with human infections were only found in the poultry-associated clade, some of which were acquired through prophages and plasmids. Furthermore, the ovine-associated clade showed signs of genome decay, but the poultry-associated clade did not. Those genomic changes most likely led to differences in host range and disease type. Surveillance using the newly identified genetic markers will be vital for tracking Salmonella Abortusovis transmission in animals and to humans and preventing future outbreaks.

Evolutionary analysis and biological characterization of a novel alphabaculovirus isolated from Mythimna separata.

Baculoviruses are insect-specific pathogens. Novel baculovirus isolates provide new options for the biological control of pests. Therefore, research into the biological characteristics of newly isolated baculoviruses, including accurate classification and nomenclature, is important. In this study, a baculovirus was isolated from Mythimna separata and its complete genome sequence was determined by next-generation sequencing. The double-stranded DNA genome was 153 882 bp in length, encoding 163 open reading frames. The virus was identified as a variant of Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and designated Mamestra brassicae multiple nucleopolyhedrovirus CHN1 (MbMNPV-CHN1) according to ultrastructural analysis, genome comparison and phylogenetic analysis. Phylogenetic inference placed MbMNPV-CHN1 in a clade containing isolates of MacoNPV-A, MacoNPV-B and MbMNPV, which we have designated the Mb-McNPV group. The genomes of isolates in the Mb-McNPV group exhibited a high degree of collinearity with relatively minor differences in the content of annotated open reading frames. The development of codon usage bias in the Mb-McNPV group was affected mainly by natural selection. MbMNPV-CHN1 shows high infectivity against seven species of Lepidoptera. The yield of MbMNPV-CHN1 in the fourth- and fifth-instar M. separata larvae was 6.25×109-1.23×1010 OBs/cadaver. Our data provide insights into the classification, host range and virulence differences among baculoviruses of the Mb-McNPV group, as well as a promising potential new baculoviral insecticide.

Host range differences between two species of freshwater horsehair worm (Nematomorpha: Chordiidae) Chordodes japonensis and C. formosanus in Japan

Horsehair worms (Nematomorpha: Gordioidea) are endoparasites of terrestrial arthropods, of which two species, Chordodes japonensis and C. formosanus, may have been historically confused due to their morphological similarity. In this study, we conducted field surveys and laboratory parasite inoculation experiments to clarify the characteristics and differences in host range and specificity between the two species. The field surveys revealed that the host ranges of the two species are markedly different. C. formosanus parasitized diverse mantids, while C. japonensis infected only species in the genus Tenodera, as predicted in previous studies. However, the two species had one mantid host, T. angustipennis in common. Parasite inoculation experiments using three species of mantids, revealed parasitism in each of the host-parasite combinations same as in the field study. These results suggest that the observed differences in host patterns are due to physiological factors between the host and parasite. Furthermore, cross-testing conducted on the shared host T. angustipennis showed that the two species coexisted within the same host in some cases, suggesting that the competitive exclusivity of the two species is low. This study also presents a tentative list of host ranges for the two species, comparing and integrating information from this and previous studies. However, the possibility remains that the host ranges of both species may be more diverse.

Maintenance of divergent lineages of the Rice Blast Fungus Pyricularia oryzae through niche separation, loss of sex and post-mating genetic incompatibilities.

Many species of fungal plant pathogens coexist as multiple lineages on the same host, but the factors underlying the origin and maintenance of population structure remain largely unknown. The rice blast fungus Pyricularia oryzae is a widespread model plant pathogen displaying population subdivision. However, most studies of natural variation in P. oryzae have been limited in genomic or geographic resolution, and host adaptation is the only factor that has been investigated extensively as a contributor to population subdivision. In an effort to complement previous studies, we analyzed genetic and phenotypic diversity in isolates of the rice blast fungus covering a broad geographical range. Using single-nucleotide polymorphism genotyping data for 886 isolates sampled from 152 sites in 51 countries, we showed that population subdivision of P. oryzae in one recombining and three clonal lineages with broad distributions persisted with deeper sampling. We also extended previous findings by showing further population subdivision of the recombining lineage into one international and three Asian clusters, and by providing evidence that the three clonal lineages of P. oryzae were found in areas with different prevailing environmental conditions, indicating niche separation. Pathogenicity tests and bioinformatic analyses using an extended set of isolates and rice varieties indicated that partial specialization to rice subgroups contributed to niche separation between lineages, and differences in repertoires of putative virulence effectors were consistent with differences in host range. Experimental crosses revealed that female sterility and early post-mating genetic incompatibilities acted as strong additional barriers to gene flow between clonal lineages. Our results demonstrate that the spread of a fungal pathogen across heterogeneous habitats and divergent populations of a crop species can lead to niche separation and reproductive isolation between distinct, widely distributed, lineages.

Peanut stunt virus movement protein is the limiting factor in Capsicum annuum infection

Cucumber mosaic virus (CMV) is one of the most devastating plant viruses, with more than 1,200 species of host plants. The host range and economic importance of peanut stunt virus (PSV) are mostly limited to legumes, despite the similar taxonomy and genome structure with CMV. Since no data are available on the background of the limited host range of PSV, RNA 3 recombinant and reassortant viruses were generated (C12P3, P12C3, C12CP3, C12PC3, C12PΔC3) to study their infection phenotype on a common host (Nicotiana benthamiana) and on a selective host (Capsicum annuum cv. Brody). The PSV movement protein (MP) was not able to function with the coat protein (CP) of CMV unless the C-terminal 42 amino acids were deleted from the PSV MP. As a result of the inoculation experiments, MP was considered the protein influencing symptom phenotypes on N. benthamiana and responsible for the host range difference on the pepper. Since plasmodesmata (PD) localization of viral MPs is essential for cell-to-cell movement, subcellular localization of GFP-tagged MPs (CMV-MP-eGFP, PSV-MP-eGFP) was observed. In the case of CMV-MP-eGFP, clear colocalization with PD was detected in both hosts, but PSV-MP-eGFP was not tightly connected to the PD in N. benthamiana and barely localized to the PD in C. annuum epidermal cells. Measuring Pearson correlation coefficients (PCCs) also supported the visual observation.

The Emergence of Fusarium oxysporum f. sp. apii Race 4 and Fusarium oxysporum f. sp. coriandrii Highlights Major Obstacles Facing Agricultural Production in Coastal California in a Warming Climate: A Case Study.

Currently, Fusarium oxysporum f. sp. apii (Foa) race 4 in celery and F. oxysporum f. sp. coriandrii (Foci) in coriander have the characteristics of emerging infectious plant diseases in coastal southern California: the pathogens are spreading, yield losses can be severe, and there are currently no economical solutions for their control. Celery, and possibly coriander, production in these regions is are likely to have more severe disease from projected warmer conditions in the historically cool, coastal regions. Experimental evidence shows that Foa race 4 causes much higher disease severity when temperatures exceed 21°C. A phylogenomic analysis indicated that Foa race 4, an older, less virulent, and uncommon Foa race 3, and two Foci are closely related in their conserved genomes. These closely related genotypes are somatically compatible. Foa race 4 can also cause disease in coriander and the two organisms readily form “hetero” conidial anastomosis tubes (CAT), further increasing the likelihood of parasexual recombination and the generation of novel pathotypes. A horizontal chromosome transfer event likely accounts for the difference in host range between Foci versus Foa races 4 and 3 because they differ primarily in one or two accessory chromosomes. How Foa race 4 evolved its hyper-virulence is unknown. Although the accessory chromosomes of Foa races 3 and 4 are highly similar, there is no evidence that Foa race 4 evolved directly from race 3, and races 3 and 4 probably only have a common ancestor. Foa race 2, which is in a different clade within the Fusarium oxysporum species complex (FOSC) than the other Foa, did not contribute to the evolution of race 4, and does not form CATs with Foa race 4; consequently, while inter-isolate CAT formation is genetically less restrictive than somatic compatibility, it might be more restricted between FOSC clades than currently known. Other relatively new F. oxysporum in coastal California include F. oxysporum f. sp. fragariae on strawberry (Fof). Curiously, Fof “yellows-fragariae” isolates also have similar core genomes to Foa races 4 and 3 and Foci, perhaps suggesting that there may be core genome factors in this lineage that favor establishment in these soils.

Prevalence of genetically similar Flavobacterium columnare phages across aquaculture environments reveals a strong potential for pathogen control.

SummaryIntensive aquaculture conditions expose fish to bacterial infections, leading to significant financial losses, extensive antibiotic use and risk of antibiotic resistance in target bacteria. Flavobacterium columnare causes columnaris disease in aquaculture worldwide. To develop a bacteriophage‐based control of columnaris disease, we isolated and characterized 126 F. columnare strains and 63 phages against F. columnare from Finland and Sweden in 2017. Bacterial isolates were virulent on rainbow trout (Oncorhynchus mykiss) and fell into four previously described genetic groups A, C, E and G, with genetic groups C and E being the most virulent. Phage host range studied against a collection of 227 bacterial isolates (from 2013 to 2017) demonstrated modular infection patterns based on host genetic group. Phages infected contemporary and previously isolated bacterial hosts, but bacteria isolated most recently were generally resistant to previously isolated phages. Despite large differences in geographical origin, isolation year or host range of the phages, whole‐genome sequencing of 56 phages showed high level of genetic similarity to previously isolated F. columnare phages (Ficleduovirus, Myoviridae). Altogether, this phage collection demonstrates a potential for use in phage therapy.

Natural Shifts in Endosymbionts' Occurrence and Relative Frequency in Their Ciliate Host Population.

The role of bacterial endosymbionts harbored by heterotrophic Paramecium species is complex. Obligate intracellular bacteria supposedly always inflict costs as the host is the only possible provider of resources. However, several experimental studies have shown that paramecia carrying bacterial endosymbionts can benefit from their infection. Here, we address the question which endosymbionts occur in natural paramecia populations isolated from a small lake over a period of 5 years and which factors might explain observed shifts and persistence in the symbionts occurrence. One hundred and nineteen monoclonal strains were investigated and approximately two-third harbored intracellular bacteria. The majority of infected paramecia carried the obligate endosymbiotic “Candidatus Megaira polyxenophila”, followed by Caedimonas varicaedens, and Holospora undulata. The latter was only detected in a single strain. While “Ca. M. polyxenophila” was observed in seven out of 13 samplings, C. varicaedens presence was limited to a single sampling occasion. After the appearance of C. varicaedens, “Ca. M. polyxenophila” prevalence dramatically dropped with some delay but recovered to original levels at the end of our study. Potential mechanisms explaining these observations include differences in infectivity, host range, and impact on host fitness as well as host competitive capacities. Growth experiments revealed fitness advantages for infected paramecia harboring “Ca. M. polyxenophila” as well as C. varicaedens. Furthermore, we showed that cells carrying C. varicaedens gain a competitive advantage from the symbiosis-derived killer trait. Other characteristics like infectivity and overlapping host range were taken into consideration, but the observed temporal persistence of “Ca. M. polyxenophila” is most likely explained by the positive effect this symbiont provides to its host.

Temporal Stability and Genetic Diversity of 48-Year-Old T-Series Phages.

ABSTRACTT-series phages have been model organisms for molecular biology since the 1940s. Given that these phages have been stocked, distributed, and propagated for decades across the globe, there exists the potential for genetic drift to accumulate between stocks over time. Here, we compared the temporal stability and genetic relatedness of laboratory-maintained phage stocks with a T-series collection from 1972. Only the T-even phages produced viable virions. We obtained complete genomes of these T-even phages, along with two contemporary T4 stocks. Performing comparative genomics, we found 12 and 16 nucleotide variations, respectively, in the genomes of T2 and T6, whereas there were ∼172 nucleotide variations between T4 sublines compared with the NCBI RefSeq genome. To account for the possibility of artifacts in NCBI RefSeq, we used the 1972 T4 stock as a reference and compared genetic and phenotypic variations between T4 sublines. Genomic analysis predicted nucleotide variations in genes associated with DNA metabolism and structural proteins. We did not, however, observe any differences in growth characteristics or host range between the T4 sublines. Our study highlights the potential for genetic drift between individually maintained T-series phage stocks, yet after 48 years, this has not resulted in phenotypic alterations in these important model organisms.IMPORTANCE T-series bacteriophages have been used throughout the world for various molecular biology researches, which were critical for establishing the fundamentals of molecular biology, from the structure of DNA to advanced gene-editing tools. These model bacteriophages help keep research data consistent and comparable between laboratories. However, we observed genetic variability when we compared contemporary sublines of T4 phages to a 48-year-old stock of T4. This may have effects on the comparability of results obtained using T4 phage. Here, we highlight the genomic differences between T4 sublines and examined phenotypic differences in phage replication parameters. We observed limited genomic changes but no phenotypic variations between T4 sublines. Our research highlights the possibility of genetic drift in model bacteriophages.

A single gene in Fusarium oxysporum limits host range.

Fusarium oxysoporum f. sp. radicis‐cucumerinum (Forc) is able to cause disease in cucumber, melon, and watermelon, while F. oxysporum f. sp. melonis (Fom) can only infect melon plants. Earlier research showed that mobile chromosomes in Forc and Fom determine the difference in host range between Forc and Fom. By closely comparing these pathogenicity chromosomes combined with RNA‐sequencing data, we selected 11 candidate genes that we tested for involvement in the difference in host range between Forc and Fom. One of these candidates is a putative effector gene on the Fom pathogenicity chromosome that has nonidentical homologs on the Forc pathogenicity chromosome. Four independent Forc transformants with this gene from Fom showed strongly reduced or no pathogenicity towards cucumber, while retaining pathogenicity towards melon and watermelon. This suggests that the protein encoded by this gene is recognized by an immune receptor in cucumber plants. This is the first time that a single gene has been demonstrated to determine a difference in host specificity between formae speciales of F. oxysporum.

Phylogeny, taxonomics, and ovipositor length variation of thePteromalus albipennisspecies group (Hymenoptera: Chalcidoidea: Pteromalidae: Pteromalinae)

The Pteromalus albipennis species group currently comprises 29 Palaearctic parasitoid wasp species associated with tephritid fly larvae developing in flower heads of Asteraceae. The species group is taxonomically challenging, because its putative species are morphologically extremely similar to each other except for some conspicuous differences in gaster length, which is a reliable proxy for the ovipositor length. Ovipositor length dictates the accessibility of host larvae to the parasitoid female and thus largely determines the host range of a species. In this study, we infer the phylogenetic relationships of currently recognized species of the P. albipennis species group by analyzing the nucleotide sequences of 124 protein-coding genes spanning more than 310,000 bp. The data were analyzed by applying the maximum likelihood optimality criterion (concatenated sequence data) as well as a multispecies coalescent approach (analyzing gene trees). Based on the inferred species trees, we (a) re-evaluate previous species hypotheses in comparison with the new phylogenomic results and (b) assess whether or not ovipositor length variation is phylogenetically constrained. Our results show some incongruence between morphology-based and phylogenomics-based species delimitations and suggest the existence of paraphyletic species. Furthermore, our study suggests that ovipositor length is no reliable predictor of phylogenetic relatedness, but a labile trait, possibly reflecting rapid evolution of host range differences. Taxonomic ambiguities and the scarce knowledge on the biology of the analyzed species highlight the difficulties of studying parasitoid wasps, even in the Western Palaearctic, and despite the application of integrative taxonomics approaches (i.e., the use of hundreds of genes for taxonomy).

Genomic differences between the new Fusarium oxysporum f. sp. apii (Foa) race 4 on celery, the less virulent Foa races 2 and 3, and the avirulent on celery f. sp. coriandrii

BackgroundMembers of the F. oxysporium species complex (FOSC) in the f. sp. apii (Foa) are pathogenic on celery and those in f. sp. coriandrii (Foci) are pathogenic on coriander (=cilantro). Foci was first reported in California in 2005; a new and highly aggressive race 4 of Foa was observed in 2013 in California. Preliminary evidence indicated that Foa can also cause disease on coriander, albeit are less virulent than Foci. Comparative genomics was used to investigate the evolutionary relationships between Foa race 4, Foa race 3, and the Foci, which are all in FOSC Clade 2, and Foa race 2, which is in FOSC Clade 3.ResultsA phylogenetic analysis of 2718 single-copy conserved genes and mitochondrial DNA sequence indicated that Foa races 3 and 4 and the Foci are monophyletic within FOSC Clade 2; these strains also are in a single somatic compatibility group. However, in the accessory genomes, the Foci versus Foa races 3 and 4 differ in multiple contigs. Based on significantly increased expression of Foa race 4 genes in planta vs. in vitro, we identified 23 putative effectors and 13 possible pathogenicity factors. PCR primers for diagnosis of either Foa race 2 or 4 and the Foci were identified. Finally, mixtures of conidia that were pre-stained with different fluorochromes indicated that Foa race 4 formed conidial anastomosis tubes (CATs) with Foci. Foa race 4 and Foa race 2, which are in different somatic compatibility groups, did not form CATs with each other.ConclusionsThere was no evidence that Foa race 2 was involved in the recent evolution of Foa race 4; Foa race 2 and 4 are CAT-incompatible. Although Foa races 3 and 4 and the Foci are closely related, there is no evidence that either Foci contributed to the evolution of Foa race 4, or that Foa race 4 was the recent recipient of a multi-gene chromosomal segment from another strain. However, horizontal chromosome transfer could account for the major difference in the accessory genomes of Foa race 4 and the Foci and for their differences in host range.

Ranking the host range of biological control agents with quantitative metrics of taxonomic specificity

The need to prevent negative impacts of importation biological control on native biodiversity has made evaluating the specificity (host range) of natural enemies a central issue for the application of biological control programs against pest organisms. Thus, when there are a number of candidate species being considered for introduction as biological control agents against a particular target pest, it is important to compare their relative host ranges. These comparisons are usually made informally using categorical terminology (e.g., generalist, specialist, oligophagous). However, relative differences in natural enemy host range are better expressed quantitatively, with both ecological (how many host species a natural enemy is capable of exploiting) and evolutionary (how the host species are related phylogenetically) components. We propose using two previously developed quantitative taxonomic and phylogenetic metrics (taxonomic host range – STD; phylogenetic species variability – PSV), in combination with the number of associated host species (host species richness), as heuristic tools to rank the relative host specificity of candidate biological control agents. We first show that although STD and PSV were developed independently, they are directly related mathematically and convey exactly the same information except on different numerical scales. We then apply the metrics to case studies regarding the host range of candidate biological control agents for: (i) an invasive whitefly in greenhouse crops, Aleurotrachelus trachoides (Hemiptera: Aleyrodidae), and (ii) an invasive pest of grapes, Lobesia botrana (Lepidoptera: Tortricidae). We also provide open source code to easily allow the calculation of these metrics for any natural enemy for which taxonomic host range information is available. Finally, we describe potential uses of these metrics in applied biological control as well as important caveats and limitations that require further theoretical work to address. We conclude that while these metrics should not be considered as absolute or infallible measurements of host range, their application should encourage biological control practitioners to explicitly consider the phylogenetic component of host range when ranking prospective candidate biological control agents.

New subspecies of fir dwarf mistletoe (Arceuthobium abietinum: Viscaceae) from the western United States and northern Mexico

Two new subspecies of fir dwarf mistletoe (Arceuthobium abietinum, Viscaceae) are described herein: Arceuthobium abietinum subsp. mathiasenii (Mathiasen’s dwarf mistletoe) and Arceuthobium abietinum subsp. grandae (grand fir dwarf mistletoe). The former dwarf mistletoe—Mathiasen’s dwarf mistletoe—parasitizes Rocky Mountain white fir in Nevada, Utah, and Arizona and Durango fir in northern Mexico; whereas, grand fir dwarf mistletoe parasitizes grand fir as well grand fir × white fir hybrids in California, Oregon and Washington. The recognition of these subspecies and, hence, their classification is based on morphological discontinuities and host range differences when compared to white fir dwarf mistletoe (Arceuthobium abietinum subsp. abietinum), red fir dwarf mistletoe (A. abietinum subsp. magnificae), and Wiens’ dwarf mistletoe (A. abietinum subsp. wiensii). The combined analyses of plant height, third internode length and width, fruit dimensions, staminate spike and petal dimensions, and anther distance to tip, contributed most to the discrimination of subsp. grandae and mathiasenii from subsp. abietinum and magnificae. The flowers of Mathiasen's dwarf mistletoe were larger than all of the other taxa classified under A. abietinum. The shoot color of Mathiasen’s dwarf mistletoe was frequently blue-green, brown, yellow-brown, or red-brown, and plants were often highly glaucous, while the shoot color of the other subspecies were typically yellow-green or yellow, except for Wiens' dwarf mistletoe whose shoots are often green-brown or red-brown. Differences in host specificity and geographic distribution also distinguish the new subspecies from the other taxa of A. abietinum.

The recombinant isolate of cucurbit aphid‐borne yellows virus from Brazil is a polerovirus transmitted by whiteflies

AbstractThe severe yellowing disease (amarelão) on melon plants is a serious problem in Brazil, although the causative agent remained unknown for a long time. Recently, recombinant isolates of cucurbit aphid‐borne yellows virus (CABYV) were reported as the possible causative agents of this disease on melon plants. Although aphids are known to be the vectors of the common type of CABYV isolates, almost no aphid colony was observed in the major melon fields in Brazil with high incidence of the severe yellowing disease. In contrast, whiteflies are often abundant. Based on this observation, the hypothesis of the transmission of recombinant CABYV by whiteflies was evaluated. After thorough transmission experiments, we found that this recombinant CABYV isolate was transmitted by the whitefly Bemisia tabaci MEAM1, but not by Aphis gossipii. Furthermore, the host response by whitefly‐based inoculation in cucurbits and other indicator plants showed differences in host range when compared to the common type of CABYV. Due to its transmissibility by the whitefly and the distant relationship of the P3/P5 protein to CABYV, the name “cucurbit whitefly‐borne yellows virus” is proposed for this recombinant CABYV. This is the second report of polerovirus transmission by the whitefly B. tabaci, following the report of pepper whitefly‐borne vein yellows virus.

Related mobile pathogenicity chromosomes in Fusarium oxysporum determine host range on cucurbits.

Fusarium oxysporum f. sp. radicis‐cucumerinum (Forc) causes severe root rot and wilt in several cucurbit species, including cucumber, melon, and watermelon. Previously, a pathogenicity chromosome, chrRC, was identified in Forc. Strains that were previously nonpathogenic could infect multiple cucurbit species after obtaining this chromosome via horizontal chromosome transfer (HCT). In contrast, F. oxysporum f. sp. melonis (Fom) can only cause disease on melon plants, even though Fom contains contigs that are largely syntenic with chrRC. The aim of this study was to identify the genetic basis underlying the difference in host range between Fom and Forc. First, colonization of different cucurbit species between Forc and Fom strains showed that although Fom did not reach the upper part of cucumber or watermelon plants, it did enter the root xylem. Second, to select candidate genomic regions associated with differences in host range, high‐quality genome assemblies of Fom001, Fom005, and Forc016 were compared. One of the Fom contigs that is largely syntenic and highly similar in sequence to chrRC contains the effector gene SIX6. After HCT of the SIX6‐containing chromosome from Fom strains to a nonpathogenic strain, the recipient (HCT) strains caused disease on melon plants, but not on cucumber or watermelon plants. These results provide strong evidence that the differences in host range between Fom and Forc are caused by differences between transferred chromosomes of Fom and chrRC, thus narrowing down the search for genes allowing or preventing infection of cucumber and watermelon to genes located on these chromosomes.

Confirmation of the wide host range of Parvilucifera corolla (Alveolata, Perkinsozoa)

Marine parasites of the genus Parvilucifera have been described as endoparasitoids of dinoflagellates. Recently, the species Parvilucifera corolla was described, but its host range was not examined. Here, the host selectivity of P. corolla was screened, including 110 strains of dinoflagellates (24 genera) and other microalgal groups as potential hosts. Infections and the full life cycle of the parasitoid were observed in 73 strains (16 genera) of dinoflagellates. Parvilucifera corolla did not infect most chlorophytes, cryptophytes, chrysophytes, diatoms, haptophytes and raphidophytes but one strain of Pyramimonas (chlorophyte) was infected, although without viable sporangia. In Symbiodinium natans, a transition to the coccoid stage was induced above a certain parasite:host ratio. These results confirm P. corolla as a generalist parasitoid of dinoflagellates, with important differences in host range regarding other species of the genus.