Ribosomal DNA sequence analysis places the yeast-like genus Symbiotaphrina within filamentous ascomycetes

Rediscovering a Forgotten System of Symbiosis: Historical Perspective and Future Potential

While the majority of symbiosis research is focused on bacteria, microbial eukaryotes play important roles in the microbiota and as pathogens, especially the incredibly diverse Fungi kingdom. The recent emergence of widespread pathogens in wildlife (bats, amphibians, snakes) and multidrug-resistant opportunists in human populations (Candida auris) has highlighted the importance of better understanding animal-fungus interactions. Regardless of their prominence there are few animal-fungus symbiosis models, but modern technological advances are allowing researchers to utilize novel organisms and systems. Here, I review a forgotten system of animal-fungus interactions: the beetle-fungus symbioses of Drugstore and Cigarette beetles with their symbiont Symbiotaphrina. As pioneering systems for the study of mutualistic symbioses, they were heavily researched between 1920 and 1970, but have received only sporadic attention in the past 40 years. Several features make them unique research organisms, including (1) the symbiont is both extracellular and intracellular during the life cycle of the host, and (2) both beetle and fungus can be cultured in isolation. Specifically, fungal symbionts intracellularly infect cells in the larval and adult beetle gut, while accessory glands in adult females harbor extracellular fungi. In this way, research on the microbiota, pathogenesis/infection, and mutualism can be performed. Furthermore, these beetles are economically important stored-product pests found worldwide. In addition to providing a historical perspective of the research undertaken and an overview of beetle biology and their symbiosis with Symbiotaphrina, I performed two analyses on publicly available genomic data. First, in a preliminary comparative genomic analysis of the fungal symbionts, I found striking differences in the pathways for the biosynthesis of two B vitamins important for the host beetle, thiamine and biotin. Second, I estimated the most recent common ancestor for Drugstore and Cigarette beetles at 8.8-13.5 Mya using sequence divergence (CO1 gene). Together, these analyses demonstrate that modern methods and data (genomics, transcriptomes, etc.) have great potential to transform these beetle-fungus systems into model systems again.

Made for Each Other: Ascomycete Yeasts and Insects

Fungi and insects live together in the same habitats, and many species of both groups rely on each other for success. Insects, the most successful animals on Earth, cannot produce sterols, essential vitamins, and many enzymes; fungi, often yeast-like in growth form, make up for these deficits. Fungi, however, require constantly replenished substrates because they consume the previous ones, and insects, sometimes lured by volatile fungal compounds, carry fungi directly to a similar, but fresh, habitat. Yeasts associated with insects include Ascomycota (Saccharomycotina, Pezizomycotina) and a few Basidiomycota. Beetles, homopterans, and flies are important associates of fungi, and in turn the insects carry yeasts in pits, specialized external pouches, and modified gut pockets. Some yeasts undergo sexual reproduction within the insect gut, where the genetic diversity of the population is increased, while others, well suited to their stable environment, may never mate. The range of interactions extends from dispersal of yeasts on the surface of insects (e.g., cactus-Drosophila-yeast and ephemeral flower communities, ambrosia beetles, yeasts with holdfasts) to extremely specialized associations of organisms that can no longer exist independently, as in the case of yeast-like symbionts of planthoppers. In a few cases yeast-like fungus-insect associations threaten butterflies and other species with extinction. Technical advances improve discovery and identification of the fungi but also inform our understanding of the evolution of yeast-insect symbioses, although there is much more to learn.

Expansion of the Candida tanzawaensis yeast clade: 16 novel Candida species from basidiocarp-feeding beetles

A major clade of new yeast taxa from the digestive tract of basidiocarp-feeding beetles is recognized based on rRNA gene sequence analyses. Almost 30 % of 650 gut isolates formed a statistically well-supported clade that included Candida tanzawaensis. The yeasts in the clade were isolated from 11 families of beetles, of which Tenebrionidae and Erotylidae were most commonly sampled. Repeated isolation of certain yeasts from the same beetle species at different times and places indicated strong host associations. Sexual reproduction was never observed in the yeasts. Based on comparisons of small- and large-subunit rRNA gene sequences and morphological and physiological traits, the yeasts were placed in Candida ambrosiae and in 16 other undescribed taxa. In this report, the novel species in the genus Candida are described and their relationships with other taxa in the Saccharomycetes are discussed. The novel species and their type strains are as follows: Candida guaymorum (NRRL Y-27568(T)=CBS 9823(T)), Candida bokatorum (NRRL Y-27571(T)=CBS 9824(T)), Candida kunorum (NRRL Y-27580(T)=CBS 9825(T)), Candida terraborum (NRRL Y-27573(T)=CBS 9826(T)), Candida emberorum (NRRL Y-27606(T)=CBS 9827(T)), Candida wounanorum (NRRL Y-27574(T)=CBS 9828(T)), Candida yuchorum (NRRL Y-27569(T)=CBS 9829(T)), Candida chickasaworum (NRRL Y-27566(T)=CBS 9830(T)), Candida choctaworum (NRRL Y-27584(T)=CBS 9831(T)), Candida bolitotheri (NRRL Y-27587(T)=CBS 9832(T)), Candida atakaporum (NRRL Y-27570(T)=CBS 9833(T)), Candida panamericana (NRRL Y-27567(T)=CBS 9834(T)), Candida bribrorum (NRRL Y-27572(T)=CBS 9835(T)), Candida maxii (NRRL Y-27588(T)=CBS 9836(T)), Candida anneliseae (NRRL Y-27563(T)=CBS 9837(T)) and Candida taliae (NRRL Y-27589(T)=CBS 9838(T)).

Metschnikowia chrysoperlae sp. nov., Candida picachoensis sp. nov. and Candida pimensis sp. nov., isolated from the green lacewings Chrysoperla comanche and Chrysoperla carnea (Neuroptera: Chrysopidae)

Fourteen yeast isolates comprising three taxa were cultured from digestive tracts of adult Chrysoperla species (Neuroptera: Chrysopidae) and their eggs. The yeast taxa were distinguished based on an estimated molecular phylogeny, DNA sequences and traditional taxonomic criteria. The new yeasts are closely related to Metschnikowia pulcherrima but are sufficiently distinguished by sequence comparison of rRNA gene sequences to consider them as novel species. Here, three novel species are described and their relationships with other taxa in the Saccharomycetes are discussed. Metschnikowia chrysoperlae sp. nov. (type strain, NRRL Y-27615T=CBS 9803T) produced needle-shaped ascospores and was the only teleomorph found. Large numbers of chlamydospores similar to those observed in M. pulcherrima were also produced. The other two novel species are asexual yeasts, Candida picachoensis sp. nov. (type strain, NRRL Y-27607T=CBS 9804T) and Candida pimensis sp. nov. (type strain, NRRL Y-27619T=CBS 9805T), sister taxa of M. chrysoperlae and M. pulcherrima. A specialized relationship between yeasts and lacewing hosts may exist, because the yeasts were isolated consistently from lacewings only. Although M. chrysoperlae was isolated from eggs and adult lacewings, suggesting the possibility of vertical transmission, no yeast was isolated from larvae.

Insect symbiosis: derivation of yeast-like endosymbionts within an entomopathogenic filamentous lineage

Yeast-like endosymbionts (YLSs) of insects often are restricted to specific hosts and are essential to the host's survival. For example, in planthoppers (Homoptera: Delphacidae), endosymbionts function in sterol utilization and nitrogen recycling for the hosts. Our study, designed to investigate evolutionary changes in the YLS lineage involved in the planthopper association, strongly suggests an origin of the YLSs from within the filamentous ascomycetes (Euascomycetes), not the true yeasts (Saccharomycetes), as their morphology might indicate. During divergence of the planthopper YLSs, dramatic changes would have occurred in the insect-fungus interaction and the fungal morphology that have previously been undescribed in filamentous ascomycetes. Phylogenetic trees were based on individual and combined data sets of 2.6 kb of the nuclear small- and large-subunit ribosomal RNA genes for YLSs from three rice planthoppers (Laodelphax striatellus, Nilaparvata lugens, and Sogatella furcifera) compared with 56 other fungi. Parsimony analysis placed the planthopper YLSs within Cordyceps (Euascomycetes: Hypocreales: Clavicipitaceae), a genus of filamentous insects and a few fungal pathogenic ascomycetes. Another YLS species restricted to the aphid Hamiltonaphis styraci (Homoptera: Aphididae) was a sister taxon to the planthopper YLSS: Filamentous insect pathogens (Metarhizium and Beauveria) specific to the same species of insect hosts as the YLSs also formed lineages within the Clavicipitaceae, but these were distinct from the clade comprising YLS species. Trees constrained to include the YLSs in families of the Hypocreales other than the Clavicipitaceae were rejected by the Kishino-Hasegawa test. In addition, the results of this study support a hypothesis of two independent origins of insect-associated YLSs from among filamentous ascomycetes: the planthopper YLSs in the Clavicipitaceae and the YLSs associated with anobiid beetles (Symbiotaphrina species). Several lineages of true yeasts (Saccharomycetes) also formed endosymbiotic associations with beetles, but they were not closely related to either group derived from the filamentous ascomycetes.