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Using mating-type loci to improve taxonomy of the Tuber indicum complex, and discovery of a new species, T. longispinosum.

ABSTRACT: Black truffles that morphologically resemble Tuber indicum have been known to occur in Japan since 1979. Our previous studies showed that there are two phylotypes of these truffles, both of which fell into the T. indicum complex (hereinafter "Tuber sp. 6" and "Tuber sp. 7"). However, their taxonomic treatment is still unclear. In this study, we conducted morphological and phylogenetic analyses for a total of 52 specimens from Japan (16 Tuber sp. 6 and 13 Tuber sp. 7), China (10 T. himalayense and 8 T. indicum), and Taiwan (5 T. formosanum). We compared ascospore ornamentation, size, distribution of asci with average number of spores per ascus, spine size and shape of the Japanese specimens with their allied taxa. For phylogenetic analysis, we sequenced two mating loci (MAT1-1-1 and MAT1-2-1) and three commonly used loci (ITS, β-tubulin, and TEF1-α). Three distinct lineages were recognized by phylogenetic analyses based on the sequences of the two mating-related loci and three independent loci. The Tuber sp. 6 sequences clustered with those of T. himalayense and T. formosanum, and there was no clear difference in morphology among them. Tuber sp. 7 formed a distinct lineage in each phylogram. The specimens tended to have five-spored asci more frequently than other allied species and could be characterized as having ascospore ornamentation with longer spines and narrower spine bases. We therefore described Tuber sp. 7 as a new species (T. longispinosum), and treat Tuber sp. 6 and T. formosanum as synonyms of T. himalayense.

SUBMITTER: Kinoshita A

PROVIDER: S-EPMC5874008 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Tuber spp. are filamentous ascomycetes which establish symbiosis with the roots of trees and shrub species. By virtue of this symbiosis they produce hypogeous ascocarps, known as truffles. Filamentous ascomycetes can reproduce by homothallism or heterothallism depending on the structure and organization of their mating type locus. The first mating type locus in a truffle species has been recently characterized in Tuber melanosporum and it has been shown that this fungus, endemic in Europe, is heterothallic. The availability of sequence information for T. melanosporum mating type genes is seminal to cloning their orthologs from other Tuber species and assessing their reproductive mode. Here we report on the organization of the mating type region in T. indicum, the black truffle species present in Asia, which is the closest relative to T. melanosporum and is characterized by an high level of morphological and genetic variability. The present study shows that T. indicum is also heterothallic. Examination of Asiatic black truffles belonging to different genetic classes, sorted according to the sequence polymorphism of the internal transcribed spacer rDNA region, has revealed sequence variations and rearrangements in both coding and non-coding regions of the mating type locus, to suggest the existence of cryptic species within the T. indicum complex. The presence of transposable elements within or linked to the mating type region suggests a role of these elements in generating the genotypic diversity present among T. indicum strains. Overall, comparative analyses of the mating type locus have thus allowed us to tackle taxonomical and phylogenetic issues within black truffles and make inferences about the evolution of T. melanosporum-T. indicum lineage. Our results are not only of fundamental but also of applied relevance as T. indicum produces edible fruit bodies that are imported also into Europe and thus may represent a biological threat for T. melanosporum.

Project description:BACKGROUND:Volvox carteri f. nagariensis is a model taxon that has been studied extensively at the cellular and molecular level. The most distinctive morphological attribute of V. carteri f. nagariensis within V. carteri is the production of sexual male spheroids with only a 1:1 ratio of somatic cells to sperm packets or androgonidia (sperm packet initials). However, the morphology of male spheroids of V. carteri f. nagariensis has been examined only in Japanese strains. In addition, V. carteri f. nagariensis has heterothallic sexuality; male and female sexes are determined by the sex-determining chromosomal region or mating-type locus composed of a >?1 Mbp linear chromosome. Fifteen sex-specific genes and many sex-based divergent shared genes (gametologs) are present within this region. Thus far, such genes have not been identified in natural populations of this species. RESULTS:During a recent fieldwork in Taiwan, we encountered natural populations of V. carteri that had not previously been recorded from Taiwan. In total, 33 strains of this species were established from water samples collected in Northern Taiwan. Based on sequences of the internal transcribed spacer 2 region of nuclear ribosomal DNA and the presence of asexual spheroids with up to 16 gonidia, the species was clearly identified as V. carteri f. nagariensis. However, the sexual male spheroids of the Taiwanese strains generally exhibited a 1:1 to >?50:1 ratio of somatic cells to androgonidia. We also investigated the presence or absence of several sex-specific genes and the sex-based divergent genes MAT3m, MAT3f and LEU1Sm. We did not identify recombination or deletion of such genes between the male and female mating-type locus haplotypes in 32 of the 33 strains. In one putative female strain, the female-specific gene HMG1f was not amplified by genomic polymerase chain reaction. When sexually induced, apparently normal female sexual spheroids developed in this strain. CONCLUSIONS:Male spheroids are actually variable within V. carteri f. nagariensis. Therefore, the minimum ratio of somatic cells to androgonidia in male spheroids and the maximum number of gonidia in asexual spheroids may be diagnostic for V. carteri f. nagariensis. HMG1f may not be directly related to the formation of female spheroids in this taxon.

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Using mating-type loci to improve taxonomy of the Tuber indicum complex, and discovery of a new species, T. longispinosum.

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