Abstract
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.
Highlights
Sexual reproduction, by virtue of its essential role in determining genetic variability and eliminating deleterious mutations, is at the origin of eukaryotic evolution
In order to evaluate putative morphological differences between truffles belonging to the different RFLP classes, a detailed analysis of ascospore morphology was performed with scanning electron microscope (SEM, Figure 1) and light microscope (Figures S1, S2 and S3)
In this study we show that T. indicum, the Asiatic truffle species which is the closest relative to the Perigord black truffle T. melanosporum, is heterothallic since its mating type (MAT) locus is organized in two idiomorphs harbored by different strains
Summary
By virtue of its essential role in determining genetic variability and eliminating deleterious mutations, is at the origin of eukaryotic evolution. Filamentous ascomycetes are haploid fungi that have two main sexual reproductive modes: homothallism and heterothallism. In homothallic species, both master MAT genes are present in the haploid nucleus of each strain. Both master MAT genes are present in the haploid nucleus of each strain These fungi do not have distinctive mating types and are capable of haploid selfing as well as of crossing with any other strain [6]. In heterothallic species, the two master MAT genes are located in different strains, haploid selfing is prevented and sexual reproduction can occur only between individuals of opposite mating type. The two alternative forms of the MAT locus in these fungi have been termed idiomorphs (MAT1-1 and MAT1-2) rather than alleles because of their dissimilar sequences [7]
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