Reevaluation of the life cycle of Tuber magnatum

Abstract

Tuber spp. are ectomycorrhizal ascomycetes that produce ascocarps known as truffles. Basic aspects of Tuber biology have yet to be fully elucidated. In particular, there are conflicting hypotheses concerning the mating system and the ploidy level of the mycorrhizal and truffle hyphae. We used polymorphic microsatellites to compare the allelic configurations of asci with those from the network of the surrounding hyphae in single Tuber magnatum truffles. We then used these truffles to inoculate host plants and evaluated the microsatellite configurations of the resulting mycorrhizal root tips. These analyses provide direct evidence that T. magnatum outcrosses and that its life cycle is predominantly haploid. In addition to its scientific significance, this basic understanding of the T. magnatum life cycle may have practical importance in developing strategies to obtain and select nursery-produced mycorrhizal plants as well as in the management of artificial plantations of this and other Tuber spp.

FIG. 1.

Revised life cycle of Tuber magnatum. (1) Asci are released following ripening of the ascocarp. (2) In close proximity to host plant root tips, ascospores germinate to produce primary homokaryotic (haploid) mycelia. (3) The primary mycelia colonize the apical root tips and form ectomycorrhizae. The mycorrhizae colonizing roots can arise from different primary mycelia, so different fungal genotypes may colonize a common host root. From the mycorrhizae, extramatrical primary mycelia develop and spread into the soil. (4) Details of the fertilization process are unknown, but by analogy to other Ascomycetes organisms, a spermatization process seems likely. The function of a male gamete may be filled by any detached cell, e.g., a hyphal fragment, conidium, or ascospore. (5) Ascocarp primordia develop. The ascogenous heterokaryotic hyphae resulting from the fertilization process are surrounded by homokaryotic maternal vegetative hyphae. (6) Adult ascocarps are composed of asci and sterile hyphae. Stages not directly observed in this study are indicated in light gray.