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.Tuber spp. are Ascomycetes fungi that establish an ectomycorrhizal symbiosis with trees and shrubs. As a result of this mutualistic symbiosis, ascocarps known as truffles are produced. Some Tuber spp. produce edible truffles that, given their distinctive taste and aroma, are highly valued by gourmets. Research on these fungi has focused on promoting the cultivation of these fungi to meet increasing worldwide demand and to provide replacements for the catastrophic decline in their natural production (10). Truffle cultivation is no longer an agronomic practice confined to Europe, where the most profitable species are endemic. Truffle plantations have been established in various countries worldwide, including New Zealand and Israel. Nevertheless, the understanding of many basic aspects of truffle biology is still in its infancy, and the ecological requirements for some of these species are still not known. One of the most elusive goals has been discerning the reproductive system of Tuber spp. The reproductive structures of these species in pure cultures have not been reported, and axenic spore germination remains an unresolved problem (26). Furthermore, the mating-type genes have never been characterized in truffles.Molecular markers are being developed to type each truffle species to overcome the difficulty of identifying these species solely on their morphological traits (1,8,11,17,18,19,22,23). By combining molecular markers with an appropriate sampling strategy, we may be able to critically evaluate the truffle reproductive system and life cycle even without reproducing the entire life cycle in the lab. To date, Tuber melanosporum Vittad. and Tuber magnatum Pico, the finest black and white truffle species, respectively, have been regarded as selfing species. When codominant markers were evaluated, heterozygous ascocarps were not detected (2,3,7,15,16). These studies proceed from the assumption that the ascocarps are diploid (dikaryotic) structures.We recently used simple sequence repeat (SSR) markers and a large survey of...

Soil analysis reveals the presence of an extended mycelial network in a Tuber magnatum truffle-ground

“…Moreover, it has been possible to widen the range of known truffle hosts: they are traditionally associated with trees and shrubs, but have now also been located inside orchid roots (Selosse et al, 2004), suggesting that truffles possess a more flexible host range. Only recently has the truffle's complex life cycle been elucidated, underlining that they are primarily haploid and reproduce by outcrossing (Paolocci et al, 2006). Unlike T. melanosporum, which can be cultivated in orchards by inoculating oaks and hazelnuts with spore suspensions (Chevalier, 1994), Tuber magnatum, the most expensive and appreciated truffle species, has so far resisted the charms of domestication.…”

Forests and non wooden products in Italy

“…The white truffle (Tuber magnatum) is the most valuable species and fortunately peculiar to our country. It is however difficult to achieve in truffle for the hard artificial mycorhization (Mello et al, 2001;Paolocci et al, 2006). This is definitely an advantage for domestic production, which may qualify as the right ecological conditions (Zambonelli et al, 2012).…”

Unresolved Problems in the Life Cycle of Truffles

“…Contrary to the above, Paolocci et al [22] and Rubini et al [23] using microsatellite markers, demonstrated that some, and possibly considerable, outcrossing takes place in T. magnatum. According to the authors, the vegetative infecting mycelium is homokaryotic, originating from a single ascospore germination.…”

“…In addition, Paolocci et al [22] and Rubini et al [23] demonstrated that single mycorrhized root tips harbored only one combination of marker alleles, indicating that the mycorrhizing agent is a homokaryotic, monosporic mycelium. This is certainly compelling (though the use of microsatellites as markers could be viewed as problematic owing to the possibility of instability of microsattelite markers, especially during meiosis).…”