Proceedings of the XLVI Italian Society of Agricultural Genetics - SIGA Annual Congress

Giardini Naxos, Italy - 18/21 September, 2002

ISBN 88-900622-3-1

Poster Abstract - 3.45

Towards the understanding of the mating system in Tuber spp.

Rubini A., Riccioni C., Topini F., Paolocci F.

Istituto di Ricerche sul Miglioramento Genetico delle Piante Foraggere CNR, Via G. Pennetti Pennella 22, 06128 Perugia

ITS, b tubulin, elongation factor 1-a, mating system, gene genealogies

Tuber spp. are ectomycorrhizal fungi that establish mutualistic symbiosis with shrubs and trees and produce highly–prized fruit bodies, known as truffles. Some Tuber spp., such as T.aestivum/T.uncinatum and T.mesentericum spp., show ascocarps with a high rate of morphological variability. The presence, within the fungal species of strains exhibiting a continuum of morphological traits between two taxa, makes it difficult to differentiate between varieties and species. Moreover, the difficulties to grow these fungi and the impossibility to mate them in controlled conditions have been major obstacles to the understanding of the reproductive system of these hypogeous fungi. In order to gain some insight into the genetic structure and modes of reproduction of the morphologically related species T.aestivum, T.uncinatum and T.mesemtericum, the ITS region of the nuclear rDNA as well as the b tubulin and elongation factor 1-a genes were amplified and sequenced on about 100 ascocarps. The high degree of sequence homology among T.aestivum and T.uncinatum samples for the three loci considered, irrespective of the morphological classification, likely supports the thesis that these two taxa belong, indeed, to the same species. Therefore, it is tempting to speculate that specific and still uncharacterized environmental stimuli induce T.aestivum to fruit at different times and that the different ripening season, in turn, affects the sporal morphology of the ascocarps, giving rise to the T.aestivum (harvested in summer) and T.uncinatum (harvested in autumn) morphotypes.

Most importantly, the consensus trees based on ITS, b tubulin and elongation factor 1-a genealogies on samples of the T.aestivum/T.uncinatum complex were congruent, providing compelling evidence about a clonal propagation system within this taxon.

Within T.mesentericum spp., two truffle classes each one with its own morphological and molecular pattern have been identified. As matter of the fact, these two classes differed not only for the ITS, as proved by RFLP and sequence analysis of these spacers, but also display a high rate of polymorphism both for b tubulin and elongation factor 1-a genes. These data open the way to a possible species revision process to elevate one of theses two taxa to the formal rank of new species. Differently, two clearly distinctive and sexually separated botanical forms are grouped within T.mesentericum. In addition, two T.mesentericum samples showing incongruent genealogies between b tubulin and ITS/ elongation factor 1-a genes and an intriguing b tubulin similarity with T.melanosporum have been identified. It is likely that, although it is quite a rare event, outcrossing or even an interspecific cross may occur in T.mesentericum.