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Tirmania nivea
Photo: Ascocarps of Tirmania nivea (by Asunción Morte - University of Murcia)

Within the framework of the JGI Mycorrhizal Genomics Initiative (MGI) and the 1000 Fungal Genomes project, we are sequencing a phylogenetically and ecologically diverse suite of mycorrhizal fungi (Glomeromycotina, Ascomycota, Basidiomycota), which include the major clades of symbiotic species associating with trees and woody shrubs. Analyses of these genomes is providing new insights into the diversity of mechanisms leading to the development of mycorrhizal symbioses. These genomic resources are also used to address fundamental questions in the evolution of the symbiotic species from their saprotrophic and endophytic cousins.

Tirmania nivea

The hypogeous fungus Tirmania nivea (Desf.) Trappe (Pezizaceae, Ascomycota) is an edible desert truffle widespread throughout arid areas in the north of Africa as well as in the Near and Middle East, where it is one of the most appreciated desert truffles. Its peridium is white when removed from the ground but upon contact with the air it becomes yellowish. With fungal smell and mild flavour, it reaches a size greater than 10 cm in diameter, which makes it very valued in the market. It is very fleshy and the glebe is whitish or light pink with paler veins. Microscopically it presents amyloid asci and smooth or minutely roughened spores of broadly ellipsoid shape. It forms ectendomycorrhizal symbiosis with the perennial host plant Helianthemum lippi and the annual Helianthemum salicifolium (Cistaceae) in alkaline soils. This species is adapted to survive at levels of heat and water stress in dry deserts, which would be very unfavourable for other fungi.

T. nivea, as well as other Terfezia species, presents a post-transcriptional maturation of its LSU rRNA producing a cleavage of five nucleotides (TCATC) that generates a hidden gap located in domain D7. LSU splits into two molecules of 1.6 and 1.8 kb, corresponding to the 5' and 3' ends. This hidden gap has been recently described in this species for the first time in the kingdom Fungi, even when it seems to be widespread in other kingdoms. This ribosomal inactivating control mechanism could be involved in the response to stress conditions. T. nivea and Terfezia species, which are also adapted to extreme environments, display high resistance to drought and high temperatures. Whether Tirmania and Terfezia 28S rRNAs influence translation efficiency and what impact this can have on their biological cycles is still unknown and deserves further research. The T. nivea genome sequence will contribute to the study of the molecular mechanism behind the introduction of this hidden break and the possible biological role of this post-transcriptional maturation during its life cycle, as well as to understand the drought tolerance of the symbiosis to drought extreme conditions.

Researchers who wish to publish analyses using data from unpublished CSP genomes are respectfully required to contact the PI (Francis Martin) and JGI to avoid potential conflicts on data use and coordinate other publications with the CSP master paper(s).

Reference:

Alsheikh A (1996) Pure culture mycorrhizal synthesis of Cistaceae with desert truffles in the genera Terfezia and Tirmnia. In Abstract Book of the First International Conference on Mycorrhizae (ICOM1), University of California, Berkeley, August 4-9, 1996, pp.18-19.