First map of chromosome terminals of higher fungi

Chromosomes (stores of genetic information in the cells) have special structures known as telomeres at their ends, the size and integrity of which are essential for the survival of the cell. In all the cells of higher organisms (including humans) the size of the telomeres gets smaller after each cell division, to the point of reaching a minimum size which triggers the death of the cell and contributes to the ageing of the organism. Thus, for the life of a cell, the length of the telomeres is something akin to the upper part of an hourglass, a timepiece so well conserved over time that we humans share it with organisms as evolutionary distant as fungi.

The oyster fungus, together with the common mushroom, is the fungus with the greatest production and consumption worldwide. Likewise this fungus is of great biotechnological interest for its capacity to produce enzymes and degrade industrial and agricultural waste. Over the last ten years, the Genetics and Microbiology Research Group (GIGM), financed under projects by the National Research Plan, has made the oyster fungus a model system for studying this type of fungus -- P. ostreatus being the first edible fungus for which the genome has been sequenced, in a joint project between the UPNA as coordinator, and the Joint Genome Institute at the Department of Energy of the United States of America, and in which more than 20 laboratories worldwide are taking part.

Their function has special relevance in tumour processes and in ageing: under normal circumstances, when the cell ages the telomeric DNA gets shorter; the moment arrives when the cell cannot divide further and so triggers a series of responses that leads to the death of the cell. Tumour cells, however, continue to divide indefinitely. Avoiding this indefinite division of cancer cells would be perfect and, in fact, much of current research targets precisely this area.

The most relevant results of the research on the telomeres of the oyster fungus are as follows. Firstly, it was observed that these sequences are identical to the human ones and are repeated between 25 and 150 times. Nothing was known about the telomeres of this basidiomycete, P. ostreatus, and they first had to see how the telomeric DNA sequences in humans were organised in order to tackle the sequencing of this fungus. Having the pattern of the former, they discovered that it was similar for P. ostreatus.

Secondly, it was observed that in the regions adjacent to the telomeres of the fungus, which are more complex, dynamic and variable than the telomeres themselves, genes similar to those described in other higher organisms were found, including some associated with diseases involving premature ageing and the Werner syndrome in humans.

Finally, it was shown that, the regions adjacent to the telomeres, genes participating specifically in the type of life of the fungus (laccase genes which code for lignin-degrading enzymes, essential for the biology of this kind of white rot fungi in wood). The presence of these specific genes in the variable regions adjacent to the telomeres may explain the capacity for adaptation of the organisms in different environments and help in understanding evolutionary processes.

The results of this PhD are the first of their kind published and will enable advances to be made in the genetic improvement programmes of edible mushrooms and in the study of the comparative genomics of fungi and higher organisms.