Leandro Ferreira Moreno successfully defended his thesis by Webmaster News 2019-01-18 09:26:07

Leandro Moreno successfully defended his PhD Thesis on Thursday November 15 at the University of Amsterdam, with the title ¬†‘The biology of black yeast genomes’. It is an interesting mix of factual and extensive explorations of the genomic diversity in black yeasts and relatives, with some more speculative chapters, like the intriguing question on understanding an infectious brain disease from the desert.

Leandro focuses on the order Chaetothyriales (Pezizomycotina, Ascomycetes). Members of this order ‘exhibit a complex ecological variation, and species are found in habitats characterised by extreme and adverse conditions, like rock surfaces in hot arid climates, in toxic niches with hydrocarbons and heavy metals and remarkably often in vertebrates as opportunistic pathogens.’
Some species cause horrific mutilating or even fatal infectious diseases in humans, often in apparently healthy individuals. Exactly this combination of properties makes the black yeasts and their relatives so interesting.

Leandro makes an ancestral reconstruction in his thesis and places the emergence of an early black yeast lineage 75-50 million years ago, during or after the Cretaceous-Paleogene extinction event.
His data suggest that from this time onward, gene duplications took place. This seems to be the most important evolutionary process that has shaped the genome of Chaetothyriales. Gene duplication and mutation is associated with adaptation to extreme environments, the preferred habitat of black yeasts.

Some of these fungi are human pathogens or opportunists. The hypothesis is that some genes that enable them to survive under extreme conditions have dual functions. Primarily these are meant for life in their natural habitat, protecting them for instance against natural stressors, while by coincidence such mechanisms could also be used to invade and colonize vertebrate hosts.
Especially these dual mode genes seem to have been duplicated in black yeasts. Particularly genes associated with the assimilation to aromatic compounds have expanded in black yeast genomes.

In the very interesting chapter five of the thesis Leandro studies the genome of Rhinocladiella mackenziei, a black fungus that accounts for the majority of fungal brain infections in the Middle East and which is fatal if untreated. He compares the genome with that of other known neurotropic fungi (neurotropism is the affinity of a pathogen for the central nervous system).
The natural origin of the fungus is unknown, yet the infections only appear in the arid climate zone. The gene composition and metabolic potential indicate extremotolerance and aromatic hydrocarbon assimilation, suggesting a possible environmental habitat of oil polluted desert soil. The isolated and sequenced strains of R. mackenziei in this chapter come from brain biopsies of patients from Saudi Arabia and Qatar.
How the brain gets infected with the pathogen is not certain, presumably by inhalation, but brain infections by R. mackenziei are primary cerebral infections, i.e. with the brain as the first site of symptoms, like that by other Herpotrichiellaceae, such as Cladophialophora bantiana and Fonsecaea monophora.
There are indications for a link between aromatic hydrocarbon assimilation and fungal neurotropism; neurotransmitters, found in the central nervous system and the brain are structurally similar. Having analyzed the genome of R. mackenziei Leandro Moreno comes to the conclusion the fungus differs from true pathogens, it is really an opportunist.

The same goes for other black yeasts. The fact that some known potentially deadly black yeasts are quite common, some of them being found in practically every dishwasher or bathroom, and nevertheless very few humans acquire an infection, suggests that they are not specialized for infecting humans. It seems that some people are genetically prone to infections with these yeasts.