Many Candida yeast pathogen species are rarely but increasingly found and we call them “emerging pathogens”. But, from where do they emerge? We found that in one such, Candida orthopsilosis, hybrids between the same two parents had been formed multiple independent times Although most of the clinical strains were hybrids, one of the parents—we called it parent A—could be rarely found too, but we’d never seen its soulmate (parent B) among clinical samples. We started a search for C. orthopsilosis strains isolated from the environment, and finally got our hands on isolates from the warm sea around Qatar. We found that, although most of the marine strains were hybrids, some of them corresponded to the elusive parent B. This unexpected finding gave us a complete overview of the evolution of this emerging pathogen. We discovered that parent B grew at high temperatures, an ability that parent A lacked completely and that hybrids had inherited to some extent. This is a very important virulence trait, as our first barrier of defence against fungal infections is our basal body temperature. To our surprise, we found that parent B and
marine hybrids were as virulent as strains isolated from the clinics. We also saw hybrid strains isolated from the clinics or from the marine environment did not differ much. This, which might seem unimportant at first, suggests an environmental origin of this species and tells us that these pathogens likely did not evolve to specifically infect us. But instead, those features enabling them to survive in the marine environment might also be useful for proliferating inside us. These results suggest that hybridisation, which we hypothesise happening in warm marine waters, could play a key role in the emergence of new hybrid pathogens. This could lead to the selection of thermotolerant strains capable of breaching our thermal barrier and causing disease, especially when our immune systems—our second defence barrier—are debilitated.
