''Wallemia ichthyophaga'' is one of the three species of

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

in the genus ''Wallemia'', which in turn is the only genus of the class Wallemiomycetes. The phylogenetic origin of the lineage was placed to various parts of Basidiomycota, but according to the analysis of larger datasets it is a (495-million-years-old) sister group of

Agaricomycotina The subdivision Agaricomycotina, also known as the hymenomycetes, is one of three taxa of the fungal division Basidiomycota (fungi bearing spores on basidia). The Agaricomycotina contain some 20,000 species, and about 98% of these are in the cla ...

. Although initially believed to be asexual, population genomics found evidence of recombination between strains and a mating type locus was identified in all sequenced genomes of the species. Only a limited number of strains of ''W. ichthyophaga ''have been isolated so far (from hypersaline water of solar salterns, bitterns (magnesium-rich residual solutions in salt production from sea water) and salted meat). ''W. ichthyophaga'' requires at least 1.5 M NaCl for in-vitro growth (or some other osmolyte for an equivalent

water activity Water activity (''aw'') is the partial vapor pressure of water in a solution divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most often defined as pure water at the same tempe ...

), and it thrives even in saturated NaCl solution. This makes it the most

halophilic The halophiles, named after the Greek word for "salt-loving", are extremophiles that thrive in high salt concentrations. While most halophiles are classified into the domain Archaea, there are also bacterial halophiles and some eukaryotic species, ...

fungus known and distinguishes it from halotolerant (e.g. ''

Aureobasidium pullulans ''Aureobasidium pullulans'' is a ubiquitous and generalistic black, yeast-like fungus that can be found in different environments (e.g. soil, water, air and limestone). It is well known as a naturally occurring epiphyte or endophyte of a wide ra ...

'') and extremely halotolerant fungi (e.g. '' Hortaea werneckii''), which are able to grow well even in the absence of salt in the medium. Inability to grow without salt is an exception in the fungal kingdom, but is common in halophilic Archaea. The fungus grows in the form of sarcina-like structures, or compact multicellular clumps. These increase in size almost four-fold when exposed to high salinity, and the cell wall experiences a three-fold thickening. This results in a substantially decreased functional cell volume and is thought to be one of the halotolerance mechanisms of this species. The

whole genome sequencing Whole genome sequencing (WGS), also known as full genome sequencing, complete genome sequencing, or entire genome sequencing, is the process of determining the entirety, or nearly the entirety, of the DNA sequence of an organism's genome at a ...

of ''W. ichthyophaga'' revealed that it has one of the smallest of all sequenced basidiomycetous genomes (9.6 Mbp, only 4884 predicted proteins). Contrary to what was observed for the extremely halotolerant '' H. werneckii'', in ''W. ichthyophaga'' there are almost no expansions in metal cation transporter genes and their expression is not salt-responsive. On the other hand, there is a vast enrichment of

hydrophobin Hydrophobins are a group of small (~100 amino acids) cysteine-rich proteins that are expressed only by filamentous fungi that are lichenized or not. They are known for their ability to form a hydrophobic (water-repellent) coating on the surface ...

s (proteins of cell wall with diverse functions and many biotechnological uses), which contain an unusually high proportion of acidic amino acids. High proportion of acidic amino acids is thought to be an adaptation of proteins to high concentrations of salt. After sequencing the genomes of nearly all known strains of ''W. ichthyophaga,'' population genomic analysis showed that the species forms a single recombining population.

References

{{Taxonbar, from=Q10719177 Wallemiales Fungi described in 1887