Abstract
Hortaea werneckii, ascomycetous yeast from the order Capnodiales, shows an exceptional adaptability to osmotically stressful conditions. To investigate this unusual phenotype we obtained a draft genomic sequence of a H. werneckii strain isolated from hypersaline water of solar saltern. Two of its most striking characteristics that may be associated with a halotolerant lifestyle are the large genetic redundancy and the expansion of genes encoding metal cation transporters. Although no sexual state of H. werneckii has yet been described, a mating locus with characteristics of heterothallic fungi was found. The total assembly size of the genome is 51.6 Mb, larger than most phylogenetically related fungi, coding for almost twice the usual number of predicted genes (23333). The genome appears to have experienced a relatively recent whole genome duplication, and contains two highly identical gene copies of almost every protein. This is consistent with some previous studies that reported increases in genomic DNA content triggered by exposure to salt stress. In hypersaline conditions transmembrane ion transport is of utmost importance. The analysis of predicted metal cation transporters showed that most types of transporters experienced several gene duplications at various points during their evolution. Consequently they are present in much higher numbers than expected. The resulting diversity of transporters presents interesting biotechnological opportunities for improvement of halotolerance of salt-sensitive species. The involvement of plasma P-type H+ ATPases in adaptation to different concentrations of salt was indicated by their salt dependent transcription. This was not the case with vacuolar H+ ATPases, which were transcribed constitutively. The availability of this genomic sequence is expected to promote the research of H. werneckii. Studying its extreme halotolerance will not only contribute to our understanding of life in hypersaline environments, but should also identify targets for improving the salt- and osmotolerance of economically important plants and microorganisms.
Highlights
Salinization of soil as a form of land degradation is rendering large areas of arable lands useless for crop cultivation and is an increasingly important cause of agricultural losses [1,2]
Even in the seven species belonging to the same order as H. werneckii (Capnodiales) the genome sizes are very variable
In H. werneckii, the proportion of repetitive sequences remains low at only 1.02%, despite its large genome size
Summary
Salinization of soil as a form of land degradation is rendering large areas of arable lands useless for crop cultivation and is an increasingly important cause of agricultural losses [1,2]. Attempts at breeding salt-tolerant lines or cultivars of crops have failed to overcome this problem and genetic engineering has yet to yield the desired results [4]. The majority of genes that have been used for this purpose to date have originated from salt-sensitive donors. Demands considerably more effort compared to conventional model organisms with sequenced genomes. The draft genome sequence of one such fungus, Hortaea werneckii, can open new avenues for experimental exploitation of its genetic resources. These may prove to be useful for crop improvement, and for industrial microorganisms.
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