Oxylipin production and novel ascospore release mechanisms in the yeast Dipodascus

English: Certain ascomycetous yeasts produce "lubricated" 3-hydroxy (3-OH) oxylipin-coated, micron-scale sexual spores in a variety of shapes, sizes, colors and sometimes with nano-scale surface ornamentations. In past literature, these oxylipin-coated ornamentations are only mentioned for use in classification and no thought was given to their possible purpose or function. These 3-OH oxylipins were found to be associated with the sheathed ascospores of some species representing the genus Dipodascus. However, these compounds have not yet been studied in detail in this genus. In addition, no thought was given to the mystery behind the release mechanics of sheathed ascospores from enclosing bottle-shaped asci and the role of these 3-OH oxylipins during dispersal. Therefore, it became the aim in this study to determine 3-OH oxylipin structure, distribution and function in D. albidus UOFS Y-1445T and D. geniculatus UOFS Y-1144 and to reveal the secrets behind the release mechanics of sheathed ascospores from bottle-shaped asci in these two species. Using electron microscopy, confocal laser scanning microscopy, gas chromatography-mass spectrometry (GC-MS) and digital live imaging, the release mechanisms of ascospores of various lengths from bottle-shaped asci, and produced by different yeasts, are compared in this study. It was found that Dipodascus albidus produces oval ascospores that are surrounded by compressible, oxylipin-coated sheaths enabling ascospores to slide past each other when forced by turgor pressure and by possible sheath contractions towards the narrowing ascus-neck. Usually elongated ascospores, as found in Dipodascopsis uninucleata var. uninuc/eata, are linked by means of oxylipin-coated interlocked hooked ridges on the surfaces of neighbouring ascospores, thereby keeping them aligned while being pushed towards the ascus-tip by turgor pressure. Interestingly, it was uncovered in this study that Dipodascus geniculatus produce elongated, oxylipin-coated sheathed ascospores that are effectively released from bottle-shaped asci without alignment. This is possible because the ascus neck and opening have the same diameter as the length of the ascospore, thus allowing the ascospores to turn sideways without blocking the ascus upon release. Interestingly, it was found that increased concentrations of acetylsalicylic acid (ASA), inhibit both ascospore release and 3-OH oxylipin production in these yeasts, thereby implicating this oxylipin in sexual reproduction. Using GC-MS analysis, the oxylipin was characterized as a3-OH metabolite. The same mass spectrum, implicating the presence of the same 3-0H oxylipin structure, was found to be present in other Dipodascus species. This implicates that this 3-0H metabolite might be conserved on genus level.