Allomyces macrogynus
Allomyces macrogynus is a species of fungus in the family Blastocladiaceae. It was first described by mycologist Ralph Emerson in 1941 as a variety of Allomyces javanicus, and later given distinct species status in 1954. Its genome has been sequenced by the Broad Institute.
Genome studies
The genome of Allomyces macrogynus has been Whole [genome sequencing|sequenced] and this makes it desirable to review an organism of interesting structure and one which is responsive to environmental changes in easily observable ways. In Seattle 1969, at an informal meeting of Emerson, Machlis, Olson, Seale and Youatt, Youatt agreed to study chemical aspects of the fungus so that when the genome was known gene activity could be related to what the genes governed.Normal growth
Allomyces macrogynus features defined by Emerson and Emerson and Wilson were of immediate interest for research and teaching because the organism had such clear and interesting structures. The vegetative growth showed the formation of rhizoids, hyphae and branching and then in the diploid cultures two kinds of fruiting body, zoosporangia ZS that reproduced the diploid organisms and resting or resistant sporangia RS that led to the haploid organism. Then on the haploid hyphae gametangia were produced with small terminal male gametangia containing carotene and larger female gametangia below.Olson reviewed studies to 1984 that included chemotaxis of male gametes to female gametes, the identification of the hormone sirenin, studies of the chemistry of walls and discharge plugs and methods for classroom demonstrations. His comprehensive monograph also compares Allomyces with other fungi in detail.
The organism which lives in tropical ditchwater has a range of survival mechanisms which can be studied in the laboratory. These include chemotaxis of zoospores to amino acids, especially leucine and lysine and to some peptides and to oxygen, and a minicycle where a germinated spore, deprived of nutrients, can produce another zoospore to move on to better conditions. Allomyces macrogynus also shows chemotropism in the growing hyphal organisms by which the rhizoids can grow towards amino acid sources and the hyphae to a better oxygen supply. The diploid organisms can produce zoosporangia ZS when conditions are good and the resistant or resting sporangia RS when they are unfavourable. The RS can survive desiccation for years.
Methodology
Study requires synchronous culture in defined media. Allomyces macrogynus is commonly grown in media with casein hydrolysate and yeast extracts as the source of nitrogen and growth factors but can be grown in a variety of chemically defined media. The simplest of these had ammonium salt as the only source of nitrogen. Defined media allow selection of ZS or RS in diploid plants and male or female gametangia in haploid plants, the chief factor being the ratio of amino acids to glucose.Interpretation of results is always easier if organisms are grown in chemically defined media and media could be very simple as would be expected for a saprophytic organism first isolated from ditch water. In this context it is worth noting that, although methionine is supplied in all culture media, the organisms can synthesise methionine and in their natural environment they probably use sulfide available at low concentration. Methionine is required for branching and, if added just before branching of a growing culture, hydrogen sulfide, cysteine and homocysteine can all be used.
Methods based on vortex mixing and osmotic shock cause death of many spores. Casein hydrolysate CH or mixtures of leucine and lysine can also be used. Small peptides in hydrolysed CH were also effective.
Casein hydrolysate CH was good for producing synchronous germination. Zoospores encysted and attached to the unshaken glass vessel and the CH could then be removed and replaced with defined medium. As wall development began the organisms detached from glass and with suitable shaking grew as suspensions of single organisms, ideal for observation. With new ways to produce RS synchronous cultures of haploid organisms could now be grown in the same way from selectively produced mature RS. For chemically defined induction of germination mixtures of leucine and lysine or phenylpyruvate were the best of many compounds tested.