Phylum Thallophyta - The Algae - Ascomycetes - Saccharomycetales
The organisms included in this order are of somewhat diverse character, but they are all predominantly unicellular. They have probably originated from more than one group of Fungi and are now classed together because of their similarity of form. Some Fungi which are not at all closely related to this order can adopt under certain conditions a yeast-like habit of growth, so that the unicellular condition may have been arrived at along ore than one evolutionary line. It is at any rate reasonably certain that this feature is secondary or derived, so far as this group is concerned and not primitive in the evolutionary sense.
Some members of the order possess a definite sexual process, but in many it is wholly lacking. In the latter case reproduction is entirely vegetative. It may be that such species are really heterothallic, in which case the chances are very great against the two strains meeting to conjugate, except in artificial culture.
Saccharomyces cerevisiae (The Beer Yeast)
The cells of Saccharomyces are very minute and either spherical or slightly oval. They are about 10u in diameter and are clothed with a delicate membrane, which appears to be a compound of two polysaccharides, glycogen and mannan, in combination with phosphoric acid. Cellulose is not present. There is abundant cytoplasm containing several small vacuoles in which are often grains of volutin (so called from its having been discovered in the bacterium Spirillumvolutans), which is ribo-nucleic acid or one of its salts.
During fermentation glycogen increases in the vacuoles so that they stain dark brown with Iodine. This carbohydrate is the chief metabolic product and reserve material of the yeast cell.
The method of nuclear division is still uncertain, some authorities maintaining that it is entirely amitotic, while others claim the existence of a simple form of mitosis with two chromosomes. The latter view seems the more probable in view of the life-cycle described below.
Species of yeast have been kept in cultivation by mankind since early times, and are widely used in fermentation processes, e.g., in beers, wines and milks, and for baking. Many species are, however, to be found wild on the surfaces of higher plants, especially on the fruits. When the latter drop in autumn the yeasts pass into the soil where they spend the ,,-inter. In spring they are splashed by rain back on to the plants, thus completing the cycle. We will deal with the chemistry of the fermentation of sugars by yeasts under Respiration in Plant Physiology.
ASEXUAL REPRODUCTION
Reproduction in Sacchawmyces takes place in two ways. The first is by the formation of buds on the vegetative cells, and the second is by means of ascospores, the true nature of which is open to question. Budding takes place very rapidly when the food supply is abundant and is in many species the only known method of multiplication. A bud begins as a small protrusion of the wall of the mother cell, usually near one end, which rapidly enlarges until it is as large as the mother cell itself. The junction of the
two cells is by a very narrow neck, and buds are therefore usually detached easily. Sometimes, however, when growth is most vigorous a bud will itself produce another bud or buds before it is detached, so that a short chain of cells is formed. It is not infrequent to find that the young cells remain attached to each other and to the parent cell for some time, so that quite large
groups may be found, containing up to a hundred cells, which remind us thai the yeasts, although normally unicellular, may be closely related to types of fungi which produce regular mycelia. When a bud is formed the nucleus of the cell divides and one portion passes into the bud, but whether this division is a mitotic process has not been fully established.
SEXUAL REPRODUCTION
Ascus formation occurs when the food supply, and especially when the water supply, is scanty. The commonest method of inducing the formation of asci is by so\ying an actiye culture on a damp plaster block in a closed glass dish. Another way is to spread the yeast on a sterilized slice of carrot which is kept dry. The asci are slightly enlarged vegetative cells, the contents of which divide and round off into four ascospores with rather dense and highly refractive walls.
When an ascus breaks down the four spores germinate, and now they fuse together in pairs with fusion of nuclei to produce two zygote, which start budding immediately and produce vegetative which will in time give rise to asci again. It sometimes happens, ho that an ascospore germinates without conjugation, and in this case it rise to much smaller, almost spherical cells, which also multiply by budding and form a " dwarf" strain. These cells cannot produce asci until two of them have conjugated to form a zygote, which by budding produces the normal, large-celled strain again. Frequently many such conjugations occur simultaneously among the dwarf cells so that it would appear as if the dwarf strain had almost wholly turned into the large-celled strain.
These facts find their explanation readily enough when viewed as an alternation of generations. The normal large-celled strain is diploid, i.e., is sporophytic, and gives rise to sporangia (asci) in which presumably meiosis occurs at spore formation. The ascospores would therefore, as in other cases, be monoploid. If they germinate singly they give rise to a smallcelled gametophytic strain. This forms no definite sexual organs, but conjugation takes place, as in some other Ascomycetes, between vegetative cells, producing diploid zygotes, which reproduce the diploid strain once more. It happens, however, that in most cases the gametophyte phase is short-circuited, and the first cells produced from the germinating ascospores, instead of developing vegetatively, conjugate immediately to form a diploid zygote, so that the ascospores in such a case are the only monoploid cells in the life-cycle. whether there is any advantage in such a procedure, or whether it merely is the result of the propinquity of the ascospores we cannot say, but we know that such short-circuiting of an alternating cycle does in fact occur in very various types of organisms, e.g., in Puccinia among the Fungi; in Fucus among the Algae, and in the phenomenon of Apospory among Ferns.
The discovery of the conjugation between germinating ascospores in Yeast has opened up very remarkable and valuable possibilities of artificial hybridization, and already a considerable number of such hybrids have been created in the search for improved capabilities of fermentation.
Schizosaccharor.nyces octosporus
We may contrast the account of Saccharomyces with the condition in another member of the Saccharomycetaceae, Schizosaccharomyces octosporus. In this species the vegetative cells are more or less spherical and, like all yeasts, are capable of multiplication by budding. Under certain conditions, however, reproduction by a sexual process may occur. Two adjacent cells of similar size put out processes which come into contact and fuse, forming a conjugation tube. The nucleus from each cell passes into the conjugation tube and there they fuse together. The two conjugating cells now enlarge, and eventually a single, large oval structure is formed, which is an ascus. The zygote nucleus divides to form usually eight nuclei, around which ascospores become organized. Sometimes the remains of the conjugating cells can be distinguished after the formation of the ascospores, in which case four appear to lie in each of these cells.
In this species, therefore, it is clear that the ascus is a post-fertilization structure agreeing in the method of its production with that of a typical Ascomycete, and there is some evidence that meiosis occurs during the formation of the ascospores. Hence in this type the true ascomycetaceous nature of the yeasts becomes more apparent.
