Phyllum Thallophyta - The Algae - Phycomycetes - Mucorales
The l\Iucorales are Phycomycetes in which the sex organs are relatively simple, and in which sexual reproduction may only occur at very infrequent intervals. When it takes place it consists of the fusion of two non-motile gametes. In most forms, including Mucor, these are morphologically identical, but in some other types they differ in size. Asexual reproduction is by means of aerially distributed spores, which may either be enclosed in a sporangium or may be abstricted in chains from conidiophores.
Consider only one example of this order, Mucor.
Mucor mucedo (The Common Bread Mould or Pin Mould)
This Fungus lives exclusively as a saprophyte and is frequently found producing a whitish-grey down on bread, cheese, jam, and other food material. It is also very widely distributed in the humus of the soil. It may be cultivated by keeping damp horse-dung or bread under a bell-jar for a few days.
The mycelium of Mucor consists of long, slender hyphae, generally white or colourless, which ramify over the surface of the food material and send down absorptive hyphae into it, for it is by this means that the essential nourishment is taken into the mycelium. These hyphae are coenocytic, though cross walls may occur at intervals in old hyphae.
The Fungus grows very rapidly, covering the substance on which it is feeding. Concurrently with growth there is abundant provision for reproduction. Normally this is effected asexually by means of spores, though sexual reproduction occurs very occasionally and under special conditions, which we shall discuss later.
ASEXUAL REPRODUCTION
Within a few days of the appearance of the mycelium asexual reproduction begins. Vertical hyphae (sporangiophores) grow out from the mycelium, which are destined to produce terminal sporangia. The sporangium is formed by the enlargement of the apex of the hypha into a spherical vesicle containing many nuclei. The protoplasm of the sporangium is divided by cleavage planes into uninucleate masses, from each of \yhich an ellipsoidal spore is formed. By a similar cleavage plane a dome-shaped septum is formed in the basal part of the sporangium, called the columella, which separates the body of the sporangium from the sporangiophore. The wall of the sporangium hardens and becomes brittle, and the entire sporangium darkens in colour until it is almost black when mature. The wall is covered with a deposit of small crystals of calcium oxalate. Kot all species of Mucor produce terminal sporangia. In some they may be developed on branched hyphae or in clusters. It is on such characters as these that the species are chiefly separated.
vVhen mature the sporangium wall is surrounded by a drop of water, secreted by the sporangiophore, and breaks round the base, leaving a ragged collar attached to the sporangiophore. The spores are not immediately shed but remain clinging together in a wet mass, and the means of their dispersal is uncertain. It has been claimed that insects are the chief agents, but it seems probable that eventually, when the whole mycelium has died and dried up, that some spores at least are scattered in the air. Their wide dispersal, especially in the soil, is othenyise difficult to understand. Spores germinate immediately on reaching a suitable substratum and produce a hypha from which a new mycelium develops.
SEXUAL REPROUCTION
It is well known that zygospores are only rarely found in cultures of Mucor, but for long the reason for this was not understood. It was shown by Blakeslee in 190+ that the mycelium of the Fungus is unisexual and not capable of producing both male and female gametes. Sexual reproduction, therefore, can only occur when different mycelia intermix. Two distinct
types exist, which are called" plus» and" minus» respectively. Zygospores can only be formed when mycelia of both types meet. There is no morphological distinction between the mycelia, though obviously there must be a physiological one. Such a condition, where the thalli are morphologically alike but physiologically different, is termed heterothallism. It is widespread in the Fungi, and many species are found to exhibit this phenomenon.
When the hyphae of the two strains come into contact, short lateral branches, called progametangia, are developed, which adhere together by their tips. They swell and a transverse septum appears in
each, cutting off a terminal cell or gametangium. The remainder of the progametangium is known as the suspensor. These gametangia are multinucleate, and when mature the wall between them breaks down and a single fusion cell is formed. The wall of the fusion cell becomes thickened and dark coloured, and develops a rough warty surface. This cell is now termed the zygospore. The thick wall forms the exospore, while within it is a thin membrane called the endospore. The suspensors wither and the zygospore is set free.
Various interpretations of the nuclear behaviour inside the zygospore have been suggested, but owing to the very small size of the nuclei there is no agreement bet\yeen the views expressed. It may be that there is general pairing of the nuclei, so that a number of diploid nuclei are produced, or, as seems more likely, that only one nucleus from each gametangium actually fuses.
The zygospore retains its vitality for a considerable time and undoubtedly will resist unfavourable conditions. Since, however, zygospore formation is rare, it must not be regarded as the chief method of tiding the Fungus over bad conditions, for the asexual spores appear perfectly capable of doing this. When germination of the zygospore occurs the wall splits open and a single hypha grows out which terminates in a sporangium. Considerable importance is attached by some to this hypha, which is called a prornycelial hypha, and to the sporangium which it produces. Howeyer, the structure of the sporangium differs in no way from the typical sporangium produced asexually, and the spores produced in it give rise to fresh mycelia in due course.
No definite information is available as to the position of meiosis, though various workers have studied the problem. According to some the zygote
nucleus divides into four prior to germination, a condition analogous with what occurs in Spirogyra. Many suggest that reduction division occurs in the sporangium borne on the promycelial hypha. It is on account of this latter view that the hypha produced by the zygospore assumes special importance for, if reduction division occurs in the sporangium it produces, this hypha must be diploid, in contrast to the rest of the mycelium. Moreover, the sporangium formed on it must also be regarded as distinct, since, unlike asexual sporangia, it is the seat of meiosis. In the light of our present knowledge, however, we cannot say where meiosis occurs, nor are we sure that there is actually any nuclear fusion in the zygospore, though it is reasonable to assume that there is. If so, then at some subsequent stage there must be a meiosis to bring back the nuclei to a monoploid state.
As in the previous genera we can trace the gradual evolution of a conidiospore from the sporangium of Mucor. The conidiospore condition seems to have been reached by the gradual reduction in the number of spores produced in a sporangium until there is only one, when the body may be truly regarded as a conidiospore, since it germinates by a germ tube.
The l\Iucorales are Phycomycetes in which the sex organs are relatively simple, and in which sexual reproduction may only occur at very infrequent intervals. When it takes place it consists of the fusion of two non-motile gametes. In most forms, including Mucor, these are morphologically identical, but in some other types they differ in size. Asexual reproduction is by means of aerially distributed spores, which may either be enclosed in a sporangium or may be abstricted in chains from conidiophores.
Consider only one example of this order, Mucor.
Mucor mucedo (The Common Bread Mould or Pin Mould)
This Fungus lives exclusively as a saprophyte and is frequently found producing a whitish-grey down on bread, cheese, jam, and other food material. It is also very widely distributed in the humus of the soil. It may be cultivated by keeping damp horse-dung or bread under a bell-jar for a few days.
The mycelium of Mucor consists of long, slender hyphae, generally white or colourless, which ramify over the surface of the food material and send down absorptive hyphae into it, for it is by this means that the essential nourishment is taken into the mycelium. These hyphae are coenocytic, though cross walls may occur at intervals in old hyphae.
The Fungus grows very rapidly, covering the substance on which it is feeding. Concurrently with growth there is abundant provision for reproduction. Normally this is effected asexually by means of spores, though sexual reproduction occurs very occasionally and under special conditions, which we shall discuss later.
ASEXUAL REPRODUCTION
Within a few days of the appearance of the mycelium asexual reproduction begins. Vertical hyphae (sporangiophores) grow out from the mycelium, which are destined to produce terminal sporangia. The sporangium is formed by the enlargement of the apex of the hypha into a spherical vesicle containing many nuclei. The protoplasm of the sporangium is divided by cleavage planes into uninucleate masses, from each of \yhich an ellipsoidal spore is formed. By a similar cleavage plane a dome-shaped septum is formed in the basal part of the sporangium, called the columella, which separates the body of the sporangium from the sporangiophore. The wall of the sporangium hardens and becomes brittle, and the entire sporangium darkens in colour until it is almost black when mature. The wall is covered with a deposit of small crystals of calcium oxalate. Kot all species of Mucor produce terminal sporangia. In some they may be developed on branched hyphae or in clusters. It is on such characters as these that the species are chiefly separated.
vVhen mature the sporangium wall is surrounded by a drop of water, secreted by the sporangiophore, and breaks round the base, leaving a ragged collar attached to the sporangiophore. The spores are not immediately shed but remain clinging together in a wet mass, and the means of their dispersal is uncertain. It has been claimed that insects are the chief agents, but it seems probable that eventually, when the whole mycelium has died and dried up, that some spores at least are scattered in the air. Their wide dispersal, especially in the soil, is othenyise difficult to understand. Spores germinate immediately on reaching a suitable substratum and produce a hypha from which a new mycelium develops.
SEXUAL REPROUCTION
It is well known that zygospores are only rarely found in cultures of Mucor, but for long the reason for this was not understood. It was shown by Blakeslee in 190+ that the mycelium of the Fungus is unisexual and not capable of producing both male and female gametes. Sexual reproduction, therefore, can only occur when different mycelia intermix. Two distinct
types exist, which are called" plus» and" minus» respectively. Zygospores can only be formed when mycelia of both types meet. There is no morphological distinction between the mycelia, though obviously there must be a physiological one. Such a condition, where the thalli are morphologically alike but physiologically different, is termed heterothallism. It is widespread in the Fungi, and many species are found to exhibit this phenomenon.
When the hyphae of the two strains come into contact, short lateral branches, called progametangia, are developed, which adhere together by their tips. They swell and a transverse septum appears in
each, cutting off a terminal cell or gametangium. The remainder of the progametangium is known as the suspensor. These gametangia are multinucleate, and when mature the wall between them breaks down and a single fusion cell is formed. The wall of the fusion cell becomes thickened and dark coloured, and develops a rough warty surface. This cell is now termed the zygospore. The thick wall forms the exospore, while within it is a thin membrane called the endospore. The suspensors wither and the zygospore is set free.
Various interpretations of the nuclear behaviour inside the zygospore have been suggested, but owing to the very small size of the nuclei there is no agreement bet\yeen the views expressed. It may be that there is general pairing of the nuclei, so that a number of diploid nuclei are produced, or, as seems more likely, that only one nucleus from each gametangium actually fuses.
The zygospore retains its vitality for a considerable time and undoubtedly will resist unfavourable conditions. Since, however, zygospore formation is rare, it must not be regarded as the chief method of tiding the Fungus over bad conditions, for the asexual spores appear perfectly capable of doing this. When germination of the zygospore occurs the wall splits open and a single hypha grows out which terminates in a sporangium. Considerable importance is attached by some to this hypha, which is called a prornycelial hypha, and to the sporangium which it produces. Howeyer, the structure of the sporangium differs in no way from the typical sporangium produced asexually, and the spores produced in it give rise to fresh mycelia in due course.
No definite information is available as to the position of meiosis, though various workers have studied the problem. According to some the zygote
nucleus divides into four prior to germination, a condition analogous with what occurs in Spirogyra. Many suggest that reduction division occurs in the sporangium borne on the promycelial hypha. It is on account of this latter view that the hypha produced by the zygospore assumes special importance for, if reduction division occurs in the sporangium it produces, this hypha must be diploid, in contrast to the rest of the mycelium. Moreover, the sporangium formed on it must also be regarded as distinct, since, unlike asexual sporangia, it is the seat of meiosis. In the light of our present knowledge, however, we cannot say where meiosis occurs, nor are we sure that there is actually any nuclear fusion in the zygospore, though it is reasonable to assume that there is. If so, then at some subsequent stage there must be a meiosis to bring back the nuclei to a monoploid state.
As in the previous genera we can trace the gradual evolution of a conidiospore from the sporangium of Mucor. The conidiospore condition seems to have been reached by the gradual reduction in the number of spores produced in a sporangium until there is only one, when the body may be truly regarded as a conidiospore, since it germinates by a germ tube.
HETEROTHALLISM
The phenomenon of heterothallism is so important in the Fungi that we must say something more about it. Though the process was first discovered in the :\Iucorales by Blakeslee in 1904, it has now been shown to occur in many families of Fungi. In many species there is nothing in the structure of the mycelia to distinguish them from one another, but where they meet a line of zygospores is formed. In such cases it is customary to speak of one strain as plus ( + ) and the other as minus ( - ).
In a few species of Mucor it has been found that zygospores are produced by the interaction of hyphae in a mycelium produced from one spore, and such species are said to be homothallic. In some instances imperfect hybridization may occur between opposite strains of different heterothallic species and between both (+) and (-) strains of heterothallic species on the one hand and homothallic species on the other. In such cases only the early stages are gone through and the mature zygospore is never produced.
In certain other genera the gametangia of homothallic species are either not quite alike, or they may even differ markedly from one another. Such a species is said to be a heterogamic homothallic one. One such example is Absidia spinosa in whicG the gametangia are of different sizes. Here the larger is regarded as equivalent to the female and the smaller to the male gamete. It has been found possible to produce a hybrid between Absidia spinosa and 1VIucor hiemalis, which has identical gametangia, that is, it is homogamic but heterothallic. As a result of such a cross it has been discovered that the (-) strain of Mucor hiemalis united with the larger gametangium of Absidia spinosa, while the ( +) strain of M. hiemalis united with the smaller gametangium. It is therefore concluded that the ( + ) strain of a heterothallic homogamic species is the female strain, while the ( - ) strain is male. It may be mentioned that, as a result of biochemical tests, these conclusions of Blakeslee have been substantiated.
