Phyllum Thallophyta - The Algae - Phycomycetes - Monoblepharidales
The lVIonoblepharidales are saprophytic Phycomycetes which live submerged in fresh water on undecorticated twigs. The hyphae are filamentous and coenocytic. The protoplasm forms an extremely regular network which makes them easily recognizable. Rhizoids pass into the substrate upon which the Fungi live, and through them food material is presumably absorbed. The vegetative mycelium is sparsely branched, is rigid and possesses no septa, except in relation to the reproductive organs.
Asexual reproduction is by zoospores produced in filamentous terminal zoosporangia. The zoospores possess a single posterior flagellum. Sexual reproduction is oogamous. The antheridia produce motile antherozoids with a single posterior flagellum. The oogonium produces a single oosphere which is non-motile. The result of fertilization is a thick-walled, goldenbrown oospore.
The order is a small one with a single family, the lVIonoblepharidaceae, which contains only one important genus, Monoblepharis.
Monoblepharis
It is not proposed to describe in detail anyone species, since none is relatively more common than the others and there are differences in the sexual reproduction features which make a single description incomplete.
Until quite recently species of Monoblepharis were regarded as extremely rare Fungi and little was known about their life-histories. From the time when they were first described by Cornu over seventy years ago particular interest has attached to them because they are the only oogamous Fungi known in which a motile male gamete is employed. Recent work on the group has shown that the reason for their apparent scarcity was primarily a matter of understanding the rather peculiar requirements necessary for their active growth, without which they are usually swamped by more vigorous members of the Saprolegniaceae. It is desirable here briefly to indicate these conditions.
The Fungus favours the shallower parts of fresh-water ponds and ditches which are not liable to disturbance, resulting in the deposition of silt. Many and various are the submerged twigs upon which the Fungus will grow, and the host plant appears to have little influence upon the growth of Monoblepharis. When collected such twigs are not likely to show any obvious signs of the presence of the Fungus, but after being washed in water in the laboratory they are placed in glass jars filled half-full of sterile water. The jars are then placed for several weeks in a refrigerator working a few degrees above freezing-point, at the end of which time a copious growth of the Fungus may be expected.
From this fact it may be deduced that the Fungus grows best during the autumn months when the temperature is low and when other aquatic Fungi are less vigorous, and passes the spring and summer in a resting condition, probably in the form of the oospores.
On a well-infected twig the mycelium will appear pale grey in colour, forming a tuft of hyphae about 2 mm. in length. These hyphae are easily recognizable by the regular vacuolation of the protoplasm which gives the coenocytes a net-like appearance. Branching is monopodial and occurs rarely.
ASEXUAL REPRODUCTION
Under favourable conditions, that is between 8° and I2° C., asexual reproduction occurs. The zoosporangia arise as slightly swollen terminal portions of hyphae. They possess at first a homogeneous cytoplasm and many nuclei, and are separated from the rest of the hypha by a septum. By means of cleavage furrows the contents become divided up into uninucleated zoospores. The apex of the zoo sporangium dissoh-es a\yay and the spores creep out by amoeboid movement. These zoospores, even at this stage, possess a single flagellum, three or four times as long as the diameter of the spores, and either immediately, or after a period of amoeboid movement, each swims away. Zoospores may frequently fail to gain the outside of the zoosporangium, but even under such conditions they germinate, producing a single hypha from which a fresh mycelium is formed eventually.
SEXUAL REPRODUCTION
Sexual reproduction is favoured either by warm conditions or by freezing.
The process differs somewhat in the species, firstly in the relative positions of the antheridia and oogonia, and secondly in the position where the oospore matures.
In some species, e.g., lvlonoblepharis polymorpha, M-insignis, M. fasciculata and M. sp. nov., * the antheridium appears to be inserted on the oogonium, but a study of their development shows that in reality the oogonium is formed beneath a terminal antheridium, although as the oogonium matures the antheridium becomes pushed to one side. In others, e.g., Monoblepharus sphaerica and M. hypogyna, the antheridium may develop below the oogonium.
THE ANTHERIDIUM
The antheridium is an oval structure cut off by a septum and is from the first multinucleate. The cytoplasm is homogeneous and when mature is divided transversely into from four to eight uninucleate antherozoids. A small opening appears in the apex of the antheridium, and towards this the antherozoids crawl by amoeboid motion, each trailing its single long flagellum
behind it. In shape these antherozoids resemble the zoospores, but they are smaller and their amoeboid movement is more pronounced.
THE OOGONIUM
After the antheridium has been cut off by a septum, the proximal portion of the hypha below the septum becomes distended and forms a literal projection, which finally enlarges into a clavate body which is also cut off by a cross wall. The oogonium gradually becomes more rounded in shape. The contents then move away from the wall, forming a central spherical oosphere.
In certain species the oogonium is formed terminally at the end of the hypha, and after it has been separated by a septum the portion of the hypha below forms a lateral projection which grows out as an hypogynous antheridium. In one species, Monoblepharis macrandra, the oogonium and antheridium occupy terminal positions on separate hyphal branches. In this case, as growth continues, additional antheridia or oogonia may develop below the terminal ones, which therefore appear to be produced in chains.
When the oosphere is mature the oogonium opens at the top to allow entry for the antherozoid, and mucus is secreted through the opening by means of which the antherozoid becomes attached and its union with the oosphere follows immediately.
THE OOSPORE
The genus Monoblepharis is sometimes divided into two sub-genera which are distinguished by \yhether the oospore develops within the oogonium or whether it leaves the oogonium immediately after fertilization and completes its maturation outside the mouth of the oogonium. In either case a thick golden-brown wall quickly develops around the zygote, forming a very conspicuous structure. The oospore may either remain attached to the oogonium for a considerable time or it may be shed soon after fertilization. It germinates, usually after a period of rest, to produce a hypha from which a new mycelium will be formed.
RELATIONSHIPS
The chief interest of the Monoblepharidales lies in the fact that they are the only group of Phycomycetes, or in fact of the Fungi, in which a relatively large non-flagellate oosphere is fertilized by a small motile, flagellated antherozoid. By those who would relate the Phycomycetes directly with the Algae, this group is used to point out a close similarity to the Chlofophyceae, and in particular to Vaucheria and Oedogonium. On the other hand, those who maintain that the higher Phycomycetes are derived from simpler isogamous Chytridiales see in the antherozoid a similarity to the isogamous gamete of a type like Rhizophidium, pointing out at the same time that its amoeboid character is also a primitive feature. Similarly they consider that in the oogonium and oosphere there is a connecting link with the Saprolegniales and Peronosporales. 'While the possibility of an algal relationship is certainly attractive, it must be admitted that most mycologists now consider the latter view is more probable.
