The following points highlight the three ways for accomplishment of plasmogamy in lower fungi. The ways are: 1. Planogametic Copulation 2. Gametangial Contact 3. Gametangial Copulation.
Way # 1. Planogametic Copulation:
It is characteristic of the primitive planogametic lower fungi (Chytridiomycetes and Plasmodiophoromycetes). In this kind of gametic union, one or both the gametes of a uniting pair are motile (flagellate) and are called the planogametes. The planogametes in the Chytridiomycetes are uniflagellate.
The single flagellum is of whiplash type. It is inserted at the posterior end (opisthocont). The Plasmodiophoromycetes produce biflagellate gametes. Both the flagella are of whiplash type but of unequal length. They are inserted at the anterior end. The shorter one has blunt tip and the longer one has a pointed tip.
(i) Isoplanogametic Copulation (Fig. 3.8 A-C):
The gametic union in the primitive chytrids (Olpidium) and Plasmodiophoromycetes (Plasmodiophora) takes place between two morphologically similar motile gametes known as the isoplanogametes (A). Sexual fusion between isoplanogametes is described as isoplanogametic copulation (B-C).
(ii) Anisoplanogametic Copulation (Fig. 3.9):
Members of the order Blastocladiales belonging to the genus Allomyces produce two types of planogametes which are morophologically similar but differ in size. These are termed anisoplanogametes (A, B). The gametic union between anisoplanogametes (C) is known as anisoplanogametic copulation (D). It is illustrated by Allomyces.
(iii) Ooplanogametic Copulation (Fig. 3.10):
Monoblepharis illustrates the most advanced type of planogametic copulation. It produces small opisthocont male gametes in large numbers in the sex organ called antheridium (A). These are known as the sperms. The oogonium produces a single large immobile female gemete, the egg or ovum (A).
The sperm enters the oogonium through the apical pore to fertilize the egg (B). The gametic union between a motile male gemete (sperm) and an immobile female gamete (egg or ovum) is called ooplanogametic copulation. It is significant to note that Monoblepharidales are the only oogamous fungi with a motile male gamete (sperm).
Way # 2. Gametangial Contact (Fig. 3.11):
A group of aplanogametic lower fungi (Oomycetes) produce non-motile gametes (aplanogametes) that are always oogamous. The aplanogametes are never released from the gametangia to the outside. The gametangia in which the gemetes are produced are highly differentiated. The male gametangium is called the antheridium and the female oogonium. No sperms are organised.
The oogonium may contain one to many eggs. At the time of sexual act, the antheridium and the oogonium come in contact (A). They do not actually fuse and do not lose their identity. The antheridium in this position puts out a slender, tubular outgrowth at the point of contact with the oogonium. It is known as the fertilization tube.
The latter pierces the thin spot in the oogonial wall and dips deep into the oogonium. It then ruptures at its tip to introduce the male nucleus (or nuclei) into the oogonium (B) where it fuses with the female nucleus (or nuclei) to accomplish fertilisation. In rare cases, the male nucleus may enter the oogonium through a pore formed by the dissolution of the intervening walls at the point of contact.
Way # 3. Gametangial Copulation:
In another group of aplanogametic lower fungi which are strictly terrestrial (Mucorales), gametic union is brought about by the fusion between the gametangia. The uniting gametangia lose their identity in the sexual act. This kind of gametic union is called gemetangial copulation. The uniting gametangia are usually morphologically similar (isogamous). Rarely they are dissimilar and unequal in size (anisogamous).
(i) Isogametangial Copulation (Fig. 3.8 D-F):
Typical examples of this method of gametic union are Rhizopus and Mucor. Some species of these genera are homothallic and certain other are heterothallic.
In the latter case sexual reproduction takes place only when two hyphae of opposite strains come in contact. Morphologically these are similar. At this point of contact, each of them produces a short, lateral copulating branch, the progametangium. The tips of two progametangia enlarge.
Later a septum appears near the swollen tip of each progametangium dividing it into a terminal gametangium and a basal suspensor (D). The intervening walls between the two gametangia finally dissolve forming a common fusion cell (E). The multinucleate protoplasts (coenogametes) of the two copulating gametangia mingle in the fusion cell.
The nuclei of opposite strains in the heterothallic species fuse in pairs to form diploid nuclei. Within the parent gametangial wall, the young zygospore secretes around it a cell wall consisting of two layers. The outer spore wall layer is thick, dark and sculptured. As the zygo spore increases in size, the parent gametangial wall ruptures exposing the dark, rugged exospore of the zygospore (F).
(ii) Anisogametangial Copulation (Fig. 3.12, A-C):
In another genus of the Mucorales called Zygorhynchus, the copulating gametangia and their suspensors are unequal in size (B). Fusion between unequal gametangia is termed anisogametangial copulation.
(iii) Anisogametangial Contact (Fig. 3.12, D-G):
Endogone lactiflua exhibits distinct anisogamy. Of the two progametangial hyphae which come in contact, one is larger in diameter the other (D). Both are multi-nucleate. Multinucleate end cells known as gametangia are delimited at their tips. All the nuclei except one degenerate in the two contacting gametangia (E) which are unequal.
They do not actually fause or lose their identity during the sexual act. The surviving nucleus of the smaller male gametangium migrates into the larger female gametangium through a pore formed by dissolution of the intervening gametangial walls at the points of contact (F).
Later both the nuclei, male as well as female, migrate into an outgrowth of the female gametangium (G). There they fuse to accomplish fertilisation. In Phycomycetes, plasmogamy is immediately followed with karyogamy.
