In this article we will discuss about the hyphal structure of allomyces. This will also help you to draw the structure and diagram of allomyces.
The hyphal wall consists chiefly of Chitin, B glucan and ash. Within it, is the plasma membrane which closely investes the hyphal cytoplasm. Embedded in the cytoplasm are the numerous nuclei. Besides the nuclei there are mitochondria and roughly unique organelles termed the concentric granules.
They are found in abundance in each hyphal segment. The concentric granules move about in a slow random fashion separate from the normal cytoplasmic streaming. According to Robert (1987) concentric granules act as plugs for septal pores and thus prevent protoplasmic loss of following damage.
The vegetative thalli in Allomyces are of two types, gametothalli and sporothalli. The former are haploid and the latter diploid. In the vegetative phase the two are indistinguishable. Towards maturity the gametothalli bear gametangia and sporothalli produce sporangia. Thus, the two types of thalli can be distinguished only when they begin to form the reproductive organs.
1. Gametothallus (Fig. 4.9):
It is concerned with sexual reproduction and is homothallic (B, C). Sexual reproduction like simple chytrids is by planogametic copulation but unlike chytrid the planogametes in Allomyces are anisogometes and the two types of gametes are produced in distinctly distinguishable male and female gametangia.
Anisogamous sexual reproduction takes place by the union of uniflagellate, anisoplanogametes. Reaching a certain stage of maturity, the gametothallus bears gametangia terminally on the side branches in close proximity in pairs or short chains.
The two kinds of gametangia are more or less globose and usually occur in the ratio of 1: 1, the orange male gametangium alternating with the colourless female gametangium Gametangial development. It was studied by Morrison (1977 a) in A. macrogynus.
He reported that a septum appears near the hyphal tip to delimit the male gametangium. A second septum appears immediately before the first to cut off the female gemetangium into more or less globose gametangia in pairs (B) or short chains. The male and female gametangia alternate with each other.
(i) Female gametangium:
It is grey or hyaline and may be borne above the male gemetangium (A. arbuscula, A) or below it (A. macrogynus, C). The multinucleate gametangial contents undergo progressive cleavage to form uninucleate, uniflagellate, female gametes, and 10 µm in size (Fig. 4.10 C2). They are about double the size of the male gametes.
(ii) Male gametangium:
It is smaller in size than the female gametangium and is orange or red in colour. The colour is due to the presence of carotenoid pigment in the cytoplasm. The uninucleate male gametes formed by progressive cleavage of the male protoplast are pigmented. They are about half the size of the female gametes and are uninucleate and uniflagellate (Fig. 4.10C).
Gametes. (Fig. 4.10C):
Both types of gametes (male and female) are motile, uninucleate and posteriorly unflagellale. The single flagellum is of whiplash type. The gamete nucleus has a prominent nucleolus. The male gamete is smaller than female but has the same structure.
(iii) Gamete Discharge (Fig. 4.10 B):
Following gametogenesis (differentiation of gametes) within the respective gametangia, the motile gametes of both kinds are discharged singly one after the other into the water at about the same time through the exit papilla one or more in number located at the various positions in the gametangial cell wall. After discharge the orange or red male gametes swim away quickly from the male gametangia.
The female gametes in A. macrogynus emerge from the gametangia. Some disperse and swim away but remain together in groups, small or large. From the clusters, they disperse with time. The separated female gametes do not swim for an extended period. Soon they settle down at the bottom. The female gemetes thus are not very active in A. macrogynus.
(iv) Plasmogamy:
The discharged female gametes produce a sexual hormone called sirenin. It serves as a chemoattractant causing directed movements of the male gametes towards the female gametes. According to Pommerville (1978), the contact of the male with the female gamete is still by a random hit. Sirenin, however, greatly increases the chances that a hit will not go amiss.
The male and the female gametes copulate in pairs (D). Pairing of the anisoplanogametes is soon followed by the fusion of their membranes and mingling of their cytoplasm. It is termed plasmogamy. Plasmogamy thus results in the formation of a binucleate fusion cell (E).
(v) Karyogamy:
Soon after plasmogamy the two nuclei (male and female) fuse to accomplish karyogamy.
(vi) Zygote:
The resultant fusion cell with a diploid nucleus is called zygote. It is motile and biflagellate. It moves sluggishly for 5 to 10 minutes (F). It then comes to rest, retracts the flagella, rounds up and secretes a wall around it (G).
Germination of Zygote (Fig. 4.10 H-I):
The zygote germinates immediately. There is thus no resting sexual spore in the life cycle. The function is taken over by the resting sporangia. At the time of germination the zygote puts out a slender basal germ tube (H), which grows and branches to form the rhizoidal system (I).
The body of the zygote enlarges and elongates in the opposite direction to form the thick, trunk-like hyphal tube which grows vigorously and branches successively dichotomously to produce the diploid sporothallus (J), apparently resembling the gametothallus in form.
2. Sporothallus (Fig. 4.11):
It is concerned with asexual reproduction. At maturity it produces two types of sporangia: (i) the colourless, thin- walled zoo or mitosporangia (A3) and (ii) the thick-walled reddish brown resting or resistant sporangia (A1) (meiosporangia). Both types of sporangia are borne on the same thallus single or in catenate series on the tips of the ultimate dichotomies (A2).
(i) Mitosporangium (Fig. 4.11 A3):
The multinucleate tip of the ultimate of the hyphal branch swells. The inflated portion is then separated from the parent hypha by a basal transverse wall. The inflated multinucleate structure meiosporangium (A3). The latter increases in size. The increase in size is accompanied by sporothallus. Increase in the number of nuclei by mitosis.
The mature mitosporangium is a colourless, thin-walled multinucleate structure containing diploid nuclei. The sporangial contents are then divided by progressive cleavage into uninucleate daughter protoplasts by inward furrowing of the plasma membrane. Each uninucleate daughter protoplast metamorphoses into a diploid, uniflagellate, opisthocont zoospore.
It is known as the mitozoospore (C1). The diploid mitozoospores escape through pores, one or more in number developed in the sporangial wall (A3). The discharged mitozoospore swims about for a while, comes to rest, retracts the flagellum, rounds up and secretes a wall. The encysted zoospore germinates immediately.
Zoospore Structure (Fig. 4.12):
It is a naked, uninucleate tiny mass of protoplast furnished with a single whiplash type of flagellum inserted at the posterior end. It has no cell wall and thus is termed naked. Within the plasma membrane, the cytoplasm contains a posteriorly located nucleus containing a distinct nucleolus. A single large basal mitochondrion forms a cap-like structure around the posterior region of the nucleus.
The flagellum within the plasma membrane integrates with a basal granule known as the functional Kinctosome. The latter is situated near the posterior end of the nucleus enclosed by the cup-like basal mitochondrion. A large, somewhat, horse shoe-shaped membrane bound organelle, the nuclear cap is located in the anterior region of the zoospore.
It surrounds the major portion of the posteriori located nucleus. The zoospore cytoplasm lacks the ribosomes and endoplasmic reticulum. The former, however, are abundant in the nuclear cap cytoplasm.
Embedded in the cytoplasm of the anterior region of the zoospore are numerous lipid bodies, mitochondria and membrane bound vesicles. Mitochondria mostly occur along the nuclear cap membrane. The double-membrane envelope of the nuclear cap is perforated at intervals like the nuclear membrane.
During encystment of zoospore the nuclear cap membrane breaks. The ribosomes become dispersed throughout the zoospore cytoplasm. The mitochondria become more numerous and are also found at the periphery of the encysted zoospore.
Germination of Encysted Mitozoospore (Fig. 4.11 C1-C2):
The encysted zoospore (Fig. 4.13) germinates immediately. It contains stored mRNA which is used up during germination. In the germinating cyst, the nuclear cap loses its identity and is no longer visible as such. The ribosomes become dispersed in the cyst.
At the time of germination, the cyst puts out a slender basal germ tube. The ribosomes become dispersed in die emerging germ tube which grows and branches to form the branched rhizoidal system.
The main body of the cyst then elongates and grows in the opposite direction to form a thicker germ tube. The later as it grows, branches repeatedly to produce the dichotomously branched part of the diploid sporothallus (Fig. 4.11 C2). The diploid mitozoospores thus serve to reduplicate the sporophyte generation in the life cycle. They play no role in the phenomenon of alternation of generations.
(ii) Meiosporangium (Fig. 4.11 A1):
The early development of meiosporangia is similar to that of the mitosporangia. There is, however, formation of thick, pitted wall internal to the original sporangial wall in addition.
The mature meiosporangia are thus thick-walled and pitted, reddish brown in colour, the multinucleate diploid sporangial contents undergo a resting period of 2-6 weeks or even more prior to the cleavage of the protoplast into zoospores, hence the name resting or resistant sporangia.
After the resting period the meiosporangia germinate. There is increase in the number of nuclei. The nuclear division, however, is meiotic. Meiotic Prophase I occurs during resting sporangium development.
The two meiotic divisions take place after induction of resting sporangium germination. Meiosis is followed by the cleavage of the sporangial contents into uninucleate, uniflagellate, opisthocont zoospores which are haploid and slightly smaller than the diploid mitozoospores.
They are known as the meiozoospores or gonozoospores. Emerson and Wilson (1954) reported the basic or haploid chromosome number of 14, and strains with 28, 50 (probably 56) for A. macrogynus indicating diploid and tetraploid series. The wall of the mature meiosporangium cracks (A1) and the meiozoospores escape in the water. They swim about for a while, then come to rest, retract their flagellum and round off.
Each quiescent meiozoospore secretes a wall around it and then germinates immediately in the same way as the mitozoospore but produces an alternate haploid plant in life cycle called the gametothallus (B2) and not the sporothallus. The meiozoospores or gonozoospores thus play an important role in the phenomenon of alternation of generations in the life cycle of Allomyces.