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In this article we will discuss about the life cycle of achlya with the help of suitable diagrams.
Mycelium in Achlya:
It is in the form of a white, tangled mass consisting of branched hyphae growing in all directions. Some of the hyphae, which are short, penetrate the substratum. They anchor the mycelium and absorb nutrient materials. The hyphae are aseptate, sparingly branched and coenocytic.
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The septa normally remain suppressed during the somatic phase. The hyphal wall contains cellulose. However, it may not form the predominant material According to some investigators; glucans may well be the predominant material. The numerous nuclei he in the peripheral layer of cytoplasm surrounding a central vacuole.
Reproduction in Achlya:
Achlya reproduces by asexual and sexual methods of reproduction.
1. Asexual Reproduction:
It takes place by vegetative methods and sporulation.
(а) Vegetative reproduction:
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Like Saprolegnia, Achyla reproduces vegetatively by fragmentation and gemmae (chlamy-dospore) formation. The gemmae are formed under unfavourable conditions. These are dense aggregations of cytoplasm in certain portions of the coenocytic hyphae.
They become cut off by septa. The gemmae may be terminal or intercalary in position and may be produced singly or in chains. They serve as a means of perennation during periods of unfavourable conditions.
(b) Sporulation (Fig. 6.10 A-E):
It takes place by zoospores. They are produced in long, cylindrical zoosporangia borne singly and terminally on somatic hyphae. Sporangium formation starts with a slight inflation of the hyphal tip (Fig. 6.10 A).
The swelling grows into a tubular or club-shaped structure by the inflow of cytoplasm and nuclei from the lower part of the hypha. Finally the swelling is cut off as a zoosporangium by a cross wall appearing at its base (Fig. 6.10 B). The zoosporangium is poorly differentiated from the somatic hyphae.
It can only be distinguished by its dense contents and basal septum. Its diameter may also slightly more than the hypha which bears it. The zoosporangium tapers slightly more than the hypha which bears it. The zoosporangium- tapers slightly both towards the base as well as apex.
Later a central vacuole appears in the dense greyish contents of the zoosporangium which markedly contrast with the hyaline watery contents of the parent hypha (Fig. 6.10 B).
Cleavage of protoplast starts from the centre. Fine, intersecting lacunae extend from the vacuole into the multinucleate peripheral protoplast dividing it into polygonal masses (Fig. 6.10 C). A papilla-like outgrowth may develop at the tip of zoosporangium.
By a peculiar change the protoplast of the zoosporangium again becomes homogeneous (Fig. 6.10 D). The homogeneous protoplast is differentiated into uninucleate, daughter protoplasts (Fig. 6.10 E) Hawker (1967) reported that in A. racemosa uninucleate daughter protoplasts are formed by aggregation of cytoplasm around the numerous nuclei found scattered in the cytoplasm.
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The daughter protoplasts get metamorphosed into biflagellate primary zoospores. The soil dwelling species of Achlya lack flagella. Flagellated primary zoospores in Achlya may be pyriform or rounded with apically attached flagella or reniform with laterally inserted heterokont flagella. Before discharge they are seen jostling one another within the zoosporangium.
The apical papilla of the mature zoosporangium dissolves. The primary zoospores escape en masse in a cluster through the apical pore (Fig. 6.10 F). Immediately after their discharge, they stop swimming and become arranged in the form of a hollow sphere just outside the mouth of the empty zoosporangium and encyst (Fig. 6.11 A).
Thus, in Achlya there is a tendency to eliminate the first motile or swarming stage. The first period of diplanetism is extremely short as compared with Saprolegnia.
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After a short resting period of a few hours each cyst (Fig. 6.10 B) germinates to give rise to a secondary zoospore (Fig. 6.11 C-D) which is reniform and has two laterally inserted flagella (Fig. 6.11 D). The flagella are unequal. The shorter is of tinsel type and is directed forward. The secondary zoospores swim about actively for a time, disperse and encyst in turn.
From secondary cysts (Fig. 6.11 E-F) emerge the tertiary zoospores (Fig. 6.11 G). They are identical to the secondary zoospores in every respect. Each tertiary zoospore germinates by a germ tube to develop into a new mycelium (Fig. 6.11 I-J). In A. racemosa the cysts of secondary zoospores germinate by germ tubes to develop into new mycelia.
Salvin (1940) reported the occurrence of 5 successive swarming stages in a nonsexual Achlya. Achlya thus is an example of polyplanetism or repeated zoospore emergence. This phenomenon is more marked in the genus Dictyuchus. The primary, secondary and tertiary zoospores are all similar in structure and shape and are, therefore, monomorphic as compared to the dimorphic zoospores of Saprolegnia.
Usually there is no internal proliferation of zoosporangia in Achlya. A new zoosporangium may, however, grow up from the same hypha at the side of emptied zoosporangium.
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Hawker (1967) reports that zoosporangia may arise from the tip of the same hypha. Some of these may grow through the old empty ones. The internal proliferation of sporangia is considered a primitive character. It denotes poor differentiation of sporangia from the somatic hyphae.
2. Sexual Reproduction:
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It is oogamous. The sex organs (antheridia and oogonia) are developed on some of the somatic hyphae. A few species of Achlya are homothallic (A. racemosa) but the majority such as A. bisexualis are heterothallic (Fig. 6.13). In the latter a single Achlya plant produces either male or female sex organs not both.
(a) Development of sex organs in the Heterothallic species (Fig. 6.13):
Investigations of Raper (1939-1951) of sexual reproduction in the heterothallic species, A. bisexualis and A. ambisexualis have revealed that the development of sex organs is governed by a set of hormones. When grown separately neither male nor female thalli form sex organs. Instead the hyphae bear sporangia.
A male mycelium is or has been growing there and vice versa. It means the development of male and female structure is controlled by the hormones that pass between the plants of opposite sexes.
The hormones produced by the male plant stimulate the development of oogonia in the female plant. The hormones produced by the female stimulate production of antheridia on the male plant. Raper has termed this kind of mixed sexuality as gynandromixis.
The production of antheridia and oogonia are governed by four hormones as follows:
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1. The female plant of A. bisexuals secretes hormone A (Antheridial). It induces the male plant to produce antheridial hyphae called antheridial initials (B).
2. The male secretes a hormone B. It induces the formation of sac-like oogonial initials (C).
3. The oogonials now secrete a third hormone C. It directs the growth of antheridial initials or hyphae towards the sources (oogonial initials) and plaster themselves around the latter. The antheridia are finally delimited at the tips of antheridial hyphae (D).
4. Upon contact with the oogonial initials the antheridia secrete a fourth and final hormone D. It induces the oogonial initials to form oogonia and cause the differentiation of eggs within them (E).
(b) Development of Sex Organs in the Homothallic species (Fig. 6.14 A-G):
The two kinds of sex organs in the homothallic species (A. racemosa) are developed on the same mycelium.
Oogonium:
The oogonia are developed at the tips either of short lateral hyphae or on a main hypha. The tip of the female branch swells to form a sac-like oogonial initial (A). It is filled with dense homogeneous contents (cytoplasm and nuclei). A central vacuole then appears in the oogonial initial (Fig. 6.14 B). Subsequently the swollen structure is cut off by a cross wall at its base (C). It is the oogonium.
The oogonia are globose or pyriform in shape and have a thin, smooth hyaline wall. Some of the nuclei in the oogonium degenerate. The surviving ones divide mitotically.
The entire multinucleate protoplast now becomes differentiated into one to ten, generally two to six eggs (D). Each mature egg or oosphere is uninucleate and is usually spherical, occasionally ellipsoidal in form. It lacks a surrounding wall. The nucleus is centrally located (D).
Antheridium:
The antheridia are developed on thin hyphal branches which arise from the female hypha. The tip of the antheridial hypha enlarges (B). A number of nuclei and some cytoplasm migrate into the inflated tip.
The swollen portion is then cut off by a cross wall as an antheridium which is more or less club-shaped.
Some nuclei in the antheridium degenerate. The survivors divide mitotically. Subsequently the antheridium comes in contact with the oogonial wall which gets thinner at the point of contact.
Fertilisation:
The mature antheridium applies itself chaemotropically to the oogoniumm and develops a slender penetration tube at the point of contact. It is known as the fertilisation tube (D). The latter pierces through the oogonial wall and reaches an egg.
The fertilisation tube may give off branches to the other Antheridium eggs in the oogonium or more than one antheridia become attached to the oogonium each sending a fertilisation tube. The latter delivers one male nucleus into each oosphere. It fuses with the nucleus of the oosphere. The cytoplasm of fertilised egg secretes a wall (E) which is generally not very thick.
Germination of oospore (Fig. 6.14 F-G):
The oospores are released by the degeneration of the oogonial wall. They enter upon a resting period. With the return of favourable conditions the oospores germinate. The outer layer of the oospore wall ruptures.
The protoplast still surrounded by the inner layer emerges through the split in the form of a simple or sparingly branched slender germ tube (F) which terminates in a zoosporangium(G).The letter produces zoospers, each of which germinates to form a new plant. Meiosis takes place at the time of oosper germination. Barkesdale (1968) showed that meiosid in Achlya is gametangial.
Phylogeny and Origin of Saprolegniales:
Divergent views have been expressed by mycologists about the origin and evolution of Saprolegniales to which belong Saprolegnia and Achlya. Many hold that Saprolegnia has evolved from the algal ancestors much like Vaucheria.
This viewpoint is supported by the following similarities that exist between the two:
1. Aquatic habit.
2. The thallus in both consisting of branched, aseptate, coenocytic filaments.
3. Apical growth.
4. Similarity of serum reactions in both.
5. Thallus attached to the substratum; In Vaucheria by colourless branched filaments forming the holdfast and in Saprolegnia by rhizoidal hyphae which penetrate the animal debris on which it grows in fresh water.
6. Asexual reproduction by motile, asexual spore’s zoospores produced within zoosporangia. The zoosporangia of both are similar in form but zoospores are biflagellate in Saprolegnia and multi-flagellate in Vaucheria. The latter, however, are considered to be synzoospores formed by the fusion of many biflagellate zoospores similar to those of Saprolegnia.
7. Sexual reproduction oogamous in both.
8. The antheridia and oogonia similar in form in both. The main difference is in the number of eggs in each oogonium. It is one in Vaucheria and more than one in Saprolegnia.
The main difference between Saprolegnia and Vaucheria are the mode of nutrition, manner of fertilisation and diplanetic zoospores in Saprolegnia. The adherents of this view envisaged the evolution of Saprolegnia from a Vaucherial ancestor by the loss of chlorophyll and change in the mode of nutrition.
This happened before the Vaucheria-like ancestor had developed compound zoospores from many biflagellate zoospores. It is not hard to believe that certain flagellates under certain conditions develop chlorophyll and synthesize their own food or lose it and live as saprobes. Therefore, by the loss of chlorophyll and holdfast would evolve a thallus very similar to an Oomycete like Saprolegnia.
Diplanetic zoospores, characteristic of Saprolegnia, probably existed in the Vaucherial ancestor. The similarity between the zoospores of Saprolegnia and the sperms of Vaucheria which are colourless in spite of their algal origin tempted some mycologists to consider that the zoospores of present-day Vaucheria correspond to the first motile stage and the sperms to the second motile stage of Saprolegnia.
The above-mentioned view of the origin of Saprolegniales now receives but little support from the modern mycologists as it emphasises only on morphological resemblance between Saprolegnia and Vaucheria.
It ignores completely the physiological differences and type of flagellation. Many mycologists consider these resemblances to be superficial indicating merely parallel development along different lines rather than close relationship.
The opponents of the algal origin of primitive Oomycetes like Saprolegnia hold that the origin of Saprolegnia like ancestors must be sought among the colourless flagelltes.
This viewpoint similarities with the Myxomycetes. The latter do not appear to be far removed from the flagellate ancestors. In the Archimycetes there are forms which may be interpreted as leading from the simplest fungi to the simple Oomycetes such as Saprolegnia.
