The life cycle of Achyla species involves asexual or sexual modes of reproduction. The genus Achyla includes approximately 47 species and belongs to the group of oomycetes or water mould. Few species of Achyla are phytopathogens like Achlya conspicua and Achlya klebsiana.
Most of the Achyla species are aquatic, while few grow on damp soil. They reproduce vegetatively by means of thick-walled chlamydospores, asexually via zoospores and sexually through the formation of oospore.
Content: Life Cycle of Achyla
Features of Achyla
Before moving on to the life cycle of Achyla, let us discuss the structural features:
Mycelium: Its mycelium appears as a white, tangled-mass. Each mycelium consists of branched hyphae that may emerge out in an indiscriminate directions.
Hyphae: The hyphae appear slender, which is broader at the base and gradually narrower at the tip. It may be coenocytic or aseptate. Hyphal projections help the Achyla species to penetrate the substratum.
Achyla reproduces vegetatively through the formation of dense or aggregated mass on the hyphae in the form of gemmae or thick-walled chlamydospores. The cytoplasmic mass of vegetative hypha transforms into a mass of spherical cells either at the terminal or intercalary position.
Chlamydospores are the resting spores, which are densely filled with protoplasm and formed on the coenocytic hyphae. On favourable condition, the chlamydospores detach out of hyphae via transverse septa, latter each chlamydospore may form a new vegetative structure.
It occurs by the method of sporulation through the following successive stages:
Firstly, a sporangium form by the gradual swelling of the protoplasm towards the apex of the hypha. Then, the sporangium initial being filled up with the multinucleate protoplasm.
Later, the vacuole appears towards the centre, and the protoplasm propels towards the periphery. A sporangium then separates from the hypha by a basal septum.
At the time of the first preliminary division stage, the central vacuole forms some ridge-like structures inside the dense protoplasm. The ridges are arranged in the form of intersecting lacunae, by which the sporangium divides into polygonal masses. After that, a stage appears where the protoplasm achieves a homogeneous phase.
The spore initials of polygonal masses are then ready to discharge out. Each spore initials are uninucleate. On maturity, the spore-initial increases in size and becomes separated from each other and termed as primary zoospores.
The papilla of zoosporangium dissolves, and the primary zoospores liberate one after one. The primary zoospores (non-motile) are then arranged over the hollow sphere, where they encyst.
Later each encysted primary zoospore will break and develop into secondary zoospore after a few hours. Firstly, a primary encysted zoospore remains in a resting phase for a while, and then the zoospore tends to emerge out of it.
Finally, the entire mass is discharged out to form secondary zoospore. The secondary motile zoospore (biflagellate) swims for some time and then again encyst except in Achyla aplanes. Finally, the zoospore will germinate on favourable conditions and give rise to the new hyphal system.
It is of oogamous type. In Achyla species, the male or female sex organs, i.e. antheridia and oogonia are found on the similar somatic hyphae. Majority of Achyla species are heterothallic, while few are homothallic. Achyla species undergo sexual reproduction through a method of gametangial contact.
The oogonia are the spherical structures that grow at the apical portion, either of short lateral hypha or on the main hypha. Thus, the vegetative hyphae that give rise to the formation of oogonia called a female or oogonial branch.
The female branch propels to develop a sac-like oogonial initial. The oogonial initials contain cytoplasm and nuclei in a homogeneous state, and later a central vacuole appears. After successive stages, the oogonium or the swollen body separates out of hypha by a basal septum.
The oogonia appear annular in shape and possess a thin, smooth, hyaline wall. Inside the oogonium, few nuclei get degenerate and the remaining divide mitotically to develop at least 2-6 oosphere. The mature oosphere appears spherical and is uninucleate. It is devoid of a surrounding wall.
The antheridia develop on the slender vegetative hyphae. It originates either from the main hyphae or from the oogonial stalks. The antheridial protrusions grow in size, and the dense multinucleated protoplast shifted towards the swollen tip.
The swollen portion or antheridium separates by the transverse septum. A shape of antheridium is more or less club-shaped. Like oogonia, nuclei in the antheridium undergoes mitotic divisions. Consequently, the antheridium interacts with the oogonial wall.
It occurs by the interaction of the mature antheridium and oogonium. An antheridium develops a fertilisation tube to penetrate the oogonial cell and reaches an oosphere or egg.
More than one antheridium can contact with the oogonial wall via a fertilisation tube. Then karyogamy takes place by the fusion of the male nucleus with the nucleus of the oosphere. The fertilised egg of oogonium develops a thin wall around it.
Germination of Oospore
The oospores release out after the dissolution of the oogonial wall. Then the oospore goes into the resting phase for a while and on favourable conditions they germinate. Upon germination, a thin layer surrounding the oospore bursts out.
The germ tube comes out of the inner protoplast into a simply branched and slender tube, after several meiotic cell divisions. After germination of oospore, the formation of zoosporangium takes place that produces zoospores, each of which germinates to develop the new vegetative body.