In this article we will discuss about the life cycle of pyronema with the help of suitable diagrams.
Mycelium of Pyronema:
It is well developed, filamentous and superficial. The hyphae are hyaline and profusely branched. They ramify over the surface of the substratum to form a cottony layer. The glistening white mycelium forms frost-like masses on the substratum.
The septate hyphae consist of short, multinucleate cells. Claussen (1912) recorded 6 to 12 nuclei in each cell.
Reproduction in Pyronema:
1. Asexual Reproduction:
Asexual reproduction by means of asexual spores is usually lacking. L. R. Tulasne and C. Tulasne (1865) however reported that asexual reproduction may sometimes take place by the formation of oidia which are produced in chains on certain erect hyphae of the mycelium.
2. Sexual Reproduction (Fig. 12.6):
It is oogamous and the species is homothallic. The male sex organ is called an antheridium and the female ascogonium. Both are multinucleate. The hyphae bearing sex organs occur closer together on the mycelium and are short and erect (A).
They fork usually once or twice or even more times. Each branch is 2 to 4 cells in height. The end cell of one of the paired branches develops into an antheridium and the other into an ascogonium.
P. domesticum requires exposure to light to initiate gametangia formation and for the continued maturation of apothecia under normal cultural conditions.
(a) Antheridium:
The male branch consists of two to four cells. The terminal cell of this branch becomes enlarged into a clavate (club-shaped) structure called the antheridium (A). It is multinucleate.
The male nuclei divide and redivide to form a hundred or more nuclei. The lower two or three cells of the branch constitute the stalk. All the cells are multinucleate. The antheridium is closely appressed to the ascogonium (A).
(b) Ascogonium:
It is developed on a two to four-celled sister branch of the dichotomy (A3). The terminal cell of the female branch swells up to form a rounded or subglobose ascogonium. The lower cells of the branch constitute the stalk.
The ascogonium is multinucleate. It contains about 100 or more nuclei. From the upper end of the spherical ascogonium arises a short, slender outgrowth. It elongates to form a slender, tubular, curved structure called the trichogyne.
The trichogyne is a unicellular but multinucleate structure. It is separated from the ascogonium by a cross wall formed at its base.
(c) Plasmogamy:
The trichogyne grows towards the antheridium and makes contact with its apex (A2) or the flank (A1). The intervening walls at the point of contact dissolve. A pore is thus formed (A₃).
The nuclei of the trichogyne disappear. A majority of male nuclei with some cytoplasm pass into the trichogyne through the pore.
They pass down the trichogyne and finally enter the ascogonium through the central pore in the septum at its base. Thereafter, the pore becomes blocked by some granule.
According to some, the male and the female nuclei in the ascogonium become arranged in pairs to form the dikaryons (C).
They do not fuse. Fusion between them is delayed. It takes place only in the ascus mother cell. The young ascus comes to possess the fusion nucleus or the synkaryon. It is diploid.
Gwynne Vaughan, Williamson (1931) and others (Harper, 1900; Tandy, 1927) hold that the male and the female nuclei in the ascogonium fuse to form diploid nuclei. These diploid nuclei or their derivatives pass into the ascogenous hyphae.
Another fusion between the two diploid nuclei takes place in the ascus mother cell. The young ascus thus comes to possess a tetraploid nucleus.
The third view is advanced by Dangeard (1907) and his followers They hold that no communication between the trichogyne and the antheridium is established.
The male nuclei do not pass into the trichogyne. The antheridium is functionless. The male nuclei degenerate and disappear in the male organ. The female nuclei in the ascogonium arrange themselves in pairs to form the dikaryons.
The sexual process in Pyronema confluens thus needs a thorough and a critical study to solve the riddle. Anyway the consensus of opinion is against fusion of male and female nuclei in the ascogonium. They simply arrange themselves in pairs to form dikaryons.
Post Plasmogamy Changes:
(a) Development of ascogenous hyphae (Fig. 12.6 C):
Following plasmogamy a number of usually 15-20 tubular outgrowths arise from the sides of the ascogonium (C). These outgrowths develop into ascogenous hyphae. Into each ascogenous hypha migrates a pair of nuclei (dikaryon) from the ascogonium.
During their further development the ascogenous hyphae become multicellular by the appearance of cross walls but the nuclei in the cells remain in pairs (Fig. 12.7 B).
The cells of the ascogenous hyphae particularly those towards the tips are binucleate (Fig. 12.7 B). The ascogenous hyphae may fork at their tips.
(b) Formation of Asci (Fig. 12.8):
The binucleate terminal cells of the ascogenous hyphae or their branches become crozier-like (A). The two nuclei in the crozier divide to form four daughter nuclei (B).
Later cell walls appear in the crozier so as to form a terminal uninucleate hook cell, a sub-terminal binucleate arch cell, and an anti-penultimate uninucleate stalk cell (C). The binucleate arch cell functions as the ascus mother cell.
The two nuclei in ascus mother cell fuse to form a fusion nucleus. The cell containing the fusion nucleus (synkaryon) is now called the young ascus (D). According to some, this fusion nucleus is tetraploid. This view is no longer held valid.
(c) Dinerentlation of Ascospores (Fig. 12.8 D-H):
With the elongation of the young ascus cell, the fusion nucleus in the young ascus cell undergoes three successive nuclear divisions to form eight daughter nuclei (E-G). They are always haploid.
In case the synkaryon is considered a diploid structure, the first and second divisions constitute meiosis and the third is mitotic. If the fusion nucleus is believed to be tetraploid out of the three divisions two are meiotic.
This double reduction division is called brachymeiosis. The ascus elongates as the ascospores are being formed by the accumulation of cytoplasm around each daughter nucleus.
Each daughter protoplast secretes a wall around it to become an ascospore. The mature asci are club-shaped in form. The lower portion of the ascus is comparatively thinner than the upper part. Hirsch (1955) reported that there is no brachymeiosis in P. confluens.
A cluster of asci (Fig. 12.9 A-G) may be produced at the end of an ascogenous hypha or its branch by the formation of new croziers as follows:
(a) The binucleate arch cell may proliferate t6 form another crozier.
(b) The terminal hook cell hypha fuses with the stalk cell and the nucleus of the latter passes into the former (C) or the nucleus of the hook cell passes into the stalk cell (E).
The binucleate new fusion cell by crozier formation produces a new ascus. The process may be repeated leading to a cluster of asci at the end of an ascogenous branch (G).
(d) Formation of Ascocarp (Fig. 12.10):
Immediately after plasmogamy, vegetative hyphae grow up from the mycelium. They branch and intertwine to form a compact network of horizontally running and intercrossing hyphae.
It is hypothecium. From the hypothecium arise narrow vertically growing sterile hyphae. They lie parallel to each other and are called the paraphyses. The cylindrical asci push up through the paraphyses.
The paraphyses and asci borne on a tuft of ascogenous hyphae which are seated on a cushion-like hypothecium together form a disc-like structure. The hymenium in Pyronema is exposed from the start. The apothecium lacks the rim or peridium.
The mature fructification is a small, salmon-pink, open, disc-shaped ascocarp called the apothecium (Fig. 12.10). The asci and the paraphyses constitute the hymenium or thecium.
Since the sex organs in P. confluens are produced close together the sterile hyphae of one incipient ascocarp intertwine and coalesce with those of the other in the vicinity. In this way several ascocarps become enveloped in a conunon sheath to form a compound ascocarp.
From this tendency of the fruiting bodies to become confluent when crowded, the plant derives its specific name confluens. The compound ascocarp is pink in colour when mature and 1 to 3 mm across.
Asci:
The asci are long, club-shaped structures. Each ascus contains eight unicellular, hyaline ascospores. They are arranged in a uniseriate manner in the central cavity surrounded by the unused portion of the ascus cytoplasm constituting the epiplasm.
Hung (1977) reported that the spore wall of the ascospores within the ascus of P. domesticum shows 3 distinct wall layers namely:-
(i) Inner endospore,
(ii) Middle epispore and
(iii) The outer perispore.
Concurrently with the development of the ascospore wall, an apical ring differentiates around the ascus just below the tip.
The small portion of the ascus above the apical ring constitutes the lid or operculum. The apical ring around the ascus serves as a weak area that eventually ruptures to allow the escape of ascospores.
Dehiscence of Asci:
Hung (1977) reported that just prior to the liberation of mature ascospores the perispore membrane starts blebbing. The vesiculation process continues until the perispore completely disappears.
Thus at the time of liberation the ascospore wall consists only of two layers—epispore and endospore. At this stage the ascus epiplasm degenerates.
In the vacuole thus formed ascospores are arranged either in a uniseriate manner (P. confluens) or suspended in the upper portion of the vacuole {P. domesticum).
The glycogen released from the disintegrated epiplasm changes into sugars of high osmotic value. This transformation results in the absorption of water and setting up of high pressure in the ascus.
According to Ingold, the walls of the mature asci are highly distended under the hydrostatic pressure thus set up. The apical ring breaks, with the lid either dispersed or hanging backwards at the top providing an open pore for ascospore liberation.
With the release of pressure the ascus wall contracts both longitudinally and transversely ejecting ascospores violently through the apical pore into the air.
Hung (1977) reported that in P. domesticum all the ascospores in the ascus are ejected at the same time by a single spurt of force. In P. confluens the ascospores escape one by one.
There is a minute fraction of a second between the escape of successive ascospores and thus they appear to be discharged simultaneously. Under suitable conditions each liberated ascospore germinates to form a new haploid mycelium of Pyronema.