TYPES OF LIFE CYCLES AND ALTERNATION OF GENERATIONS

Phytophthora:

This fungus fits into the division Oomycota. There is around 75 species in this genus, most of which live as parasites on the flowering plants. The species Phytophthora infestans is of big economic significance. It causes a serious potato disease termed as potato blight or late blight of the potato. 

Morphology:

The mycelium of Phytophthora is abundantly branched and comprises of aseptate, hyaline and coenocytic hyphae. The hyphae ramify in the intercellular spaces of the host tissues. The mycelium generates haustoria that penetrate the host cell wall and enter the cells to draw nourishment. The haustoria might be branched or simple. Phytophthora reproduces both sexually and asexually. 

Asexual Reproduction:

In humid and warm weather it generally reproduces asexually. Throughout this phase a tuft of slender, branched hyphae generally occur from the internal mycelium. They come out via the stomata or pierce via the epidermal cell on the lower surface of the leaf.  In tubers they emerge via the injured parts of the skin. Such aerial hyphae are hyaline and branched. They bear a sporangium at their tip. The sporangia are thin-walled, hyaline and lemon-shaped and encompass a bear-like projection or papilla at their tips.

The mature sporangia can simply be separated from the sporangiophore. The sporangiophore is branched. It bears nodular swellings that indicate the point of detachment of sporangia. Wind, rain drops or contact by neighboring leaves detach and spread out the ripe sporangia on the neighboring potato plants. They might fall on the ground and get dispersed into the soil. The sporangia lose their viability when they fail to germinate in a few hours.

If the sporangia fall on the leaf of a host plant, they germinate. Temperature and Moisture are the determinants for germination. In the presence of water and low temperatures (up to 12°C) the sporangium acts as a zoosporangium. The protoplast splits into 518 uninucleate daughter protoplasts that convert into zoospores.

The zoospores are biflagellate and uniform of the two flagella one is of the whiplash kind and the other is of the tinsel kind. The zoospores are set free however the apical papilla into the vesicle in several species. The vesicle soon bursts open to release the zoospores. The discharged zoospores swim for some time and later settle on the substratum losing the flagella and germinate.

Throughout germination, the zoospore puts out a short hypha termed as appressorium. The appressoria assist to fix the fungus on the surface of the host leaf. From the appressorium, a narrow, peg-like infection hypha builds up that force its way into the host leaf.

At temperature up to 24°C, and low relative humidity the sporangium germinates directly acting similar to a conidium. It germinates generating a germ tube or a short hypha that enters into the host leaf. The sporangia that are washed into the soil germinate and infect the tubers. As an outcome the tubers rot through harvest time or all through storage. Beneath favorable conditions a number of asexual generations might be generated in one growing season. This outcome in rapid propagation of the fungus to spread disease.

Sexual Reproduction:

Sexual reproduction is of the oogamous kind. The male sex organs are antheridia and the female oogonia. They occur at the tips of short lateral branches as antheridial and cogonial initials correspondingly. Phytophthora infestans is heterothallic.

The antheridium is a club-shaped structure having one or two nuclei to start with. Later the nuclei split and produce around 12 nuclei. At the time of fertilization just one functional nucleus persists and the others degenerate. The oogonium builds on a neighboring hypha of the antheridial branch. It grows across the antheridium and enlarges to make a pear-shaped or spherical structure. It includes dense cytoplasm and lots of nuclei (around 40).The protoplast of the oogonium becomes distinguished into an outer multinucleate periplasm and a central uninucleate ooplasm. The central nucleus splits into two and one of them vanishes. The surviving nucleus performs as the egg nucleus. Later, the nuclei of the periplasm degenerate. The oogonial wall swells out at a particular point to form a receptive spot. The oogonial wall disintegrates at this spot. Via this opening, the antheridium pushes a short fertilization tube. The fertilization tube penetrates the periplasm and reaches the ooplasm. Here it opens and delivers the male nucleus all along with the surrounding cytoplasm. The male and female nuclei fuse, therefore bringing out the fertilization.

The fertilized egg produces a thick wall around itself and becomes the oospore. If the conditions are favorable the oospore germinates. It is assumed that meiosis occurs all through germination. The germination of oospore occurs after the decay of the host tissue. A germ tube builds up from the oospore and might directly build up into a mycelium or oospore might bear a terminal sporangium. Within the sporangium zoospores are generated which after liberation build up into new mycelia.

In the life-cycle of Phytophthora there is an asexual cycle which might repeat throughout favorable conditions. The sexual cycle occurs prior to the onset of unfavorable conditions forming a resting spore. Such cycles usually alternate with one other.

Rhizopus:

Rhizopus is a member of division Zygomycota. It is generally termed as bread mould as it is often found growing on stale bread. It is a saprophytic fungus. It as well grows on decaying vegetables, fruits and other food materials.

The mycelium is white cotton-like fluffy mass having many, slender, branched hyphae. The mycelium consists of three kinds of hyphae: (a) rhizoidal (b) stolons and (c) sporangiophore.

The rhizoidal hyphae are a group of brown, slender and branched rooting hyphae that occur from the lower surface of the stolon at some points which are the apparent nodal points. Such hyphae assist in anchorage and in the absorption of water and nourishment from the substratum.

The aerial hyphae that grow horizontally above the surface of the substratum are termed as stolons. Such hyphae are comparatively big and slightly arched. The stolons grow quickly in all directions, totally filling the surface of the substratum.

The third type of hyphae termed as sporangiophores build up throughout the reproductive stage. The sporangiophores occur from the apparent nodal areas, opposite to the rhizoidal hyphae in the cluster. They grow up vertically bearing sporangia at their tips.

Asexual Reproduction:

Rhizopus reproduces asexually through multinucleate, non-motile spores that are generated in round, small and black sporangia. The sporangia are borne terminally and singly on unbranched sporangiophores. A mature sporangium is distinguished into two areas, a central less dense, vacuolated area having fewer nuclei termed as columella and a peripheral dense area having lots of nuclei termed as sporoferous area. The protoplast in the columella is nonstop having the sporangiophore.

The sporeferous area experiences cleavage to form a number of multinucleate parts. Such parts round off and secrete walls around them to become the sporangiospores. These are unicellular, multinucleate, non-motile aplanospores, globose or oval in shape. As the spores mature the sporangium explodes open discharging the spore mass. A part of the wall remains as a collar-like fringe at the base of the sporangium.

The spores are scattered away through the wind. Falling on an appropriate substratum, beneath appropriate conditions a spore germinates generating a short germ tube which grows further and branches profusely to generate three kinds of hypae.

Under adverse conditions Rhizopus generates chlamydospores.

Sexual Reproduction:

To the end of the growing season Rhizopus reproduces sexually. Sexual reproduction is of conjugation kind. Here the two gametangia fuse. A few species of Rhizopus are homothallic whereas others are heterothallic. In heterothallic species the mycelia fit into two mating kinds or strains one plus and the other minus.

Throughout sexual reproduction the hyphae of the two mating types (+ and -) termed as zygophores are attracted towards one other. They produce copulating branches termed as progametangia that meet up at their tips. The tips of the progametangia expand due to the accumulation of cytoplasm and nuclei and are cut off from the basal part through cross walls. The terminal part is termed as suspensor. The whole gametangium converts into an aplanogamete. The two gametangia might be of the similar size or one of them slightly smaller than the other.

If the gametangia mature the intervening, walls dissolve and the two gametes and their nuclei fuse generating a zygospore. The zygospore rises in size and secretes a thick two layered wall around it. The outer layer is warty and dark. It is termed as exospore or extine. The inner layer is thick and is termed as endospore or intine. As the zygospore rises in size, the wall of fusion cell having the zygospore breaks and it is set free.

Proceeding to germination of the zygospore, the diploid nuclei split meiotically producing many haploid nuclei. Throughout meiosis segregation of strains occurs. The zygospore throughout germination absorbs water and swells. As an outcome the outer wall extine breaks open. The internal wall intine with the inner contents grows out as a germ tube or promycelium. The promyceliurn is of limited growth and generates a terminal sporangium. It is termed as zygosporangium or germ sporangium. Within the sporangium, numerous non-motile germ spores termed as meiospores are produced. They are discharges at maturity that build up into new mycelia.

The life-cycle of Rhizopus comprises of two stages: asexual and sexual. The asexual phase comprises of myelium, sporangiophores, sporangia and the sporangiospores. This stage in the life-cycle serves to spread the haploid stage of the fungus all through favorable conditions. The sexual stage comprises of mycelia of the plus and minus strains, the progametangia, gametangia, aplanogametes, promycelium, zygospore, germ sporangium and the germ spores. Among such, the zygospore is the only diploid structure. All others are haploid. Such a sexual cycle is termed as haplontic characterized through zygotic meiosis and haploid mycelium as the merely adult fungi Rhizopus shows heterothallism in which the mycelia of a single species are morphologically identical however physiologically dissimilar. There is no evident difference among female and male mycelia apart from in their sexual behavior.

Such a difference is chosen by the terms plus and minus. This was first introduced by Blakeslee in the year 1904. This is the first sign of the origin of dioecious condition of sexual phase in an organism.

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