Members of the Zygomycotina take the name from the type of sexual reproduction that occurs in them. The word ‘zygo’ is a prefix which is translated as ‘pair’. Fungi here undergo sexual reproduction that involves the production of conjugating gametangia which leads to the production of zygospores/zygosporangia. Asexual reproduction is by non-motile aplanospores contained in sporangia. The spores maybe violently dispersed but in many species, the spores are passively dispersed by rain, wind or animals. Zygomycotina fungi are terrestrial, filamentous with coenocytic hyphae and chitinous cell wall. They are very invasive, ecologically diverse and of great economic importance. Members of this group include the organisms you find growing on your left over ‘eba’ and bread.
Classification of Zygomycotina
The sub-division is divided into two classes which are: class Trichomycetes and class
Members of the trichomycetes have affinity with the guts of arthropods and they are studied under animal science. They are found growing commensally in the guts of animals like crustaceans, insects and millipedes.
Many members of this class are saprophytes while a few are parasites of higher plants and animals. You can only find terrestrial moulds in this class existing in a wide range of habitats. They possess the following distinctive features:
This class consists of two orders namely: Order Mucorales and Order Entomophthorales (parasitic on animals).
Members of the mucorales constitute the largest and most studied of the zygomycetes. They are sometimes called pin-moulds because of the way the sporangium is positioned on the upright sporangiophore. They are generally fast growing saprophytes while a few are facultative parasites of plants and animals. They are commonly found in soil and dung (coprophilous fungi). In many genera numerous asexual spores are contained in a globose sporangium which surrounds a central core of columella.
Figure 6.1: Example of columella bearing mucoraceae as found in Mucor spp. (Mucoraceae) Some members possess few spores which are usually dispersed as a unit termed ‘SPORANGIOLA’. An example is found in Thamnidium elegans.
Figure 6.2: Example of sporangiola found in Thamnidium elegans (Thamnidiaceae) Others may possess unicellular sporangia called CONIDIA (one-celled)
Figure 6.3: One celled conidium as found in Choanephora cucurbitarum (Choanephoraceae). While there may be some members with no columella and the spores are termed MEROSPORES. These spores are arranged in rows inside a cylindrical sac termed MEROSPORANGIUM.
Figure 6.4: Example of merosporangium found in Syncephalastrum rasemosum
Sexual reproduction is by isogamous (identical gametes) gametangial conjugation in which equal gametangial derived from branched tips fuse to form thick-walled zygospores. The zygospores are resistance survival spores of the mucorales. Members of this group utilize a wide range of sugars. Although, starch can be decomposed by most spp, cellulose is not used by most members of the mucorales because they lack cellulose degrading enzymes. Under anaerobic conditions, ethanol and numerous organic acids can be produced by members in culture media.
In 1961, Martin, W.W. divided the order into 9 families of:
Family Mucoraceae e.g. Mucor
Family Pilobolaceae e.g. Pilobolus
Family Thamnidiaceae e.g. Thamnidium
Family Cunninghamellaceae e.g. Cunninghamella
Family Choanephoraceae e.g. Choanephora
Family Piptocephalidaceae e.g. Syncephalis
Family Kickxellaceae e.g. Kickxella
Family Mortierellaceae e.g. Mortierella
Family Endogonaceae e.g. Endogone
Members of the family Endogonaceae have been identified as mycorrhizae fungi and they now belong to the Glomeromycota group.
Fungi in this family are saprobes that possess columellate sporangia with numerous spores which have thin-non persistence membrane. The spores are liberated by the breakage of the sporangial wall. The sporangiophores are erect with aerial hyphae. Members are abundant in soil, dung and moist fresh organic matters in contact with the soil. They occur in early succession on dung or soil substratum. Species like Rhizopus stolonifer causes spoilage of food stuff. Some are opportunistic in human suffering from diabetes, leukemia, cancer and immune-suppressed systems where they cause the fungal disease known as mucormycosis. This can lead to lesions in the brain, lungs and the digestive system.
Morphologically, this group of fungi has coarse, coenocytic mycelium with numerous branches which usually end in fine tips. Septum may appear in older culture and thick-walled chlamydospores may be formed and cut off from the mycelium by such septa. Chlamydospores are survival structures for this group of fungi. Members of this family include Mucor, Rhizopus, Absidia, Actinomucor and Phycomyces.
Figure a: Chlamydospores development.
Figure b: An example of a mucoraceae. (Source: http://en.wikipedia.org/wiki/file:Sporangium)
This is by non-motile spores called aplanospores contained in spherical or pear-shaped sporangium which may be borne singly at the tip of an erect sporangiophore or it may occur on branched sporangiophores. The sporangiphores usually arise from the mycelium. In Cunninghamella elegan the sporangia may be arranged in whorl on aerial branches while in Rhizopus spp the sporangiophores arise in a group to form a clump of rhizoids. The sporangiophores of many species of Phycomyces and Mucor are phototrophic. Phycomyces have been extensively used in the study of phototrophism.
few members of the mucoraceae are homothalli producing zygospores in cultures derived from a single sporangiospore e.g. is Rhizopus sexualis. The majority of the mucoraceae are heterothalli requiring two opposite strains for zygospore formation. When two compatible strains (positive and negative strain) are inoculated in opposite sides of a solid culture medium, they grow towards each other and zygospores are produced where they meet. The diagram below gives a representation of sexual reproduction in mucoraceae.
Figure : sexual reproduction in mucoraceae (Source: http://en.wikipedia.org)
The formation of zygospore follows several steps divided into three (3) main stages A-C.
Stage A = Zygophore Formation
In this stage, aerial hyphae are induced to come together at their tips by trisporic acid which is produced in abundance in heterothalli species when the positive and negative strains are in contact. Trisporic acid assists in the identification of compatible sexual partner. It pulls two potentially compatible hyphae to grow towards each other. Club-shaped zygophores or protuberances containing β-carotene are produced at this stage. This stage is represented by diagrams 1 and 2 above.
Stage B = Zygotrophic reaction
In the stage, the zygophore are directed to grow towards each other in response to a volatile chemical stimulus until they are in contact with each other at their tips. This is represented in diagram 3 above.
Stage C = Thigmotrophic Reaction
This stage shows the events that occur after the positive and negative zygophores are in contact. The tip of each zygophore develops into a progametangium or gametangium proper and each become cut off by a septum to separate the multinucleate distant gametangium from the suspensor. After a while, the wall separating the two gametangia breaks down and the cytoplasmic content fuses. The nuclei fuse to produce a diploid nucleus. The resulting zygote swells and develops a dark, thick, slimy outer layer. After resting for a period, the zygospore may germinate by producing a sporangiophore bearing a sporangium at its tip or may develop directly into mycelium.
Development of Sporangiophore
In mucoraceae, a sporangiophore develops as a long, blunt-tipped aerial hypha growing from the mycelium away from the substratum. The tip expands to form the sporangia initial (the beginning of the sporangia). This is then cut off by a septum from a central spore free columella. The sporangial wall darkens and may develop a spiny surface. In many spp. of Mucor the sporangia wall dissolves and the spores absorb water, so that the tip of the sporangiophore bears a drop of liquid containing spores. This is termed ‘sporangial drop’ and it adheres to the columella. The remnant of the sporangial wall can be seen as a frill at the base of the columella. In Mucor mucedo and Phycomyces spp., the spores are embedded in mucilage and when the sporangium is touched, a slimy content is exuded causing the sticky spores to be dispersed by insects or rain splash. In M. plumbens, the sporangial wall breaks into pieces and air current or mechanical agitation readily liberates the spores while in R. stolonifer the sporangium dries, the columella collapses and becomes inverted at the tip of the sporangiophore. The sporangial wall then breaks into fragments and dry spores become dispersed by wind. The sporangiospores germinate on fresh medium to produce mycelium with new crop of sporangiophores
Morphology of fungi
The thallus which is the body of a filamentous fungus is made up of a network of fine thread- like structures called hyphae. These can be found growing on/in suitable substratum. The filamentous fungi can be septate or non-septate. When the thallus is segmented you say the fungus is septate and when not segmented you either say such a fungus is non-septate or coenocytic. Fungi can be unicellular (one-celled) or multicellular (multiple-celled organisms).
The vegetative phase of a fungus is a thallus. Thallus is the body of an organism that is not differentiated into leaves, stem and roots. This may be unicellular or filamentous.
There are two forms of unicellular fungi; these are the yeast and the yeast-like fungi. The yeast is more or less a spherical, single-celled structure that becomes a reproductive unit at the stage of reproduction. It can produce both asexual and sexual cells. In the lower fungi group a fungus with this type of thallus is said to be holocarpic. Mycelium is absent in this type of thallus. Examples include Synchytrium and yeasts. The yeast-like fungi produces pseudohyphae when the physiological conditions are not conducive. A typical example is the genus Candida.
Fungus with filamentous thallus is commonly called a mould. There are two groups of fungi with this type of morphology. One which is purely filamentous e.g. Aspergillus niger and the other which is dimorphic. A dimorphic fungus exists in two different morphological forms at two different environmental conditions. It exists as yeast in tissue and in vitro at 37°C while as a mould in the natural environment and in vitro at room temperature, e.g. Blastomyces dermatidis. The filamentous thallus is a long strand of cells joined head to head. This usually originates from the germination of spores. In some species, after germination the spores produce a short, tubular
structure of limited growth called a hypha. The spores of most fungi however give rise to fluffy thallus consisting of a mass of fine, branched filaments. These fine long filaments are the hyphae. Some of the hyphae extend into the air at maturity to bear the reproductive bodies. Collectively, the hyphae make up the mycelium and this grows on substratum (medium). It consists of a thin, transparent wall filled or lined with a layer of cytoplasm. In the life cycle of a fungus, the mycelium obtains food from the substratum and it also carries out the general activities of a plant cell such as absorption, digestion, respiration, excretion and growth. The hyphae exhibit apical growth and spread in all directions within and over the substratum to form a loose and ramifying network. The hyphae in some group form various colourations when they are matured or during reproduction and these can be modified to serve various purposes.
Kinds of Mycelium
The mycelium can be septate or non-septate. Septate mycelium is that with cross walls called septa which divide the hyphae into segments. The septa appear at regular intervals behind the hyphal tip. They may be uninucleate or multinucleate; transverse/longitudinal/oblique. Each septum usually has a central pore to allow continuous flow of cytoplasmic content. The non- septate mycelium lacks internal partitioning and the mycelium is a continuous mass. As stated earlier, non-septate mycelium is also called coenocytic mycelium.
Modification of Hyphae
In most filamentous fungi, the mycelium aggregate in different ways in order to perform various functions. Some of these modifications include:
Plectenchyma: This is a false tissue formed by aggregation of hyphae. All fungal tissues come under this term. There are two types of plectenchyma which are:
Prosenchyma: This is a loosely woven tissue that runs more or less parallel to one another and composed of elongated cells with each hypha retaining its individuality.
Pseudoparenchyma: These are hyphae that become woven and intertwined into a compact mass. The walls of the hyphae fused and each loses its individuality. This gives the hyphae mass the appearance of a continuous structure consisting of isodiametric or oval cells.
Rhizomorph: This is a thick strand or root-like aggregation of vegetative hyphae. The hyphae lose their identity and the whole mass behaves as an organized unit.
Sclerotium: This is a compact globose or elongated structure formed by the aggregation and adhesion of hyphae. This can survive for a long period and thus represent the resting stage of the fungus. Sclerotia usually germinate to form hyphae or reproductive structures.
Stromata: These are fungal tissues that form reproductive structures and they are compact somatic structures like mattresses.
Pseudosclerotium: These are sclerotia-like bodies formed at the base of fruiting structures in higher fungi.
Appressorium: Are commonly found in parasitic fungi mostly ecto-parasites. It is a terminal simple or lobed swollen structure of germ-tube or infection hyphae that adhere to the surface of the host and helps the pathogen to penetrate the host cell wall.
Haustorium: This is mostly produced as intracellular absorbing structures of obligate parasites. It varies in shape and secretes specific enzymes to hydrolyze the host cell wall components, thereby absorbing nutrients from the host without killing it.
Reproductive Phase of Fungi
The reproduction phase starts when the mycelium reaches the maturity stage and had accumulated reserved food. There are three kinds of reproduction in fungi; these are vegetative, asexual and sexual. They all vary from one group of fungi to the other.
In some fungi the vegetative phase consists of a naked, multi-nucleate, amoeboid mass of protoplasm called plasmodium. There are three types of plasmodium namely:
This is the smallest, simplest and most primitive among the myxomycetes. It is homogenous, microscopic and lacks veins. Its entire structure is transformed into a single sporangium during the fruiting phase.
This plasmodium is transparent with individual strands being 5 to 10 mm wide. This type resembles the protoplasmodium in the early stage but as it matures, it elongates and enlarges into a network of delicate strands. Its venation is not obvious and the protoplasm undergoes reversible streaming movement.
This type of plasmodium is the most common and largest. A matured phaneroplasmodium is a massive, highly granular structure of several centimeters in diameter. Its veins are highly differentiated into a network of tubular strands and conspicuous while the protoplasm shows rhythmic and reversible streaming. The networks of strands you see in Figure below are the veins
of the plasmodium these are thickening towards the margin. You can find an example of this in the Order Physarales
Phaneroplasmodium of Physarum polycephalum
The sporangium (pl. sporangia) is a type of fruiting structure that houses the spore. This is usually formed at the tip of the hyphae. Sporangia and spores are formed in the presence of light on the surface of substratum based on species. They also possess various forms and colour. Sporangium is multi-nucleated and consists of a tough network of plasmodium known as capillitium which encloses numerous spores in its meshes. In primitive forms of fungi, plasmodium migrates and feeds for a period of time before being converted into numerous sporangia during favourable conditions. Capillitia show hygroscopic movement and the sporangia usually burst irregularly at the apex to release uninucleate spores. As seen in the Figure below the sporangia are opened showing that the spores have been dispersed.
Types of sporangium
There are four types of sporangium namely:
Simple multi-sporous sporangium
This is formed when the protoplasm of the plasmodium becomes raised into a large number of crowded but discrete sporangial initial/primodia. There are various forms of this type of sporangium which include:
(a.) Stipitate type
This type of sporangium is formed as a result of the upward flow of protoplasm. As the protoplasm move from the primodium (an early tissue growth) it deposits a central shaft of non- living materials which forms the sporangiophore/stipe. Sporangiophore is a reproductive thread of fungus from which the spore-forming sac develops. When the protoplasm reaches the tip, an outer layer is secreted around the developing sporangium. This layer is called the peridium. The protoplasm becomes vacuolated and divides into several portions. The nucleus then undergoes mitosis and a haploid nucleus enters into each of the protoplasmic portion where walls are then secreted around each protoplasmic portion to form a spore.
Physarium polycephalum Order Physarales and Family Physaraceae
Diachea leucopodia of the order Physarales and family Didymiaceae .
In some myxomycetes, the capillitium develop throughout the interior development of the sporangium. These capillitia are composed of waste chemical materials especially CaCO3 derived from the protoplasm. Different genera have different type of capillitium. Some maybe filamentous and usually develop with spores within the sporangium.They help to retain the spores in the sporangium. Therefore, allowing a gradual dispersal of the spores over a long period. Most are often ornamented.
A filamentous capillitium showing spores.
Ornamented capillitium showing stained spore as found in Trichia favoginea
(b.) Sessile type
Some other type of simple multi-sporous sporangia may be sessile. Example of this is Trichia favoginea as shown above.
Simple monosporous sporangium
This type of sporangium is found in the order Ceratiomyxales where delicate branching of sporangiophores arise from the substratum at interval. Each sporangiophore/stipe bears a single mono-sporous sporangium at the tip. The spores are borne externally and each gives rise to eight myxoamoebae.
This is the type sporangium where you find the protoplasm becoming concentrated around some of the major veins as found in phaneroplasmodium. As shown in Figure below a plasmodiocarp usually retains some of the branching habit of the plasmodium veins and are covered by peridium.
Plasmodiocarp showing the tubular strands.
An aethalium is a sporangium formed as a result of the aggregation of the entire plasmodium into a single sporangial initial. Aethalia are large, sessile, spherical or hemi-spherical and covered by peridia. They are compound sporangia.
Aethalium of Lycogala epidendrum
Members of this division are generally known as slime moulds. They are also called false fungi because they lack cell wall. It is a division that serves as an intermediate between the plant and the animal kingdom in the evolution of living things. Members of this division are found on decaying wood, leaves, in moist shady places, on ground and aerial litters, humus and damp soil. There are about 1250 known species.
Classification of myxomycota
This division consists of a single sub-division which is MYXOMYCOTINA. The myxomycotina comprises four classes namely:
Acrasiomycetes (cellular slime mould)
Myxomycetes (true slime mould)
You also call this group the true slime mould. It is a group of fungi-like organisms with approximately 800 well known species. They are animal-like in the vegetative phase and plant- like in the reproductive phase. This is because they are amoeboid in the vegetative phase and the entire structure is converted to spores during the reproductive phase. They ingest food materials by means of phagocytosis. Phagocytosis is a process whereby an organism engulfs a food material by pseudopodia and secretes hydrolytic enzymes to digest the food which is then ingested into the system.
Characteristics of myxomycetes
They are saprophytes.
They are usually colourless but some species maybe variously coloured.
They exhibit both asexual and sexual mode of reproduction.
Asexually they grow by fission of the plasmodium whereby each grows into a new individual.
Members are used for biological research because of the pure protoplasm which they possess.
They possess naked slimy body called plasmodium. The plasmodium is made up of a mass of protoplasm with many nuclei. The plasmodium behaves as vegetative and reproductive cells (both cannot be distinguished) and possess pseudopodia for amoeboid movement.
The spores in most genera of the class myxomycetes are normally globose, uninucleate and haploid. Spores are small uninucleate asexually reproductive structures produced mostly by fungi and are capable of developing into a new individual. Spores surfaces range from almost smooth to reticulate. The wall of a spore is composed mainly of cellulose. In myxomycetes, the fruiting structure produces dry spores which are dispersed by wind. When a spore lands on a suitable substratum, it germinates by producing one or more myxoamoeba or zoospores. Each zoospore has two unequal anterior whip-like flagella (one obvious and the other not visible). A zoospore may loss its flagella to become a myxoamoeba while myxoamoeba may gain flagella to become a zoospore. Both types of cells may become encysted under unfavourable conditions. They may also act as gamete and fuse in pairs with their respective types to produce a diploid cell. The diploid nucleus undergoes repeated mitosis to produce a mass of multinucleate protoplasm which grows and expand into a diploid plasmodium. The plasmodium is amoeboid and feeds by phagocytosis of bacteria, yeast, fungal spores. It lacks a true cell wall.
This class consists of organisms which are endo-parasites or obligate parasites of higher plants. Endo-parasites are parasites that live in the host. They share the following characteristics with the myxomycetes.
They produce plasmodia.
They produce zoospores with a pair of unequal anterior flagella.
They differ from the myxomycetes in following ways:
They lack sporangium.
They are obligate parasites.
The plasmodium does not form fructification at maturity.
The thick spore wall is chitinous while that of myxomycetes contain cellulose
Many fungi in this group are of economic importance as they cause the diseases in crop plants. A common disease is club-rot of onion, garlic and beet root. An example is Spongospora subterranea which causes powdery scab of potato.
members of this division the ‘true fungi’. It is a group of fungi that possess cell wall which is made up of chitin. The lower forms of fungi in this division have holocarpic thallus while the higher forms have eucarpic thallus. The characteristics of fungi in this group vary from one sub- division to another and there are five sub-divisions in Eumycota namely:
In the recent reclassification of fungi, members of this group have been reclassified into three major divisions of Chytridiomycota, Neocallimastigomycota and Blastocladiomycota. However, the conventional system of classification is retained for the time being because the new system is trailed with controversies. Members of this sub-division have coenocytic mycelium and reproduce by means of flagellated spores. These spores are termed ‘zoospores’, they are asexual
and haploid. Members produce three distinct types of zoospores and show centric nuclear division.
These zoospores are:
oospore with a single posterior flagellum that is the whiplash type
oospore with a single anterior flagellum that is the tinsel type
(c.) zoospore which is bi-flagellated. This can be apically or laterally attached flagella. One of the flagella is th whiplash type while the other is the tinsel type.
The members of the third group may possess pear-shaped or kidney-shaped zoospore.
Based on the type of flagellation, members of the mastigomycotina are grouped into three classes of:
Chytridiomycetes with whiplash flagellum
Hyphochytridiomycetes with tinsel flagellum
Oomycetes with bi-flagella of kidney or pear shape.
Fungi found in this class are literally called chytrids. The class consists of mostly aquatic fungi which reproduce by the production of a single posterior flagellum. Members are found in soil, fresh water, estuaries, saline. You find them growing as saprophytes on plant and animal debris or as parasites in the cells of algae and small aquatic animals like zooplanktons. Some members may attack the underground part as well as the aerial parts of higher plants and cause diseases of economic significant. For example Phyosderma maydis causes brown spot disease of maize. However, some species of Allomyces and Blastocladiella are used as research tools in studying morphogenesis.
The class consists of five orders which are: Order- Chytridiales
Order- Monoblepharidiales Order- Spizellomycetales Order- Neocallimastigales Order- Blastocladiales
Members of this order either lack a vegetative system or possess a specialized rhizoidal vegetative system. The zoospore usually has a single conspicuous oil globule flagellum and germination is mono-polar. In this order, the morphology of the thallus varies.
Morphology of the thallus in Chytridiales
Two types of thallus exist in the order:
Holocarpic: This is coined from two Greek words Holos meaning ‘entire’ and karpos which means ‘fruit’. It is a type of unicellular thallus whereby the whole vegetative structure is reproductive in function. A matured thallus is usually a spherical or cylindrical sac which is
(b.) Endobiotic: In this type, the entire structure (sporangium and rhizoid) are formed within the host cell.
(c.) Polycentric: In this type a complete system of sporangia develop at the tip of a complex rhizoidal system. This is represented in the diagram below where you see one of the sporangia dispersing the spores.
(Source: Webster, J. and Weber, R. W. S. Cambridge University Press, New York. (2007).
Reproduction in Chytridiales
In this order, the sporangium usually bears one or more discharge tubes. This can either be inoperculate or operculate sporangium. Sporangia with the inoperculate thalli form a discharge tube that penetrates to the exterior of the host cell. This discharge tube becomes gelatinous and dissolves for the zoospores to escape. In genus like Chytridium that have the operculate thallus. The tip of the discharge tube either cuts open at a special line of weakness, then becomes detached or one side of the tube opens causing the operculum to fall like a hinge to allow the zoospores to escape. The number of zoospores formed in a sporangium varies with the size of the zoospores. The period of zoospore movement also varies, some flagellated zoospores swim while some seem to be incapable of active swimming so they assume amoeboid movement. In some, swimming may last for only a short period while others can swim for several hours. After awhile, the zoospores come to rest or encyst. During germination the flagellum maybe cut off. The behaviour of the encysted zoospores differs in different species. In holocarpic parasites where the zoospores encyst on the host cell, its wall and parts of the host cell become dissolved and the cytoplasmic content of the zoospores enter the host cell. However, the structure is different and varies in eucarpic organisms.
Class – Oomycetes
This class is commonly called ‘white moulds’ or ‘white rust’. Most members are aquatic but a few are terrestrial. The cell wall lack chitin but some amount of cellulose is present however the chief component is glucans. Members possess zoospores with two flagella and the thallus varies in different genera. They usually reproduce both asexually and sexually through oogamy. Oogamy is a type of reproduction involving gametes whereby a small motile male cell fuses with
a large female sessile cell. The gametes are the only haploid structure in the life cycle of members of this class.
There are four orders in this class namely: Order: Lagenidales (inoperculate) Order: Saprolegniales (operculate) Order: Leptomitales (operculate) Order: Peronosporales. (Operculate)
In this session you shall only be studying the Saprolegniales.
Fungi found in this order have either holocarpic thallus or eucarpic thallus. They are abundant in wet soil, lakes, marsh and fresh water as saprophytes on plant and animals debris. A few can be found in brackish water. Members are known to attack algae, other fungi, crayfish, zooplankton; they sometimes cause disease epidemics in zooplanktons. Some species of the genus Saprolegnia are predominately parasites of fish and fish eggs. A species of Aphanomyces causes root rot of peas.
Two types of zoospores are found in some genera. The first type is usually the pear shaped type with apically attached flagella. This is often referred to as the primary zoospore. After swimming for sometimes, the zoospore encyst and withdraws the flagella. The cyst then germinates to produce the secondary type of zoospore which is the bean-shaped type with laterally attached flagella. This swims for a short period, encyst and germinates to form either another secondary zoospore or a germ-tube in which a filament or thallus develops.
An example of asexual reproduction of the Saprolegniales (indirect germination).
Sexual reproduction in saprolegniales is oogamous where the female reproductive organ is called oogonium and the male organ is called the antheridium. The oogonium is a large spherical and non-motile organ. It contains one to several eggs depending on the species. The unfertilized egg is termed oosphere whereas the fertilized egg is known as oospore.
During reproduction, the antheridium produces branches which adhere themselves to the wall of the oogonium and penetrate through a fertilization tube. A single male nucleus then enters each egg and fertilization takes place. The oospore formed is a survival structure which grows, matures and become released from the oogonium. Under favourable environmental conditions, the oospore germinates to form a new fungus.
Sexual reproduction in Saprolegniales
These are mostly terrestrial fungi, living in soil or on vascular plants as obligate parasites. The zoospores are laterally bi-flagellated and the zoosporangia are the parasitic forms which function as conidia. Many species cause the disease known as downy mildew which is of great economic importance. Example is late blight of potato, black spot of cocoa, downy mildew of grape vine. Some species are obligate parasites. There are three families in this order namely:
Family: Albuginaceae Family: Peronosporaceae Family: Pythiaceae
You will find only one genus in this family with about 240 known species of obligate parasites of land plants especially crucifers. They cause the disease known as ‘white blisters’ or ‘white rust’. The disease is detected by the destruction of the stem and by the appearance of shinning white blisters on the leaves, stem and fruits before the host epidermis is ruptured. Dry pustules (fungi tissues) are exposed as the epidermis ruptures. The mycelia in host tissues are intercellular but at interval may send small spherical haustoria into the cells. The intercellular mycelia aggregate beneath the host epidermis to form palisade of cylindrical sporangiophores. The sporangiophores bear one-cell conidia or sporangia in chains. These form white powdery masses and are dispersed by wind. If the sporangia fall on suitable host leaf, they germinate within a few hours in a film of water to form about eight bi-flagellated zoospores. After swimming for sometimes the zoospores encyst and later germinate to form germ-tubes which penetrate the host epidermis. A new set of sporangia maybe produced within ten days of germination.
Albugo candida showing detached conidia/sporangiospores escaping from host edpidermis. (Source: Vashishta, B. R. and Sinha, A. K. Raghi Publications, New Delhi. 2005).
Reproduction in Albuginaceae
The oogonium of the genus Albugo grows in the intercellular spaces of the stem and leaves of the host. At first, both the oogonium and the antheridium are multinucleated but there is only one functional male and one functional female nucleus. After fusion of the male and female nuclei, a thin membrane first develops around the oospore. This is followed by several mitotic divisions resulting in as many as 32 nuclei in a matured oospore. A brown warty thick wall then develops around the oospore. The oospore germinates after a resting period to form a spherical thin-walled vesicle which maybe sessile or maybe suspended by a wide cylindrical tube. Later, 40-60 zoospores differentiate within the vesicle and are released as the vesicle ruptures.
Generally, you are likely to find the obligate or biotrophic parasites of higher plants in this family. They cause the disease known as ‘downy mildews’. These organisms cannot be cultured easily in the laboratory but some have been grown on tissue cultures. The mycelia in the host tissues are coenocytic and they are found in the intercellular spaces of the living host cells. This group of organisms penetrates the host cells by means of haustoria. Sporangiophores are from the mycelia within the host tissue and go outside the host through the stomata bearing sporangia at the tips. The sporangia are dispersed by wind. A few species germinate by means of bi- flagellated zoospores while majority directly produce germ-tubes. Genera in this family include: Peronospora, Plasmospora, Sclerospora and Bremia.
Members of this genus cause disease of high economic importance. For example P. pisi attacks peas; P. destructor attacks onion. A diseased plant is detected by the appearance of swollen
distorted stems which bear white hair-like sporangiophores. They cause yellow patches on the upper surface of leaves while white sporangiophores protrude from the under surface. The sporangiophores usually emerge singly or in groups. They consist of a stout main axis which branches dichotomously to produce oval-shaped sporangia at the tip as the branches curve. The sporangia become detached as a result of hygroscopic twisting of the sporangiophore induced by changes in humidity.
In reproduction, a thick wall oospore is formed after fertilization. Oospores of some species germinate directly but others produce zoospores. Some species are homothallic while others are heterothallic. In P. parasitica, when a sporangium falls on a suitable host, it germinates directly by means of germ-tube. The germ-tube then penetrates the host epidermal wall. This is seen in the diagram below where the conidiophore is emerging from the stoma and bearing conidia at the tip of its branches.
(Source: Vashishta, B. R. and Sinha, A. K. Raghi Publications, New Delhi. 2005).
Members of this family are non-obligate parasites or saprophytes. The parasitic species can live as saprophytes while the saprophytic species can live as parasites. They include both aquatic and
terrestrial species. Members produce conidiophores/sporangiophores that are not differentiated from the mycelium. For example, they may branch intermittently and resume growth after the production of conidium or sporangium either below or within the previous empty sporangium. The saprophytic members can grow on plant and animal remains while as parasites of plant and animals, they grow within the host tissues. Important genera in this family are Pythium and Phytophthora.
Phytophthora literally means a plant destroyer. Many species are parasites of flowering plants causing leaf blight and fruit diseases. An important example is P. infestans which causes Late blight of potato, a condition which resulted in an epidemic in Ireland in 1848. It consists of profusely branched, coenocytic, thick mycelia. Glucan is the chief component of the hyphae wall.
Phytophthora showing dichotomously branched sporangiophores. (Source: Vashishta, B. R. and Sinha, A. K. Raghi Publications, New Delhi. 2005).
Formation of Haustoria in Phytophthora
This forms both intercellular and intracellular hyphae. The intracellular hyphae emerge from the host cell into an intercellular space or directly into a neighbouring host cell. They both have similar ultra-structure. Haustoria develop on the intercellular hyphae when they come in contact with primary walls of the host mesophyll cells. The haustoria are variously shaped in this genus. For instance, in P. infestans they are small globes, occasionally short, straight or curved pegs. There may be one or more haustoria in each host cell. The intracellular haustoria are connected to the intercellular hyphae by a neck-like constriction at the penetration site. Thus the long haustorial neck usually associated with this organ is lacking. The haustoria are more commonly found in the tubers. In severe cases of infection the entire plant above the ground is killed.
Under favourable conditions, aerial hyphae arise from the stoma of leaves or from the lenticels or injured portion of the tuber. These hyphae have indeterminate growth and are called sporangiophores (conidiophores). Phytophthora differ from other members of the family in having sporangiophores distinct from the somatic hyphae. Sporangium is then formed by the inflation of the tip of the side branch of the sporangiophores. This is then cut off by a transverse septum. The hyphal branch having the young terminal sporangium continues to grow. As it reaches maturity, the branched tip swells slightly and proliferates pushing the sporangium, this process may be repeated. Sporangia are then borne terminally and are subsequently shifted to a lateral position. The sporangia are lightly attached and bear nodular swellings that show the point of detachment. The sporangia are dispersed by wind, rain splash or contact with other leaves.
They may also be washed into the soil. Under favourable conditions of moisture and temperature they germinate after landing on healthy leaf of a host plant either indirectly by producing zoospores (12-15°C) thus functioning as zoosporangia or directly into germ-tubes (20-25°C) thus functioning as conidia. They lose their viability if they fail to germinate within a few hours. The librated zoospores swim for sometimes in a film of water, encyst and germinate to produce germ- tube. This grows over the epidermis of the leaf and produces appressorium at its tip which develops a fine tubular, peg-like outgrowth, this is the infection hypha that pierces the cuticle and penetrates the epidermis into the host tissue to cause infection.
Sexual reproduction in Phytophthora is oogamous. Some species are homothallic while some are heterothallic. All species in the genus are monoecious that is, the mycelium is capable of producing both antheridia and oogonia. The antheridium arises from a short lateral hypha from a mycelium with its tip inflated while the oogonium arises from a short, lateral hypha without an inflation of the tip. The antheridium and oogonium grow towards each other. When they come in contact, the oogonium penetrates the antheridium to the other side where it forms a globose structure. The antheridium then forms a collar-like structure surrounding the base of the oogonium (amphigynous condition). There is only one functioning nucleus in the oogonium and one functioning nucleus in the antheridium. The antheridium forms a fertilization tube that penetrate the oogonial wall to transfer its nucleus, fertilization of the oosphere occurs thereafter. The fertilized egg secretes a thick wall around it and becomes an oospore. The oospore serves as a survival structure and increases with age. It germinates under favourable conditions after the decay of the host tissues.