Some dikaryan anamorphs (especially coelomycetes) grow in dead leaves and stems of desert plants, and other moulds are the most drought tolerant of all organisms, able to grow at water activities below 0.70 (for example, on jams, salt fish and other substrates of extremely high osmotic pressure - see Chapter 20).

While many zygomycetes can assimilate only 'accessible' substrates like sugars and starch, ascomycetes can often exploit cellulose, and many basidiomycetes can digest both cellulose and lignin, carbon sources that are available to remarkably few other organisms. Though fungi cannot fix atmospheric nitrogen (this talent seems to be restricted to the prokaryotes), dikaryan fungi can use many different forms of combined nitrogen: some ascomycetes even specialize in metabolizing the protein keratin, which is the main component of hair and skin. In case you were wondering if members of this group constitute a health hazard -- they do. Some other orders of ascomycetes are obligate parasites of plants. Remember the 'downy mildews' caused by oomycetes? Well, there are also plant diseases called 'powdery mildews' that are caused by ascomycetes.  The similarity of terminology is unfortunate, but try to remember the difference, because although the groups of fungi involved are both obligately biotrophic, the diseases are different in many important ways, such as host ranges and methods of control. This is just one example of how taxonomy has practical implications (see Chapter 12).

Thousands of basidiomycetes, and a quite a few ascomycetes, establish intimate mutualistic symbioses (mycorrhizas) with the roots of trees, especially conifers (see Chapter 17).  Nearly 18,000 ascomycetes, and a few basidiomycetes, have domesticated algae, thus becoming lichens, which can live in some of the world's harshest climates, and colonize the barest and most inhospitable substrates (see Chapter 7). Some dikaryan fungi have even re-entered the water and, lacking motile cells, have evolved other mechanisms, such as long appendages, to aid spore dispersal. Dikaryan fungi range from unspecialized, almost omnivorous saprobes, to fungi so specialized and ecologically demanding that they are found only on one particular leg of one species of insect. Some dikaryan fruit bodies are microscopic (as in many ascomycetes), but often (especially among the basidiomycetes), they are large and complex, and most of the common names applied to fungi refer to the visible teleomorphs of basidiomycetes, and in a few cases, ascomycetes. You may already be acquainted with some of these: I will introduce you to many more in the pages ahead.

Phylum ASCOMYCOTA - Characteristics of Teleomorphs

Most dikaryan fungi share a number of important features -- (1) chitinous cell walls; (2) hyphae with regular cross-walls called septa (centrally perforated to allow movement of cytoplasm, and sometimes nuclei, between compartments): (note that many yeasts are unicellular, so don't produce hyphae); (3) the ability of somatic, assimilative hyphae to fuse with one another (anastomosis) and to exchange nuclei; and (4) the occurrence in their life cycles (or at least in those which produce a teleomorph) of a unique nuclear phenomenon called the dikaryon. After sexually compatible nuclei from different mycelia have been brought together by anastomosis, they pair off, but don't fuse immediately to form a diploid zygote. Instead, they go on dividing synchronously to populate what are called dikaryotic hyphae, in which each compartment has two sexually compatible haploid nuclei. Oh yes, they do fuse eventually, but not before some remarkable developments have taken place, and in basidiomycetes, perhaps not for years. Read on.

If ascomycetes and basidiomycetes share all these things, how do they differ? Actually, in many ways, and with experience it's usually easy to tell their sexual fructifications apart with the naked eye. But their microscopic, unicellular meiosporangia are most diagnostic of all (compare the two sets of diagrams below).

But most ascomycetes interpolate a dikaryophase, during which the number of pairs of compatible nuclei is multiplied, often enormously, as dikaryotic hyphae (often called ascogenous hyphae, as in the diagram above) grow and branch within a mass of monokaryotic (haploid) tissue which is the framework of the fruit body (the ascoma).   Eventually, the ultimate branches of the dikaryotic hyphae, of which there may be millions in larger ascomata, reach their ordained positions in the future hymenium and the long-delayed sexual fusions take place. The genome is reshuffled during the ensuing meiosis in each ascus (this genetic recombination is due to crossing-over, which is explained in Chapter 10). Each meiosis will produce a somewhat different arrangement of the genome. In this way the products of a single encounter are first multiplied, then meiosis generates a lot of genetic diversity. Not only is the dikaryon itself an unusual phenomenon, but during the dikaryophase an effectively diploid mycelium is growing within, and drawing nourishment from, the haploid ascoma tissue. This phenomenon has interesting parallels in the red algae, though molecular evidence doesn't support a close relationship between the two groups (both lack motile gametes and appear to have simply hit upon the same solution to the problem posed by the rarity of sexual encounters).

Ascospores are not motile, in the sense of self-propelling, but most ascomycetes nevertheless send their ascospores on their way with a burst of kinetic energy. The ascus is a tiny spore-gun, which works by building up internal pressure, then releasing it through the tip (see animation below). The job of most asci is to get their ascospores into the turbulent airflow above the ascoma. Before the ascus bursts open at the tip and shoots its spores skywards, the pressure inside the ascus is cranked up by osmolytes such as glycerol.  It builds up to about 3 atmospheres before the apex blows.

Mature asci of the dung-inhabiting Ascobolus (below) project above the hymenium and point toward the light before discharging their spores. In this way they ensure that the spores will not run into any obstacles on their upward flight (see Chapter 8).

Four kinds of Ascoma.   The multicellular structures (ascomata) that produce the asci, and act as the platforms from which the spores are launched, come in four main designs, sectional views of which are shown in the diagrams below.

(1) APOTHECIAL ASCOMATA [below]
The construction of the ascoma may allow several or many asci to discharge simultaneously because the entire fertile layer or hymenium is exposed...
The diagram on the left shows a vertical section through a very small apothecial ascoma.
The photomicrograph on the right shows three ascomata of Ascobolus: the dark dots are unitunicate-operculate asci (see next section) containing dark spores, arranged in a flat superficial hymenium. 

Four kinds of Asci.  Before we go on to explore the many orders of ascomycetes, we must take a closer look at the ascus itself. All asci are not the same. 
There are four flavours...

The animation on the right (courtesy of Dr. Jim Worrall) shows how operculate asci develop and shoot.   

In any modern consideration of the ascomycetes, we cannot ignore their asexual reproductive phases, many of which are called moulds. You already know that zygomycetes have diverse asexual phases. So you won't be surprised to discover that many ascomycetes have comparable asexual (anamorphic) phases during which they reproduce rapidly, and often relatively cheaply, by means of mitospores called conidia. 

The asexually reproducing phase of the ascomycete life cycle was more or less ignored for many years in favour of teleomorph studies. But when we consider that the anamorph is an important (and sometimes the only) phenotypic expression of many ascomycete genotypes, we realize that it has much to tell us.   Besides, one can get DNA and RNA from anamorphs just as easily as from teleomorphs, so we are beginning to understand the relationships of anamorphs better, even in many cases where no teleomorph is known.

Though they play essentially the same role in the life cycle, the anamorphs of ascomycetes differ from those of zygomycetes in two very important respects:

(A) While zygomycete mitospores commonly originate by free-cell formation inside a sporangium, many spores cleaving from a single mass of cytoplasm, the mitospores (conidia) of ascomycetes are basically modified bits of hyphae, either budded out as a new structure, or converted from a whole existing cell.

(B) In zygomycetes, anamorph and teleomorph often occur together (especially in homothallic species) and always share the same binomial. In ascomycetes, anamorph and teleomorph often develop at different times, and on different substrates. Each phase has often been collected in total ignorance of the existence of the other, and because of this, the International Code of Botanical Nomenclature maintains that it is legal to give them separate binomials - a useful option but one giving rise to a great deal of confusion in some students' minds (How can one organism have two names?)

Even though several thousand anamorph-teleomorph connections have now been established in the ascomycetes (you can now find them at this web site: 
http://www.cbs.knaw.nl/databases/index.htm ) these represent only a small proportion of the total number of taxa, either of anamorphs or teleomorphs (we know about 30,000 of each). Because anamorphs so often occur alone, it is still normal and accepted practice to use separate binomials for them, as you will see. Nevertheless, I find it completely unacceptable to talk about conidial fungi, as many other texts do, as if they constituted a separate major high-level taxon called the 'Deuteromycotina.' This ignores both the evidence that they are all expressions of dikaryomycotan genomes, and the thousands of connections that have already been established with teleomorphs (and the number grows every year).

Of about 30,000 known ascomycetes, about 5,000 have so far been connected to their anamorphs. What about the many thousands of conidial (anamorphic) fungi that are still 'orphans'? I think there is good reason to believe that many of them have given up sex altogether, and become 'anamorphic-holomorphs,' though they seem to have retained some genetic flexibility by: (a) having more than one kind of nucleus in their mycelia (heterokaryosis) as a result of occasional hyphal fusions (anastomoses); (b) sometimes undergoing a complex parasexual process involving rare somatic diploidization, mitotic crossing-over, and finally a return to the haploid condition (this process is more fully explained in Chapter 10).

It turns out that the conidial fungi are a mixed bag.  Although most of them are (or were) part of ascomycete life cycles, some are (or were) connected with basidiomycetes. Despite this mixed ancestry, we have had to set up a single classification for all of them, because it is often impossible to tell in which subphylum the connection lies. Unfortunately, our scheme for classifying the anamorphs has so far been able to make little reference to teleomorphs, for several reasons:
(1) teleomorphs are known for only 10-15% of anamorphic species;
(2) members of what seems to be a single anamorph genus may have teleomorphs in many different holomorph genera (even from different orders). This must be due to convergent evolution among anamorphs;
(3) anamorphs belonging to several different anamorph genera can have sexual phases in a single holomorph genus.  This must be due to radiative evolution among anamorphs.

There are about 1,500 described genera of conidial fungi, and almost 30,000 described species, and these numbers are increasing rapidly. The first really useful classification of dikaryomycotan anamorphs, established in the late 19th century, was based on mature morphology. The principal characteristics used were:
(1) colour, septation and shape of conidia;
(2) conidiophore aggregation, or lack of it;
(3) the production of conidia in enclosed structures or the absence of such enclosure.   
Don't worry, all these characters are illustrated below. 
The division of anamorphs into two large groups is informal, is based on morphology, and is really just for convenience.   

The easiest decision to make is usually whether a conidial fungus is a hyphomycete or a coelomycete. You can recognize a hyphomycete because its conidiophores can be single or aggregated in various ways, but are never enclosed within a covered conidioma. Coelomycetes form their conidia in enclosed conidiomata (below), which usually develop just beneath the surface of their plant substrate.  I am not going to discuss coelomycetes in detail (though there are thousands of them, and they cause many important diseases of crop plants), but I must mention a few basic facts.

Coelomycetes  = covered conidiomata

The pressure of accumulating conidia, and often of accessory mucilage, eventually splits the host epidermis and allows the conidia to escape. These acervuli have developed in the surface layers of an avocado.

'Mostly Moulds'

Hyphomycetes  = exposed conidiophores or conidiomata

Most of what we call moulds are hyphomycetes (the anamorphic or asexual reproductive phases of many Ascomycetes and some Basidiomycetes).  As of May 2005, we recognize almost 1,400 form-genera and almost 13,000 species of hyphomycetes.

A small minority of moulds are anamorphic phases of Zygomycetes (things like Rhizopus and Mucor), or microscopic ascomycetes such as Chaetomium. Weeds are plants growing where we don't want them. Moulds are sometimes the weeds of the fungal world -- fungi which have a habit of fruiting on substrates that are of interest to us, such as food, paper, leather and wood.  

What do Hyphomycetes look like to the naked eye?  Well, I must admit that they are very unimpressive -- look at the picture below, of a log with some black mould colonies.

But put a bit of that black stuff under the microscope, and a new world opens up...  Among the hyphomycetes, conidiophores are usually solitary [below, left], though they sometimes form columnar aggregations called synnematal conidiomata [below, centre], or cushion-shaped masses called sporodochial conidiomata [below, right].

On the right, a synnematal    conidioma of  Gangliostilbe 
(a number of conidiophores growing up in a parallel bundle)

 

More recently, it was discovered that conidial fungi use a number of different techniques to produce their spores. Since these represent genuine `embryological' differences, they have become important characters in our classification. Spores which look alike often develop in different ways.  We begin by checking an anamorph to see which of two basic patterns of development - blastic or thallic - it exhibits.

We will examine eight different kinds of conidium development: six blastic, two thallic.

Type I - blastic-acropetal or blastic-synchronous conidiogenesis

The sequence of blastic-synchronous conidium development is shown in the three accompanying photomicrographs...

(the first two are taken by Nomarski interference contrast, the third by phase contrast)

In the Spilocaea anamorph [right and below] of Venturia inaequalis, the apple scab fungus, each seceding conidium leaves a ring-like scar, an annellation, around the conidiogenous cell, which then grows on through the scar ('percurrently') to produce the next conidium. Conidiogenous cells that have produced x spores bear x annular scars -- hence the name annellidic. Scopulariopsis (far right) has several annellidic conidiogenous cells on each branched conidiophore. The photo below is an oil-immersion picture of a tape-lift from one of my apples (which is unfortunately not resistant to scab). You can see flame-shaped conidia with truncate bases, and several annellations on the central conidiogenous cell.

Many common moulds produce conidia in rapid basipetal succession from the open end of special conidiogenous cells called phialides. Important genera such as Penicillium, Aspergillus, Fusarium, Stachybotrys, Trichoderma and  Chalara are all phialidic.

Below is Trichoderma harzianum, a well-known mycoparasite.

In Basipetospora (a thermotolerant fungus used in Indonesia in the preparation of a red food colouring), a conidium forms at the tip of the relatively undifferentiated conidiogenous hypha and is delimited by a cross-wall; then a short zone of the hypha just below the conidium balloons out to produce the second conidium. After this has been delimited by a septum, the next segment of the hypha plasticizes and blows out, and so on. As the chain of conidia elongates, the conidiogenous hypha becomes shorter.

Watch the animation several times. Focus first on the elongation of the chain, then on the shortening of the conidiogenous hypha.  Similar development occurs in Trichothecium and Cladobotryum, but this is an unusual kind of conidiogenesis.

In the Oidium anamorph of Erysiphe graminis, whitish chains of conidia (the 'powdery mildew') cover the host leaves. Each chain consists of a graded series of gradually maturing conidia, the oldest at the tip, the youngest barely differentiated from the hyphal cell just below it. New material is added at the base of the chain in a form of intercalary growth, arising from a sometimes swollen mother cell which appears to be a highly modified phialide.

In the Geotrichum anamorphs of Dipodascus spp. (Saccharomycetes) (right), an assimilative hypha stops growing, then becomes divided up into short lengths by irregularly arising septa. These are double septa which split apart schizolytically to give a 'chain' of short cylindrical 'fission arthroconidia' that disarticulates and appears jointed (hence 'arthric').

The Microsporum anamorphs of Nannizzia (Prototunicatae: Onygenales), which can digest keratin, and cause skin diseases in humans (see Chapter 23), develop large thallic phragmospores at the ends of hyphae. These conidia are liberated rhexolytically.

Why is it important to be able to identify conidial fungi? Some of them literally grow on you. The most prevalent fungal diseases of humans (mycoses) are caused by conidial fungi. Various superficial mycoses, ranging from ringworm of the scalp, through jock itch (more vulgarly known as crotch-rot), to athlete's foot, are caused by keratin-attacking species of Microsporum, Epidermophyton, and Trichophyton (see Chapter 23).

When I compiled the fungi causing important plant diseases, I found that 62 were conidial fungi (hyphomycetes and coelomycetes), as compared to 111 from all other fungal groups combined. One of the most serious outbreaks of plant disease in recent years was the southern corn blight, caused by Bipolaris (Helminthosporium) maydis, the anamorph of Cochliobolus heterostrophus (Bitunicatae: Dothideales), which devastated the U.S. corn crop in 1970. The special 'Texas male sterile' strain of corn becoming widely used for seed at that time was highly susceptible to the fungus, which produces a toxin that disrupts membranes, especially those of the mitochondria, reducing the production of ATP. The toxin also reduces photosynthesis: it inhibits uptake of potassium by the guard cells of the stomates, causing the stomates to close and thereby reducing the intake of carbon dioxide. Southern corn blight was brought under control by changing the strain of seed corn used by growers. Plant diseases are covered in more detail in Chapter 12.

Here, colonies of Penicillium italicum are shown growing on oranges: this exemplifies the moulds that spoil food in storage (e.g.,  in your refrigerator). Food spoilage is the subject of Chapter 20.

Worse yet, Aspergillus flavus (of which a single highly magnified phialide-bearing conidiophore is shown here), grows on peanuts and many other substrates, producing a mycotoxin called aflatoxin, which contaminates food and causes liver damage even at very low concentrations.  It is the most potent carcinogen (cancer-inducing) substance known.

The subject of mycotoxins is explored more fully in Chapter 21.

On the positive side, hyphomycetous and coelomycetous anamorphs are among the prime colonizers and decomposers of plant debris, playing a vital role in the carbon and nitrogen cycles. Hyphomycetes dominate the soil mycota in most forests. The economy of many streams is based on the dead leaves of land-based plants. These are colonized by aquatic hyphomycetes, which usually form tetraradiate (four-armed) conidia, and are tolerant of low temperatures so can grow during the winter and even under ice. These fungi make the dead leaves much more palatable and nutritious for the various detritivorous invertebrates which eat them, and thus the fungi act as vital intermediaries in energy flow in northern stream systems. The terrestrial and aquatic ecology of conidial fungi is explored in Chapter 11.

Some soil-inhabiting hyphomycetous anamorphs have evolved specialized traps with which they catch small animals -- nematodes, rotifers, tardigrades, amoebae and even springtails -- these truly predatory fungi can be visited in Chapter 15.

Conidial fungi are not just out there doing their own thing. We have also learned to exploit some of them for our own purposes.  The enzymes of Penicillium camembertii produce the soft, smooth texture of Camembert and Brie cheeses. Penicillium roquefortii puts that inimitable zippy flavour in blue cheeses such as Roquefort, Danish Blue, Stilton and Gorgonzola. Now there's even Cambozola, which blends the buttery texture of Camembert with the assertive flavour of Gorgonzola. 

Aspergillus oryzae is used in the Far East to turn soya protein into such delicacies as soy sauce (or its sweet Indonesian variant, Ket-jap, the word which became Ketchup in English)(see Chapter 19).  Other anamorphs are also exploited in the orient to prepare a variety of "fermented foods."

Finally, despite the insidious threat of mycotoxins, other secondary metabolites have come to play important roles on the human stage: (1) a substance which gave Penicillium chrysogenum an edge over competing bacteria in the natural habitat became one of our most potent weapons against bacterial disease - penicillin.

It has enormously improved the success rate of organ transplant operations by preventing recipients' immune systems from rejecting the implant, but not leaving them totally defenceless against infection.  This substance or its derivatives also hold out some hope for treatment of severe auto-immune diseases such as juvenile diabetes, rheumatoid arthritis, multiple sclerosis, myasthenia gravis, aplastic anaemia, Addison's disease, Hashimoto's thyroiditis, systemic lupus erythematosus and Henoch-Schonlein purpurea.

See Chapter 24 for more.

Here is a quiz which will tell you how well you have retained the information about development of conidia.  See how many of the 12 methods you can recognize by name.

Make a list, then check it by clicking here.

Find and enlarge the picture of the anamorph of Coccidioides immitis (the second down on the page) which causes coccidioidomycosis, a serious human disease (see Chapter 23), and decide what kind of conidium development is shown there.