1.6 Classification of fungi
In your previous classes, I assured you that you have learnt about the classification of living organisms. You may remember that in the most precise words classification is the grouping of organisms in particular taxa on the basis of their most similar characters. It gives information about how an organism is similar or dissimilar from other organisms. However, classification has three functions –
i) It provides a framework of recognizable features by which an organism under examination can be identified,
ii) It is an attempt to group together organisms that are related to each other, and
iii) It assists in the retrieval of information about the identified organisms in the form of a list or
catalogue.
Today I am going to discuss about classification of fungi.
In the above, we have discussed different aspects of fungi such as cell structure, thallus structure, mode of nutrition, etc. There, we learnt some of the characteristics that are different from the other plants. Some are different from animals. You might also be able to explain some characters have affinities with plants and some have affinities with animals. Therefore, such characteristics are reasons to place fungi in another group in the classification system.
R. H. Whittaker (1969) proposed a five-kingdom system of classification in which fungi were placed in separate groups.
He kept them in the kingdom “Fungi” considering all the multicellular eukaryotic
fungi with a few unicellular fungi (Eg. Yeast). Likewise, different authors
have attempted to classify fungi most appropriately at their level best. But,
still, no concrete classification module of fungi has been fixed. However,
here, I am going to discuss the two most acceptable forms of fungal
classification.
In earlier
days fungi were classified on the basis of some morphological and microscopic
characteristics such as –
a) Differentiation into
plasmodium or mycelium,
b) Presence or absence of
septa,
c) Structure and production
of sexual spores,
d) Nature of reproductive
structure or fruiting bodies etc.
The classification based on such characters
may be called a traditional system of fungal classification. In such a classification
system, characters were elaborated by the observations and identified the fungi
on the basis of observed characters. In such a classification system,
phylogenetic characters were not studied and, so sometimes related species were
kept in another group or apart. Even, non-fungal members have been included in
the kingdom of fungi. Such a classification system was dominant till the middle of
twenty century.
From the later part of the twentieth century,
new approaches in fungal taxonomy have evolved with inputs from recent
technologies, ultrastructure studies, DNA and RNA sequencings, biochemical
analysis and new phylogenetic systems have come up.
Like other groups of plant classification, fungal taxonomy and nomenclature are also
governed by the International Code for Botanical Nomenclature (ICBN).
Fungi classification can be
categorised as Traditional classification and Modern classification.
1.6.1 Traditional methods of classification
The traditional methods of fungal classifications can
be considered the works carried out by Bessey (1950), G. W. Martin (1961), C.
J. Alexopoulos (1962) and G. C. Ainsworth (1971). The basic outline of their
classifications is given below. Here, we are going to discuss in detail of
traditional classification done by C. J. Alexopoulos (1962) and G. C. Ainsworth
(1971).
1.6.1 (a) Bessey (1950)
Let us discuss the old traditional classification system
of fungi proposed by Bessey in 1950. As I read he divided true fungi into lower
and higher fungi and then into classes on the basis of the extent of thallus
development, life cycle patterns, and flagellation of reproductive structures. Just
remember that he simply classified the fungi into two groups –
i)
Lower fungi,
ii)
Higher fungi
i) Lower fungi group contained –
Class I: Phycomycetes. He placed fungal members in this class on the basis of the following characteristics –
a)
Unicellular thallus,
b)
Coenocytic mycelium,
c) Sexual
reproduction forms oospore or zygospore
ii) Higher fungi: The fungi having one celled, not producing planocytes
or producing cellular mycelium were placed under this group. This group is divided
into three classes –
Class I : Ascomyceteae – Fruiting body is ascus,
Class II: Basidiomyceteae – Fruiting body is basidium,
Class III: Fungi Imperfecti – Sexual reproductive stage not known
Bessey did not consider Myxomycetes as fungi. so he
placed them under the name Mycetozoa outside the limits of the vegetable kingdom. Let us see the classification outline of Bessey
(1950) in Figure 1.
1.6.1 (b) G. W. Martin (1961)
From the Bessy classification, you might
get an idea of how fungi were classified on the basis of a few characters. But you
remember that one classification process could not satisfy another as
knowledge is growing up among researchers every day. So from the esteemed end of
every researcher, they try to give their best classification system and still, the same working trend is going on.
Let us see G. W. Martin and how he classified
the fungi into different divisions or classes etc.
In his classification system, he included slime moulds in the
fungi. He divided the division fungi Mycota into two subdivisions.
He had divided the entire fungi (Mycota) group into two
sub-division-
1. Sub-division: Myxomycotina – in this sub-division, he
included all slime moulds,
2. Sub-division: Eumycotina – in this sub-division, he included the true fungi.
1. Sub-division-
Myxomycotina – This sub-division contained one sub-class, 5- orders,
2. Sub-division – Eumycotina
– This sub-division contained 4 classes
i) Class I- Phycomycetes
with 3 sub-classes and 16 orders,
ii) Class II – Ascomycetes
with 2 sub-classes and 25 orders,
iii) Class III-
Basidiomycetes with 2 sub-classes and 11 orders
iv) Class IV- Deuteromycetes
with no sub-class and 4 orders.
Now, let
us see the classification outline of W.
Martin (1961) in Figure 2.
Figure 2. Outline of, G. W. Martin (1961), the traditional system of fungal classification 1.6.1 (c) C. J.Alexopoulos (1962)
Now, I am going to discuss
the Alexopoulos (1962) classification system. From the previous two
classification systems, namely Bessey and Martin, you have observed the
developing trends of classification methods. Subsequent workers got more
exposoure to incorporate new characters in their classification system.
Gradually classification became more updated. Again, we are going to discuss
the little more advanced classification method that was forwarded by C. J.
Alexopoulos in 1962.
Alexopoulos first divided the Mycota into
two sub-division. Do you remember who has done it so likely? His two
sub-division were 1. Myxomycotina and 2. Eumycotina. In the sub-division
Myxomycotina, he included all the fungi having no wall in the hyphae and in the
sub-division Eumycotina, he included fungi that have walls and true fungi.
Sub-division Myxomycotina has only one class, namely
Myxomycetes, while sub-division Eumycotina has 8 classes and one form
class.
Let us see
the classification outline of Alexoulos (1962) classification in Figure 3.
Figure 3. Outline of C. J. Alexopoulos (1962)
classification
However, in 1979 Alexopoulos
with C. W. Mims modified the previous classification. They removed fungi from
the kingdom Plantae and gave them a kingdom status called Myceteae which was
further divided into Divisions, Sub-division, Classes, etc. They included all
fungi including slime moulds in the kingdom Myceteae of the super kingdom
Eukaryonta.
You may go for further study of the modified classification proposed by C. J. Alexopoulos and C. W. Mims (1979).
1.6.1 (d) G. C. Ainsworth (1971)
Now, we are going to discuss latest
traditional classification proposed by G. C. Ainsworth in 1971. He has
published his this claasification method in his book “Dictionary of Fungi”.
This classification is based on the following features –
a) Presence or absence of
cell wall,
b) Presence or absence of
motile cells,
c) Nature and type of
asexual spores,
d) Number of flagella and
their position on the zoospores,
e) Nature of sporophores,
f) Presence and absence of
fruiting bodies and their nature of the present.
Let us see brief outline (Figure 4)) and in details outline (Figure 5.) of G. C. Ainsworth (1971) classification.
Figure 5. Outline of G. C. Ainsworth (1971)
classification
Check
Your Progress
a) According
to your understanding which traditional classification is most
acceptable and why?
b) How
Ainsworth classification is best among the other classification methods.
justify
it?
c) Why term
traditional is used in fungal classification?
1.6.2 Modern classification
Dear students, now I am going to discuss the modern classification of fungi. Before going to the topic, we would like to
review the traditional methods of fungal classification. I hope you are now
able to explain about traditional classifications methods of fungi that
forward by different workers. Starting from the Bessey (1950) to Ainsworth
(1971) of different traditonal methods
of fungal classification, we have seen all the classification systems were
based on basic characteristics such as cell structure, present or absence of
wall, presence or absence of motile cell, nature of asexual and sexual spores,
number of flagella and position, unicellular cell structure, etc. In
traditional classification methods, phylogenetic relationships among the different
groups were ignored. Even, non-fungal members were also included in the fungal
groups or fungal groups were kept apart from fungi. In traditional
classification, we have not seen any genetic or biochemical characteristics
among the same or different groups.
To overcome the demerits of traditional
classification, with the aids of modern technologies, some researchers were
forwarded the modern classification of fungi.
As we discuss in previous classes from the later part of the twentieth century, new approaches in fungal taxonomy have evolved with inputs from recent technologies, ultrastructure studies, DNA and RNA sequencings, biochemical analysis and new phylogenetic systems have come up. However, the traditional methods are not ignored as they provide detailed descriptions and morphological features of different species and groups. In this section, we are going to discuss the modern classification of fungi.
Therefore, considering the confusion in the groups, deviation from the ICBN rules, and the new features gained from the molecular phylogeny, several workers have attempted in a coordinated manner since the last decade of the twentieth century to decide the phylogenetic classification of fungi with a broad scope by formulating the following principles –
i) Classification should not be misleading about the phylogeny of the organisms they represent,
ii) Methods of classification should efficiently promote the translation of phylogenetic hypotheses into classification. It says that once a monophyletic group has been discovered by a systematist, it should be possible to name it quickly and simply,
iii) Classification should
be stable.
Lets us discuss some
classification methods based on these principles which are considering modern
classification –
1.6.2(a) Webster and R.W.S.
Weber (2007)
Let
us discuss one of the most recent classifications that put forwarded by Webster
and R.W.S. Weber in 2007 and published in the 3rd edition of their
book “Introduction to Fungi”. They have included the slime moulds in the
division Myxomycota under Protozoa kingdom.
Now we are going to see the outlines of their classification –
According to their classification, the fungi and fungus-like
organisms have been segregated in the three kingdoms Protozoa, Straminipila and
Eumycota.
Under the kingdom Protozoa – there are 2 divisions which contain unicellular
fungi-like organisms. These 2 divisions are –
Division 1: Myxomycota: It has 4 classes - i) Acrasiomycetes, ii) Dictyoteliomycetes,
iii) Protosteliomycetes, and iv) Myxomycetes.
Division 2: Plasmodiophoromycota:
It has 2 orders
Plasmodiophorales and Haptoglossales.
Under the kingdom
Starminipila – there are 3
divisions.
Division 1: Hyphochytriomycota,
Division 2: Labyrinthalomycota,
Division 3: Oomycota with 8 orders.
Under the kingdom Eumycota – there are 4 divisions.
Division 1: Chytridimycota with 1 class (Chytridiomycetes)
Division 2: Zygomycota with 2 classes (Zygomycetes and
Trichomycetes)
Division 3: Ascomycota with 5 classes (Archiascomycetes,
Hemiascomycetes,
Plectomycetes, Hemenoascomycetes and Ustilaginomycetes).
Division 4: Basidiomycota with 4 classes (Homobasidiomycetes,
Heterobasidiomycetes, Urediniomycetes and
Ustilaginomycetes).
1.6.2 (b) D. S. Hibbett et.
al. (2007)
Let us
discuss another most recent method of fungal classification that was
forwarded by Hibbett and his team
in 2007. This is a comprehensive phylogenetic classification of fungi. The basic features of their
classification are –
a) Fungi and animals are
considered sister groups. They have closed relatives and share a common
ancestor known as the opisthokont clade,
b) The phylum Zygomycota is
replaced by the term Glomeromycota. Because phylum Zygomycota is not accepted in
the most recent classification because doubt raised about the relationships
between the groups that have been traditionally placed in this phylum,
c) They have treated
evolutionary characters of different groups in their classification. Aquatic
final groups such as Chytridiomycota are considered to be the most primitive,
d) They expressed that the
evolution of higher fungi took place first by the loss or withdrawal of chytrid
flagellum and the development of branching, aseptate fungal filaments,
e) Septate filaments evolved
by divergence from the Glomeromycota as a combined pre-Basidiomycota or
pre-Ascomycota clade about 500 million years ago,
f) They considered
Basidiomycota and Ascomycota as sister groups that diverged about 300
million years ago and hence placed under the sub-kingdom Dikarya,
g) Aerobic chytrids have
given rise to symbiotic fungi residing in the lumens of grazing mammals helping
in their digestion recently perhaps 60-80 million years ago,
h) The members whose
phylogenies have not been ascertained are placed in that
class or phylum,
i) They have separated the
pseudo-fungi-like organisms from the true fungi and classified them under the sub-kingdom Chromista and sub-kingdom Protozoa,
j) The traditional class
Phycomycetes have been removed and similarly Myxomycetes of traditional
classification is considered under the kingdom Protozoa,
k) True fungi which make up
the monophyletic clade are called Kingdom Fungi and have been classified into
7 phyla.
Now, in our discussion, we are going to discuss
only the kingdom of Fungi. As we are now aware that kingdom fungi have been divided into 7
phyla by Hibbett et. al. (2007)
They have divided the kingdom of fungi into 7
phyla.
Phylum 1: Chytridiomycota with 2 classes, 4 orders
Water
moulds and aquatic fungi produce motile zoospores, simple posterior flagellum,
unicellular/ filamentous, holocarpic/eucarpic, and zygotic meiosis, considered most
ancestral.
Phylum 2: Neocallimastigomycota with 1 class and 1 order
The thallus
mono/polycentric, anaerobic, found in digestive systems of large herbivores
mammals, and zoospores positively uniflagellate/polyflagellate.
Phylum 3: Blastocladiomycota with 1 class and 1 order
Water
moulds, and aquatic fungi, all have zoospores, saprophytes, parasites
on algae or plants and invertebrates, thallus monocentric/polycentric, and sometimes mycelia.
Phylum 4: Microsporidia
Unicellular
parasites of animals, considered as sister group of rest of fungi.
Phylum 5: Glomeromycota with 5 sub-phylum and 9 orders
Symbiotic fungi. They do not form zygospores.
Phylum 6: Ascomycota with 3 sub-phylum, 15 classes and 1 order
This is the
largest group of fungi and the lifestyles adopted to cover the complete range
from saprotrophs to symbionts and parasites. Sexual spores are known as
ascospores formed endogenously within the ascus. Except for yeasts, asci are
produced in complete fruiting bodies. The basic type of the genus is Pezzia.
This group contains about 64000 species under 6355 genera.
Phylum 7: Basidiomycota with 3 sub-phylum, 13 classes and 47
orders
This group of fungi produce basidiospores
within the basidium. They are generally saprophytes, but parasites on plants
and insects. Thallus filamentous with septate hyphae. Septa have central pores
which are called dolipore. The filaments are 2 types – primary homokaryotic
(having 1 nucleus) and secondary heterokaryotic with 2 nucleus or dikaryotic
cells. Asexual takes place by fragmentation of the thallus, oidia, conidia etc.
Sexual reproduction performs by somatogamy and basidiospores. It contains about
1600 genera and 32000 species.
Check
Your Progress
a) Why modern classification
methods are brought up by various mycologists?
b) How modern classification
methods are more reliable than traditional ones?
c) Among the modern
classification methods, which one will you refer to for your
better understanding?
d) What informations are
being carried out in the word “Phylogeny”?
e) How phylogeny helps in
classification?
Figure 7: Outline of fungal classification forwarded
by D. S. Hibbett et. al. (2007)
1.7 Let us sum up-
Dear students, we have discussed several aspects of fungi in
your introductory class. From our discussion, we are now in a position to
define fungi, explained to thallus organization of fungi, and fungal nutrition and
later part of the unit we have discussed both traditional and non-traditional
methods of fungal classification.
It is again reminding you that fungi are
heterotrophic organisms. They have certain affinities with plants and animals.
But from the point of plant taxonomy, they differ from both plants and
animals. Due to their enormous similarity with
plants, they are placed under the plant kingdom. Like plants, they have cells, which is
the major difference from animal cells. We have discussed that fungi draw their
nutrition in different ways. Some fungi draw their nutrition from dead and decaying
organic matter – called saprophytes; some fungi draw their foods
from living organisms – called parasites; some fungi live by sharing
foods and other elements with organisms in mutual benefits- called symbiosis
and some fungi prey on other organisms – called as predacious.
In the classification section, we have discussed different methods
of fungal classifications forwarded by different authors. I assured you that you
are now able to explain traditional and non-traditional methods of
fungal classification. In traditional methods, only external characters were
considered for grouping the fungi. In such classification methods, taxonomists ignored phylogenetic relationships among the different groups. To overcome
such problems, in recent years mycologists have classified the fungi by adding
the phylogenetic characters apart from characters used in traditional methods.
The most recent modern classification methods are forwarded by Webster and
R.W.S. Weber (2007), D. S. Hibbett et. al. (2007), etc.
For more study, you may go through the latest editions of mycology books.
1.8
Glossary –
Chitin: a
complex polysaccharide, a polymer of N-acetylglucosamine, found in the exoskeletons of arthropods and in the
cell walls of fungi; thought to be responsible
for some forms of asthma in humans
Hypha: a long,
branching, filamentous structure of a fungus that is the main
mode of vegetative growth
Mycelium: the
vegetative part of any fungus, consisting of a mass of branching, threadlike
hyphae, often underground
Thallus: the vegetative
body of a fungus
Saprophyte:
any organism that lives on dead organic matter, as certain fungi and bacteria
Septum: cell wall division between hyphae of a fungus
1.9 Further readings
1. Alexopoulos,
C.J., Mims, C.W., Blackwell, M. (1996). Introductory Mycology,
John Wiley & Sons (Asia) Singapore. 4th edition
2.
Webster, J. and Weber, R. (2007). Introduction to Fungi, Cambridge University
Press, Cambridge. 3rd edition.
Answer to check your progress-
1. What do you mean by
heterotropic mode of nutrition?
Ans- The organisms which cannot prepare their foods
and depends on other sources
are called the heterotrophic mode of
nutrition. Example – Man, Fungi etc.
2. What is the saprophytic mode
of fungal nutrition?
Ans- The fungal groups who
draw their nutrition from dead and decaying organic
material is called saprophytic fungi
and this mode of nutrition is saprophytic
mode of nutrition.
3. What are the characters
considered in traditional classification methods?
Ans- In traditional
classifications methods superficial characters such as vegetative
structures, hyphal structure, structure
and number of reproductives bodies are
considered.
Bibliography
1. Alexopoulos,
C.J., Mims, C.W., Blackwell, M. (1996). Introductory Mycology,
John Wiley & Sons (Asia) Singapore. 4th edition
2.
Webster, J. and Weber, R. (2007). Introduction to Fungi, Cambridge University
Press, Cambridge. 3rd edition.
3.
Sethi, I.K. and Walia, S.K. (2011). Textbook of Fungi and Their Allies,
Macmillan Publishers India Ltd.
5.
Sharma, P.D. (2011). Plant Pathology, Rastogi Publication, Meerut, India.
Probable
Questions
Very short questions-
1. What is the reserve food materials of
fungi?
2. How many fungal groups are placed in the Bessey classification?
3. Why fungus body is called thallus?
4. Who has forwarded the phylogenetic
classification of fungi?
Short questions (Write within 100 – 150
words)
1. Write
the principles of modern fungal classifications?
2. Write the different modes of fungal
nutrition?
3. Write a note on fungal cell wall
compositions?
4. Write the thallus organisation in
fungi?
