Problems and Solutions in Mycology

Topic 1

Degeneration  of sex in fungi 

        In Fungi, simplification of sex organs is observed from lower groups to upper groups:-

      1.In Mastigomycotina,a lower group of fungi,isogamy,anisogamy or oogamy resulted into zygote. 

       2.In Ascomycotina & Basidiomycotina, the sexual reproduction process is progressively simplified in different directions with well- established sexuality.

        3 Whereas ,sex organs are absent in few higher fungi of basidiomycotina,though sexual reproduction is well established.

Degenaration sex in non - septate and ceonocytic fungi :- 

         1. The members of Oomycetes of Mastigomycotina , like Pythium , Phytophthora etc.female oogonium is fused by male antheridium to develop either oospore or zygote. It is considered as an advanced type of sexual reproduction.

        2. In the members of Zygomycotina like Mucor,Rhizopus etc, morphologically similar but physiologically disimilar (+ and -) gametangia copulate to from zygospore .

It means that advanced oogamy of Mastigomycotina is simplified in Zygomycotina where male and female sex organs are abolished.

Degenaration of sex in Ascomycotina :- 

    The members of Ascomycotina show gradual degenaration of sex :- 

            1.In Pyronima confluens , and in others oogamy is observed where antheridial nuclei migrate into the ascogonium and lateron  ,at the tip of ascogenous hyphae crozier is formed.At last in the penultimate cell of the crozier opposite nuclei fuse to from zygote, which performs meiosis and mitosis respectively to from eight ascospores.

          2. In Penicillium vermiculatum, ascogonium and antheridium simultaneously developed but it has been observed that antheridial is non-functional and female nuclei arrange in pairs and lastly ascospores are formed by direct development method.

            3.In Aspergillus herbariorum ,though antheridium coils round the oogonium but fails to make a content by the trichogyne,but female nuclei paired in the oogonium .

       4.In Collema pulposum ( an ascomycetes lichen ) ,the function of antheridium is replaced by asexual spores like conidia,oidia etc. That is ,oogonium  and asexual spores fuse to each other .whereas , in Humaria granulata  somatic hyphae unite with the physiologically different Ascogonium.

        5.In Humaria rutilens sexually different two somatic hyphae unite instade of the sex organs .

     In Ascobolus degenaration of sex is well established by its members. 

       a) In Ascobolus scatigenous and A. strobilinus well developed antheridium spirally coiled around the ascogonium and transfer male nuclei through trycogyne. 

      b) But in Ascobolus carbonarius , antheridia is replaced by specialised spermatium to perform spermatization. 

    C) In A.citrinus oogonial nuclei pass to the  ascogenous hyphae by autogamy due to the absence of antheridia. 

  D) In  A. furacurs both male and female sex organs are absent ,but copulation is performed by somatogamy.

 E) In A.equines  appomix is performed as sexual act is completely abolished.

     Degenaration of sex in Basidiomycetes:- 

1.In Polyporus and  Agaricus  ( class :Hymenomycetes) monokaryotic mycellium is well - developed and extensively branched. 

2. In Ustilago zeae, ( of class : Taleomycetes) monokaryotic stage is poorly  developed. 

3. In Ustilago sagetum infectous thread developed from basidial cells and plasmogamy and dikaryotic condition are established between the infectous thread of opposite meeting types.

 4. In Tillatia caris, mycellial involvement is replaced by sporodia. 

Topic 2

Parasexuality in Fungi

Several members of Deuteromycetes, a few Ascomycetes & Basidiomycetes perform plasmogamy,karyogamy & meiosis (haplodisation) in their life cycle but not at a specific time and specified points . This abnormal behaviour of such fungi is termed as parasexuality .

Pontecarvo & Roper ,1952 first discovered parasexuality in Aspergillus nidulans and also coined this term (Greek , para=beside of sex ).A few fungi which perform parasexuality are :

Aspergillus nidulans, A.niger, A fumigatus ,A. oryzae, Penicillium chrysogenum , P.expansum, P.italicum,Fusarium oxysporum,Ustilago maydis,etc.

Mechanism:

Pontecarvo,1956 observed the following steps of parasexuality in Aspergillus  nidulans:

(i) Formation of heterokaryotic mycelium:  Anastomosis of somatic hyphae of different genetic constitution , fusion of some nuclei and their subsequent division and mutation of one or more nuclei of a homokaryotic mycelia in some Ascomycetes resulted into heterokaryotic mycelium.

(ii) Karyogamy: Fusion of two haploid nuclei resulted into the formation of 2n nuclei .

(a) Fusion of like nucli (n) resulted into homozygous 2n nuclei 

     A  X  A -----> AA

         Or,

     B  X  B -----> BB

(b) Fusion between two genetically dissimilar nuclei :

     A X  B -------> 5 types of gametes:

 A (haploid nuclei) , 

 B (haploid nuclei ) ,

 AA (homozygous diploid nuclei),

 BB (homozygous diploid nuclei),

 AB ( heterozygous diploid nuclei).

(iii) Multiplication of the nuclei :

The above said 5 types of nuclei multiply and the ratio of the resulted nuclei is :

Heterozygous 2n nuclei: n nuclei =1:1000.

(iv) Occasional Mitotic crossing over:

Mitotic crossing over occasionally take place during the multiplication of diploid nuclei and resulted in the formation of new gene combination.

 In Aspergillus nidulans, it is very rare ,but in case of Panicillum chrysogenum and A. niger it is as frequent it is as frequent as during meiosis .

(v) Sorting out of 2n nuclei :

Sorting of 2n nuclei takes place in uninucleate conidia producing fungi .

(vi) Occasional haplodisation of 2n nuclei 

Occasionally haploid conidia are formed from some hyphae of  2n mycelia .Such haploid conidia germinate to form haploid mycelia ,.This phenomenon indicates the occasional halodisation of some 2n nuclei.

(vii) Sorting of new haploid strains :

Incorporation of n nuclei in uninucleate conidia indicates that new haploid strains are being sorted which are genetically different from their parents due to mitotic recombination.

Hence , after parasexuality a mycelium may contains:

(a) haploid nuclei similar to both of the parents.

(b) haploid nuclei with new genetic recombination.

(c) several types of diploid homozygous nuclei and

(d) several types of diploid heterozygous nuclei.

Significance of Parasexuality:

(i) New and better strains of industrially important Deuteromycetes are obtained by mutation through parasexuality.

(ii) It may be applied in the analysis of genetic and physiological process of fungi.

(iii) During the genetic control of pathogenicity and host range of Fusarium it can be applied .

Topic 3 General characters of Phycomycetes

(1) Either mycelium absent /rudimentary (e.g.Synchytrium ,Olpidium ) or well-developed (e.g. Pythium, Mucor, etc ).

(2) Mycelium is non-septate & coenocytic .

(3) Asexual reproductive units are endogenously formed sporangiospores (zoospores e.g. Synchytrium or aplanospores )

(4) In Oomycetes sexual reproduction is oogamous ,e.g. Pythium,Phytophthora,etc & isogamous in Zygomycetes .

(5) Zoospores are flagellated and motile .

(6) Dikaryophase is absent due to the fact that karyogamy follows plasmogamy immediately .

(7) Zygote produced sexually is simple and unicellular .

Topic 4: Sexual Representation in Penicillium 

Fig: Stages of sexual reproduction in Penicillium vermiculatum

 Dangeard in 1907 observed the sexual reproduction in P. vermiculatum , which is as follows:

A long, straight and club-shaped ascogonium (female) developed from mycelium.

It’s nucleus divides mitotically into 64 nuclei.

Simultaneously, a uninucleate male antheridial branch arises and coils around the female body.

Then a septa forms to make terminal antheridia which swells , turns club- shaped and uninucleate.

The antheridial tip touches the wall of the ascogonium & the point of contact dissolve but antheridial nucleus does not pass into the ascogonium i.e. remains non- functional .

· Meanwhile , ascogonial nuclei get paired (termed autogamy) & after that segmented leaving dikaryon in each chamber .

In P. glaucm instead of autogamy it performs plasmogamy i.e. the content of antheridium i.e. 32 -64 nuclei formed by repeted mitosis in trichogyne migrate in the female ascogonium.

From each bi-nucleate chamber new ascogenous initial developed which are binucleate and turned into ascogenous hyphae which are branched .

Development of Ascus:

 In P. vermiculatum all binucleate cells of ascogenous hyphae developed into globose ascus mother cell in which n+n turned into 2n & the structure termed as young ascus. It is called direct method .

In other species tip of ascogenous hyphae turns hook -like & segmented into 3 cells :-

(i) Tip ultimate with a nucleus

(ii) Penultimate cell with 2 nuclei

(iii) Uninucleate basal cell . 

  The 2 nuclei of penultimate cell turned 2n .It is called indirect method).

Then in both cases meiosis is performed & then mitosis take place .Hence the 2n of the young ascogonium turned into 8 haploid nuclei in a mature ascus .

Formation of cleistothecium:

Meanwhile, numerous sterile hyphae inter-woven to form the fruit body enclosing many ascus .

Liberation of spores :

When spores mature , the asci dissolves and ascospores released within the fruit body.Then the peridium decayed to liberate the spores in air.

 

Topic 5

Clamp Connections in Basidiomycetes Fungi

In most genera of Basidiomycetes clamp connections are formed during the divisions of the cells of the secondary mycelium. Usually in the terminal cells of the hyphae clamp connections are formed.

The stages of clamp connection formation are given below (Kniep,1917 and Bensaude,1918 ):

(i)At the time of cell division of the terminal two nucleated cell a lateral backwardly directed outgrowth develops between the two nuclei a and b . This outgrowth turns hooklike and known as clamp.

(ii)Out of the two nuclei a and b , b migrates towards the outgrowth and the other i.e., remains within the cell.

(iii)Both the nuclei (a and b) divide mitotically to form four nuclei ( a,a',b and b’).

(iv) Out of b and b’ , nuclei b remains in the clamp and b’ goes back to the dividing cell.

(v) Simultaneously, a' and a remain at the apical and basal region of the apical dividing cell.

(vi) During the time , the clamp bents over and the free tip of the clamp fuses with the dividing cell.

(vii)Then the zone of fusion dissolves, the b nucleus migrates towards a nucleus and remains within the cell.

(viii)In the next step the clamp turns closed permanently by the formation of a septum at the point of origin of the clamp.

(ix)Next a vertical septum forms just below the level of the outgrowth of the clamp. Thus , the apical dikaryotic cell turns into two dikaryotic cell. The terminal cell contains nuclei a’ and b' , whereas the sub-terminal cell contains nuclei a and b .

  

 Thus from one dikaryotic cell two dikaryotic cells are formed and the process repeats many times and thus the growth of the secondary mycelium continues.

Topic *

Fig: Polyporus fruit body

Polyporus

Systematic Position    

Kingdom Mycota

Division. Eumycota

Sub-division. Basidiomycotina

Class. Hymenomycetes

Sub-class. Holobasidiomycetodae

Order. Aphyllophorales

Family. Polyporaceae

Genus. Polyporus

Common name : Bracket Fungi ,because the outer margin of the basidiocarp looks like first brackets or Pore fungi , because on the under-surface of the fruit body / basidiocarp many fine pores are present.

Indian species : India represents 159 species of Polyporus like P. betulinus , P. agariceus ,P. circinatus , P.gilvus , P.cubensis , etc 

Economic importance

The harmful members:

(i) P. sulphureus / Sulphur mushroom causes heart rot of oak .

(ii) P. betulinus /Birch Polyporus causes heart rot of birch.

(iii) P. abietinus causes decay of Abies sp.

(iv) P. cinnabarinus causes decay of oak branches .

(v) P.squamosus causes heart rot of elm & sycamore.

(vi) P. glomerata causes wood decay of red maple .

(vii) P. versicolor is a wood rotter .

The useful members :

(i) Young fruit body of P. frondosus ,P. umbellatus ,P. leucomela , P. sulphureus and sclerotia of P. mylittae/ black fellow's bread are edible .

(ii) Reddish-brown basidiocarp of P. lucidus is used in decoration .

Vegetative structure :

(i) Body is mycelial .Hyphae are slender , branched and septate .

(ii) Primary monokaryotic hyphae developed from basidiospore is short lived and soon diplodised to perrenial fruit body of secondary dikaryotic mycelia .

(iii) Clamp connection is formed at the septa of secondary mycelium.

Asexual reproduction:

It is rare and occasionally conidia fromed on dikaryotic mycelium or on curious sterile fruit body germinate into dikaryotic mycelium.

Sexual reproduction:

It is somatogamous and heterothallic . Two somatic hyphae of + & - strain respectively diplodised into secondary dikaryotic mycelium.At favourable time this perrenial mycelium gives fruit body.

Development of fruit body :

At first tender fruit body is a sherical hyphal knot like structure.It gradually developed into a young knot with cap-like pileus with or without a short stipe.When young it is soft , pliable and covered by downy layer of mycelium but at maturity it is hard ,leathery or corky with many pores on under-surface .

Structure of mature basidiocarp:

On maturity fruit body turns hard, leathery /corky.A pileus may or may not be stipitate. If stipitate ,then it may be centric, lateral or dorsal.

 A few variations are :

P. adustus..........sessile

P. borealis..........sessile

P. anceps...........effused reflexed

P. squosus........sub-sessile

P. brumalis.......stipitate

Upper surface is flat,slightly undulating or smooth.Lower surface is smooth ,flat and porous .

Anatomy:

There are three types hyphae to constitute the fruit body :

(a) Generative hyphae : Thin walled , dense cytoplasmic ,branched with or without clamp connection .

(b) Binding hyphae : Thick-walled , narrow & much branched .

(c) Skeletal hyphae : Thick walled , narrow , unbranched & actually these are the lateral branches of generative hyphae .

Types of Fruit body: 

On the basis of hyphal composition fruit body are of three types :

(i)Monomitic : Made up of only generative hyphae ,e.g.P.adustus

(ii) Dimitic : Made of generative hyphae along with either binding or skeletal hyphae , e.g. P.sulphureus .

(iii) Made of all three kinds of hyphae .e.g. P.versicolor .

V.S. of the basidiocarp :

On v.s. the fruit body consists of the following layers :

(i) Pileus surface : This upper surface is made of a thin layer of thick-walled hyhae .

(ii) Context : Just below the pileus there are anastomosing hyphal zone with large & irregular spaces. It is also called duplex when differentiated into upper soft and lower hard & firm layer .

(iii) Tube layer : Third layer is made of vertical tubes .The connective tissue in between tubes when made of generative & skeletal hyphae , called dissepiment .

(iv) Pore surface : In this porous surface tubes open .

(v) Hymenium : It is the inner fertile layer of pores and composed of club-shaped basidia , sterile paraphyses,cystidia & setae which grow right angles to the length of the tube .Mature basidia bears four sterigmata ending in basidiospore.

Dispersal of  spores :

Basidiospores are discharged in the pores & on germination each basidiospore gives rise to monokaryotic primary mycelium on suitable substratum .