Chapter 31 fungiby johnny elvir

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22 Φεβ 2013 (πριν από 4 χρόνια και 8 μήνες)

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Fungi are a huge and important component of
the biosphere.


Their diversity alone is
staggering:although

100,000 species have been described, it is
estimated that there are actually as many as 1.5
million species of fungi


Fungi breakdown and recycle nutrients.


Almost all plants depend on a symbiotic
relationship with fungi that helps their roots
absorb minerals from the soil.


Two species of fungi
decomposing a log


Like
animals,fungi

are
heterotrophs

they cannot
make their own food as plants and algae can.


But unlike
anials,fungi

do not ingest their food the
digest their food while it is still in the environment
by secreting powerful hydrolytic
enzymes,called

exoenzymes,into

their surrounding.


Exoenzymes

breakdown complex
molecules to smaller organic compounds
that the fungi can absorb into their bodies
and use.


Among the fungi are species that live as
decomposers (saprobes),
parasites,and

mutualstic

symbionts
.


Saprobic

fungi break down and absorb
nutrients from nonliving organic
material,such

as fallen
logs,animal

corpses,and

the wastes of
living organisms.


Parasites fungi are
pathogenic,including

species that infect human lungs and other
species that are responsible for about 80% of
plant diseases.


Mutualistic

fungi also absorb nutrients from a
host
organism,but

the
mutualistic

fungi
reciprocate with functions beneficial to the host
organism,but

the
mutualistic

fungi reciprocate
with functions beneficial to the host in some
way,such

as aiding a plant in the uptake of
minerals from the soil.


The bodies of these fungi typically form a
network of tiny filaments called
hyphea
.


Hyphea

are composed of tubular cell walls
surrounding the plasma membrane and
cytoplasm of the cells.


Unlike the cellulose walls of
plants,fungal

cell
walls contain
chitin
a strong but flexible
nitrogencontaining

polysaccharide that is also
found in the external skeletons of insects and
other arthropods.


Fungal
hyphae

form an interwoven mass called
a
mycelium
that surrounds and infiltrates the
material on which the fungus feeds.



In most
fungi,the

hyphea

are divided into cells
by
crosswalls,or

septa
.


Coenocytic

fungi,these

organisms consist of a
continuous
cytoplasmic

mass
contining

hundreds or thousands of nuclei.


Some species have specialized
hyphea

called
haustoria

that enable them to penetrate the
tissue of their hosts.


Mutually beneficial relationships between such
fungi and plant roots are called
mycrrhizae
.


Ectomycorrhizal

fungi
form sheaths of
hyphae

over the surface of a root and also grow into
the extracellular spaces of the root cortex.


Endomycorrhizal

fungi extends their
hyphae

through the root cell wall and into tubes
formed by
invagination

of the root cell
memebrane

.



fungi propagate themselves by producing vast
numbers of
spores,ethier

sexually or asexually.


Spores can be carried long distances by wind or
water,and

if they land in a moist place where
there is food, they
germinate,producing

new
mycelia.


Generalizes the many different life cycles that
can produce fungal spores.


Generalized life cycle of fungi.


The nuclei of fungal
hyphae

and spores of most
species are
haploid,except

for transient diploid
stages that form during sexual life cycle.


Generally,the

sexual reproduction in fungi
begins when
hyphae

from two distinct mycelia
release sexual signaling molecule called
pheromones
.


If the mycelia are of different mating types the
pheromones from each partner bind to receptor
on the surface of the other and the
hyphea

extends toward the source of the pheromones.


When the
hyphea

meet they
fuse,this


compatibilty

test”contributes

to genetic
variation by preventing
hyphea

from fusing
with other
hyphea

from the same mycelium or
another genetically identical mycelium.


The union of the cytoplasm of the two parent
mycelia is known as
plasmogamy
.


In many
fungi,the

haploids nuclei contributed
by each parent do not fuse right away.


Instead parts of the mycelia contain
coexisting,genetically

different nuclei.


Such a mycelium is said to be a
hetrokaryon
(meaning different nuclei)


In some
species,heterokaryotic

mycelia become
mosaics,with

different nuclei restricted to
different parts of the network.


In other
species,the

different nuclei mingle and
may even exchange chromosomes and genes in
a process similar to crossing over.


In some fungi ,the haploid nuclei pair off two
to a
cell,one

from each parent.



Such a mycelium is said to be
dikaroyotic
(meaning two nuclei).


As a
dikaryotic

mycelium
grows,the

two nuclei
in each divide in tandem without fusing.


Hours,days,or

even centuries may pass
between
plasmogamy

and the next stage in the
sexual
cycle,
karyogamy
.


During
karyogamy,the

haploid nuclei
contribute by the two parent
fuse,producing

diploid cells.


Zygotes and other transient structure form
during
karyogamy

the only stage in which
diploidy

exists in most fungi.


The
heterokaryotic

condition also offer some of
the advantages of
diploidy

in that one haploid
genome may be able to compensate for harmful
mutation in the other.


In addition to sexual reproduction ,many fungi
can reproduce asexually.


Clones are produced by mitotic production of
spores which can be spread by air or water.


Some species only reproduce asexually.


Some fungi that can reproduce asexually grow
as
mold
.


Mold grow rapidly as mycelia and produce
spores.


Many species that can grow as mold can also
reproduce sexually if they come in contact with
other mating types.


Another asexual fungi is
yeast
.


Yeast inhabit liquid or moist
habitats,including

plant sap and animal tissue.


Instead of producing
spores,yeast

species can
also grow as filamentous
mycelia,depending

on the
availabilty

of nutrients.


Sometimes yeast can produce sexually.


Penicillium

a mold commonly encountered as a
saprobe on food.


Yeat

saccharomyces

cerevisiae

in various stages
of budding(SEM).


Many molds and yeasts have no known sexual
stage.


Mycologists,have

traditionally called such
fungi
deuteromycetes
(traditional classification
for a fungus with no known sexual stage)or
imperfect fungi.


Whenever a sexual stage of a so
-
called
deuteromycete

is
discovered,the

species is
classified in a particular
phylum,depending

on
the type of sexual structure is formed.


Fungi and
animalia

are sister kingdom.


While the majority of fungi lack
flagella,the

linages

of fungi thought to be the earliest to
diverge do have flagella.


Most of the
protist

that share a close common
ancestor with animals and fungi also have
flagella.


These three groups of eukaryotes
-
the
fungi,animals,and

their
protistan

relatives
-
are
called
opishokonts,members

of the
clade

opisthokonta
.



Phylogenetic

evidence also suggests that the
ancestor of fungi was
uncellular
.


Based on molecular clock
analysis,scientists

have estimated that the ancestors of animals
and fungi diverged into separate
linages

l.5
billion years ago.


However the oldest undisputed fossils of fungi
are only about 460 million years old.


One possible explanation for this discrepancy
is that the microscopic ancestors of terrestrial
fungi fossilized poorly.


Fossil fungal
hyphae

and spores from the
ordovician

period (about 460 million years
ago;LM
).


Much of the fungal diversity we observe today
may have had its
phylogenetic

origin in
adaptive radiation when life began to colonize

land.


Fossils of the earliest known vascular plants
from the late
silurian

period contain evidence
of
mycorrhizae,the

symbiotic relationship
between plants probably existed in this
symbiotic relationship from the earliest periods
of colonization of land.


Fungi classified in the phylum
chytridiomycota,called

chytrids
,are

ubiquitous
in lake and soil.


Some are
saprobes;other

parasitize
protists,plants,or

animals.


Like other
fungi,chytrides

have cell walls made
of
chitin,and

they also share certain key
enzymes and metabolic pathways with other
fungal group.



Phylogency

of fungi


Chytrides

are unique among fungi in having
flagellated
spores,called

zoospores
.


Until
recently,systematists

thought that fungi
lost flagella only once in their
history,after

chytrids

had
divergance

from other linage.


Molecular data indicate that
somechytrides

are
actually more
closly

related to another fungal
group the
zygomycetes,if

this is true then
flagella were
loston

more than one occasion
during the evolution of fungi.


The
approximatly

1,000 known species of
zygomycetes
(fungi in the phylum
zygomycota
)exhibit a considerable diversity of
life histories.


This phylum includes fast growing molds
responsible for rotting produce such as
peaches,strawberries,and

sweet
potatos

during
storage.


Horizontal
hyphae

spread out over the
food,penetrate

it and absorb nutrients.


The
hyphae

are
coenocytic,with

septa found
only where reproductive cells form.


The parents in a sexual union are mycelia of
different mating types that possess different
chemical
markers,although

they may appear
identical.


Plasmogamy

produces a sturdy structure
called a
zygosporangium
,in

which
karyogamy

and the meiosis occur.


Zygosporangosporangia

are resistant to
freezing and dying and are metabolically
inactive.


When condition
improve,a

zygosporangium

undergoes
meiosis,germinates

into a
sporangium,and

releases genetically diverse
haploid spores that may colonize a new
substrate.


Microsporidia

are unicellular parasites of
animals and
protists
.


They are often used in the
bilogical

control of
insect pests.


While
microsporidia

do not normally infect
humans,they

do pose a risk to people with
AIDS and other immune
-
compromised
conditions.

Host cell

nucleus

Developing

microsporidian

Spore


The
glomeromycetes
,fungi

assigned to the phylum
glomeromycota,were

formally thought to be
zygomycetes
.


In spite of their small numbers
-
only 160 species
have been identified to date
-
the
glomeromycets

are an
ecoligically

significant group.


All
glomeromycetes

form a distinct type of
endomycorrhizae

called
arbuscular

mycorrhizae
.


The tips of the
hyphae

that
puch

into plant root
cells branch into tiny treelike structure known as
arbusculares
.



Abusculare mycorrhizae


Mycologists have described more than 32,000 of
ascomycetes
(fungi in the phylum
ascomycota
)from
a variety of
marine,freashwater,and

terrestrial
habitats.


The defining feature of
ascomycetes

is the
production of sexual spores in saclike
asci
(
singular,ascus
);thus they are commonly called
sac fungi
.


Unlike
zygomycetes,most

ascomycetes

bear their
sexual stages in fruiting
bodies,or

ascocarps,which

rang in size from microscopic to macroscopic.


Unlike
zygomycetes,most

ascomycetes

bear
their sexual stages in fruiting
bodies,or

ascocarps,which

range in size from microscopic
to macroscopic.


Ascomycetes

vary in size and complexity from
unicellular yeasts to elaborate cup fungi and
morels.


They include some of the most devastating
plant pathogens.

Morchella

esculenta,
e

tasty morel

Tuber melanosporum,
a truffle


Altough

the life cycle of various
ascomycete

group differ in details of their reproductive
structure and
processes,there

are some
common element.


Ascomycetes

reproduce asexually by
producing enormous number of asexual spores
called
condia
.


Condia

are formed inside
sporangia,as

are the
asexual spores of most
zygomycetes
.


Incontrast

to the life cycle of
zygomycetes,extended

dikaryotic

stage of
ascomycetes
(and also
basidiomycetes
)
provides increased opportunity for genetic
recombination.



Approximtely

30,000
fungi,including

mushrooms and shelf
fungi,are

called
basidiomycetes

and are classified in the
phylum
basidiomycota
.


The name of the phylum derives from the
basidium
(
latin

for “little pedestal”),a cell in
which a transient diploid stages occurs during
the fungal life cycle.


The club
-
like shape of the
basidium

also gives
rise to the common name
club fungus
.

Puffballs emitting

spores


(
Dic
)


The life cycle of a
basidiomyete

usually include
a long
-
lived
dikaryotic

mycelium .


Periodically,in

response to
enviormental

stimuli the mycelium reproduces sexually by
producing elaborate fruiting bodies called
basidiocarps
(a mushroom is a
fimulare

example).


The numerous
basidia

in a
basidiocarp

are the
sources of sexual spores called
basidiospores
.


The mushroom’s cap supports and protects a
large surface area of
basidiaon

gills.


Fungi are well adapted as decomposers of
organic material, including the cellulose and
lignin of plant cell walls.


Fungi and bacteria are primarily responsible
for keeping ecosystems stocked with the
inorganic nutrients essential for plant growth.


Without these
decomposers,carbon,nitrogen,and

other
elements would become tied up in organic
matter.


Mycorrhizae

are enormously important in
natural ecosystem and agriculture.


It is relatively easy to demonstrate the
significance of
mycorrhzae

by comparing the
growth of plants with and without
mycorrhizae
.


Foresters commonly inoculate pine seedlings
with
ectomycorrhizal

fungi to promote
vigorous growth in the tree.


Some fungi share their digestive service with
animals,helping

breakdown plant material in the
guts of cattle and other grazing mammals.


Many species of ants and termites take advantage
of the digestive power of fungi by raising them in
farms.


The insect scour tropical forests in search of
leaves,which

they carry back to their nest and feed
it to the fungi.


The fungi have been so dependent on their
caretakers that in many cases they can no longer
survive without the insect.


Ant taking a stem of a plant to take it to the
fungus.


Lichens are a symbiotic association of
millios

of
photosynthetic microorganisms held in a mass
of fungal
hyphae
.


They form a surface
-
hugging carpet that can be
found growing on
rocks,rotting

logs,trees,and

roofs in various shrub
-
like,leaf
-
like,or

encrusting forms.


The merger of fungus and alga or
cyanobacterium

is so
comeplete

that lichens are
actually given scientific names as though they
were single organism.


More than 13,500 species have been
described,accounting

for a fifth of all known
fungi.


In most lichens that have been
studied,each

parter

provides something the other cannot
obtain on its own.


The fungi of many lichens reproduce sexually
by forming
ascocarps

or
basidiocarps
.


Soredia

are small cluster of fungal
hyphea

with embedded
algea
.


Free
-
living fungus that provides the antibiotic
penicillin,for

example,is

thought to have
descended from a lichen fungus.


As tough as lichens
are,many

do not stand up
very well to air population.


Their passive mode of mineral uptake from
rain and moist air makes them particularly
sesitive

lichens and an increase in the number
of hardier species in an area can be an early
warning that air quality is deteriorating.



Of the 100,000 known species of
fungi,about

30%
make their living as
parasites,mostly

on or in
plants. For
example,ophiostoma

ulmi,the

ascomycete

that causes
dutch

elm
disease,has

drastically changed the landscape of the
northeastern united states on logs that were sent
from
europe

to help pay world war 1
debts,the

fungus is carried from tree to tree by bark beetles.


Another
ascomycete,cryphonectria

has killed 4
billion native
american

chestnut trees in the
eastern united states.


Some of the fungi that attack food crops are toxic
to humans.


Certain species of the
ascomycete

mold
aspergillus

contaminate improperly stored grain and peanuts
by secreting carcinogenic compounds called
aflatoxins
.


Of diseased rye is inadvertently milled into flour
and then
consumed,poisons

from the ergots can
cause
ergotism
,
acondition

charcterize

by
gangrene,nervous

spasms,burning

sensation,hallucination,and

tempory

insanity.


Animals are much less susceptible to parasitic
fungi then are plants.


Only about 50 species of fungi are known to
parsitize

humans and other
animals,but

these
relativity few species do considerable damage.


The general term for such a fungal infection is
mycosis
.


Skin mycoses include the disease
ringworm,so

named because it appears as circular red areas
in the skin.


Some mycoses are opportunistic ,
occuring

only
when a change in the body’s
microbiology,chemistry,or

immunology allows
fungi to grow unchecked.


Under certain circumstances ,
candida

can grow
too rapidly and become pathogenic, leading to
so
-
called yeast infection.


Many other opportunities mycoses in human
have become more common in recent
decades,due

in part to AIDS which
compromiss

the immune system.


The danger posed by fungi should not
overshadow the immense benefits we derive
from these remarkable eukaryotes.


many yeast and fungi are used for
acholic

beverages medical supplies and food.


Fungi is used for
reasearhing

in biotechnology .