Hyphae
ofvegetatively compatible fungal
strains canfuseduring growth,
and the fusion cell survives and may grow in some
species.
Isolates that arevegetatively compatible belong
to a commonvegetative compatibility
group(VCG). However,
ifhyphae
of the two strains do notfuse,
or if one or both of the fused cellsdie, creating
a"barraqe"
zone between the twomycelia,
thenthe strains are considered to be
vegetatively incompatible (Puhalla, 1979).
Thus,
strains within aspecies
may be classifiedaccording
to theirvegetative
reaction with other isolates from the same
species. Vegetative compatibility
iscontrolled
by
the action of a set ofvegetative incompatibility (vic
orv-c)
orheterokaryon incompatibility (het)
loci inAscomycetes (Glass
andKuldau, 1992),
where the sexualstages
of Fusarium spp.belong. Incompatibility
reaction may be
homogenic
orallelic,
in which a stableheterokaryon
is formedonly
when the twointeracting
strains carry the same alleles at all vicloci,
orheterogenic,
in which the alleles at one locus interact with alleles of other loci(Puhalla, 1981).
Thetypes
of interaction demonstrated for Fusarium are of thehomogenic type.
Two strains in the sameVCG carry the same alleles at all vic loci. Studies conducted on thegenetic
control ofvegetative compatibility
inFusarium estimated that 10 or more unlinked vic loci control
vegetative compatibility
in F. moniliforrne(PuhaUa
andSpieth, 1983;
Puhalla andSpeith, 1985; Sidhu, 1986).
The formation of a
heterokaryon
between twogenetically
differenthaploid
strains is an essentialpart
of the lifecycle
in most heterothallicfungi.
Heterokaryon
formation between differentfungal
individuals is animportant
14
component
in manyfungi
lifecycles. Complementation
orheterokaryosis
may differ from it constituents inaggressiveness
or host range; some of theseaspects
have beenpreviously
reviewed(Christensen
and DeYay. 1955;
Parmeter et
ai., 1963; Isaac, 1967;
Thinline andMcNeil, 1969; Boone, 1971;
Webster, 1974; Ogoshi, 1987; Rayner, 1991;
Glass andKuldau, 1992).
In mostcases,
vegetative compatibility
ishomogenic
thatis,
twofungi
arevegetative compatibility
if the alleles ateach oftheircorresponding
vic loci are identical.Sexual and
vegetative compatibility
orheterokaryons
arequite
distinctfrom one another in many
fungi.
Strainscapable
offorming
a successful sexualheterokaryon
may be unable toform a successfulvegetative heterokaryon
andvice versa. Strains that are
vegetatively compatible
with one another arefrequently
described as members of the samevegetative compatibility
orVCG.Sexual
compatibility
isusually governed by
one or moremating-type
loci thatmay have two or more alleles
(Fincham
etai., 1979;
Glass andKuldau, 1992).
In
Ascomycetous fungi
such asAspergillus, Cryphonectria,
Fusariumand
Neurospora, vegetative compatibility
reactions wereextensively
studied(Leslie, 1993).
Thefungi
served as models for the basicstudy
of thegenetic
mechanisms
controlling vegetative compatibility
andthey
can be used toillustrate some of the ways in which
vegetative compatibility
may be used in thestudy
offungal populations (Christensen
and DeYay, 1955; Brasier, 1983).
Classification
by
means ofVCGs, does, however,
have limitations.Firstly generation
of nit -mutants necessary for VCGassignment,
is laborious and timeconsuming
forsome isolates(Correll
etai., 1987a).
Generation of NitM or nit-3 mutants is very
difficult,
if notimpossible
in some isolates.Secondly.
self-incompatibility
can make the VCGassignment
of an isolateimpossible
15
(Jacobson
andGordon, 1988).
Certain isolates have been found thatthey
arenot
vegetatively compatible
with themselves or otherisolates,
and thus can not beplaced
into the VCG. Thisphenomenon
ofvegetative self-incompatibility
must be taken into accountwhen
examining
thepopulation biology
of Fusariumspecies.
Thisphenomenon
has been found among certain isolates of F.oxysporum
(Puhalla, 1984a;
Correll etal., 1987a;
Basland andWilliams, 1987;
Jacobson and
Gordon, 1988).
In a
sexually reproducing fungi, vegetatively compatible
strains are morelikely
to begenetically
similar thanvegetatively incompatible
strains. Forexample,
strains offungus
those arevegetatively compatible
arequite
similarwith
respect
to traitscolony
size(Croft
and Jinks1977;
Correll etal., 1986b),
antibiotic
production (Croft
andJinks, 1977), sanguinarine sensitivity (Puhalla
and
Hummel, 1983),
virulence(Correll
etal., 1985; 1986b;
Gordon etal., 1986)
and
isozyme patterns (Bosland
andWilliams, 1987).
Vegetative compatibility systems generally
act to restrict the transfer of nuclear andcytoplasmic
elementsduring growth.
Nit mutants were used asforcing
markers forheterokaryon
tests and VCG served as a natural means toanalyze fungal populations (Leslie, 1993).
In thefungal population,
strains canbe classified into different
vegetative compatibility
groups(VCGs)
based ontheir
ability
to formheterokaryons
with one another(Puhalla
andSpieth, 1985).
The allelic
vegetative compatibility
reaction has been described in differentascomycetous fungi,
such asAspergillus, Cryphonectria,
Fusarium andNeurospora.
At least 10 differentvegetative incompatible (vic)
loci(termed
hetloci)
have been identified and five have beenmapped
inNeurospora (Mylyk, 1975; Perkins, 1975)
whileeight
vic loci are known inAspergillus
nidulans16
(Croft
andJinks, 1977).
There also is evidence forgenetic segregation
of vicloci in both F. moniliforme
(perfect stage
GibberellafujikuroJ) (Puhalla
andSpieth, 1983)
and F.graminearum (perfect stage
Gibberellazeae) (Bowden
and
Leslie, 1992).
At least 10 vic loci areexpected
and one vic locus(vic 1)
hasbeen
mapped
in F. moniliforme. To be in the sameVCG,
two strains must be identical of each other atof least 10 different vic loci. Differencesatasingle
viclocus are sufficientto block the formation ofa stable
heterokaryon (Leslie
etai., 1992).
Vegetative incompatibility
serves toregulate genetic variability by controlling heterokaryosls
andparasexual
recombination(Leslie
etai., 1993).
Ithas
generally
been assumedby analogy
with sexualincompatibility systems.
This
vegetative incompatibility
willmarkedly
reduce thespread
ofsuppressive cytoplasmic genetic elements, including, viruses,
and from strain to strain in nature.Caten (1971)
havesuggested
thatvegetative incompatibility might
serve to
protect mycelia
from invasionby suppressive cytoplasmic
determinants
following hyphal anastomosis,
and that its role is therefore one of cellulardefenceagainst genetic
infection.Using
VCGs to determine theidentity
of a distinctpopulation
mayprovide
valuableinsight
intorelationship
between new and establishedinfestation,
andpatterns
of diseasespread. Although
virulence has been anextremely
useful characteristicfordifferentiating
isolates withinspecies
such asFusarium,
it is stillonly
asingle
trait.Moreover,
virulence has been shown to be influencedby
a number of variablesincluding temperature (Pound
andFowler, 1953),
method ofinoculation and{Kraft
andHagland, 1978}
and host age(Hart
and
Endo, 1981)
and17
At the
present time,
mostpopulation genetic
studies of Fusarium spp.such as F. monilifonne and F. oxysporum have been conducted
using
thevegetative compatibility
group(VCGs)
as a marker forgenotyping fungal
isolates
(Farrokhi-Nejad
andLeslie, 1990; Campbell
etai., 1992;
Kedera etai., 1994).
Strains thatarevegetative compatible,
Le.belong
to the sameVCG,
canform a stable
heterokaryon,
and share an identical set of alleles at about 10 vicloci
(Leslie, 1993).
The VCGtechnique
isparticularly
suitable forpopulation genetic
studies of Fusarium such as F.moniliforme,
because field isolates of thisfungus belong
to many VCGs(Leslie
etai., 1992).
Isolates of F.moniliforme the
belonging
to the same VCG arepresumed
to beclones,
and VCGanalysis might
therefore be used for strain identification(Kedera
etai.,
1994).
Most
fungi
can utilize nitrate as anitrogen
sourceby reducing
it toammonium via nitrate reductase and nitrite reductase
(Garraway
andEvans, 1984),
but thehigher Basidiomycetes,
theSaprolegniaceae,
and theBlastocladiales
apparently
can notsynthesize
nitrate reductase(van Alfen, 1982).
The reduction of chlorate to chloriteby
nitrate reductase canpresumably
results in chlorate
toxicity
in theseorganisms.
Ingeneral,
thegrowth
of chlorate sensitive strains is restrictedby
the chlorate resistant strains that either do not take up chlorate or are unable to reduce chlorate to chlorite. Nitrate nonutilizing
mutants(nit mutants)
areusually
unable to reduce chlorate to chlorite because of a lesion of one or more of the loci that control nitratereductase,
thus
rendering
them chlorate resistant.Nit mutant have also been recovered from a number of other
fungi, including
N. crassa(Marzluf
et al.1985),
F.graminearum (Puhalla, 1985;
18
Puhalla and
Spieth, 1985),
F. oxysporum(Puhalla, 1985; Correll,
etal'J 1987a),
Verticil/um albo-atrum
(Gordon
elaI., 1986),
A. flavus(Papa, 1986),
F.moniliforme
(Kistler
etaI.,
1987; Bowden andLeslie, 1992)
and F. poae(Liu
and
Sundheim, 1996).
2.1.2.2a
Using
VCGs to Assess LifeCycle
Many Ascomycetes
arecapable
ofperforming
both sexual and asexualreproduction.
The lack of VCGdiversity
within apopulation
can be due to thelack of sexual recombination or the selection ofa
particularly
fitgenotype,
as in the asexualproliferation
of agiven genotype during
anepidemic (Klein
andCorrell, 2001). Consequently,
the extent of VCGdiversity
in apopulation
mayserve as an indicator of the relative
frequencies
of sexual and asexualreproduction
in thepopulation.
Forexample,
in a fieldstudy
whereasymptomatic
corn wassampled,
but 100 VCGswere identified in apopulation
of F. moniliforme with very few
(3%)
of the VCGsrepresented
more than once(Klein
etaI., 1995) suggesting
that sexualreproduction
may beimportant
inshaping population
structure. Incontrast,
apopulation study
of F.proliferatum
from asparagus found 20 VCGs among a
sample
of 110isolates,
but most of the isolates(88
out of110) belong
to one of three common VCGs(Elmer,
1991).
Theseexamples
indicatethat,
unlessonly
certain VCGs arebeing selected,
asexualreproduction
may be thepredominant
factoraffecting population
structure.19
2.1.2.2b VCG
Diversity
inEpidemics:
FormaeSpeciales (f. spp.)
Many investigators
assume that a formaspecialis (f. sp.) implied
somedegree
ofgenetic
orevolutionary relationship
among isolates within a group.Puhalla
(1985)
used nit mutants toexaminevegetative compatibility
within andbetween several f. spp. of F. oxysporum. Studies followed
evaluating vegetative compatibility
as a tool forassessing genetic diversity
withinpopulation
of agiven
f. sp. or therelatively large non-pathogenic portion
of apopulation
and itsutility
forpathogen
and race identification(Correll
etaI., 1986a;
Correll etal., 1986b;
Basland andWilliams, 1987). Vegetative compatibility
tests have beenwidely
used to characterizegenetic diversity
in F.oxysporum and often
provide
the framework for ·studies on hostspecificity,
molecular
phylogenetic relationship,
andpopulation diversity.
VCGdiversity
has been examined in over 30 f. spp. of F. oxysporum and a
systematic numbering system
was established(Kistler
et ai.,1998), although
studies vary in the extent ofgeographic diversity
of thesamples
and utilization of other biochemical and molecular markers. Molecular markers and VCGusually
arenot
independently
associated. These results areinterpreted
to mean VCG in F.oxysporum
represent clones,
orclosely
strains descended of a common ancestor(Anderson
andKohn, 1995;
Gordon andMartyn, 1997).
Therelationships
betweenVCGs,
races, and molecularhaplotypes
in various f. spp.has been examined
(Gordon
andMartyn, 1997).
There are a fewexamples
inwhich a f. sp. contains
only
oneVCG,
and isolatesbelong
to asingle
race e. g.f. sp. laclucum and albedinis
(Hubbard
andGerik, 1993;
Tantaoui etai., 1996).
20
In F. oxysporum, Puhalla
(1984b; Puhalla, 1985)
showed that thevegetative compatibility
group(VCG)
may be ahandy
tool fordifferentiating
f.sp. of F. oxysporum. This
study
indicated thatvegetative compatibility grouping
can
be,
atleast,
used to SUbstitutive the mean forpathogenicity
tests. As theperfect stage
of F. oxysporum is not known(Snyder
andToussoun, 1965), vegetative compatibility
is theonly
mean to show theexchange
ofgenetic
information between two strains of F. oxysporum
(Correll, 1986a;
Correll etal., 1986b;
Correll etal., 1987a).
All strains within asub-group (VCG) readily
formed
heterokaryon
with eachother,
whereas strains from different groups would not. It was demonstrated that strains that werevegetatively compatible
were much more
likely
to begenetically
similar thanvegetatively incompatible
strains. The merger has been
exploited
in studies of F. oxysporumto determine thegenetic
relatedness of isolates whichbelong
to the same race or f. sp.(Puhalla, 1985;
Correll etal., 1986a;
Correll etal.,
1987a; Jacobsen andGordon, 1988;
Katan andKatan, 1988;
Eimer andStephens, 1989).
Heterokaryosis
has beenrecognized
in severalspecies
of the genusFusarium, including
F.subgJutinans
and F.sporotrichioides (Cullen
etaI., 1983),
F. moniliforme(Puhalla
andSpieth, 1983),
F. oxysporum(Puhalla, 1985;
Correll et
aI., 1987a),
F.graminearum (Adams
etai., 1987;
Bowden andLeslie, 1992),
F. poae(Liu
andSundheim, 1996)
and F.proliferatum (Eimer
etai., 1999).
21