6
Population and Chemotype Shifts
Combined analyses of multilocus genotyping and neutral molecular markers permit
a large-scale analysis of the diversity, mycotoxigenic potential, and population
structure among Fusarium species [217–223]. For instance, such analyses have
exposed two dominant populations of F. graminearum in North America – NA1
and NA2 populations. The NA1 population is genetically diverse and comprises
of native isolates which typically represent the 15-AcDON chemotype, whereas the
NA2 population characterizes an invasive population that has undergone a bottleneck and is related with the 3-AcDON chemotype [24]. Recently, isolates possessing
a novel NX-2 chemotype have been found in F. graminearum populations in
southern Europe and in the north of the USA, which are sympatric with the NA1
and NA2 populations [77, 78, 223]. F. graminearum with NX-2 chemotype has
undergone toxin diversification in response to the variations in selection pressure
acting on the cytochrome P450 enzyme which is encoded by TRI1 [24]. Kelly et
al. (2016) suggested that adaptive constrains on the molecular evolution
of trichothecene biosynthetic genes might be population- or niche-specific and,
moreover, have shown that the variation of particular mycotoxins might be significant in niche adaptation [78].
Extensive research has provided tremendous insight into the genetic basis of the
chemotype variation among Fusarium strains. On one hand, chemotype variation
relates to the differences in the presence and/or absence of biosynthetic genes. For
example, TRI16 is present and functional in T-2 toxin-producing Fusarium species
(F. sporotrichioides Sherb.), whereas it is not occurring or pseudogenized in the
species producing NIV or DON. Similarly, the presence or absence of a functional
TRI13 is responsible for the DON and NIV chemotype polymorphism observed in F.
graminearum and associated species [85, 86]. However, on the other hand, trichothecene chemotype variation results from the differences in function of allelic
variants of the same TRI1 gene [77]. In some F. graminearum strains, TRI1 adds
a hydroxyl group both at C-7 and C-8, resulting in the formation of DON and
NIV [99], whereas in F. sporotrichioides, TRI11 adds a hydroxyl group at C-8 only,
leading to the formation of the T-2 toxin [89, 90].
The particular mycotoxin variant (chemotype) produced by an unknown isolate
or a novel Fusarium species can readily be inferred using DNA-based methods. For
instance, the TRI5 gene which encodes trichodiene synthase [74] was one of the first
ones to be used in designing the “generic trichothecene” marker [224]. Based on this
knowledge, gene-specific markers were designed for identifying the particular
chemotype variants of F. culmorum, F. cerealis, and F. graminearum. TRI3, TRI7,
and TRI13 genes were the targets in designing chemotype-specific markers which
are helpful in detecting the DON, 3-AcDON, 15-AcDON, and NIV chemotypes, as
well as the TRI5 and TRI4 for the discriminating type A versus type B trichothecene
producers [224–228]. Moreover, the zearalenone chemotype has been detected in
F. culmorum and F. equiseti populations using PKS4 and PKS13 genes from the ZEA
gene cluster [125, 229, 230]. Additionally, the fumonisin chemotype was identified
based on FUM1 and FUM8 gene-based markers among F. verticillioides,
F. anthophilum, F. fujikuroi, and F. proliferatum species [2, 7, 12, 102, 231]. The
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
235
Population and Chemotype Shifts
Combined analyses of multilocus genotyping and neutral molecular markers permit
a large-scale analysis of the diversity, mycotoxigenic potential, and population
structure among Fusarium species [217–223]. For instance, such analyses have
exposed two dominant populations of F. graminearum in North America – NA1
and NA2 populations. The NA1 population is genetically diverse and comprises
of native isolates which typically represent the 15-AcDON chemotype, whereas the
NA2 population characterizes an invasive population that has undergone a bottleneck and is related with the 3-AcDON chemotype [24]. Recently, isolates possessing
a novel NX-2 chemotype have been found in F. graminearum populations in
southern Europe and in the north of the USA, which are sympatric with the NA1
and NA2 populations [77, 78, 223]. F. graminearum with NX-2 chemotype has
undergone toxin diversification in response to the variations in selection pressure
acting on the cytochrome P450 enzyme which is encoded by TRI1 [24]. Kelly et
al. (2016) suggested that adaptive constrains on the molecular evolution
of trichothecene biosynthetic genes might be population- or niche-specific and,
moreover, have shown that the variation of particular mycotoxins might be significant in niche adaptation [78].
Extensive research has provided tremendous insight into the genetic basis of the
chemotype variation among Fusarium strains. On one hand, chemotype variation
relates to the differences in the presence and/or absence of biosynthetic genes. For
example, TRI16 is present and functional in T-2 toxin-producing Fusarium species
(F. sporotrichioides Sherb.), whereas it is not occurring or pseudogenized in the
species producing NIV or DON. Similarly, the presence or absence of a functional
TRI13 is responsible for the DON and NIV chemotype polymorphism observed in F.
graminearum and associated species [85, 86]. However, on the other hand, trichothecene chemotype variation results from the differences in function of allelic
variants of the same TRI1 gene [77]. In some F. graminearum strains, TRI1 adds
a hydroxyl group both at C-7 and C-8, resulting in the formation of DON and
NIV [99], whereas in F. sporotrichioides, TRI11 adds a hydroxyl group at C-8 only,
leading to the formation of the T-2 toxin [89, 90].
The particular mycotoxin variant (chemotype) produced by an unknown isolate
or a novel Fusarium species can readily be inferred using DNA-based methods. For
instance, the TRI5 gene which encodes trichodiene synthase [74] was one of the first
ones to be used in designing the “generic trichothecene” marker [224]. Based on this
knowledge, gene-specific markers were designed for identifying the particular
chemotype variants of F. culmorum, F. cerealis, and F. graminearum. TRI3, TRI7,
and TRI13 genes were the targets in designing chemotype-specific markers which
are helpful in detecting the DON, 3-AcDON, 15-AcDON, and NIV chemotypes, as
well as the TRI5 and TRI4 for the discriminating type A versus type B trichothecene
producers [224–228]. Moreover, the zearalenone chemotype has been detected in
F. culmorum and F. equiseti populations using PKS4 and PKS13 genes from the ZEA
gene cluster [125, 229, 230]. Additionally, the fumonisin chemotype was identified
based on FUM1 and FUM8 gene-based markers among F. verticillioides,
F. anthophilum, F. fujikuroi, and F. proliferatum species [2, 7, 12, 102, 231]. The
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
235