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Constraints to the evolution of both insect and pathogen resistance in a wild
crucifer
Thure Hauser et al.University of Copenhagen
Barbarea vulgaris, flea beetles and white rust
Phyllotreta nemorumBarbarea vulgaris
Albugo candida
P
G
Insect resistance• Caused by specific tri-terpenoid saponin compounds
– Especially hederagenin cellobiocide
• Lethal to larvae– Rupture membrane of gut cells– Deterrent to adults if they taste it
• These saponins affect– Diamond Back Moth (Plutella xylostella)– Yellow-stiped flea beetle (Phyllotreta nemorum)– Some Pieris species and strains
– And probably other species as well
• The insect-susceptible plant type (P) produces other saponins with unknown function
OCH2OH
COOH
O
OH
HOO
HOH2C
O
OH
HOHO
HOH2C
White rust resistance • Resistance mechanism not known
– No correlation between Albugo and insect resistance in F3 hybrids
Frequency of plants with white rust symptoms
• Both the resistant and susceptible plant-types occur in Denmark– Why?
• If it is so great to be resistant, – why don’t we find only dually resistant
plants?
?
Historical constraints on evolution of dually resistant Barbarea plants?
• G and P type have different geographical distributions
Trait distributions
West East
Inse
ct re
sist
ance
Old historical divergence
• Thus, the different resistances of the G and P type are embedded in a larger divergence of the two types
• Diverged in different refugia during ice age?– Met later in Scandinavia and Finland
Reproductive barrier
G and P type is separated by a partial hybridisation barrier
Fully developed
Partially developed
Aborted
OCH2OH
COOH
O
OH
HOO
HOH2C
O
OH
HOHO
HOH2C
Proposed biosynthesis of saponins
oxidosqualene cyclases (OSC)
UDP-glycosyltransferases (UGT)
cytochrome P450 monooxygenases (P450)
O
HO HO
COOH
O
COOH
O
OH
HOHO
HOH2C
2,3-oxido-squalene
β-amyrinoleanolic
acid
hederagenin
hederageninmonoglucoside
oleanolic acidcellobioside
hederagenincellobioside
OSC
P450
UGT
oleanolic acidmonoglucoside
UGT
UGT
Common in all plants
O
COOH
O
OH
HOO
HOH2C
O
OH
HOHO
HOH2C
OCH2OH
COOH
O
OH
HOHO
HOH2C
P450 UGT
Recombining the insect and disease resistances into dually resistant plants is hindered by– Hybridisation barrier– Complex biosynthetic pathway that may be disrupted in hybrids
• A matter of time?
But is it really so great to be resistant?
!
?
Effect of resistances on F3 hybridsD
isease score in field
Bite
hol
esLe
af m
ines
Insect resistance
Pathogen resistance
Performance of F3 hybrids
• No fitness benefit of being insect resistant• Strong benefit of being disease resistant
Pathogen resistanceInsect resistance
Impact of flea beetles and white rustG-plants P-plants
New questions
• No obvious benefits of saponin-based insect resistance– Spectrum vs impact ?– Costs vs benefits?– Conditional on environment ?
• Why not only the pathogen-resistant P-type?– Disease incidence?– Time?
• Hybridisation barrier + trait associations– May have spread from the P-type to the G-type ?
– Genotype x environment interactions
?
Implications
• Present ecological interactions may be constrained by historical biogeography
• Defense traits and genes are not always beneficial for fitness– Use of resistances in biotech, breeding vs spraying ?
• Resistance-resources in wild crop-relatives
!
The Barbarea group
P-type
0
100
200
300
400
500
Contr Herb Path H&P
Sap
onin
3 (
peak
are
a)
.
0
20
40
60
80
Contr Herb Path H&P
G-type
0
50
100
150
200
250
300
Contr Herb Path H&P
Hederagenin cellobioside .
(peak area)
0
20
40
60
80
Contr Herb Path H&P
a
b
b
b
a a
c
b
Contr +Ins +Path +Both Contr +Ins +Path +Both
Induction of saponins (and glucosinolates)
• Saponins increase also in insect-resistant type• Combination of insect and pathogen gives highest induction
Oecologia, in press