1) Landraces of maize attract parasitoids to plants with stemborer eggs through induced volatile emissions, whereas commercial hybrids do not show this induced response. 2) A genome-wide association study identified single-nucleotide polymorphisms associated with the traits of parasitoid attraction and volatile induction in response to stemborer eggs. 3) Developing molecular markers linked to these traits could allow breeding of maize varieties with both increased yield and the ability to attract natural enemies of stemborers via herbivore-induced plant volatiles.

1. THE CALL OF THE WILD -
tracking an indirect defence trait in maize
TOBY BRUCE
Rothamsted Research, UK

2. Domesticated from Zea mays ssp. parviglumis
8,700 years ago
Photo by Nicolle Rager Fuller, NSF

3. High yielding
varieties
(HYVs) bred
from landraces
Nature Genetics 44, 808–811 (2012)

4. African smallholders
mostly grow
LANDRACES not HYVs
Why?

5. HYPOTHESIS: Landraces are MORE RESISTANT TO
STRESSES such as stemborer attack…

6. are parasitoids more attracted to
HIPVs from landraces?

7. ±
24h

9. Bioassay of response to volatiles from plants WITH and
WITHOUT eggs

10. Parasitoid response - landraces
ATTRACTED to plants with eggs
Tamiru et al. (2011) Ecology Letters 14: 1075

11. Volatile profiles - landraces
INDUCTION with plants with eggs
(a) (E)-ocimene, (b) (R)-linalool, (c) (E)-4,8-dimethyl-1,3,7, nonatriene (DMNT), (d) methyl salicylate,
(e) decanal, (f) methyleugenol, (g) (E)-(1R,9S)-caryophyllene, (h) (E)-β-farnesene, (i) (E,E)-4,8,12-
trimethyl-1,3,7,11-tridecatetraene (TMTT).
Tamiru et al. (2011) Ecology Letters 14: 1075

12. Parasitoid response – commercial hybrids
Commercial hybrid lines
NOT attracted to plants with eggs
Tamiru et al. (2011) Ecology Letters 14: 1075

13. Volatile profiles – standard commercial hybrids
NO induction with plants with eggs
(a) (E)-ocimene, (b) (R)-linalool, (c) (E)-4,8-dimethyl-1,3,7, nonatriene (DMNT), (d) methyl salicylate,
(e) decanal, (f) methyleugenol, (g) (E)-(1R,9S)-caryophyllene, (h) (E)-β-farnesene, (i) (E,E)-4,8,12-
trimethyl-1,3,7,11-tridecatetraene (TMTT).
Tamiru et al. (2011) Ecology Letters 14: 1075

14. Ethanol extract of elicitor
Effect is systemic + ELICITOR involved
Tamiru et al. (2011) Ecology Letters 14: 1075

15. African landraces
a = 2-heptanol (stereochemistry undefined), b = decane, c = (R)-limonene, d = methyl benzoate, e = (R)-linalool,
f = (E)-4,8-dimethyl-1,3,7-nonatriene, g = cis-p-mentha-2,8-dien-3-ol*, h = methyl salicylate, i = decanal, j = 2,3-
dimethylacetophenone*, k = neryl acetate, l = eugenol, m = copaene*, n = germacrene D*
Tamiru et al. (2012) J. Chem. Ecol. 38: 213

16. New Project: markers for the trait

18. Egg induced HIPV emission trait only found in
2 out of 44
commercial hybrid maize lines tested so far

20. Landraces
Reference genome
(B-73 maize lines)

21. Why is this indirect defence trait
so rare in HYV lines ?

22. Could it be the legacy of selecting for
yield and quality in a pesticide treated
background?

23. Identifying
linked
molecular
markers using
GWAS (genome-wide
association
study)
maize

24. Parasitoid attraction
Quantile-quantile (QQ) –plot of P-values for both parasitoid attraction to the volatiles induced
and volatiles induction in response to exposure to the parasitoids. The Y-axis is the observed
negative base 10 logarithm of the P-values, and the X-axis is the expected observed negative
base 10 logarithm of the P- values under the assumption that the P-values follow a uniform [0,1]
distribution. The dotted lines show the 95% confidence interval for the QQ-plot under the null
hypothesis of no association between the SNP and the trait.

25. Associated SNPs for
Parasitoid attraction
Manhattan plot
generated using
137940 SNPs in
preliminary GWAS
analysis for both the
trait parasitoid
attraction and
volatile induction.
The X-axis is the
genomic position of
the SNPs in the
genome, and the Y-axis
is the negative
log base 10 of the P-values.
Each
chromosome is
colored differently.
SNPs with stronger
associations (based
on bonferronoi
correction) with the
trait will have a
larger Y-coordinate
value denoted by
parallel in light green
color
Associated
SNPs for
Volatile
induction

26. Conclusion
We are developing molecular markers
for a valuable indirect defence trait to
facilitate breeding via MAS (marker
assisted selection) of maize that has
BOTH this trait and higher yield…