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Background and Objectives:
Cereal aphids are important
pests of the commercial common wheats, Triticum aestivum L.
and durum wheat, Triticum durum Desf. Crop resistance would
be a particularly desirable method for managing cereal aphid pests
in Manitoba, where the dominant crop, spring-sown wheat, has a low
value per unit area. A diploid ancestor of modern wheats, Einkorn
wheat, Triticum monococcum L., is reported to show resistance
to the English grain aphid, the most damaging cereal aphid in
Manitoba. Most reports on resistance to cereal aphids are for
seedlings, usually winter wheat, not for adult plants of spring-sown
wheat which are the primary target of English grain aphid in
Manitoba.
The wheats that we grow in Manitoba have been
developed through 1000's of years of crop breeding, first by farmers
in the Middle East, North Africa, and southern Europe, and in the
last 100 years by professional plant breeders. The modern wheats
are derived from primitive ancestral wheats. Our modern wheat
cultivars are often susceptible to insect pests, for example cereal
aphids. Some agricultural scientists believe that the ancestral
wheats are less susceptible because resistance genes that were
present in the wild have been lost during the domestication of our
modern cultivars. This possible effect of domestication has two
conflicting implications:
-
Good resistance genes to insect pests may be
available in the wild ancestors of wheat that we might transfer to
modern wheat by classical plant breeding methods.
-
Domestication itself may somehow reduce the
effectiveness of the ancestral resistant genes, and make them
unusable in modern agriculture.
The first part of this project examined these
two possibilities by comparing the levels of crop resistance to
three cereal aphid pests in a diverse collection of ancient and
modern wheats. The second part of the project examined diverse
germplasm in one of the ancient wheats, to try and identify sources
of resistance to the key cereal aphid pest of Manitoba wheat.
This project helped support the Ph.D. research
of S. M. Migui in the Department of Entomology, University of
Manitoba, which was completed successfully in the spring of 2002.
Additional financial support for the project was provided directly
to Mr. Migui’s studies in the form of a Graduate Fellowship from the
University of Manitoba. As a result of this work and the expertise
that Dr. Migui gained during the project, he was offered a 3-year
post-doctoral position with the Commonwealth Industrial Research
Organization (CSIRO) of Australia to conduct further research on
resistance in wheat to aphids. The research is being conducted at
the CSIRO European Laboratory in Montpellier, France. With this
additional training, Dr. Migui hopes to return to Canada,
particularly to Manitoba, and apply his expertise to breeding wheats
with resistance to cereal aphids and other pests.
In the first study, 41 accessions of wild and
cultivated wheats belonging to 19 wheat species were tested in the
field for resistance to three species of pest aphids, English grain
aphid, bird cherry-oat aphid, and greenbug. The plants were grown in
replicated plots covered with cages and infested artificially with
aphids as they began to head. Antibiotic resistance was estimated by
the increase in biomass of aphids over three weeks on adult plants
(the production of aphids, measured as dry weight). Overall
resistance was estimated by the plant biomass lost due to aphid
infestation (the plant production lost measured as a dry weight).
In the second study, potential sources of
resistance against the most important cereal aphid pest in Manitoba,
English grain aphid, were identified in the primitive ancestral
Einkorn wheat species, Triticum monococcum.
42 accessions of T. monococcum and three cultivated wheats were
infested with aphids. Seedlings were tested in the laboratory for six
days and adult plants were tested in the field for 21 days.
Resistance was measured as in the first study with antibiosis
estimated by the gain in biomass of aphids during infestation.
Overall resistance was estimated by the loss of foliage and head
biomass due to infestation.
Results and Discussion:
In the first study, all three species of aphids
survived and reproduced on all wheats, and reduced head biomass
compared to uninfested controls. The level of antibiosis varied
among wheat species and among accessions, with accessions from
three, five, and one species showing antibiosis to each aphid
species. Overall resistance to the three aphid species was observed
in five to seven accessions per aphid species. Resistance was
usually specific to one aphid species. The frequency of accessions
with antibiosis or overall resistance was associated with the ploidy
level (number of chromosomes) of the plant species. Resistance was
highest for primitive diploid (one set of chromosomes) species and
lowest for modern hexaploid (three sets of chromosomes) species. No
consistent relationship between resistance and level of
domestication was detected, however accessions of the wild wheats,
Triticum boeoticum Bois, Triticum tauschii (Coss.)
Schmal. and Triticum araraticum Jakubz. exhibited high levels
of resistance to aphids, as did Triticum monococcum L. which
is derived from T. boeoticum. Nevertheless, individual
susceptible or resistant accessions occurred at all levels within
the evolutionary tree of wheat.
This study had three useful outcomes:
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High levels of resistance were identified in
some ancestors of wheat.
-
One particular ancestral wheat species,
Triticum monococcum, was identified as a particularly useful
source of resistance.
-
The process of domestication of the wheats did
not reduce the level of resistance to cereal aphids.
In the second study, nearly a third of the
Einkorn wheat accessions exhibited partial resistance to aphids. No
relationship was found between seedling and adult plant resistance:
the former was antibiosis and the latter was tolerance which reflected
the plants ability to recover from the damage caused by the aphids.
Three accessions had levels of adult plant resistance that reduced
loss by 63-73%, and are promising sources of resistance. Highly
susceptible accessions also were identified, which could be useful for
investigating the inheritance of resistance.
The study had two useful outcomes:
-
Aphids do not interact with seedlings in the
same way as they do with adult plants and therefore, resistance to
aphids for Manitoba must be studied in adult plants.
-
Useful levels of both types of resistance,
tolerance and antibiosis were observed in some lines of Einkorn
wheat, but other lines were as susceptible or more susceptible than
wheats commonly grown in Manitoba.
We conclude that some ancient wheats,
particularly Einkorn wheat, Triticum monococcum, are useful
sources of resistance genes for inclusion in Manitoba wheat breeding
programs. Furthermore, there is no reason to believe that
“domesticating” or breeding these traits into our modern cultivars
will destroy the resistance. These conclusions led to our second
study, the identification of particular Einkorn wheat lines with
high levels of resistance against the most serious cereal aphid pest
in Manitoba, the English grain aphid. This knowledge has allowed us
to use Einkorn wheat to identify resistant genes that should be
useful in protecting modern Manitoba wheats from our most serious
cereal aphid pest. Once these genes have been characterized in
Einkorn wheat, they can then be transferred to adapted spring
wheats, and incorporated into our wheat breeding programs. We trust
that the eventual outcome of this project will be a reduction in
yield loss in wheat due to damage by cereal aphids, and a reduction
in the use insecticides to control these pests in Manitoba wheat.
Acknowledgements:
This project was made possible due to funding
from the Governments of Manitoba and Canada through the
Canada-Manitoba Agri-Food Research and Development Initiative
(ARDI).
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