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| Research |
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research interests are in the area of physiological and
genetic life-history trade-offs; particularly in understanding
how organisms evolve to defend themselves against attacks
from other organisms and how they trade-off the costs
of these defences with life-history traits such as longevity
and reproduction. I use a combination of behavioural,
physiological, quantitative and molecular genetic approaches
to address these questions, using insects as model systems. |
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AGEING
AND REPRODUCTIVE INVESTMENT IN THE BURYING BEETLE,
NICROPHORUS VESPILLOIDES
In nature, burying beetles bury the carcass of
a small bird or rodent and use it to provision
dependent young. Larvae solicit resources, much
like begging nestlings, and both parents take
part in parental care. During this project I will
examine the interactions between parents and offspring,
as well as those among rival larvae. In theory,
parental care strategies should change as animal
grows older and nears the end of its life.
Right: Nicrophorous vespilloides,
drawing courtesy of Steve Collett
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Despite
extensive interest in the biology of ageing,
this idea has not been subject to much testing.
I will examine both the effects of natural
ageing on reproductive investment, and
of the risk of death, which will be
simulated by inducing an immune response in
the parents. I will also examine trade-offs
between immune function responses and parental
care which has been shown to be costly in terms
of longevity and future reproduction.
Left:
A burying beetle caring for larvae. (Photo ©
Oliver Krüger) |
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COSTS
OF IMMUNITY IN INSECTS
Which factors have shaped the evolution of immunity in insects?
Immunity to parasites and pathogens is an important life history
trait. Natural selection should favour individuals with stronger
immune systems. However, trade-offs could alter the optimal
level of investment in immunity, and patterns of investment
may be further influenced by the nutrients available to the
insect. |
Solitary and crowded phases in S. littoralis |
Immunity
and density dependent melanism: Spodoptera
littoralis - Egyptian cotton leafworm.
This is a phase polyphenic species that displays different
phenotypic forms depending on population density. The
solitary phase is cryptically coloured whereas
the high density, crowded phase has a conspicuous,
highly melanised cuticle. One suggestion for the adaptive
value of this density-dependent melanism is that of density
dependent prophylaxis (Reeson & Wilson 1998), which
posits that as the risk of disease increases with population
density, so should investment in the immune system. Melanisation
of the cuticle occurs via the action of the phenoloxidase
(PO) enzyme which is also involved in a number of immune
system processes. Therefore, the dark, crowded phase larvae
may be black due to an upregulation of the immune system
in response to population density. |
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| Working
with Ken
Wilson (Lancaster University) I found that
dark phase larvae have higher levels of PO but lower levels
of antibacterial activity, indicating a trade-off within
the immune system. This was confirmed by showing that
the two traits were negatively genetically correlated.
This suggests that individuals face a constraint that
limits their ability to maximise their resistance to different
parasites and pathogens. The possibility that plasticity
in both colour and immune function are mediated by juvenile
hormone are currently being investigated. |
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| Immunity
and diet – self medication in caterpillars: Maintaining
the immune system and mounting an immune response are
expected to be costly in terms of energy or specific nutrients
that have to be diverted away from other functions. Costs
of immunity may therefore be ameliorated by the intake
of specific nutrients to compensate for the loss. With
Ken Wilson (Lancaster University) and
Steve
Simpson (University of New South Wales) I
used a “geometric approach” to examine the
nutritional costs of immune function in Spodoptera
littoralis. By giving caterpillars a choice between
diets containing different protein to carbohydrate ratios
(P:C), or restricting them to diets containing known amounts
of protein and carbohydrate, we showed that different
immune responses peaked in different regions of nutrient
space, meaning that caterpillars would have to modify
their diets differently depending on the response they
wanted to maximise. |
S. littoralis choosing between diets with different protein
to carbohydrate ratios |
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| We
also showed that the diet choice caterpillars make when
uninfected corresponds to the diet that gives them maximum
fitness, but that they will change their diet when infected
to improve their immune response. |
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Minor male and major male O. taurus from Nijhout 2003 |
Immunity
and sexual selection:
Investment in costly sexually-selected traits is linked
to individual quality such that only high quality individuals
can afford to invest in them. Dung beetles provide excellent
examples of costly sexually selected ornaments in the
form of horns that are often used by males in contests
for females. In the dung beetle, Onthophagus taurus,
males occur in two forms, “minors”, which
are hornless or have rudimentary horns, and “majors”,
which develop large horns that scale with body size. The
form an adult male will take is primarily determined by
the resources it is supplied with as a larva by its mother. |
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| Working
in collaboration with Leigh
Simmons (University of Western Australia) I found that immune investment in dung beetle larvae was
consistently higher in males destined to grow large horns,
and that the effect could not be explained by differences
in body size or condition. The relationship may instead
be mediated by hormones and future work will explore this
possibility. |
Dung beetle larva in the brood ball provided by its
mother. |
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THE
PHYSIOLOGY AND GENETICS OF HOST-USE IN PHYTOPHAGOUS INSECTS
Plants can use a vast number of physical and chemical
defences to deter insect feeding. Despite this, some species
utilise plant species from many different families, all
with a different arsenal of defences at their disposal.
A good example of this is Helicoverpa armigera which has been recorded on over 100 plant species from
36 families in Australia alone. With Owain
Edwards and James
Ridsdill-Smith (CSIRO Entomology,
Australia) I investigated the genetic basis for
the interaction between H. armigera and one of
its hosts, the chickpea Cicer arietinum, using
a combination of additive genetic and molecular genetic
techniques. I identified heritable variation in the ability
of H. armigera larvae to feed on different chickpea
genotypes and found evidence that this may be driven by
variation in gut protease activity. Molecular genetic
exploration of this interaction points to differential
expression of gut protease genes in chickpea feeders,
though work is ongoing in this area. |
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| Selected
Publications (click here for a complete
list & PDF downloads) |
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- Cotter,
SC, Simpson, SJ, Raubenheimer, D and Wilson K. Dietary preference
in insects represents a compromise between the nutritional
requirements of competing immune and life-history traits.
Functional Ecology, In Press
- Cotter,
SC, Topham, EA, Price, AJP and Kilner, RM. Fitness costs associated
with mounting a social immune response. Ecology Letters,
DOI: 10.1111/j.1461-0248.2010.01500.x
- Cotter,
SC & Kilner, RM (2010) Sexual division of antibacterial
resource defence in breeding burying beetles, Nicrophorus
vespilloides. Journal of Animal Ecology 79:35-43.
- Cotter,
SC, Myatt, JP, Benskin, CMH & Wilson, K (2008) Selection
for cuticular melanism reveals immune function and life-history
trade-offs in Spodoptera littoralis. Journal
of Evolutionary Biology 21: 1744-1754.
- Cotter,
SC, Beveridge, M & Simmons, LW (2007) Male morph predicts
investment in larval immune function in the dung beetle, Onthophagus
taurus. Behavioral Ecology 19: 331-337
- Cotter,
SC & Wilson, K (2004) Costs of resistance: genetic correlations
and potential trade-offs in an insect immune system. Journal
of Evolutionary Biology, 17: 421-429
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