This presentation will explain the history of evolution of food habits in insects along with the scientific evidence from published literature.

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INSECTS
Evolution of Life on Earth
CK-12 Foundation

3. EVOLUTION OF DIET BREADTHIN HERBIVORUS INSECTS
Suresh R. Jambagi
III Ph.D. (Entomology)
PAMB0021
UAS-GKVK
Doctoral Seminar- IV

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Introduction
Status & Evolution of
herbivores
Evolution path
Causes for diet
expansion
Causes for diet
contraction
Generalists vs
Specialists
Conclusion
Flow of Seminar

5. ANCIENT RAPID RADIATION OF INSECTS
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HERBIVORES INSECTS
 400 Mya- Plants evolved terrestrial forms
 Next 40 Mya- Dense vegetation
 300 Mya- Evolution of herbivores feeding
insect orders
 0.5 million species described- Herbivores
(Hardy et al., 2020)

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 GENERALISTS evolved first…..(Xiao et al., 2021)
 Orthopteroid group- Orthoptera, Blattodea,
Grylloblattodea, Mantodea, Mantophasmatidae,
Phasmatodea
 M.P morphological structures – Drive diet breadth
 Tailed by- SPECIALISTS
(Bernays, 1998)

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DIET EXPANSION
i. Competition to
broad diet
ii. Environmental
uncertainty to
broad diet
Causes of Diet Expansion
 Negative density dependent selection
(NDDS)- Drive evolution of niche breadth.
 Generalists- Heterogenous environments
 Evolution of niche breadth- match
environmental heterogeneity.
 Broad niches can also be adoptive without
intense competition.

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Competition
Indirect
Top-down
effect
Bottom-up
effect
Direct
Among
herbivores
i. From Competition to Broad Diets
(Hardy et al., 2020)

14.  Herbivores should evolve a broad diet in
mixed community of plants under NDDS
(If no constraints on adaptation).
 Indirect competition between
herbivorous insects implies selection of
broader diet.
 Novel hosts have lower densities of
competitors than occupied hosts.
 Proposed by- Hairston, Smith and Slobodkin
(1960)
 Hypothesis- “There will be no competition among
herbivores for food resources”
WRONG
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(Hairston et al., 1960)

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ii. From Environmental Uncertainty to Broad Diet
 Plant communities change over time
 Specialists- more expose to these changes
(Extreme case- extinct)
 Generalism- persist by virtue of being less bad
during worst of time.
 Volatility of plant communities w.r.t. geographical
variations.
 Tropic guilds more host specific than temperate
areas (Forister et al., 2015)

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 Individual plants also change over time- Danger poses to
herbivores
 Hence Diet expansion is prerequisite
 Plant communities= Hosts + Non-hosts (Embedded)
 Host finding- Important
 Generalism- common in poor host finders like Wind
dispersed scale insects (Normark and Johnson, 2011).
 Generalism- Way of minimizing the risk of not finding a
host
 Host finding mechanism involves costs
GENERALISM- Hedging against Shocks
(Hardy et al., 2020)

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DIET SPECIALISATION
- Specialists prevail in most of the natural systems
- Have narrow diets
- Specialization is better option under no short of
resources
Why..?
“Adaptation potentiality
to deal with specific
food”
(Carroll and Boyd, 1992)

18. • Combined effect
• Cost of high density
• Cost of low density
• Insufficient standing
variation
• Genetic drift & draft
• Generalists- slow to adopt
• Epistasis
• Pleiotropy
1. Genetics
Constraints
2.
Population
Constraints
4.
Constraint
Integration
3.
Ecological
Constraints
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Causes for Diet Contraction
Constraints are the basis of all theoretical models of specialization

19. Conceptual overview
(Hardy et al., 2020)
Constraints- Basis of all theoretical models of specialization 19

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1. Genetic Constraints
Mutation
Epistasis
Pleiotropy
Performance
Preference

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A.Epistasis
 Single phenotype is affected by more
than one locus.
 Potentially beneficial mutations work
only in certain environments
 Strong negative evolutionary
correlation in host use
 Negative correlation are rare acc. to
comparative phylogenetics analysis.
 Ex: Bugs, moths and butterflies:
Evolutionary correlation in host use-
POSITIVE (Peterson et al., 2016)

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B. Pleiotropy
 Single locus affects more than one phenotype
 Mutation that improves one function can
degrade another
 Drive the evolution of niche specificity
 Adoption to one host comes at the cost of
reduced performance on another.
(Hardy et al., 2020)

23. Fig.1: Trajectory for mean fitness of E. coli
during 20,000 generations in minimal glucose
medium. Each point is the mean of all 12
populations, and fitness of each population relative
to the ancestor was measured with fivefold
replication. 23
Methodology
• 12 lines of two variants of E. coli
• Media- Davis minimal media supplemented with
glucose
• Competitive fitness assayed by- 23 different C-
source
• Catabolic diet breadth assayed by- Biolog ES
plates
• Maintained till- 20,000 generations (3000 days)
Objective: To Evaluate bases for ecological
specialization in E. coli
(Cooper and Lenski, 2000)

24. Fig.2: Hypothetical trajectories for the evolution
of ecological specialization (Reflected by the
decay of total catabolic function).
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Table 1: Changes in catabolic functions, based on
comparisons btwn evolved populations and common
ancestor. [Red- catabolic functions that consistently
decayed; Green- significant gains in function].
(Cooper and Lenski, 2000)
Functional decay is inversely parallel to gain in
fitness- AP

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Phenotypic trade-offs may happen in two ways
i. Performance
 Chemical defense
 Insects- sequester toxins- reuse them
for own defense- Prevalence of
specificity Ex: Milk weed butterfly
 Metabolic trade-offs
 Specialists- detoxify their host
defense chemicals with greater
economy (Rothwell and Holesky, 2020)
ii. Preference
 Chemical communication
 Information Processing Hypothesis-
”Specialists make faster decisions than
generalists”
 Ex: Aphids- more specialized forms
more accurately settled on good hosts
(Bernays and Funk, 1999)
 Specialists: poor decisions have harsher
consequences and more strongly
selected against

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2. Population Constraints
A. Insufficient standing variation
 Natural selection requires genetic variation
 Variation required for some type of niche
evolution is absent.
Ex: i. Ophraella beetles (Futuyma et al., 1995)
ii. E. coli – not survived >43oC (Bennet and
Lenski, 1993)
Ophraella notulata

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B. Genetic Drift
 Specialization via drift expected when small
population are isolated from generalist
ancestors.
 Cryptic genetic variation- Keep population
specialized
 Demonstrated in- Chlamydomonas algae
adaptation to light and dark environment
(Reboud and Bell, 1997).

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Objective: To find evidence for adaptation of a strain of
phytophagous moth Thyrinteina leucoceraea to its novel host
plant, eucalyptus.
Methodology:
 Two strains: i. Guava strain ii. Eucalyptus strain
 Two hosts: i. Native- Guava ii. Novel- Eucalyptus
(Grosman et al., 2015)

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Fig.1: Average net reproductive rate (R0, a) and intrinsic
growth rate (rm, b) of the eucalyptus and guava strain on
a diet of eucalyptus and guava host plant foliage.
Results
 Both fitness measures show that the
eucalyptus strain performed significantly
worse on the native host plant than did
the guava strain.
 No significant trends for the eucalyptus
strain being more adapted to eucalyptus
than the guava strain.
Conclusion
‘The loss of fitness on native host seems to
have preceded the adaptation to the novel
host’.
“ECOLOGICAL TRAP…!!?”
(Grosman et al., 2015)
Specialization is an evolutionary dead
end…!!??

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C. Generalists are slow to adopt
 Generalism take long time to evolve
 Simple population genetic model: Adaptation to specific resource will be
faster in specialists.
 Conditionally neutral alleles- Genetic variants that are advantageous on one
host plant and neutral on others.
 Such alleles will be more rapidly promoted and fixed in specialists.
 Also, specialists more rapidly purge deleterious alleles on a particular host
 No strong empirical evidences
(Whitlock, 1996)

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3. Ecological Constraints
A.Costs of High Density
 Classical theory of adaptive radiation-
Antagonism between species- biggest
constraint on diet breadth.
 Strong density dependent selection
lead to niche expansion.
Niche
expansion
i. Extinction
of
antagonist
ii.
Colonization
of new
environment
iii. Evolution
of key
innovations
(McArthur et al., 1972)

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B. Costs of Low Density
 Allee effects: Could lead to positive
selection for host-use specialization.
 Ex: Finding a mate- More difficult in
generalists (rare)
 Host-use specificity gives mate searchers a
more specific target.
 Lack of evidences
(Colwell, 1986)

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VS

35. Traits Generalists Specialists
1. Host use adaptation Slow adopters Fast
2. Novel host associations Fast Slow
3. Genetic architecture
evolution
Reduce scope for epistatic
and pleiotropic constraints
Enhance
4. Phenotypic plasticity More adoptive plasticity More non-adoptive
5. Population genetics
Larger & less structured
population
Smaller & structured
6. Community interactions
i. With host plants
ii. With endosymbionts
-different w.r.t host plants
-Less pronounced
-Specific- more HIPV’s
-More specificity
7. Hedging against shocks Less Vulnerable More vulnerable
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(Hardy et al., 2020)

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Few supporting examples:
1. For genetic architecture evolution: (Gouin et al.,
2017)
-FAW, Spodoptera frugiperda- More polyphagous- had
the most diverse gene families involved in
detoxification of xenobiotics.
2. Phenotypic plasticity: (Ragland et al., 2015)
-Non-adoptive plasticity in snowberry-feeding fruit fly
species Rhagoletis zephyria
- Raised on Apple – Lost fitness
- Novel host induces changes in the gene expression
of several hundred loci

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Conclusion
Diet Breadth Evolution- Complex & Multi-
faceted contineous process

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