Ant mutualisms

EEOB 400: Lecture 15
What is coevolution?
Two (or more) species:
1) exert selective pressures on each other, and
2) evolve in response to each other
Because each species is evolving in response to the other, one important feature
of coevolution is that the selective environment is constantly changing
When does coevolution occur?
Selective pressure will be strongest when there is a close ecological relationship
“Close” ecological relationship = usually specialists rather than generalists
Important ecological relationships that give rise to coevolution:
1) predators & prey 2) parasites & hosts 3) mutualists
4) competitors
mutualist A
competitor A
mutualist B
competitor B
How do we study coevolution?
Like most evolutionary questions, it can be studied at various levels:
Adaptations of individuals  Interactions between species  Broad evolutionary patterns
Coadaptation Reciprocal adaptations of two species
Could refer to species, adaptations possessed by individuals, genotypes, etc.
Lycaenid caterpillars secrete “honeydew” that ants drink
Ants defend caterpillars against parasitic wasps
Honeydew secretion and defense are coadaptations
Does coadaptation demonstrate coevolution?
Biologists often have a strict
definition of coevolution:
evidence of parallel evolution
between taxa is required
Fig-wasp mutualism
Fig trees (Ficus)
~750 tropical species, all of which depend entirely on wasps for pollination
Figs are not fruits – they are specialized
inflorescences with hundreds of unisexual flowers
Fig-wasp mutualism
Fig wasps (Agaonidae)
Males: suited only for boring holes and mating
Females: adaptated for flying, burrowing into
figs, and laying eggs in fig oocytes
- Receptive figs produce scents that are specific to a particular pollinator species
- Shape of ostiole specific to head shape of particular wasp species (lock-and-key)
- Morphology of individual flowers specialized to a particular wasp species
Female wasp
enters via ostiole
and oviposits in
female flowers
Male flowers
Female flowers
Flower styles are Pollen
different lengths –
wasps only oviposit
in ones w/ short
Fig-wasp mutualism
Don’t worry…
the wasps leave
before the fruit
is ripe to eat
Fig-wasp mutualism
Seed dispersal
Although pollination is very host-specific, seed dispersal is usually not
Over 1200 different vertebrate species are known to eat & disperse fig seeds
Accordingly, we would expect fig-disperser coevolution to be much weaker
Fig-wasp mutualism
A twist to the story…parasitism
In addition to pollinating wasps, figs are associated with parasitic wasps
Parasitic wasps do not enter the ostiole and do not pollinate the fig’s flowers
Instead, they use a long ovipositor to puncture the fig and lay eggs from outside
Parasites reduce fitness of figs and pollinator wasps
- By ovipositing in flowers that would otherwise produce pollinator wasps
- By directly predating pollinator wasps in some species
- By ovipositing in flowers that would otherwise produce seed for the fig
Fig-wasp mutualism
Congruent phylogenies due to cospeciation – strong evidence for coevolution
Host jumping
“Missing the boat”
A statistical method known as
phylogenetic reconciliation analysis
tests the hypothesis that two phylogenies
are more different than expected by chance
Fig-wasp mutualism
Figs and pollinator wasps show a very high degree of cospeciation
Despite pressure from
parasitic wasps, fig –
pollinator specificity
is maintained
Indicates a very tight
ecological relationship
Weiblen & Bush (2002) Mol. Ecol. 11:1573-1578
Fig-wasp mutualism
Figs and parasites do not show as strong evidence for cospeciation
Host jumping
Host duplication
“Missing the boat”
Weiblen & Bush (2002) Mol. Ecol. 11:1573-1578
Fig-wasp mutualism
Figs are coadapted to both pollinators and parasites
Figs must balance their own reproductive success against the need to maintain
pollinator specificity and reduce impact of parasites
Pollinators oviposit from within the fig
and into innermost layers of oocytes
Some ancestral figs solve this problem by
producing flowers with styles of different
lengths so at least some will produce seed
Parasites oviposit through fig and into
outermost layers of oocytes
Fig-wasp mutualism
Figs are coadapted to both pollinators and parasites
Figs must balance their own reproductive success against the need to maintain
pollinator specificity and reduce impact of parasites
Functional dioecy
Some species produce figs with either
all long or all short styled flowers
A will produce pollen and pollinator eggs,
so it is functionally male (= no fig seed)
B will produce only seed (and parasite
eggs), but to do so it has to smell like A
to trick pollinator females into entering
This strategy doesn’t eliminate parasitism,
but it guarantees that seed will be set
Note that mutualism is not all “warm and fuzzy”…mutualists will always try to
maximize their benefit (pollination) and minimize their cost (loss of seed production)
Plant-insect coevolution
Cospeciation in a plant-herbivore system
Tetraopes beetles eat milkweed plants in the genus Asclepias  cospeciation
Plant-insect coevolution
Cospeciation in another plant-herbivore system
Blepharida beetles eat Bursera plants
Becerra (1997) Science
Plant-insect coevolution
Cospeciation in another plant-herbivore system
Blepharida beetles eat Bursera plants
There is a high degree of host-specificity
Then why so much host-jumping?
Why not cospeciation like in Tetraoptes
beetles and milkweed plants?
Becerra (1997) Science
Plant-insect coevolution
Host specificity is determined by the chemical defenses of the plant
Four major chemical classes of
plant defenses against herbivory
(indicated by colors)
These chemical classes do not
correspond to plant clades (top)
The bottom figure shows beetle
phylogeny with branches coded
for the chemical type of the host
The phylogenies are incongruent
because host switching can occur
as long as the beetle switches to a
new host with chemical defenses to
which it is already adapted
Becerra (1997) Science
Congruent phylogenies can arise for more than one reason
Today we are discussing congruence as a result of cospeciation
But recall that congruence is also predicted by vicariance biogeography
Congruent Incongruent
Plant-pollinator coevolution
Flower and fly or moth pollinators
Many flies and moths have outlandish proboscises
to extract nectar from similarly outlandish flowers
Darwin received a specimen of the orchid Angraecum
sesquipedale and predicted from it that there must
exist a pollinator with a proboscis measuring 10-12”
This prediction was not
confirmed until 1903 with
Xanthopan morgani moth
Plant-pollinator coevolution
Host specificity drives coevolution
A flower “wants” its pollen spread
to other flowers of the same species
A coevolutionary
Extravagant traits can
coevolve in response
Flower evolves
Fly responds
Plant-pollinator coevolution
Mutualisms can be exploited
Mutualisms can be exploited by “cheaters”
that collect benefits but avoid costs, as in the
case of the deceptive orchid Disa nivea
D. nivea mimics a
flower to fool the fly
Prosoeca ganglbaueri
Figs 2-4 = flower species that produces nectar
Figs 5-6 = mimic orchid that “cheats”
Anderson et al. (2005) Am. J. Botany 92: 1342
Ant mutualisms
Ants and insects that produce “honeydew”
Ants participate in dozens of mutualisms and show coadaptations for each
Ants tending a lycaenid caterpillar
Ant drinking honeydew from an aphid
Many different insects provide ants with “honeydew” – source of nutrition for the
ants that has no other function for the insect – specifically coevolved for ants
In return, ants defend insects from parasites and predators
Ant mutualisms
Ants and acacia trees
Pseudomyrmex ants protect acacia trees from herbivores – in return, the acacia
feeds the ant with nectar and protein rich Beltian bodies, and provides a place for
the ants to live in the acacia’s modified thorns
Ant mutualisms
Ant-fungus mutualism
Attine ants (~210 species) have cultivated fungal
gardens for over 50 million years
Benefits to the ant:
Fungi produce nutritional “gongylidia” that are
harvested by ants to feed their larvae
Fungi can digest cellulose, ants can not
Atta cephalotes collecting leaf
cuttings for their fungal garden
Benefits to the fungi:
Ants remove plants and other
fungi that compete for nutrients
and provision fungi with leaves
Ants cultivate actinomycete
bacteria that produce antiboiotics
against Escovopsis fungi, which
would otherwise parasitize the
mutualist fungi
Ant with pockets
of bacteria
Captive colony of Atta mexicana
tending to a fungal garden
Ant mutualisms
Ant-fungus mutualism
To simplify the system in a diagram:
Mutualism or
+ +
(gardened fungus)
kill parasite
Parasite kills cultivar
(Escovopsis fungi)
A four-way symbiosis – but do these species coevolve?
Ant mutualisms
Coevolution – Patterns of parallel evolution between ants and fungal cultivars
…and between these two groups and Escovopsis parasites !!
Currie et al. (2003) Science 299: 386-388
Host-parasite coevolution
Coevolution – Thus far we have seen examples from mutualism interactions
Pocket gophers (Geomyidae) are are parasitized by lice (Mallophaga)
Clear pattern of cospeciation – this example also shows how rates of evolution
can be compared (b) to provide further evidence
for coevolution (letters in b = branches in a)
Coevolutionary arms races
“Arms race”
Coevolving species have to constantly “improve” to meet each new adaptation
with a “better” adaptation of their own
Coadaptations become increasingly powerful, yet species are not any better
adapted because the selective landscape is constantly changing
This may sound familiar: it is Van Valen’s Red Queen Hypothesis:
- running as fast as possible just to stay in the same place
An inherent feature of coevolution
We often think of “arms races” as occurring between predators and prey, or
between parasites and hosts – this makes intuitive sense
But it is not really that different in mutualists – each mutualist will be best adapted
when it receives the maximum benefit while paying the minimal cost
Coevolutionary arms races
An arms race in a predator-prey interaction
Taricha granulosa newts have powerful tetrodotoxins
(TTX) that are secreted as protection from predators
Thamnophis sirtalis garter snakes are the only major predator
of this newt – they have evolved resistance to TTX
Toxins produced by newts are hundreds of times more powerful that necessary
to kill any other predator (including humans), but snakes are resistant
Can we find evidence for coevolution?
Brodie et al. (2002) Evolution 56:2067-2082
Coevolutionary arms races
Snake populations
vary in resistance
to newt toxins
Snake populations
outside of newt’s range
have low resistance
Brodie et al. (2002) Evolution 56:2067-2082
A geographic mosaic
with two coevolutionary
Coevolutionary arms races
An arms race in a predator-prey interaction
The extremely high toxicity of Taricha granulosa,
which is hundreds of times more toxic than necessary
for most predators, is a result of an escalating arms
race with one species, Thamnophis sirtalis
Snake resistance is
predicted by newt
toxicity, as expected
if these species are
Brodie et al. (2002) Evolution 56:2067-2082
Evidence for coevolution
Local coadaptation
Snakes and newts are locally coadapted:
- snakes have not evolved resistance in populations outside of the newt’s range
- populations with high newt toxicity have high snake resistance
Snails and their castrating trematode parasites
In three separate studies, parasites were better able to infect snails from their own
population than hosts from other populations – parasites are locally coadapted
Curt Lively’s research:
Inferring an arms race from fossils
Shells of fossil gastropods
Difficult to infer coadaptation from fossils because we can’t observe interactions
But we can use characteristics that reflect predator-prey interactions
When a shell is repaired
following a failed predation
attempt, it leaves a clear
pattern evident in fossils
Gastropods “cement”
themselves to the substrate
as an adaptation against
Gastropods with thickened
or narrowed apertures are
better able to survive
predation events
The incidence of shell repair
increases through time,
suggesting predation is
becoming more intense
The incidence of mobile
gastropods that lack a
means of attachment
decreases over time
The incidence of thickened
or narrowed apertures
increases over time
Fossils and the Red Queen
Probability of extinction
The fossil record also supports another important theoretical point:
Probability of extinction is constant through the course of evolution
Why is this important?
It shows that evolution is not progressive – taxa that have been around longer
have not become “better adapted” and thus better able to avoid extinction
Supports the Red Queen model and implicates coevolution as a major force:
Organisms have to keep running (evolving) just to stay in place (avoid extinction)
Coevolution and radiation
Biologist JBS Haldane was once asked by theologians:
“What could one conclude about the Creator from a study of His creation?”
Haldane’s reply: “An inordinate fondness for beetles"
Coevolution and radiation
Why are beetles so speciose?
Over half of all beetles are phytophagous (feed on plants), and a large number of
these herbivorous beetles feed on angiosperms (flowering plants)
Farrell (1998) hypothesized that specialization on different
angiosperm species led to the radiation of beetle species
The increase in herbivorous
beetle genera correlates with
the exponential increase of
angiosperms beginning in
the Cretaceous
Radiation of
Farrell (1998) Science 281: 555-559
Coevolution and radiation
Why are beetles so speciose?
Phytophagous beetles are a monophyletic group, but specialized feeding on
angiosperms has evolved multiple times within phytophagous beetles
(A) Curculionoidea
(B) Chrysomeloidea
Beetles feed on:
Angiosperms (dicots)
Angiosperms (monocots)
Many more beetle
genera occur in
clades that feed
on angiosperms
Farrell (1998) Science 281: 555-559
Coevolution and radiation
Why are beetles so speciose?
Specialization on angiosperms leads to rapid beetle speciation via coevolution
Evolutionary changes in plant host
lead to incredible beetle radiations
It is important to note that this same
pattern is observed in five different
clades, indicating that the change in
host type is driving this pattern
Radiation of
Farrell (1998) Science 281: 555-559
Why is coevolution important?
We can simplify ecology as consisting of 2 types of interactions
1) Abiotic – interactions with temperature, light, nutrients, humidity, etc.
2) Biotic – interactions with other organisms
Coevolution occurs only as a direct result of biotic interactions
A simple question about the importance of coevolution:
If change in the physical environment ceased,
would evolution come to a stop?
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