Transcript Host sanctions and pollinator cheating in the fig tree

Greg Cello and Melissa Finnell
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K. Charlotte Jandér
◦ Cornell University, Department of Neurobiology and
Behavior, Ithaca
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Edward Allen Herre
◦ Biologist at the Smithsonian Tropical Research Institute
◦ Research in co-evolution of figs and their pollinator
wasps, general host- parasite co-evolution; the
evolutionary effects of population structure on sex ratio,
sexual selection and parasite virulence, the effects of
plant pathogens on population and genetic structure if
hosts, interactions between tropical host plants and both
mycorrhizal and endophytic fungi.
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Cheaters
◦ Individuals that reap the benefits of mutualistic
interactions without paying the cost of providing
service to the mutualist
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Host sanctions
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Foundress
◦ Mechanisms that lower the fitness of uncooperative
symbionts
◦ Pollen-bearing female wasp
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Monoecious
◦ Species that have both male and female
reproductive units
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Oviposition
◦ Process of laying eggs by animals with little or no
embryonic development within mother
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Passive pollination
◦ Numerous, large male flowers that release abundant
pollen onto Fig wasp
◦ Therefore, trees invest considerable resources in
producing abundant pollen, and no aspect of pollen
transfer relies on specialized wasp behavior
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Active pollination
◦ Relatively few, small male flowers
◦ Pollen transfer is completely dependent on specialized
wasp morphology and pollination behavior.
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Previous studies show:
◦ Existence of host sanctions that lower fitness of
uncooperative symbionts
Long-term stability of the mutualism requires
mechanisms that limit or prevent cheating
Pollination is dependent on having pollen bearing
female fig wasp enter fig and pollinate the
flowers
Direct tradeoff between producing fig seeds or
wasp offspring
Pollination syndromes in figs:
◦ Passive
◦ Active
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To test whether cheating levels in symbionts
of fig tree-fig wasp system are related to
sanction strength
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Experimentally produced pollen carrying(P+)
wasps and artificially pollen-free wasps(AP-)
◦ Introduced one wasp into each fig to create
pollinated(P+) and unpollinated(P-) figs
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2 components of fig sanctions that influence
fitness:
◦ Proportion P+ and P- figs that tree aborted
◦ Reproductive success of AP- and P+ wasps in unaborted
figs
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Experimental introduction:
◦ When figs receptive, a single AP- or P+ female wasp was
introduced to random fig
◦ Prevented uncontrolled pollination by enclosing in mesh
bag
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During weeks after introduction, aborted figs
were collected
Later, non-aborted figs were collected just
before wasps emerged
As wasps emerge, collected in vials and counted
Each tree produced a value of MR, OR and WR.
◦ MR proportion of P- figs matured
◦ OR relative number offspring in unaborted Pfigs
◦ WR relative fitness of single foundress P- wasp
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Estimate proportion of wasps associated with
each species that are likely to be single
foundress
Collected unmanipulated, naturally occurring
wasps to screen for “cheaters”
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The proportion of naturally occurring pollen-free
wasps was negatively correlated with sanction
strength (1 2 WR) across actively pollinated fig species
Host sanctions were
found in all actively
pollinated fig species,
but not in passively
pollinated fig species
They found pollen free
individuals in all species
where wasps can easily
cheat by omitting any of
the time consuming
behaviors associated
with active pollination
Within these actively
pollinated fig species,
pollen-free wasps were
most common in the
species with the
weakest sanctions.
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Three novel findings:
◦ Host sanctions vary dramatically
◦ Existence of “cheaters”
◦ Association between sanction strength and
proportion of pollen-free wasps
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Host sanctions detected in all major
subgenera of Fiscus
In actively pollinating species sanction
strengths are similar to those reported
previously
In passively pollinating species, there is no
indication of host sanctions
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Pollen-free wasp species that belong to
otherwise mutualistic pollinating wasp
species
“Cheat” by omitting any time-consuming
behaviors associated with active pollination
Effective cheaters with respect to tree’s seed
production(female function)
Found in natural populations of all actively
pollinating wasp species
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Negatively correlated in active species
◦ Pollen-free wasps most common in species with
weakest sanctions
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Fig sanctions likely to be a modification of
aborting unpollinated flowers
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Strengths
◦ The article was organized in a clear and consistent
manner
◦ Covered most if not all questions that could have
been asked while also addressing further needed
studies
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Weaknesses
◦ None notable
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Further studies of the figs are needed to
identify the level of precision and mechanism
of sanctions, and to attempt to quantify the
relative costs of sanctions across species.
Further studies of the wasps are needed to
determine if naturally occurring pollen-free
wasps inherit this trait from their mothers,
and whether any fitness benefits of the
pollen-free trait are large enough to explain
its persistence despite the sanctions.
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What is the difference between passive and
active pollination?
Are host sanctions found in actively
pollination species or passively pollination
species?
What does it mean to be a “cheater”?
What is the association between sanction
strength and pollen-free wasps?
Host sanctions and pollinator cheating in the
fig tree–fig wasp mutualism
K. Charlotte Jander and Edward Allen Herre
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Department of Neurobiology and Behavior, Cornell University, Ithaca, NY
14853, USA
Smithsonian Tropical Research Institute, Unit 9100, PO Box 0948, DPO, AA
34002-9998, USA
Department of Animal Ecology, Evolutionary Biology Centre, Uppsala
University, 752 36 Uppsala, Sweden