128 research outputs found
Tradeoffs in the evolution of plant farming by ants
Diverse forms of cultivation have evolved across the tree of life. Efficient farming requires that the farmer deciphers and actively promotes conditions that increase crop yield. For plant cultivation, this can include evaluating tradeoffs among light, nutrients, and protection against herbivores. It is not understood if, or how, nonhuman farmers evaluate local conditions to increase payoffs. Here, we address this question using an obligate farming mutualism between the ant Philidris nagasau and epiphytic plants in the genus Squamellaria that are cultivated for their nesting sites and floral rewards. We focused on the antsâ active fertilization of their crops and their protection against herbivory. We found that ants benefited from cultivating plants in full sun, receiving 7.5-fold more floral food rewards compared to shade-cultivated plants. The higher reward levels correlated with higher levels of crop protection provided by the ants. However, while high-light planting yielded the greatest immediate food rewards, sun-grown crops contained less nitrogen compared to shade-grown crops. This was due to lower nitrogen input from ants feeding on floral rewards instead of insect protein gained from predation. Despite this tradeoff, farming ants optimize crop yield by selectively planting their crops in full sun. Ancestral state reconstructions across this antâplant clade show that a full-sun farming strategy has existed for millions of years, suggesting that nonhuman farmers have evolved the means to evaluate and balance conflicting crop needs to their own benefit
Mutualisms drive plant trait evolution beyond interactionârelated traits
Mutualisms have driven the evolution of extraordinary structures and behavioural traits, but their impact on traits beyond those directly involved in the interaction remains unclear. We addressed this gap using a highly evolutionarily replicated system â epiphytes in the Rubiaceae forming symbioses with ants. We employed models that allow us to test the influence of discrete mutualistic traits on continuous nonâmutualistic traits. Our findings are consistent with mutualism shaping the pace of morphological evolution, strength of selection and longâterm mean of nonâmutualistic traits in function of mutualistic dependency. While specialised and obligate mutualisms are associated with slower trait change, less intimate, facultative and generalist mutualistic interactions â which are the most common â have a greater impact on nonâmutualistic trait evolution. These results challenge the prevailing notion that mutualisms solely affect the evolution of interactionârelated traits via stabilizing selection and instead demonstrate a broader role for mutualisms in shaping trait evolution
Host sanctions and the legumeâ rhizobium mutualism
Explaining mutualistic cooperation between species remains one
of the greatest problems for evolutionary biology1â4. Why do
symbionts provide costly services to a host, indirectly benefiting
competitors sharing the same individual host? Host monitoring
of symbiont performance and the imposition of sanctions on
âcheatsâ could stabilize mutualism5,6. Here we show that soybeans penalize rhizobia that fail to fix N2 inside their root nodules. We
prevented a normally mutualistic rhizobium strain from cooperating
(fixing N2) by replacing air with anN2-free atmosphere
(Ar:O2). A series of experiments at three spatial scales (whole
plants, half root systems and individual nodules) demonstrated
that forcing non-cooperation (analogous to cheating) decreased
the reproductive success of rhizobia by about 50%. Non-invasive
monitoring implicated decreased O2 supply as a possible mechanism
for sanctions against cheating rhizobia. More generally,
such sanctions by one or both partners may be important in
stabilizing a wide range of mutualistic symbioses
Adaptation and enslavement in endosymbiont-host associations
The evolutionary persistence of symbiotic associations is a puzzle.
Adaptation should eliminate cooperative traits if it is possible to enjoy the
advantages of cooperation without reciprocating - a facet of cooperation known
in game theory as the Prisoner's Dilemma. Despite this barrier, symbioses are
widespread, and may have been necessary for the evolution of complex life. The
discovery of strategies such as tit-for-tat has been presented as a general
solution to the problem of cooperation. However, this only holds for
within-species cooperation, where a single strategy will come to dominate the
population. In a symbiotic association each species may have a different
strategy, and the theoretical analysis of the single species problem is no
guide to the outcome. We present basic analysis of two-species cooperation and
show that a species with a fast adaptation rate is enslaved by a slowly
evolving one. Paradoxically, the rapidly evolving species becomes highly
cooperative, whereas the slowly evolving one gives little in return. This helps
understand the occurrence of endosymbioses where the host benefits, but the
symbionts appear to gain little from the association.Comment: v2: Correction made to equations 5 & 6 v3: Revised version accepted
in Phys. Rev. E; New figure adde
Evolutionary maintenance of genomic diversity within arbuscular mycorrhizal fungi
Most organisms are built from a single genome. In striking contrast, arbuscular mycorrhizal fungi appear to maintain genomic variation within an individual fungal network. Arbuscular mycorrhizal fungi dwell in the soil, form mutualistic networks with plants, and bear multiple, potentially genetically diverse nuclei within a network. We explore, from a theoretical perspective, why such genetic diversity might be maintained within individuals. We consider selection acting within and between individual fungal networks. We show that genetic diversity could provide a benefit at the level of the individual, by improving growth in variable environments, and that this can stabilize genetic diversity even in the presence of nuclear conflict. Arbuscular mycorrhizal fungi complicate our understanding of organismality, but our findings offer a way of understanding such biological anomalies
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Does urbanization explain differences in interactions between an insect herbivore and its natural enemies and mutualists?
Urbanization can alter the composition of arthropod communities. However, little is known about how urbanization affects ecological interactions. Using experimental colonies of the black bean aphid Aphis fabae Scopoli reared on Vicia faba L, we asked if patterns of predator-prey, host-parasitoid and ant-aphid mutualisms varied along an urbanization gradient across a large town in southern England. We recorded the presence of naturally occurring predators, parasitoid wasps and mutualistic ants together with aphid abundance. We examined how biotic (green areas and plant richness) and abiotic features (impervious surfaces and distance to town center) affected (1) aphid colony size, (2) the likelihood of finding predators, mutualistic ants and aphid mummies (indicating the presence of parasitoids), and (3) how the interplay among these factors affected patterns of parasitoid attack, predator abundance, mutualistic interactions and aphid abundance. The best model to predict aphid abundance was the number of mutualistic ants attending the colonies. Aphid predators responded negatively to both the proportion of impervious surfaces and to the number of mutualistic ants farming the colonies, and positively to aphid population size, whereas parasitized aphids were found in colonies with higher numbers of aphids and ants. The number of mutualistic ants attending was positively associated with aphid colony size and negatively with the number of aphid predators. Our findings suggest that for insect-natural enemy interactions, urbanization may affect some groups, while not influencing others, and that local effects (mutualists, host plant presence) will also be key determinants of how urban ecological communities are formed
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