Abstract:
The obligate pollinators of figs, species-specific agaonid wasps, benefit figs only by transporting pollen between trees; larvae are seed predators. But given the high risk of mortality in flight between trees, adult wasps should prefer to pollinate and oviposit within inflorescences (syconia) at the same tree at which they developed. Flowering within individuals is tightly synchronous in most species, while different trees flower out of phase with each other, suggesting that fig phenology has evolved to assure outcrossing. However, some fig species show distinct within-tree flowering asynchrony. It has been suggested that such asynchrony is an adaptation by which figs in seasonal environments can reduce pollinator mortality, by permitting wasps to persist on individual trees at times when flight would be impossible. The authors have rejected the validity of this Seasonality Hypothesis for Ficus aurea near its northern range limit. Crops of individual trees were most, not least synchronous during the coldest, driest months of two years. Maximum asynchrony occurred in seasons that were probably most favourable for wasp transit between trees, but temporal overlap of the phenological stages that permit wasps to remain on their natal trees was always very rare, implying that consecutive cohorts of developing syconia may be spaced in time to limit this occurrence. -from Authors
PMID: 20855614;PMCID: PMC2951420;Abstract:
Understanding cooperation is a central challenge in biology, because natural selectionshouldfavor "free-loaders" that reap benefits without reciprocating. For interspecific cooperation (mutualism), most approaches to this paradox focus on costs and benefits of individual partners and the strategies mutualists use to associate with beneficial partners. However, natural selection acts on lifetime fitness, and most mutualists, particularly longer-lived species interacting with shorter-lived partners (e.g., corals and zooxanthellae, tropical trees and mycorrhizae) interact with multiple partner species throughout ontogeny. Determining how multiple partnerships might interactively affect lifetime fitness is a crucial unexplored link in understanding the evolution and maintenance of cooperation. The tropical tree Acacia drepanolobium associates with four symbiotic ant species whose short-term individual effects range from mutualistic to parasitic. Using a long-term dataset, we show that tree fitness is enhanced by partnering sequentially with sets of different ant symbionts over the ontogeny of a tree. These sets include a "sterilization parasite" that prevents reproduction and another that reduces tree survivorship. Trees associating with partner sets that include these "parasites" enhance lifetime fitness by trading off survivorship and fecundity at different life stages. Our results demonstrate the importance of evaluating mutualism within a community context and suggest that lifespan inequalities among mutualists may help cooperation persist in the face of exploitation.
PMID: 19767463;PMCID: PMC2786516;Abstract:
As polyphagous, holometabolous insects, tephritid fruit flies (Diptera: Tephritidae) provide a unique habitat for endosymbiotic bacteria, especially those microbes associated with the digestive system. Here we examine the endosymbiont of the olive fly [Bactrocera oleae (Rossi) (Diptera: Tephritidae)], a tephritid of great economic importance. "Candidatus Erwinia dacicola" was found in the digestive systems of all life stages of wild olive flies from the southwestern United States. PCR and microscopy demonstrated that "Ca. Erwinia dacicola" resided intracellularly in the gastric ceca of the larval midgut but extracellularly in the lumen of the foregut and ovipositor diverticulum of adult flies. "Ca. Erwinia dacicola" is one of the few nonpathogenic endosymbionts that transitions between intracellular and extracellular lifestyles during specific stages of the host's life cycle. Another unique feature of the olive fly endosymbiont is that unlike obligate endosymbionts of monophagous insects, "Ca. Erwinia dacicola" has a G+C nucleotide composition similar to those of closely related plantpathogenic and free-living bacteria. These two characteristics of "Ca. Erwinia dacicola," the ability to transition between intracellular and extracellular lifestyles and a G+C nucleotide composition similar to those of free-living relatives, may facilitate survival in a changing environment during the development of a polyphagous, holometabolous host. We propose that insect-bacterial symbioses should be classified based on the environment that the host provides to the endosymbiont (the endosymbiont environment). Copyright © 2009, American Society for Microbiology. All Rights Reserved.
PMID: 19845459;Abstract:
With the growing recognition of exploiters as a prominent and enduring feature of many mutualisms, there is a need to understand the ecological and evolutionary dynamics of mutualisms in the context of exploitation. Here, we model coevolution between mutualist and exploiter birth rates, using an obligate pollinating seed parasite mutualism associated with a nonpollinating exploiter as a reference system. In this system, mutualist and exploiter larvae parasitize the host plant, competing for and consuming seeds. Evolution of the mutualist determines which exploiters can invade successfully. Subsequent coevolution with an exploiter has a strong, predictable influence on mutualist-exploiter coexistence, mutualist and exploiter phenotypes, and species abundances. Weak mutualist competition promotes "evolutionary purging" of the exploiter, while weak exploiter competition leads to "evolutionary suicide" of the system. When stable, long-term coexistence occurs, we identify two main "trait-abundance syndromes" that have three novel implications. (1) Persistent, highly parasitic exploiters can be favored by coevolution. (2) Even then, the density of coevolved mutualists can be high. (3) Low plant density results primarily from the evolution of mutualist, not exploiter, birth rate and density. To evaluate these predictions, studies are needed that identify and compare populations with and without exploiters and compare life-history traits of mutualists and exploiters. © 2009 by The University of Chicago.
