Host sanctions in Panamanian Ficus are likely based on selective resource allocation

Who holds the reins? Context-dependent resource allocation in the mutualism between fig trees and their fig wasp pollinators

Mutualisms are consumer-resource interactions, in which goods and services are exchanged. Biological market theory states that exchanges should be regulated by both partners. However, most studies on mutualisms are one-sided, focusing on the control exercised by host organisms on their symbionts. In the brood-site pollination mutualism between fig trees and their symbiont wasp pollinators, galled flowers are development sites for pollinator larvae and are exchanged for pollination services. We determined if pollinator galls influenced resource allocation to fig inflorescences called syconia and considered feedbacks from the host tree. We experimentally produced syconia containing only seeds (S), only pollinator galls (G) or seeds and galls (SG) with varying number of introduced female pollinator wasps, i.e., foundress wasps. Biomass allocation to syconia was affected by foundress numbers and treatment groups; SG treatments received highest biomass allocation at low foundress numbers, and both G and SG treatments at high foundress numbers. Seeds are important determinants of allocation at low foundress numbers; galls are likely more influential at high foundress numbers. Most allocation in the G and SG treatment was to the syconium wall, likely as protection from parasitoids and temperature/humidity fluctuations. Dry mass of individual seeds and wasps (except at low foundress numbers) was unchanged between treatment groups, indicating seeds and wasps regulate resource flow into them, with lower flow into galls containing the smaller males compared to females commensurate with sexual dimorphism. We demonstrate the importance of considering the direct role of symbionts in accessing resources and controlling exchanges within mutualisms.

Diversity and metabolic potentials of microbial communities associated with pollinator and cheater fig wasps in fig-fig wasp mutualism system

Microbial symbionts can influence a myriad of insect behavioral and physiological traits. However, how microbial communities may shape or be shaped by insect interactions with plants and neighboring species remains underexplored. The fig-fig wasp mutualism system offers a unique model to study the roles of microbiome in the interactions between the plants and co-habiting insects because a confined fig environment is shared by two fig wasp species, the pollinator wasp (Eupristina altissima and Eupristina verticillata) and the cheater wasp (Eupristina sp1 and Eupristina sp2). Here, we performed whole genome resequencing (WGS) on 48 individual fig wasps (Eupristina spp.) from Yunnan, China, to reveal the phylogenetic relationship and genetic divergence between pollinator and congeneric cheater wasps associated with the Ficus trees. We then extracted metagenomic sequences to explore the compositions, network structures, and functional capabilities of microbial communities associated with these wasps. We found that the cheaters and pollinators from the same fig species are sister species, which are highly genetically divergent. Fig wasps harbor diverse but stable microbial communities. Fig species dominate over the fig wasp genotype in shaping the bacterial and fungal communities. Variation in microbial communities may be partially explained by the filtering effect from fig and phylogeny of fig wasps. It is worth noting that cheaters have similar microbial communities to their sister pollinators, which may allow cheaters to coexist and gain resources from the same fig species. In terms of metabolic capabilities, some bacteria such as Desulfovibrio and Lachnospiraceae are candidates involved in the nutritional uptake of fig wasps. Our results provide novel insights into how microbiome community and metabolic functions may couple with the fig-wasp mutualistic systems.

Pollinating fig wasps' simple solutions to complex sex ratio problems: a review

Local mate competition (LMC) favours female biased clutch sex ratios because it reduces competition between brothers and provides extra mating opportunities for sons. Fig wasps seem to fit LMC model assumptions and lay female-biased sex ratios as predicted. These female biased sex ratios increase fitness greatly. In line with predictions, their sex ratios become less female-biased as the number of mothers laying in the same fig increases. However, this variation results in comparatively small fitness benefits compared to just biased ratios and data suggest substantial mismatches with LMC theory. The mismatches are due to several factors. (1) Multiple foundresses typically lay too many daughters. (2) Single foundress sex ratios are explained by sequential oviposition and ladies-last models. (3) Mortality that typically exceeds 10% may decouple the link between primary sex ratios, the focus of model predictions, and secondary sex ratios of adult wasps that are counted by researchers. (4) Model assumptions are frequently violated: (a) clutch sizes are unequal, (b) oviposition may not be simultaneous (c) cryptic/multiple wasp species inhabit the same host, (d) foundress numbers are systematically undercounted, (e) inbreeding coefficient calculations are inaccurate, and (f) male wasps sometimes disperse. These data and calculations suggest that alternative explanations must be considered seriously. Substantial data show that wasps typically lay most of their male eggs first followed by mostly female eggs require a new approach. These "slope" strategies result in more accurate sex ratios that are automatically adjusted to foundress number, own and relative clutch sizes and to sequential clutches. This effect will alter sex ratios in all species once the egg capacity of a fig is crossed or when interference reduces clutch sizes. In addition to this passive response, the females of about half the studied species have a conditional response that reduces female bias under higher foundress numbers by laying more sons. Therefore, wasps seem to use a very simple strategy that increases their fitness. Natural selection could have optimized parameters of the slope strategy and possibly the existence of the slope strategy itself. Variation in the slope strategy that is the result of natural selection is adaptive. Research should therefore focus on quantifying variables of this slope strategy. Currently, it is unclear how much of the variation is adaptive as opposed to being coincidental by-products.

The evolution of parasitism from mutualism in wasps pollinating the fig, Ficus microcarpa, in Yunnan Province, China

Theory identifies factors that can undermine the evolutionary stability of mutualisms. However, theory's relevance to mutualism stability in nature is controversial. Detailed comparative studies of parasitic species that are embedded within otherwise mutualistic taxa (e.g., fig pollinator wasps) can identify factors that potentially promote or undermine mutualism stability. We describe results from behavioral, morphological, phylogenetic, and experimental studies of two functionally distinct, but closely related, Eupristina wasp species associated with the monoecious host fig, Ficus microcarpa, in Yunnan Province, China. One (Eupristina verticillata) is a competent pollinator exhibiting morphologies and behaviors consistent with observed seed production. The other (Eupristina sp.) lacks these traits, and dramatically reduces both female and male reproductive success of its host. Furthermore, observations and experiments indicate that individuals of this parasitic species exhibit greater relative fitness than the pollinators, in both indirect competition (individual wasps in separate fig inflorescences) and direct competition (wasps of both species within the same fig). Moreover, phylogenetic analyses suggest that these two Eupristina species are sister taxa. By the strictest definition, the nonpollinating species represents a "cheater" that has descended from a beneficial pollinating mutualist. In sharp contrast to all 15 existing studies of actively pollinated figs and their wasps, the local F. microcarpa exhibit no evidence for host sanctions that effectively reduce the relative fitness of wasps that do not pollinate. We suggest that the lack of sanctions in the local hosts promotes the loss of specialized morphologies and behaviors crucial for pollination and, thereby, the evolution of cheating.

Fitness costs for fig wasps that fail to pollinate their host Ficus perforata

Mutualisms are of fundamental ecological importance, but risk breaking down if one partner stops paying the costs yet still takes the benefits of the interaction. To prevent such cheating, many mutualisms have mechanisms that lower the fitness of uncooperative symbionts, often termed host sanctions. In mutualisms where the interacting partners are species-specific, we would expect to see coevolution of the levels of host sanctions and partner cooperation across species-pairs. In the mutualism between fig trees and their species-specific pollinating fig wasps, host sanctions vary greatly in strength, and wasp cooperation levels vary accordingly. Here I show experimentally that in Panamanian Ficus perforata (section Urostigma, Americana) there are fitness costs for wasps that do not pollinate. These fitness costs are caused by a combination of abortions of unpollinated figs and reduced proportion of wasp larvae that successfully develop to adults. The relative fitness of wasps that do not pollinate compared to wasps that pollinate is 0.59, leading to the intermediate sanction strength 0.41. Next, by screening pollinators of F. perforata I found that 1.9% of wasp individuals in natural populations failed to carry pollen. Across five actively pollinated Neotropical fig species and their pollinators, fig species with stronger host sanctions had fewer uncooperative wasps, as would be expected if sanctions promote cooperation.

An early gall-inducing parasitic wasp adversely affects the fitness of its host Ficus tree but not the pollinator

The fig tree-fig pollinator mutualism is one of the most tightly knit symbiotic systems. The research on the ecology of non-pollinators which exploit the mutualism without providing services to the host is very limited and conclusions about the role they play in the maintenance of this mutualism are full of contradictions. The non-pollinating fig wasps species are highly diverse in their feeding habit and ecological function, which may result in different consequences on the mutualism. Sycophaga testacea is an early-ovipositing galler hosted by Ficus racemosa, which is a potencial competitor to the pollinators as they use the same female flowers in the fig as their ovipositing sites. In this study, we investigate the effect of S. testacea on the production of both pollinator and fig tree with a field control experiment. Seed production is decreased significantly when the figs were parasitized, while the offspring production of the pollinator is not significantly affected, which indicates that this galler species has a harmful effect on the fitness of its host fig tree but not the pollinator. The overall development ratio of the galls is decreased significantly when the figs were parasitized, and we found that the intrinsic low development ratio of S. testacea is responsible for the decrease in the overall development ratio.

Stability in fig tree–fig wasp mutualisms: how to be a cooperative fig wasp

Fig tree–fig wasp mutualisms are diverse and underpin much biodiversity. The wasps (Agaonidae) are the sole pollinators of the trees (Ficus). Figs are enclosed inflorescences, each of which contains many small flowers. Female wasps (foundresses) enter receptive figs to spread pollen and to lay their eggs individually into fig flowers. As they oviposit, wasps also inject chemicals that transform individual flower ovaries into galls that will feed and house wasp offspring. For fig tree–fig wasp mutualisms to persist, the trees must set seed; therefore, the wasps have both to pollinate and to fail to gall all flower ovaries. However, wasps that avoid pollination costs and/or gall all flowers are predicted to outcompete more cooperative conspecifics, resulting in destabilisation of the mutualism. Here, I review the literature on why wasps pollinate by focusing on how trees reduce investment to unpollinated figs, resulting in ‘sanctions’ to wasps that fail to pollinate via reduced production of offspring. I also review the mechanisms that prevent wasps from galling all flowers, mainly those in monoecious Ficus, that also result in wasps predominantly galling longer flowers whilst leaving shorter flowers to become seeds. I make suggestions for future work and conclude by reaffirming why multiple processes promote stability in fig tree–fig wasp mutualisms.

Foliar endophytic fungi alter patterns of nitrogen uptake and distribution in Theobroma cacao

Colonization by foliar endophytic fungi can affect the expression of host plant defenses and other ecologically important traits. However, whether endophyte colonization affects the uptake or redistribution of resources within and among host plant tissues remains unstudied. We inoculated leaves of Theobroma cacao with four common colonizers that range in their effect from protective to pathogenic (Colletotrichum tropicale, Pestalotiopsis sp., Colletotrichum theobromicola, or Phytophthora palmivora). We pulsed the soil with nitrogen-15 (¹⁵ N) and then traced ¹⁵ N uptake and its subsequent distribution to whole plants and individual leaves. At a whole-plant level, C. tropicale-inoculated plants showed significantly greater ¹⁵ N uptake than endophyte-free plants did in the same pot. Among leaves within plants, younger leaves were particularly enriched in ¹⁵ N, but endophyte inoculation at the individual leaf level did not alter ¹⁵ N distribution within plants. However, leaves co-inoculated with pathogenic Phytophthora and protective C. tropicale experienced significantly elevated ¹⁵ N content as pathogen damage increased, compared with leaves inoculated only with the pathogen. Further, endophyte-pathogen co-infection also increased total plant biomass. Our results indicate that colonization by foliar endophytes significantly affects N uptake and distribution among and within host plants in ways that appear to be context dependent on other microbiome components.

Foundress numbers and the timing of selective events during interactions between figs and fig wasps

In intimate mutualisms between hosts and symbionts, selection can act repeatedly over the development times of the interacting individuals. Although much is now known about the overall ecological conditions that favor the evolution of mutualism, a current challenge is to understand how natural selection acts on the number and kinds of partners to shape the evolution and stability of these interactions. Using the obligate fig-fig wasp mutualism, our experiments showed that the proportion of figs developed to maturity increased quickly to 1.0 as the number of foundresses increased, regardless of whether the foundresses carried pollen. Selection against pollen-free wasps did not occur at this early stage in fig development. Within figs that developed, the proportion of galls producing adult wasps remained high as the number of pollen-carrying foundresses increases. In contrast, the proportion of galls producing adult wasps decreased as the number of pollen-free foundresses increased. Viable seed production increased as the number or proportion of pollen-carrying foundresses increased, but the average number of wasp offspring per pollen-carrying foundress was highest when she was the sole foundress. These results show that figs and their pollinator wasps differ in how fitness effects are distributed throughout the development of the interaction and depend on the number and proportion of pollen-carrying foundresses contributing to the interaction. These results suggest that temporal fluctuations in the local number and proportion of pollen-carrying wasps available to enter figs are likely to have strong but different effects on the figs and the wasps.

Host investment into symbiosis varies among genotypes of the legume Acmispon strigosus, but host sanctions are uniform

Efficient host control predicts the extirpation of ineffective symbionts, but they are nonetheless widespread in nature. We tested three hypotheses for the maintenance of symbiotic variation in rhizobia that associate with a native legume: partner mismatch between host and symbiont, such that symbiont effectiveness varies with host genotype; resource satiation, whereby extrinsic sources of nutrients relax host control; and variation in host control among host genotypes. We inoculated Acmispon strigosus from six populations with three Bradyrhizobium strains that vary in symbiotic effectiveness on sympatric hosts. We measured proxies of host and symbiont fitness in single- and co-inoculations under fertilization treatments of zero added nitrogen (N) and near-growth-saturating N. We examined two components of host control: 'host investment' into nodule size during single- and co-inoculations, and 'host sanctions' against less effective strains during co-inoculations. The Bradyrhizobium strains displayed conserved growth effects on hosts, and host control did not decline under experimental fertilization. Host sanctions were robust in all hosts, but host lines from different populations varied significantly in measures of host investment in both single- and co-inoculation experiments. Variation in host investment could promote variation in symbiotic effectiveness and prevent the extinction of ineffective Bradyrhizobium from natural populations.

Fitness reduction for uncooperative fig wasps through reduced offspring size: a third component of host sanctions

Mutually beneficial interactions between two species-mutualisms-are ancient, diverse, and of fundamental ecological importance. Nonetheless, factors that prevent one partner from reaping the benefits of the interaction without paying the cost are still poorly understood. Fig trees and their unique pollinators, fig wasps, present a powerful model system for studying mutualism stability. Both partners depend completely on each other for reproduction, cooperation levels can be manipulated, and the resulting field-based fitness quantified. Previous work has shown that fig trees can impose two types of host sanctions that reduce the fitness of wasps that do not pollinate: (1) fig abortion, which kills all developing larvae, and (2) reduced number of wasp offspring in figs that are not aborted. Here we demonstrate a third component of host sanctions. Through manipulative field experiments, we show that for four of five studied species, offspring of pollen-free foundresses are only 50-90% the size of offspring of pollinating foundresses. We further show that in all four studied species, smaller wasps are less likely to reach and enter a flowering fig to become foundresses themselves. Therefore, the experimentally determined size reduction of offspring is estimated to cause an additional reduction of up to 80% in fitness for a pollen-free foundress. We determine that the size reduction of pollen-free offspring acts on the level of the entire fig fruit rather than on individual flowers. These results show that estimates of the fitness effect of host sanctions on uncooperative symbionts should consider not only offspring quantity but also offspring quality. We discuss implications beyond the fig tree-fig wasp mutualism.

Carbon allocation and competition maintain variation in plant root mutualisms

Plants engage in multiple root symbioses that offer varying degrees of benefit. We asked how variation in partner quality persists using a resource-ratio model of population growth. We considered the plant's ability to preferentially allocate carbon to mutualists and competition for plant carbon between mutualist and nonmutualist symbionts. We treated carbon as two nutritionally interchangeable, but temporally separated, resources-carbon allocated indiscriminately for the construction of the symbiosis, and carbon preferentially allocated to the mutualist after symbiosis establishment and assessment. This approach demonstrated that coexistence of mutualists and nonmutualists is possible when fidelity of the plant to the mutualist and the cost of mutualism mediate resource competition. Furthermore, it allowed us to trace symbiont population dynamics given varying degrees of carbon allocation. Specifically, coexistence occurs at intermediate levels of preferential allocation. Our findings are consistent with previous empirical studies as well the application of biological market theory to plantroot symbioses.

Tracking plant preference for higher-quality mycorrhizal symbionts under varying CO2 conditions over multiple generations

The symbiosis between plants and root-colonizing arbuscular mycorrhizal (AM) fungi is one of the most ecologically important examples of interspecific cooperation in the world. AM fungi provide benefits to plants; in return plants allocate carbon resources to fungi, preferentially allocating more resources to higher-quality fungi. However, preferential allocations from plants to symbionts may vary with environmental context, particularly when resource availability affects the relative value of symbiotic services. We ask how differences in atmospheric CO 2-levels influence root colonization dynamics between AMF species that differ in their quality as symbiotic partners. We find that with increasing CO 2-conditions and over multiple plant generations, the more beneficial fungal species is able to achieve a relatively higher abundance. This suggests that increasing atmospheric carbon supply enables plants to more effectively allocate carbon to higher-quality mutualists, and over time helps reduce lower-quality AM abundance. Our results illustrate how environmental context may affect the extent to which organisms structure interactions with their mutualistic partners and have potential implications for mutualism stability and persistence under global change.

Limiting the cost of mutualism: the defensive role of elongated gynophore in the leafflower-moth mutualism

Mutualisms are interactions from which both partners benefit but may collapse if mutualists' costs and benefits are not aligned. Host sanctions are one mechanism whereby hosts selectively allocate resources to the more cooperative partners and thereby reduce the fitness of overexploiters; however, many mutualisms lack apparent means of host sanctions. In mutualisms between plants and pollinating seed parasites, such as those between leafflowers and leafflower moths, pollinators consume subsets of the seeds as larval food in return for their pollination service. Plants may select against overexploiters by selectively aborting flowers with a heavy egg load, but in many leafflower species, seeds are fully eaten in some fruits, suggesting that such a mechanism is not present in all species. Instead, the fruits of Breynia vitis-idaea have stalk-like structures (gynophore) through which early-instar moth larvae must bore to reach seeds. Examination of moth mortality in fruits with different gynophore lengths suggested that fruits with longer gynophore had higher moth mortality and, therefore, less seed damage. Most moth mortality occurred at the egg stage or as early larval instar before moths reached the seeds, consistent with the view that gynophore functions to prevent moth access to seeds. Gynophore length was unaffected by plant size, extent of moth oviposition, or geography; thus, it is most likely genetically controlled. Because gynophores do not elongate in related species whose pollinators oviposit directly into the ovary, the gynophore in B. vitis-idaea may have evolved as a defense to limit the cost of the mutualism.

Why mutualist partners vary in quality: mutation-selection balance and incentives to cheat in the fig tree-fig wasp mutualism

Mutualisms between species are ecologically ubiquitous but evolutionarily puzzling. Host discrimination mechanisms that reduce the fitness of uncooperative symbionts can stabilise mutualism against collapse, but also present a paradox - if discrimination is effective, why do uncooperative symbionts persist? Here, we test whether mutations or fitness benefits of cheating best explain the prevalence of uncooperative wasps in the fig tree-fig wasp mutualism. By combining theory with field-collected data we demonstrate that the proportions of pollen-free wasps of strongly discriminating hosts are reached with reasonable mutation rates. In contrast, in weakly discriminating hosts, the required mutation rates, assuming a single locus, are untenably high, but the required cheater advantages fall within expected ranges. We propose that when discrimination is weak, uncooperative symbionts proliferate until they reach the equilibrium proportion that balances costs and benefits of cheating. Our results suggest that mechanisms that resolve the paradox of uncooperative symbionts differ among host species.

Selective resource allocation may promote a sex ratio in pollinator fig wasps more beneficial for the host tree

Mutualisms play a key role in most ecosystems, yet the mechanisms that prevent overexploitation of the mutualistic relationship are still poorly understood. In the mutualism between fig trees and their pollinating wasps both partners depend on each other. Fig trees benefit from female wasps that disperse their pollen, whereas wasps frequently benefit from a higher ratio of male offspring. Here we use manipulative field experiments to address whether host trees (Ficus racemosa) can influence the offspring sex ratio of the pollinator wasp. We controlled wasp matings; virgin wasps can lay only male eggs. We found that virgin foundress wasps had fewer offspring than mated foundresses. This was not caused by virgin wasps having a shorter lifespan, or laying fewer eggs. Instead, male wasp larvae were more likely to die during development. Additionally, male eggs were deposited in flowers of equal style length to those of female eggs, yet emerged from galls with shorter pedicels than those of female wasps. We suggest that male larvae are either allocated less resources by the tree, or are less able to attract resources, during development. If the tree orchestrates this difference it would promote a more female-biased wasp brood, thus increasing the tree’s fitness.