Asymmetric interaction and indeterminate fitness correlation between cooperative partners in the fig–fig wasp mutualism

Abstract

Empirical observations have shown that cooperative partners can compete for common resources, but what factors determine whether partners cooperate or compete remain unclear. Using the reciprocal fig–fig wasp mutualism, we show that nonlinear amplification of interference competition between fig wasps—which limits the fig wasps' ability to use a common resource (i.e. female flowers)—keeps the common resource unsaturated, making cooperation locally stable. When interference competition was manually prevented, the fitness correlation between figs and fig wasps went from positive to negative. This indicates that genetic relatedness or reciprocal exchange between cooperative players, which could create spatial heterogeneity or self-restraint, was not sufficient to maintain stable cooperation. Moreover, our analysis of field-collected data shows that the fitness correlation between cooperative partners varies stochastically, and that the mainly positive fitness correlation observed during the warm season shifts to a negative correlation during the cold season owing to an increase in the initial oviposition efficiency of each fig wasp. This implies that the discriminative sanction of less-cooperative wasps (i.e. by decreasing the egg deposition efficiency per fig wasp) but reward to cooperative wasps by fig, a control of the initial value, will facilitate a stable mutualism. Our finding that asymmetric interaction leading to an indeterminate fitness interaction between symbiont (i.e. cooperative actors) and host (i.e. recipient) has the potential to explain why conflict has been empirically observed in both well-documented intraspecific and interspecific cooperation systems.

Spatial heterogeneity and host repression in fig-fig wasp mutualism

“…Therefore, style length is not causal factor explaining the abundance of unexploited flowers. We found higher production of galls in the cold season (Figure 2), with up to 60% of female flowers giving rise to galls, which confirms previous observations that pollinators can oviposit in most female flowers in both Ficus sycomorus [22,32] and other figs [11].…”

“…The cooperative interaction can turn into a conflictual one when cooperative partners are too numerous, i.e., N>b/c (b is the benefit for cooperation, c is the cost for cooperation), if spatial constraint is inadequate for preventing cooperative partners from overexploiting the commons at the expense of other involved partners [56,58,62]. In the fig/fig wasp mutualism, the trade-off between figs and fig wasps can be affected by the total number of female flowers and the proportion of those that are pollinated or parasitized [22,29,41]. This latter proportion is determined by foundress number and utilization efficiency of each foundress, which will be greatly affected by ecological factors including competition and interference within the syconium but also temperature and humidity.…”

“…The pollination and oviposition efficiency of foundresses might greatly affect the trade-off between the figs and pollinators wasps [22,28,29]. We also examined whether the pollination and oviposition efficiency of foundresses varies seasonally.…”

“…However, many Ficus species receive multiple foundresses, and foundress numbers can vary dramatically between species [15,21]. Density dependent interference competition between the foundresses together with high temperature may be responsible for per capita reductions in the numbers of offspring in figs entered by increasing numbers of females [16,22]. However, this hypothesis does not efficiently explain why direct conflict could be observed in both warm and cold seasons occasionally [22], and it also does not make clear on if the interference competition and high temperature could exclusively explain the fig-fig wasp mutualism stability.…”

Evolutionary stability analysis of asymmetric hawk-dove game considering the impact of common resource

“…, 从而均衡状态可能被打破, 并随之建立新的均衡状态。 经典的"鹰鸽博弈"理论模型不能解释多种 均衡状态之间的相互转化, 出现这种困境的原因很 可能是经典模型仅考虑了博弈双方对等的情况。而 在现实的合作系统中, 博弈双方大都具有非对称性 相互关系, 比如蜂后(queens)与工蜂(workers)、猴王 与次级猴、双亲与子代、强者与弱者、雌性与雄性 等 (Maynard Smith, 1982)。 大量的实验观测及数据分 析显示，几乎所有的经典合作模式系统都存在着不 同程度的非对称性相互关系 (Heinsohn & Packer, 1995;Reeve, 1992;Wang et al, 2009Wang et al, , 2010Wang et al, , 2011, 这种非对称相互关系, 很可能是不同均衡状态之间 相互转化的本质原因。 本文基于经典的"鹰鸽博弈"模型, 同时考虑 非对称性相互关系和资源压力的影响, 建立了具有 四策略(实力强且合作、实力强且不合作、实力弱且 合作和实力弱且不合作) 的非对称博弈模型。结合 演化博弈理论及动力系统稳定性理论分析该系统 的演化行为发现：在系统达到稳定状态时, 四种策 略的比例变化显著地依赖于博弈双方的强弱之比, 资源压力及冲突的单位成本收益。同时发现, 当资 源短缺时, 随着博弈双方的强弱之比的变化, 经典 的"智猪博弈"将与"鹰鸽博弈"之间形成相互 转化关系, 从而为合作系统的不同均衡状态之间相 互转化给出了一个动力学解释。 1 建立模型 Smith, 1982;Drew et al, 2002), Smith, 1982;Drew et al, 2002)。 而在现实的合作系统 中博弈双方的实力一般是不对等的(即实力强弱的 非对称) (Heinsohn & Packer, 1995;Reeve, 1992;Wang et al, 2009Wang et al, , 2010Wang et al, , 2011 …”