Analysis of Causation Behind Imbalanced and Unequal Reliance in Mutualisms

*Originally written as an assignment for a biology course. Most of these are hypotheses I tried to synthesize. Many are unsubstantiated as combined information.

Clark's Nutcracker (Nucifraga columbiana)

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Analysis of Causation Behind Imbalanced and Unequal Reliance in Mutualisms

Introduction

While many processes led to symbiotic interactions between different organisms across systems, few have captured evolutionary studies as much as those of mutualistic benefit where evolution led to an exchange of gain between two species (Dethlefsen et al. 2007). Instructors have utilized these mutualistic interactions to introduce students to current concepts of evolution, ecology and speciation at all levels of education. However, while past research and current science education have led to the superficial definition of mutualism as a doctrine of equal exchange in dependence between separate parties (West et al. 2006), the reality is much less defined than what is currently taught in collegiate classrooms. In reality, mutualistic relationships between species are often unequal, imbalanced, and may also deviate toward one party over the other. Such imbalances may still occur in the face of survival, reproduction, and speciation, with examples to support their occurrence. However, the reasoning behind such deviations and imbalances is unclear and often only inferred from single, individualized studies between various mutualists. What is known is that often mutualisms arise as a result of increased fitness in separate species with mutual benefit (Baoming & Bever 2016). Then, as time progressed onwards changes may occur between two parties which were mutualistic. Such changes may have led to one particular party becoming more dependent on the other, while the other may have adapted to a secondary and less dependent lifestyle. Analyses revealed that these changes can potentially be summarized into three main factors: coevolution, resource limitation, and speciation. It is very likely that the high numbers of unequal, imbalanced, and differentiated reliance within mutualistic relationships arose as a result of changes within these three factors via non-progressive evolutionary tinkering.

Research Analysis and Reasoning

Varied or Imbalanced Mutualist Coevolution:

One explanation for deviations in dependence between mutualists may have arisen as variations between two different parties caused by imbalanced coevolution. An example occurred between the whitebark pine (Pinus albicaulis) and the Clark’s Nutcracker (Nucifraga columbiana) (Tomback 1982). This mutualism is the predominant driving factor in the spread and reproduction of whitebark pine (Tomback & Linhart 1990), which cannot regenerate without aid of the Clark’s Nutcracker’s caching behavior (Tomback 1982). We know of the relative length of such mutualistic coevolution via the observation of the whitebark pine’s spread into formerly glaciated areas (Richardson et al. 2002). But while both parties benefit from the exchange (Tomback 1982), the dependence between the two parties is unequal. For one, whitebark pine is only found within small sections of the Clark’s Nutcracker’s range (Clague & Mathewes 1989). In areas with whitebark pine nutcrackers show favoritism toward them possibly as a result of higher seed nutrient content (Schaming 2015). However, Clark’s Nutcrackers exist in whitebark pine-absent regions. This suggests that the whitebark pine as an obligate mutualist of the Clark’s Nutcracker while nutcracker is only a facultative mutualist of the whitebark pine, being able to feed on a range of food sources (Vander Wall & Balda 1977). A limitation to the spread of whitebark pine also resulted from this inequality. Under current climates, whitebark pine is generally found in the highest elevations of western mountains (Tomback & Linhart 1990), which favors nutcrackers for the latter (and many other animal species) is exposed to food sources at highest of altitudes (Powell & Logan 2007). If mutualism was equal, then the widespread distribution of the Clark’s Nutcracker should increase spread of whitebark pine trees across much of the western mountains. However, the lack of homogeneity in whitebark pine haplotype frequencies among populations revealed nutcrackers’ failure to distribute seeds greater than 20km (Richardson et al. 2002). A similar imbalanced coevolutionary mutualism has also occurred between pinyon pines (Pinus edulis) and Pinyon Jays (Gymnorhinus cyanoephalus) (Ligon 1978). Likewise, research suggesting possible imbalanced mutualisms between co-evolved birds and bird-dispersed trees were found in up to 19 species of Strobus pine (Tomback & Linhart 1990). These explanations are also in accordance with differentiated dependence across the co-evolution of the two mutualists where one of the mutualists co-evolved to a lesser extent in dependence than the other.

Disruptions in Resource Limitations within Mutualisms:

A second explanation as to the inequalities of biological mutualisms lies in the selection for strong relationships between different species resulting from resource limitations. For example, mycorrhizal partners and plants often created mutualistic situations via the exchange of carbon and phosphorus; the latter of which was often a limiting resource in various habitats. Host plant preferentially distributed a higher amount of carbon to the roots which associate with the fungi and in return, the fungi increased delivery of phosphorus. Such helped stabilize mutualisms in resource-limited environments (Baoming & Bever 2016). However, in situations where the limiting resource’s availability increased, plants’ preferential allocation declines (Baoming & Bever 2016). The context dependence of some organisms, such as beneficial mycorrhizas, caused fluxes in populations in inverse correlation to the presence or absence of the limiting resource (Hoeksema et al. 2010). As beneficial mutualists declined, it elevated the probability that nonbeneficial competitors increase, lowering mutualism (Bever 2015). Therefore, in areas with the increase of a normally limiting resource, mutualistic relationships may falter, with one part of the partnership being naturally selected against (e.g. fungi) while the other partner continues to proliferate (e.g. the plant) due to less competition on the latter. The resulting situation is one in which one partner is discretionally reliant on the other, while the second is obligatorily reliant on the first, similar to the whitebark pine and nutcracker example. Over time, by having variations and possible fluxes in the amount of limiting resources, some mutualisms became imbalanced due to selection pressures despite beneficial exchange on both sides.

Speciation within Mutualistic Relationships:

The last and particularly recent suggestion as to the imbalance between two parties in mutualism is speciation within one party of the exchange. Van Leuvan et al. (2014) suggested a species divergence within the mutualistic relationship between cicadas of genus Tettigades and its microbial endosymbionts. Tettigades cicadas generally form important survival-based mutualistic relationships with two microbes, Sulcia and Hodgkinia. Normally, Hodgkinia contained one genome which encoded genes for histidine and methionine critical for organismal function (Van Leuvan et al. 2014; Telfer 1991). However, in some individual Tettigades cicadas the Hodgkinia within their bodies have speciated into two different versions via a novel bacterial lineage splitting event (Van Leuvan et al. 2014). Such speciation resulted in cytologically unique but metabolically interdependent species in which the two new genotypes of Hodgkinia partition coding pathways, resulting in both species being required for the cicada’s histidine and methionine production (Figure 1). The explanation lies in symbiont function degradation in long term mutualistic or endosymbionic events (Baumann et al. 1996). To compensate for such degradation, one symbiont may be replaced altogether with a new symbiont (Koga et al. 2013). Currently, such speciation was regarded as non-detrimental. However, further speciation events may lead to even greater shifts in dependence, resulting in increased deviation from equal exchanges. As shown by Kikuchi et al. (2007), many of these microbes can survive in external environments, which may also cause decreased reliance on the “host” (e.g. the cicada). This may cause health problems for the “host” species as mutualistic microbes often function to withstand colonization by pathogenic microbes (Dillon & Dillon 2004). The end result was the new dependence of two separate symbionts for survival function (as in the case of some Tettigades sp.), resulting in cicadas’ increased symbiont dependence and an elevated level of unequal mutualistic reliance.

Conclusions

Ultimately, differential reliance within mutualistic relationships could be summarized as resulting from three different factors: varied or imbalanced mutualist coevolution, disruptions or variations in resource limitations between mutualistic parties, and speciation within mutualistic relationships. Different parties which exhibit deviated mutualisms generally have one or a combination of these variables at play. More extensive research into deviated mutualisms and facultative mutualisms may potentially reveal further syntheses and variables at play, extending beyond the three premises. Additionally, they may help answer species-specific questions as to why, despite continued mutualism, some species become extremely reliant while others are less obligatory. Comprehension of these imperfect relationships could lead to better understanding of organismal evolutionary history, biomedical research (e.g. microbial imbalance), as well as proper measures in environmental policy and conservation (e.g. saving the endangered whitebark pine). Thus, mutualism should not be viewed as the perfected “mutual exchange” in benefit. Instead to properly aid in the accurate application in and beyond the sciences, biological mutualism should be viewed as a concept with the potential for inequality. This inequality though, is merely an imperfect result of evolutionary tinkering via shifts in coevolution, varied reliance in resource limitations, and convoluted situations in speciation.

Figure 1. Graphical Abstract of Speciation Event. Graphical representation of research in identifying the genome duplication event which caused creation of two separate genotypes of Hodgkinia. The sympatric speciation event resulted in partitioning of coding pathways for histidine and methionine production, leading to cicadas’ increased reliance on number of symbionts (Van Leuvan et al. 2014).

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