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Appendix: genetic correlations between mutualistic species
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The relative standard understanding of the evolution of interspecific cooperation is that
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cooperative species have a direct or indirect benefit exchange. Accordingly, these species could
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potentially increase their fitness advantage by stably maintaining a cooperative interaction
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(Bronstein 2001; Trivers 1971). The evolution of interspecific cooperation, however, may also
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be understood through the framework of genetic variation: ‘the correlation between
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mutualistic partners has an effect similar to the genetic correlations within species that form
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the standard coefficients of relatedness in kin selection models’ (Frank 1994).
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In mutualism systems, if the symbionts are vertically transmitted by the host, kin selection
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can explain why the individual symbionts decrease their virulence to benefits host, simply
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because the neighbours of cooperative individuals are genetically correlated with those
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symbionts. In vertically transmitted mutualisms, the virulence decrease of the individual
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symbionts that benefits the host will also provide more resources or lessened host resistance to
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their neighbours, potentially indirectly helping these cooperative actors to transmit their gene
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frequency since the cooperative actors are highly genetically correlated with their neighbours.
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Many mutualism systems, however, are not of the vertical transmission systems. Frank’s model
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showed that genetic variation of the symbiont and host created by a special spatial structure
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may also create a genetic correlation between the host and symbionts.
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In fig–fig wasp mutualism, the fewer foundresses (fig wasps in the cavities of fig fruits) being
natal figs, the higher probability will be for inbreeding or local mate in their offspring. Though
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other reports have found that fig wasps may have a long distance dispersal from their host figs
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(Nason et al. 1998), the predispersal mating mechanism still ensures a higher inbreeding
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possibility and local mate possibility among the offspring in lower foundress syconia. The local
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mate among the offspring of the foundresses might provide another one explanation to the
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evolution of mutualisms within the framework of genetic correlation. Individuals that decrease
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the virulence to benefit to the host will also advantage the offspring of its neighbours; however,
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such altruistic behaviour indirectly benefits its gene transmission, because its own offspring will
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mate with the offspring of their neighbours that benefited from the less parasitized host.
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Obviously, the local mate probability will greatly depend on the foundress number.
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Though the precise mechanism on how the relatedness between fig and pollinator wasp is
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created through genetic variation has not been exactly identified, fig wasps that be able to
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adjust their behaviour due to genetic variation resulting from the change of foundress number
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per syconium (e.g. sex ratio adjustment) have been widely demonstrated (Herre 1985; Herre
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1987; Moore et al. 2002). Understanding the relatedness in terms of genetic variation therefore
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should not be theoretically excluded in fig–fig wasp systems (Frank pers. Commm.).
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Bronstein, J. L. 2001. The costs of mutualism. The American Zoologist 41:825-839.
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Frank, S. A. 1994. Genetics of Mutualism - the Evolution of Altruism between Species. Journal of Theoretical
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Biology 170:393-400.
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Herre, E. A. 1985. Sex ratio adjustment in fig wasps. Science 228:896-898.
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—. 1987. Optimality, plasticity and selective regime in fig wasp sex ratios. Nature 329:627-629.
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Moore, J. C., S. G. Compton, M. J. Hatcher, and A. M. Dunn. 2002. Quantitative tests of sex ratio models in a
pollinating fig wasp. Animal Behaviour 64:23-32.
Nason, J. D., E. A. Herre, and J. L. Hamrick. 1998. The breeding structure of a tropical keystone plant resource.
Nature 391:685 - 687.
Trivers, R. L. 1971. Evolution of Reciprocal Altruism. Quarterly Review of Biology 46:35-&.
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