High Symbiont Relatedness Stabilizes Mutualistic Cooperation in Fungus-Growing Termites

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Science  20 Nov 2009:
Vol. 326, Issue 5956, pp. 1103-1106
DOI: 10.1126/science.1173462

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  1. Fig. 1

    Fungus cultivation by macrotermitine termites. (A) The inside of a colony of Macrotermes natalensis in South Africa. Fungus gardens are arranged in an interconnected network of chambers, each containing a mass of masticated plant substrate in which Termitomyces grows. (B) Close-up of a fungus garden of a Macrotermes sp. with multiple asexual fruiting bodies (nodules). The spores survive passage through the termite gut and are mixed with predigested plant material to be deposited as inoculated fresh garden substrate (21, 22). (C) Genetic diversity of Termitomyces symbionts associated with Macrotermes natalensis in South Africa. Each circle represents a fungal genotype. Only two colonies shared the same fungal genotype [larger circle marked with an asterisk (*)]. No within-colony variation was found in 13 multiply sampled nests (number of samples per nest indicated; n = 5.9 samples on average). (D) Experimental demonstration of positive frequency-dependent selection on Termitomyces strains in mixtures of asexual spores indicated by letters in (C) with one of them being inoculated at 50% (the x axis label strain and the darker bars) and the others at 12.5% each (pooled in the lighter bars). The expected frequencies of the dominant strain without frequency-dependent selection are given in parentheses. **P < 0.01; ***P < 0.001; ****P < 0.0001.

  2. Fig. 2

    The effects of symbiont relatedness on fungal spore yield and nodule number. (A) Representative examples of monoculture (right) and mixed culture of five strains (left) after we inoculated ~30,000 asexual spores per petri dish; (bottom) close-ups of plates. Normalized yield (B) and number of nodules (D) (relative to monocultures set at 100%) (19) against symbiont relatedness, the probability that two spores in a mixture are clonally related. Dots are averages of five replicates, bars are 95% confidence limits, and the lines are regressions (quadratic for yield, R2 = 0.956, P < 0.0001; linear for nodule number, R2 = 0.735, P < 0.0001) (19). (C) No direct mycelial antagonism was observed between two genetically different fungal strains inoculated on the same plate (left), relative to two strains of the same clone (right); the figure is a typical example of 21 strains tested in all 231 possible combinations) (19).

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