ScienceDaily (Jan. 14, 2008) —
All species of termites are social insects, like ants. Entomologists
have listed over 2000 species across the world and more than one-third
of them live in Africa. This continent harbours 160 from the subfamily
Macrotermitinae. Contrary to other termites, the species of this
subfamily-cannot digest either cellulose or lignin, basic constituents
of their food plants, and therefore call on the services of a symbiotic
relationship with a higher fungus. Using roughly chewed and only
slightly digested plant material, they make a small ventilated
structure, the fungus comb or garden, on which the mycelium of a
Termitomyces, a Basidiomycete fungus of the same family as the Lepiota,
will grow.
This fungus will gradually break down the ligneous and
cellulose-bearing material amassed on the comb into substances that are
easier for the termites to assimilate. The entomologists focusing on
these fascinating social insects have long considered the symbiotic
relationship between the Macrotermitinae and the fungus Termitomyces was
one of interaction specificity, in that one species of termite
associated with a single species of fungus, following a system
elaborated by co-evolution that began several tens of millions of years
ago.
A study recently published by an international research team, jointly
involving IRD scientists, brought proof that this symbiotic relationship
is much more complex and diversified than was suspected. With the aim of
understanding better how the ties between the insect and the fungus are
forged, samples of termites and associated Termitomyces were taken in
South Africa and Senegal, from 101 colonies belonging to eight species
divided over three different genera.
Analysis of molecular variance using amplification and sequencing of
highly variable genome sequences from the termites and fungi gave
measurements of the level of symbiotic interaction between fungus and
insect depending on hierarchical taxonomic level (genus, species,
colony). Strong evidence of variability in the host-symbiont association
emerged: it was high between genera, low between species and quite high
between colonies of the same species.
Among the different species studied, only the termite Macrotermes
natalensis established a symbiotic relation specificity with a single
fungus species, whatever the colony investigated. In contrast, the three
termite species of the genus Odontotermes are capable of growing several
different species of Termitomyces. Further, bilaterally low specificity
was found in the other species of the genus Microtermes, where several
different termite species were associated with a variety of fungi. This
confirmed therefore that, in most cases, no co-evolution occurred in the
symbiotic relationship which brought the fungus into interaction with
the termite.
The secret of this diversification could lie in Termitomyces’
capacity for fructification through fruiting bodies. The investigators
advanced the hypothesis that this element of a fungus, which develops
from the mycelium then reaches the surface, where it forms its visible
part, would render the organism retrievable by termites from another
colony which would take it back to their nest to establish a new growth
site. The results indicate strongly that, rather than an
intergenerational transmission mode, like the one found in
fungus-growing ants, a process of horizontal transmission operates
between of the same given fungus between many different colonies of
termites, whether or not the latter belong to the same species.
Seeing that in Africa 90% of crop damage perpetrated by termites is
the work of fungus-growing species, better understanding of the
transmission mode of the symbiont fungus in these insect hosts is
essential. The termites of the genus Odontotermes, which this study
showed to have an aptitude for growing a variety of Termitomyces
species, can for example cause yield losses of 25% in a field of sugar
cane.
In the northern areas of Cameroon, other species attack subsistence
farmers’ staple crops of millet and sorghum. The latest control
strategies implemented for limiting damage caused by these pests now
target the fungus rather than the termite. By replacing insecticide by
fungicide, these new methods thus employ a substance less harmful for
humans. Nevertheless, these insects’ quite low interaction specificity,
making them capable of growing diverse species of fungi, could keep them
a step ahead of the fungicide control methods and thus hinder this new
approach to termite control.
Journal references
Aanen D. K., Ros V. I. D., Licht H. H. D., Mitchell J., de Beer Z.
W., Slippers B., Rouland Lefèvre Corinne, Boomsma J. J. Patterns of
interaction specificity of fungus-growing termites and Termitomyces
symbionts in South Africa Bmc Evolutionary Biology, 2007, 7, p.
NIL_1-NIL_11. doi:10.1186/1471-2148-7-115
Duur K. Aanen, Paul Eggleton, Corinne Rouland-Lefèvre, Tobias
Guldberg-Frøslev, Søren Rosendahl, and Jacobus J. Boomsma The evolution
of fungus-growing termites and their mutualistic fungal symbionts, PNAS,
2002, 99: 14887-14892 doi: 10.1073/pnas.222313099
Adapted from materials provided by
Institut de Recherche pour le Développement.
