|Publication Type:||Journal Article|
|Year of Publication:||2005|
|Authors:||Bouchard, J., Bouchard-Madrelle C.|
|Journal:||Bulletin de la Societe Zoologique de France|
|Keywords:||Diptera, Insecta, Mycetophilidae : Nematocera|
Among Diptera, Mycetophilidae are often observed on mushrooms, but Polyporus are not really attractive hosts. However, we have studied an alpine species which grows on a fungus, Phellinus tremulae, a parasite of the Poplar, Populus tremula. This fungus is infrequent in France, but has been recorded by a Mycologist, P. HEINEMANN (Bruxelles) from Haute-Maurienne-Savoie near Bonneval-sur-Arc (1979; registered samples in Jardin botanique national de Belgique, August 1978). With him, we saw that larvae of a mycetophilid (tribe Sciophilini) of the genus Sciophila devour Phellinus tremulae, even though its hymenium is relatively hard. Some preliminary observations were published in 1994 (J. BOUCHARD), but it was clear that a study needed to be carried out on a longer period, because of the gradual evolution of the woodland. In the study area, a valley oriented West-East, Poplars grow among fallen rocks in the upper border of the subalpine forest, between 1820 and 1900 m. They form a woodland of 18-20 ha which has been climbing eastwards between the rocks for about twenty years. The Mycetophilid Sciophila sp. is limited to this woodland area: Phellinus tremulae is evidently absent on Poplars below Bonneval. The question arose as to whether this complex relation between three partners can be defined and we tried to determine some controlling factors. Young Poplars, 20-25 years old, are not infested by the fungus, and Phellinus cannot be observed on the western border of this woodland, exposed to rather strong winds blowing during sunny days. Carpophores are not found on Poplars growing on wet soils and particularly near the torrent, where these trees take advantage of the best conditions for growth. On the contrary, carpophores appear on the lower parts of old trees living elsewhere, the bark of which is grooved, frequently injured, or if a branch has been broken. The spores of Phellinus tremulae are extremely small (2-3 [mu]m) and smooth. A priori, anemophily seems to be the mode of dispersal of the fungus. However, this pathway is far from the only one. After studying Sciophila for more than twenty years, it is clear to us that this animal is always related to Phellinus tremulae growing just above Bonneval-Tralenta. Such a correlation has also been observed by HEINEMANN in two other parts of our Alps (pers. comm.) and we found an other example along a high valley situated in the Queyras, a southern region of the Alps, once more at the top of the subalpine forest (1950-2000 m). The larvae of the four successive instars eat the spores and the hymenium of the carpophore, where they stay for months under a solid web secreted by the salivary glands. This web constitues a good protection against small predators; small birds generally stay near the border of the forest, eating in bushes or among grassland and we have neither seen nor heard Certhiides, the only true corticols able to eat such small preys along trunks. Possessing strong mouthparts and powerful cephalic muscles, as determined from histological sections, but with a lengthy digestion, the larvae slowly dig a superficial furrow in the fungus, leaving a wet slimy trail. They are able to restore their web rapidly, in a few minutes, if it is destroyed. The gut of the larva is always stuffed with spores and debris of hymenium, but the waste is eliminated at the end of the last larval instar. Adult Sciophila walk and can sometimes jump, but cannot fly vigorously : strong winds, which frequently blow in summer, send it away from the western border of the woodland and induce a concentration of these animals in protected areas. Direct microscopic observation of imago allows the location of spores on the body and especially along veins of the wings. Histological sections made using the trichromic method (Regaud-Masson), show many spores forming clumps in the folds of the integument or trapped between bristles of the margin of abdominal segments. They are often embedded in sticky liquids, easily stained with aniline-blue and apparently produced by the fungus and certainly harves ed on wounded trunks. The results suggest that the male and female imagoes are richly supplied with numerous and diverse sensillae. Abundant on the antennae, they are prominent on the long and complex palps, where thousands of batteries of short chemoreceptor sensillae can be seen at high magnification. Long and short bristles exist elsewhere, such as on the legs. Antennae and palps are able to detect the specific odors produced by the wounded Poplars and those of the fungus (perceptible to humans). While feeding, Sciophila favours the dispersal of spores and is the real infestive agent in newly-injured trees. After a study of the various small animals wandering on the carpophores, such a dispersal does not seem to be favoured by other species such as Collembola, mites and tiny Insects. Having studied this complex relationship for more than twenty years, we found that it tends towards a climacic equilibrium, strictly limited to the upper part of subalpine forests. However, it is clear that near Bonneval the woodland tends to slowly invade screes, proliferating between rocks and climbing towards the foot of the cliff. We regard this as a consequence of increasing temperatures. Now, it is time to study the health of the young Poplars which are not yet attacked by Phellinus tremulae and thus devoid of Sciophila. In Savoie or Queyras, such poplar islets seem to be glacial relicts. Mycologists have observed Phellinus tremulae parasitizing Populus tremula in both Palearctic and Nearctic cold countries (Scandinavia, Russia, Canada...). It is to be hoped that other workers will investigate the chemical processes of the inter-relationship that we have observed : it could serve as a model study which could provide insights concerning what is probably a symbiosis between Sciophila and Phellinus, which are clearly interdependent. The complex evolutinary interactions between these three organisms is certainly worth of further study.
Relation stable, "symbiotique", entre un diptere Sciophilide et son hote subalpin, le polypore Phellinus tremulae (Bond.), parasite de Populus tremula (L.). Stable "symbiotic" relationship between a sciophilid fly and its subalpine host, the polypore Phel