Russulales News / Characteristics of the russuloid fungi

Features of basidia have suffered especially from the absence of appropriate descriptive methods. Dimensions of basidia - like those of the other hymenial cells - differ too much to permit working with mean values for practical identification purposes.

Morphological variations of basidia have always been neglected in the study of Agaricales, in sharp contrast to their important taxonomic use in Aphyllophorales. Josserand (1956: 83-84) states that "la forme de la baside est rarement importante pour le spécificateur car elle est peu vadable d'une espèce à une autre. Sa monotonie est cependant parfois interrompue, non pas quand on passe d'une espèce à une autre, mais bien d'un genre à un autre". For Amanita, Bas (1969: 321) observes "the characters of the basidia are of little taxonomic importance". Neuhoff (Die Milchlinge, 1956: 48) also notes "die Basidien sind von ziemlich einheitlicher Form und Grösse". Lennox (1979: 47), in a study of collybioid genera, concludes "rarely are the basidia distinctive enough in size or shape to be of taxonomic importance".

For Russula, Romagnesi was the first to notice the variation in dimensions of basidia, but he also observes (1967: 47) "les caractères qualificatifs sont loin de valoir ceux de la taille". He considers the basidia to be always claviform. As a result, Romagnesi - as well as Singer and most other students of Russulales - only give the upper and lower limits for basidium. dimensions and the number of sterigmata. Illustrations of basidia are usually also omitted.

It is only when one has the courage and patience to make precise illustrations of the basidia of every studied specimen that differences start to show. Illustrations permit easy comparison and correct interpretation of eventual differences. For the basidia of the Central-African Russula, the drawings revealed an unexpected morphological variation. The difficulty of expressing these differences in a text results from the poverty of the existing descriptive vocabulary (claviform, subcylindric, piriform…). I, therefore, suggest that a series of illustrations of different basidia-types should be used as a sort of guide for descriptive purposes. I presented a first series of basidium types, reflecting mainly the existing variation range in Central African Russulae at the 4th International Mycological Congress at Regensburg in 199.

Variation in basidium features for Russula. All illustrations result from a revision of Central African Russulae (Buyck, 1989).

In delimiting the different basidium types, one should make use of the size and morphology of the basidia and of the sterigmata. The final publication of an illustrated reference series of basidium types should be based on detailed statistical data and cover the existing range of basidium types in Russulales and might later eventually be extended to other genera of Agaricales as well. The here presented illustrations have not yet this ambition. I simply want to convince the reader of the existence of an unexpected variation in basidial features and stimulate better and more accurate observation. In the mean time, precise illustrations of basidia should be provided in all future descriptions of Russulales.

I would like to insist on the fact that it is not my aim to use these illustrations to characterise a certain Russula by a certain type of basidium. (although this is often possible!). My main objective consists in providing more accurate description possibilities for basidia.

From my own experience with the basidia of Russulaceae, I can safely say that the features of basidia are fairly constant for a given species, and thus often permit characterisation. Many subsections and sections are characterised by the occurrence of one dominant type of basidium. Similar accurate and comparable illustrations do not exist for the temperate species, but preliminary research on European and American taxa (Buyck, unpubl.) suggest important taxonomic and systematic implications. From my work on tropical Russulales, it is suggested that narrow, almost cylindrical and small basidia are the most primitive condition in Russulales. Progressive inflation and/or elongation of the basidia and sterigmata has resulted in several types of basidia. In some taxa a certain percentage of bisporous basidia is constantly found and the reduction of the number of sterigmata should be regarded as an evolved feature. These evolutions result in the production of a considerable lower number of sexual spores in a single basidioma. This supports earlier hypotheses on poor spore dispersal for ectomycorrhizal fungi (Buyck, 1991) and is reinforced by the fact that ectomycorrhizal fungi do not produce asexual spores (Hutchison, 1989). For saprophytic fungi, on the contrary, the evolution of a more efficient arrangement of the hymenophore (Ingold, 1939) and the production of asexual spores result in the production of a higher number of spores.

Further examples of variation in basidium features for Russula. With the exception of Russula crassotunicata, all illustrations result from a revision of Central African Russulae (Buyck, 1989).

What to observe and how to do it?

Basidia