Différences de fréquences d'occurrence entre espèces de micromycètes parasitant une même espèce de plante-hôte ; procédure d'appréciation statistique

Résumé En Fr

The differences of prevalence among species of micro-fungi that parasitize a same host-plant species ; how to evaluate their statistical significance, using a newly designed test. Beyond simply recording the list of parasitic microfungi that occur within a given region -which is a first necessity -some quantitative aspects regarding the distribution of species may also be of additional interest. Among these aspects, the differences between the occurring frequencies of different species of microfungi that parasitize a same host-plant species feature special interest. The subject, however, raises a specific issue regarding the testing for statistical significance of the differences between the recorded occurrence frequencies, since the common practice, while sampling parasitic micro-fungi, involves the census of fungi occurring on host but almost never the complementary census of the absences of the same fungi. Accordingly, enumerative census are available only and not the corresponding frequencies. As knowing those frequencies is mandatory for common statistical tests to be performed, some special adaptation of those tests has to be considered beforehand. Accordingly, a modified x2 test has then been designed (see the Appendice in English) to assess appropriately the statistical significance of the gaps between the frequencies of occurrence of different species of fungi parasitizing a same host-species. This relevant methodology is then applied to the data collected during a survey of the distributions of Erysiphales, Peronosporales, Ustilaginales and Uredinales in Southern Burgundy (mainly Saône-et-Loire department). The most obvious (statistically significant) differences between the levels of occurrence of different micro-fungi species parasitic on a same given host-plant species are thus disclosed and a particular attention is devoted to the comparison between the respective frequencies of autoecious and heteroecious Rust-species that parasitize a same host-plant species. Autoecious species are generally considered more evolved than heteroecious ones and, accordingly, are logically expected to take benefit from the easier completion of their simpler, one-host life-cycle. Surprisingly, this expected advantage does not reveal statistically significant when occurrence frequencies are compared between heteroecious and autoecious species occurring on a same host, at least on the basis of our recorded data. This trend, however, needs to be substantiate by further investigations based on additionnai similar works in the same and several other regions.

Au-delà du simple recensement des espèces de champignons parasites des plantes sauvages, des aspects plus quantitatifs de leurs distributions méritent d'être considérés. Il est en particulier intéressant de mettre en évidence les différences de fréquences d'occurrence entre espèces fongiques parasites d'une même espèce végétale-hôte et de pouvoir s'assurer objectivement de la signification statistique de telles différences. Quelques points méthodologiques préalables à une étude rationnelle de cette question sont abordés avant analyse des résultats obtenus à partir d'un premier échantillonnage de plus de 200 espèces de rouilles, oïdiums, charbons et mildious récoltés sur 286 espèces de plantes-hôtes en Bourgogne méridionale. La signification évolutive qui peut être hypothétiquement attachée aux écarts entre fréquences relatives d'occurrence des rouilles sur une même espèce-hôte est ensuite discutée en liaison avec le caractère hétéroïque ou autoïque des espèces concernées.