Ann. For. Sci. 52 (1995) 103-115
DOI: 10.1051/forest:19950202

Les hêtres tortillards en Europe occidentale. Aspects génétiques

B Démesure^a, B Comps^a, B Thiébaut^b, G Barrière^a and J Letouzey<sup>a

a</sup>  Université Bordeaux I, département de biologie des végétaux ligneux, laboratoire d'écologie génétique, avenue des Facultés, 33405 Talence cedex
^b  Université des sciences et techniques du Languedoc, Institut de botanique, 163, rue A-Broussonet, 34000 Montpellier et CNRS, centre d'écologie fonctionnelle et évolutive, BP 5051, 34033 Montpellier cedex, France

Résumé - Une forme héritable du hêtre européen (Fagus sylvatica L) décrite sous le nom de «tortillard» (var tortuosa Pépin) existe actuellement dans 3 stations européennes : Verzy (France), Süntel (Allemagne) et Dalby-Söderskogs (Suède). Dans chaque station, les 2 types de hêtre cohabitent en conservant leurs caractères morphologiques respectifs malgré des échanges de gènes, vraisemblablement limités, et forment ainsi 2 sous-populations. L'analyse génétique a été réalisée à l'aide de 12 marqueurs alloenzymatiques. Les allozymes du tortillard identifiés sont les mêmes que ceux du hêtre européen alors que d'autres espèces de hêtres analysées se distinguent de ce dernier par des allozymes différents. On peut donc penser que le hêtre européen et le tortillard ont toujours une histoire évolutive commune et que leur séparation est relativement récente et même incomplète. Ces résultats confortent la décision de Pépin (1861) de créer une variété et non une espèce nouvelle pour désigner le tortillard. Diverses comparaisons inter- et intra-stationnelles permettent d'éclairer certaines hypothèses antérieures émises sur le tortillard : i) origine géographique unique et transferts de matériel végétal, ii) prépondérance de la reproduction végétative, iii) dégénérescence par endogamie, dérive génétique et reproduction végétative, iiii) existence d'une aire géographique ancienne plus vaste et plus continue.

Abstract - A genetic study of Fagus sylvatica L var tortuosa Pépin. A form of the European beech (Fagus sylvatica L) was described as 'Tortillard' (vartortuosa Pépin) by Pépin (1861). We have named this form 'winding beech'. It exists at present in 3 European stands: in Verzy, near Reims (France, 49° 14'N, 3° 59'E, alt 288 m); in the Süntel mountains, near Hanover (Germany, 52° 12'N, 9° 17'E, alt 170-250 m); and in Dalby-Söderskogs in southern Sweden (55° 38'N, 13° 19'E). These stands are located within the optimal European range of the beech (fig 1). In each stand, common beech (F sylvatica L) and winding beech (F sylvatica L var tortuosa Pépin) coexist and, in spite of gene exchanges (surely limited) they keep their respective morphological characters and can be considered as 2 'subpopulations'. A genetic analysis of the 6 subpopulations was carried out using 12 polymorphic alloenzymatic markers. We also analysed: (i) individuals from populations of other beech species, using the same markers; and (ii) individuals from common beech populations located in the 3 regions where winding beeches are found. Interstand and intrastand allelic frequencies were compared. We also carried out: (i) a hierarchical analysis including the 6 subpopulations, using Nei's genetic distances; and (ii) a discriminant analysis including the 6 subpopulations and the other sampled beech stands. We then compared: (i) the heterozygote numbers of the 2 subpopulations within each stand, at each locus and for all loci together; and (ii) the homozygote and heterozygote distribution at 1, 2, 3,... 8 loci. Multilocus F_is values were also computed. All alloenzymes observed in common beech are present in winding beech, whereas some were not observed in other beech species (table I). Moreover, in these other species new alloenzymes appear. Thus it is possible to suppose that both forms of the European beech still have a common evolutive history and that their separation is rather recent and even incomplete. The comparison between the allelic frequencies of the 2 subpopulations within each stand shows a very small number of significant deviations (table II). On the other hand, the interstand comparison between winding beech subpopulations or between common beech subpopulations shows that most deviations are significant (table II). This result is confirmed by the dendrogram built from Nei's genetic distances (fig 2). The discriminant analysis divides the 6 subpopulations and the beech stands sampled within the 3 regions into 3 groups according to their geographical location (fig 3). There is no difference of F_is values between the 2 subpopulations within each stand (table III). The heterozygote number at each locus (table IV) and the distribution of homozygotes (0) and heterozygotes at 1,2,3... 8 loci (table V) differ very little from one subpopulation to the other within each stand. From all these results we can discuss some previous hypotheses: (i) a geographical common origin of both beech forms and material transports from one stand to another; (ii) the preponderance of vegetative reproduction in winding beech; (iii) the degeneration of winding beeches caused by endogamy, genetic drift and vegetative reproduction; and (iv) a wider and more continual former geographical area. This study questions the transport of material from 1 station to another and the degeneration of the winding beech and minimizes the influence of vegetative reproduction (table VI, fig 4). It also leads to a discussion on the origin of the winding beech; our results do not provide enough arguments in favour of either a wider former geographical area or a multiple origin.

Key words: winding beech / Fagus sylvatica L var tortuosa Pépin / allozyme / layering

Mots clés : hêtre tortillard / Fagus sylvatica L var tortuosa Pépin / allozymes / marcottage