Báo cáo khoa học: "Genetic variation of the Croatian beech stands (Fagus sylvatica L): spatial differentiation in connection with the environment"

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Original article Genetic variation of the Croatian beech stands (Fagus sylvatica L): spatial differentiation in connection with the environment M Plazibat B Thiebaut B Comps I Sugar I Trinajstic 1 Université de Bordeaux I, Département de Biologie Végétale, Avenue des Facultés, 33405 Talence Cedex; 2 Centre Louis Emberger, CNRS, BP 5051, 34033 Montpellier Cedex; Institut de Botanique, Université des Sciences et Techniques du Languedoc, Auguste Broussonnet, Montpellier, France; rue 3 Botanicki Zavod, Fakulteta Sveucilista u Zagrebu, Marulicev trg 20/11, 41000 Zagreb; 4 Sumarski Fakultet, Simunska Cesta, 4100 Zagreb, Yugoslavia 15 October 24 (Received 1990) April 1990; accepted Summary — Thirty-five beech stands located in Croatia, (Yugoslavia) have been analysed using 6 polymorphic enzymatic loci. Three of them (PX-1, PX-2, GOT-1) seem to be more or less influenced by selection since their allelic frequencies are related to climatic conditions. The total gene diversity is higher for only 1 out of 6 loci in the continental region. Discriminant analyses on allelic frequencies show that the Mediterranean Seslerio-Fagetum which grows in rather dry conditions, is an associa- tion apart; and a significant difference exists between 2 groups of Mediterranean beechwoods locat- ed in the highlands and on the plain, respectively. A general tendency towards a heterozygotic defi- cit occurs with the same significance in both regions. Multilocus F-statistics reveal that the total genotypic differentiation and its 2 components (intra- and interpopulations) do not differ between the 2 regions. genetic differentiation / beech population / Croatia / geographic variation Résumé — La variation génétique des hêtraies croates (Fagus sylvatica L) : différenciation spatiale en relation avec l’environnement. Trente-cinq hêtraies localisées en Croatie (Yougosla- vie) ont été étudiées à l’aide de 6 marqueurs enzymatiques polymorphes. Trois d’entre eux (PX-1, PX-2, GOT-1) semblent plus ou moins soumis à la sélection, en particulier parce que leurs fré- quences alléliques varient parallèlement aux conditions climatiques. La diversité allélique totale est plus élevée pour1 locus sur 6 seulement, en région continentale. Deux analyses discriminantes sur les fréquences alléliques montrent : que le Seslerio-Fagetum méditerranéen qui caractérise des stations relativement sèches repré- - sente une association originale; une différence significative entre 2 groupes de hêtraies méditerranéennes en fonction de leur loca- - lisation soit en montagne, soit à basse altitude. Une tendance générale se manifeste vers un déficit en hétérozygotes de même importance dans les deux régions. Les F-statistiques montrent que la différenciation génotypique totale, aussi bien que ses 2 composantes (intra- et interpopulations) ne sont pas différentes d’une région à l’autre. différenciation génétique / hêtraie / Croatie / variation géographique * Correspondence and reprints
methods and models have been proposed, INTRODUCTION which can best be applied to actual plant populations (Malécot, 1969; Gregorius, genetic structure of beech stands de- The 1975a, b; Van Dijk, 1987). pends on selection and the mating system, Historical factors could also play an im- in addition to gene flow and genetic drift: portant role in the genetic structure of these factors induce inter- and intra- beechwoods. After the last glacial period, population genetic differentiation over current beech stands spread out from a space and time (Kim, 1979; 1980; Müller- principal source located in the Balkans and Starck, 1985, 1989; Cuguen, 1986; Gre- from several secondary sources in south- gorius et al, 1986; Cuguen et al, 1988). western Europe (Paquereau, 1965; Beug, Beech is a climax species in most of 1967; Sercelj, 1970; Jalut et al, 1975; Europe where it grows under vari- western Triat-Laval, 1978; Pons, 1983). It appears ecological conditions. Particularly ous that beech colonized its present areas at close to the Mediterranean sea, neigh- various periods: southern countries have bouring beechwoods may develop in very been colonized since 5 000 or 4 000 BP different climates, ie Mediterranean or con- (approximately 40-50 generations) and tinental climates, depending on whether northern plains since only 3 000 or 2 500 they are located in the lowlands or in the BP (25-30 generations) (Vernet, 1981). highlands (Misic, 1957; Thiébaut, 1984). Thus there has been a higher number of This environmental diversity favours the generations in the south than in the north. genetic differentiation of beechwoods by Genetic differentiation within and among selection and genetic isolation due to beech stands seems to be higher in the phenological differences (Thiébaut et al, south where ecological conditions are 1982; Felber and Thiébaut, 1982, 1984; more heterogeneous, the stands are older N’Tsiba, 1984; Thiébaut, 1984; ; Barrière and their sources more numerous (Comps et al, 1985; Cuguen et al, 1985; Comps et et al, 1989). Our purpose was to examine al, 1987). the genetic differentiation of beechwoods Beech is an anemophilous and most- in Croatia characterized both by the Medi- ly allogamic species characterized by a terranean and continental climates. Prelim- low self-fertilization rate (Nielsen and inary results have shown that this genetic Schaffalitzky-de-Muckadell, 1954) which differentiation is higher in Croatia than in can nevertheless produce some heterozy- other countries of central Europe (Comps gotic deficit. In addition, gene flow may be et al, 1989). generally limited within populations in the optimal beech range due to the high den- sity of beechwoods: it is therefore likely that mating occurs between closely MATERIAL AND METHODS spaced individuals. According to Cuguen (1986) and Cuguen et al (1988), genetic structures should be approximately de- Sampling scribed by the "isolation by distance mod- el" (Wright, 1943, 1946). This model induc- carried out in 35 beech stands Sampling was relatedness between individuals and es a representing the various climatic conditions, the therefore a genetic differentiation within various soils and topographic locations where and among populations (Cuguen, 1986). beech grows in Croatia (fig 1, table I). In each Since Wright’s theoretical works, other beech stand, plant material (buds and twigs)
one of them located at 800 m was included in sampled from about 50 trees chosen at ran- was dom over a 3-4-ha area and in as homogene- this group due to the dryness of the station); the ous an environment as possible. latter are beechwoods which are always located above 900 m. The continental region was not With regard to the climate, we compared 2 subdivided: it includes all the forests located regions located on either side of the Dinaric along the northern slope of the Dinaric Alps, on Alps, in Mediterranean and continental climates, the plain and in the highlands of Croatia. With respectively. In the Mediterranean region, we regard to soil factors, we could only define 3 also distinguished the forests located near the classes of pH (acid, neutral and basic); we pre- littoral at low altitude from the highland forests: ferred to carry out a synthetic characterization of the former are generally oak-beech mixed fo- environment by analysing the plant associations rests characterized by the dominance of oak to which the forests under study belong. and mostly located below 500 m in altitude (only
mate oxaloacetate transaminase), PGI-1 (phos- Biochemical analysis phogluco-isomerase), MDH-1 (malate dehydrog- enase) and IDH-1 (isocitrate dehydrogenase). Extraction from buds and cortical tissues of Three of these loci (PX-1, GOT-1 and MDH-1) twigs, electrophoresis and staining were per- possess 2 codominant alleles while the others formed using the techniques described by Thié- have 3 codominant alleles (PX-2, IDH-1 and baut et al (1982) and Merzeau et al (1989). Ge- PGI-1) (Thiébaut et al, 1982; Merzeau et al, netic variability was studied using 6 polymorphic 1989). loci: PX-1, PX-2 (peroxidases), GOT-1 (gluta-
high level of polymorphism: the corresponding Mathematical analyses heterozygote excess is connected with a statisti- cal effect due to the low probability of encounter- Allelic differentiation ing homozygotes of a rare allele in the sampling (Cuguen, 1986; Cuguen et al, 1988). The total gene diversity (H was estimated ) T us- ing Nei’s method (1973, 1977) (*): RESULTS 2 i T H = 1 - Σp where p is the of the i th al- frequency mean i Allelic differentiation lele, weighted by the sample size. We also esti- mated H and D which are the weighted aver- S ST age gene diversities within and among At each locus, allele generally ap- one populations, respectively, with H= H+ D TSST. than the others in peared frequently more Allelic frequencies of the different groups of the beechwoods under study (table II). beechwoods characterized according to climate, However for PX-1, the rarer allele (PX-1- soil or plant associations were compared for 105) appeared more frequently in some each locus using an analysis of variance and the Mann-Whitney test. Gene diversities were Mediterranean forests located in the high- compared using only the non parametric Mann- lands (6 out of 7 stands) and only 2 out of Whitney test because they did not fit a normal 7 at low altitude. distribution. We then carried out a discriminant analysis including only the loci for which pre- vious comparisons displayed significant differ- Allelic differentiation according ences. Only one allele was taken into account for each diallelic locus and only 2 for each trial- regions to lelic locus. Gene diversity varied from one locus to an- Genotypic differentiation other and according to the region. It was relatively high for PX-1, PX-2, IDH-1 and Since the theoretical works of Wright (1965), MDH-1, whereas it was lower for GOT-1 genotypic structures have often been analysed and PGI-I (table III). It was higher for PX-1 using F-statistics. F an estimation of the total ITis in the Mediterranean region (P < 0.1) genotypic differentiation, and allelic diversity is and for MDH-1 in the continental region partitioned into intra- (F and inter- (F popu- ) IS ) ST lation components. Estimates of the 3 F- (P < 0.05) (table IV). In the Mediterranean statistics were made according to the method of region, the gene diversity was higher Weir and Cockerham (1984). They were weight- for 2 out 6 loci in the highlands: PX-2 ed by the stand sample size and its variance (P < 0.001) and GOT-1 (P < 0.01) and was and by the number of stands studied. For each higher on the plain for MDH-1 and PGI-1 index, a variance was estimated using a jack- (P< 0.10). knife procedure (Miller, 1974; Reynolds et al, 1983). This variance allowed us to determine diversity was distributed in the Gene whether each value was significantly different throughout, the greater part same manner from 0, and to test differences between 2 re- being within the forests (86.9-99.1%, table gions. III) as observed in mostly allogamic spe- In populations which not very polymor- are cies (Tigerstedt, 1973; Rudin et al, 1974; fre- phic, negative F values IS generally are more Lundkvist and Rudin, 1977; Hamrick et al, quent than within stands characterized by a "gene diversity" instead of heterozygosity. (*) As recommended by Nei (1973), the word will we use
A discriminant analysis was carried out 1979; Loveless and Hamrick, 1984). How- including only PX-1, PX-2 and GOT-1 alle- ever, theinter-population component var- lic frequencies for which the previous com- ied somewhat according to region and lo- parisons had shown significant deviations cus. (P < 0.05, table IV, fig 2). It confirmed the Comparison of allelic frequencies be- difference’ between the 2 groups of Medi- tween regions confirmed the contrast be- terranean beechwoods (highland and low- tween the Mediterranean and the conti- land) for which discrimination was com- nental regions for PX-1 (table IV); we also plete. Continental beechwoods constituted found 0.05 < P < 0.10 for MDH-1. In the an intermediate group between the other Mediterranean region, the deviation be- two. GOT-1 and PX-2 loci were more re- tween the lowlands and the highlands was sponsible than PX-1 for this discrimination confirmed for PX-2 and GOT-1. between these climatic groups.
Allelic differentiation according 0.10) (table IV). The greatest part of (P < to associations gene diversity was always found to be within the populations, whatever the asso- ciation group and the locus. The originality of Croatian beechwood as- Comparisons of allelic frequencies be- sociations is mostly related to the exis- tween the 2 groups of associations tence of rather dry stands. This ecological showed significant differences for PX-1, character has induced the settlement of a PX-2-39 and IDH-1 (table IV). Thus, there special association, the Seslerio-Fagetum, was a greater contrast between associa- whereas other beechwoods are more akin tions than between Mediterranean and to those of Central Europe. This explains continental regions. why we only compared this association with the others taken as a whole. A discriminant analysis including PX-1, PX-2 and IDH-1 allelic frequencies (fig 3) Gene diversity varied from one locus to confirmed the previous differences be- another (table III). In Seslerio-Fagetum it tween the 2 groups of associations; but was only found to be higher for PX-1
0, which implies the existence of genotypic their discrimination was not quite com- differentiation both intra- and interpopula- plete. The PX-2 locus was the most re- tions (table V). F value was positive, sponsible for this discrimination. IS which reveals a general tendency towards a heterozygote deficit. Genotypic differentiation The number of beechwoods and trees studied within each region were very alike, For the whole of Croatia, all multilocus es- which allowed the comparison of F- statistics values. As for the whole of Croa- timates of F-statistics were different from
DISCUSSION AND CONCLUSIONS tia, all multilocus estimates differed from 0, and F values were positive. Differences IS between the 2 regions were never signifi- The number of beechwoods studied was cant. However, to similar F tended IT values rather low as there is a variety of climate in to correspond lower F and higher F IS ST Croatia. We could have sampled a greater values in the Mediterranean region; but number of stands on the mountain sum- these differences were not significant. In mits and within the continental region the Mediterranean region, the comparison where beech is very frequent, but not between lowland and highland beech- along the littoral where beechwoods are woods became impossible due to the low rare. A lack of balance in the sampling number of stands in each group. which would have favoured the number of beechwoods studied belonging to the first Within each association group, all multi- group could have introduced a bias in our locus estimates of F-statistics were differ- analysis. However, as the total number of ent from 0. As for the regions, differences trees analysed was great in both cases, between any pair of homologous values the previous disadvantage was partially taken from each group of associations re- compensated. It would be interesting to spectively were significant. never
In Croatia, 4 loci are regularly polymor- phic (PX-1, PX-2, MDH-1 and IDH-1) and the other 2 less so (GOT-1, PGI-1) (table II). Allelic variations could be connected with climate for PX-1, PX-2, GOT-1 and with plant associations for PX-1, PX-2 and IDH-1 (table IV, figs 2 and 3). There is a greater difference between the 2 groups of associations than between Mediterranean and continental regions. This may be due to the existence in the Mediterranean re- gion of an association, Fagetum subalpi- num (4 stands) very different from the Ses- lerio-Fagetum and more hygrophilous, which tends to reduce deviations between the 2 regions. This result confirms the orig- inality of the Seslerio-Fagetum which char- acterizes the driest stands of the Croatian beechwoods. On this scale, either PX-1 or PX-2 ap- pear to be subject to genetic selection fol- lowing environmental changes. This obser- vation confirms some previous results (Thiébaut et al, 1982; Thiébaut, 1984; Bar- rière et al, 1985; Comps et al, 1987): in other southern countries, we found a posi- tive correlation between PX-2 allelic diver- sity and the extent of climatic variations connected with the altitudinal range of the stands. For PX-1, a positive correlation was shown between extreme climatic con- ditions for beech and the highest values of PX-1-105 frequency. The dry conditions to which Seslerio-Fagetum is subject seem to be the most unfavourable to the growth of the beech in Croatia and also correspond to the highest frequencies of the same al- lele in this country. On the other hand, in Southwestern Eu- rope where GOT-1 polymorphism is high, the frequency of the GOT-1-105 allele is significantly higher under severe climate conditions at high altitude (Comps et al, over a wider area along the Dinar- sample 1987). In Croatia, in spite of the low poly- ic chain as far as Macedonia, particularly morphism of this locus, its allelic frequency in the lowlands.
is also connected with altitudinal climatic population (ie a Wahlund effect) can most- ly explain the observed deficits. changes (higher diversity in the highlands). Taking into account all these observa- The arguments put forward in the Intro- tions, it should be noted that the 3 men- duction suggest that there has been more tioned loci may be subject to selection. opportunity for the development of genetic Koehn (1978) considered it is necessary to differentiation among and within popula- display the interrelations between pheno- tions close to the Mediterranean sea. To typic diversity, physiological reaction and what extent do our results confirm this hy- variations of environmental factors in order pothesis? to prove the adaptative signification of an Only the allelic frequencies of PX-1 lo- enzymatic polymorphism. However, the cus differ significantly between the conti- connections between climate and allelic nental and the Mediterranean regions variations of PX-1, PX-2 and GOT-1 have (table IV); in the latter the corresponding been exhibited in different regions, each gene diversity is higher, but only at the apart from the others. Even if the repetition 0.10 level. The discriminant analysis does of such a phenomenon does not constitute not allow us to entirely separate one region an indisputable proof of a selection effect, from the other (fig 2). Allelic differentiation it is nevertheless a favourable argument is not clearly higher within Seslerio- (Clarke, 1975; Bergmann, 1978). Fagetum which is chiefly a Mediterranean By using a greater number of loci, this association than in the other mostly conti- study complements our previous results: in nental associations (table III). However, Croatia, allelic frequencies of 2/3 newly comparisons of allelic frequencies between used loci (MDH-1, PGI-1) do not vary sub- the 2 groups of associations display a stantially as a function of environmental greater contrast than between regions factors, even if MDH-1 gene diversity dif- (table IV). fers according to the region. IDH-I locus Over the whole of Croatia, a genotypic does not to be connected with re- seem differentiation within and among popula- gional climate; however, its polymorphism tions is clearly apparent. However, com- is significantly lower in Seslerio-Fagetum parisons of the corresponding F-statistics subject to dry conditions than in the other values between the Mediterranean and more mesophilous associations. continental regions do not display any sig- Multilocus F-statistics reveal a total nificant deviations. Comparisons of F- genotypic differentiation which does not statistic values between the 2 groups of differ whatever the climate (table V). The associations lead to the same result. How- intra- is higher than the inter-component, ever, the lack of genotypic differences be- as observed by various authors (Sakai and tween 2 regions is probably increased for 2 Park, 1971; Mitton et al, 1977, 1981; Lin- 1) the methods employed may reasons : hart et al, 1981; Knowles, 1984; Knowles limiting (jackknife); 2) due to a biologi- be and Grant, 1985; Shea, 1985; Cuguen, reality which is more complex and less cal 1986). All F values are positive, which IS perceptible at the genotypic than at the al- implies a general tendency towards a het- lelic level. erozygotic deficit. These observations con- Finally, very few arguments confirm pre- firm previous results obtained throughout vious hypotheses, at least on the Croatian Europe (Cuguen, 1986). As the self- scale. Of course, on this scale, the time fertilization rate of beech is low (from 0 to which as elapsed between the settlement 0.05); isolation by distance within each
Paule L, Sugar I, Thiebaut B, Trinaj- Comps B, of Mediterranean and that of continental stic Y (1989) Genetic variability in beech- beechwoods may be insufficient. To test woods (Fagus sylvatica L) over central Eu- the validity of our hypothesis, we are carry- rope, allozymic variations in six enzyme ing out a study to compare the beech systems: spatial differentiation among and stands of Croatia, a part of Yugoslavia lo- within populations. In: Proc 3rd Symp Im- cated close to the Mediterranean, with the provement and Silviculture of Beech, 1988, IUFRO Project Group P1, Zvolen, Czechoslo- Slovak beech stands in central Europe. vakia, 10-00 Our hypothesis will be confirmed if the ge- Cuguen J (1986) Différenciation génétique inter- netic differentiation is found to be higher in et intrapopulations d’un arbre forestier aném- Croatia. ophile : le cas du hêtre (Fagus sylvatica L). Thèse de Doctorat, Université des Sciences et Techniques du Languedoc, Montpellier, ACKNOWLEDGMENTS France Cuguen J, Thiebaut B, N’Tsiba F, Barrière G (1985) Enzymatic variability of beech stands This research carried out with the financial was (Fagus sylvatica L) on three scales in Eu- support of the INRA contract 1233 A: "Ameliora- rope: evolutionary mechanisms. In: Genetic tion, sylviculture et qualité du bois des feuillus Differentiation and Dispersal in Plants (Jac- précieux". The authors aregrateful to RM Guil- quart P, Heim G, Antonovics J, eds) NATO baud, J Letouzey and S Vodichon for their tech- ASI Series, Montpellier, 17-39 nical assistance. Merzeau D, Thiebaut B (1988) Ge- Cuguen J, netic structure of the European beech stands (Fagus sylvatica L): F-statistics and impor- tance of the mating system characteristics in REFERENCES their evolution. Heredity 60, 91-100 Felber F, Thiebaut B (1982) La hêtraie méridio- Barrière G, Comps B, Cuguen J, N’Tsiba F, nale française : structure génétique en rela- Thiebaut B (1985) The genetical ecological tion avec les conditions écologiques : In: variability of beech (Fagus sylvatica L) in Eu- Struktur and Dynamik von Wäldern, Ber der rope - an alloenzymatic study: genetic isola- 25 intern Symposien der intern Vereinig für tions of beechwoods. In: Proc 1st Symp Im- Vegetationskunde Rinteln 1981. AR Gartner provement and Silviculture of Beech. IUFRO Verlag, Vaduz, Liechtenstein, 459-473 Project Group P1 10-00, Grosshansdorf Felber F, Thiebaut B (1984) Étude préliminaire 1984, 24-50 sur le polymorphisme enzymatique du hêtre Bergmann F (1978) The allelic distribution at an (Fagus sylvatica L) variabilité génétique de acid phosphatase locus in Norway spruce deux systèmes de peroxydases en relation (Picea abies) along similar climatic gradients. avec les conditions écologiques. Oecol Plant Theor Appl Genet 32, 57-64 5, 133-150 Beug HJ (1967) On the forest history of the Dal- Gregorius HR (1975a) A model for genetic rela- matian coast. Rev Paléobot Palynol 2, 271- tionship among offspring from open- 279 pollinated plant populations. Theor Appl Gen- et 46, 109-115 Clarke B (1975) The contribution of ecological genetics to evolutionary theory: detecting the HR (1975b) A model of the determi- Gregorius direct effects of natural selection on particu- nation of the variance in genetic relationship lar polymorphic loci. Genetics 79, 101-113 among offspring from open-pollinated plant populations. Theor Appl Genet 46, 157-163 Comps B, Barrière G, Merzeau D, Letouzey J (1987) La variabilité alloenzymatique des hê- Gregorius HR, Krauhaussen J, Müller-Starck G traies dans les sous-domaines médio- et eu- (1986) Spatial and temporal differentiation among the seeds in a stand of Fagus sylvati- atlantiques d’Europe. Can J For Res 17, ca L. Heredity 56, 255-262 1043-1049
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