Báo cáo khoa học: "Intra- and interpopulational genetic variation in juvenile populations of Quercus robur L and Quercus petraea Lieb"

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Original article Intra- and interpopulational genetic variation in juvenile populations of Quercus robur L and Quercus petraea Liebl G Müller-Starck S HH Hattemer 1 Herzog 1 Institut für Forstgenetik und Forstpflanzenzüchtung der Universität Göttingen, 3400 Göttingen-Weende, Germany; 2 Eidgenöss, Forschungsanstalt für Wald, Schnee und Landschaft, Zürcherstrasse 111,8903 Birmensdorf, Switzerland Summary — In each of 5 2-year-old populations of Quercus robur and Q petraea (single and multi- population samples), genetic variation was quantified with respect to 13 polymorphic enzyme coding gene loci. Genetic control and inheritance of isoenzymes was verified beforehand by means of anal- yses of full-sib families. The observed average heterozygosities were 21.3% Q robur and 21.9% for Q petraea (conditional heterozygosities of 56.6 and 56.7 respectively). The mean number of alleles per locus is 3.2 for Q robur and 3.1 for Q petraea. The relatively small genetic diversities indicate mi- nor polymorphisms. The genetic distances between pairs of samples indicate remarkable differenc- es between populations. Most of the single population samples could be proven to share a smaller proportion of the entire gene pool than each of the multipopulation samples. There is a strong genet- ic similarity between Q robur and Q petraea in terms of common alleles. It is concluded that, more than in other species, large genetic variation must be incorporated into oak population in order to maintain the ability of these species to adapt to heterogeneous environments. petraea / alloenzymes / heterozygosity / diversity/ genetic distahce / gene- Quercus robur / Q tic differentiation Résumé — Variabilité génétique intra- et interpopulation dans de jeunes populations de Quer- cus robur L et de Quercus petraea Liebl. La variablité générique a été estimée dans 5 popula- tions de Quercus robur et 5 populations de Q petraea à partir de 13 loci polymorphes contrôlant l’expression d’enzymes. L’hérédité mendélienne des isozymes a été au préalable vérifiée par l’étude * Send all correspondence to address 2.
de ségrégation dans les croisements contrôlés. Les valeurs moyennes de l’hétérozygotie observée sont de 21,3% chez Q robur et 21,9% chez Q petraea. Les mêmes valeurs pour les hétérozygoties conditionnelles sont de 56,6% et 56,7%. Le nombre moyen d’allèles est de 3,2 pour Q robur et 3,1 pour Q petraea. Les diversités génétiques sont relativement peu élevées. Les distances génétiques entre populations indiquent de très fortes différences entre elles. Les populations prises individuelle- ment partagent une partie plus faible de l’ensemble du pool génétique que les populations regrou- pées entre elles. Les allèles communs indiquent une très forte similarité entre Q robur et Q petraea. En conclusion, il est recommandé de conserver une variabilité génétique élevée dans les chênaies de manière à maintenir leur aptitude à s’adapter à des milieux hétérogènes. Quercus robur / Quercus petraea / allozymes / hétérozygotie / diversité / distance génétique / différenciation génétique INTRODUCTION MATERIALS AND METHODS Quercus robur L (penduculate oak) and Samples Quercus petraea Liebl (sessile oak) be- long to the major deciduous tree species For each species, 5 populations were grown in Germany. Like Fagus sylvatica L, oaks from commercially utilized seed lots (see table I) are carrier tree species of complex forest which are commonly used for artificial regenera- ecosystems which range from the low- tion in Germany. Two categories of commercial reproductive material are involved: 1) mixtures lands to the submountainous or even the of seed lots which originate from harvest in dif- moutainous regions. Oaks are extremely ferent stands which all together belong to the long-lived species with forest rotation cy- same region of provenance (’multipopulation cles of 200 or more years. Oaks are ex- samples’); and 2) material which originates from posed to more heterogeneous environ- single oak stands which cover areas of 50-100 ha. All stands are supposed to be predominantly time than any other ments over indigenous. A total of 1605 individuals were tree species. In the study of predominant genotyped at the age of 2 yr. For location of the genetic variation and its implications on origin of the studied samples, see Müller-Starck the ability of tree populations to survive in and Ziehe (1991). complex environmental situations, oaks may function well as model organisms. The objective of the present study was Genotyping in the description of the genetic proceed to variation in oak populations and thus in the Genetic control and inheritance of isoenzymes characterization of the natural variability of verified beforehand by utilizing full-sib fami- was forest ecosystems. Data on patterns of ge- lies and their parents of Q robur and Q petraea netic variation will contribute to a better un- (Müller-Strack and Hattemer, 1990). For extrac- tion of bud and leaf tissues, enzymes were sep- derstanding of principles of adaptation and arated by starch-gel electrophoresis and isoelec- survival of oaks and are needed as criteria tric focusing mean of genotyping see Müller- for the choice of reproductive material, for Starck and Ziehe (1991). Enzyme systems with silvicultural treatment as well as for declar- environmentally dependent expression of isoen- ation and conservation of genetic resourc- zymes, such as acid phosphatases or esteras- es. es, were excluded from further studies. Ten en-
berds, 1986); for Müller-Starck zyme systems were studied (abbreviations and a see summary EC No / in brackets): Aminopeptidase (AP, and Gregorius (1986). 3.4.11.1, leucine-and alanine-AP), diaphorase (DIA, 1.8.1.4), glutamate-oxaloacetate transa- mininase (GOT , 2.6.1.1 (= aspartate amino- RESULTS AND DISCUSSION transferase, AAT)), isocitrate dehydrogenase (IDH, 1.1.1.42), malate dehydrogenase (MDH, 1.1.1.37), peroxidase (PER, 1.11.1.7), 6- Intrapopulational variation phosphogluconate dehydrogenase (GPGDH, 1.1.1.44), phosphoglucose isomerase (PGI, in Quercus robur and Q petraea 5.3.1.9), phosphoglucomutase (PGM, 5.4.2.2), shikimate dehydrogenase (SKDH, 1.1.1.25). Average degrees of heterozygosity Genotypes were scored at 13 polymorphic gene loci: AP-A, DIA-A, GOT-B,C, IDH-A, MDH-B,C, PER-B, 6PGD-A,B, PGI-B, PGM-A, SKDH-A. Results are summarized in tables II and III; The conditional heterozygosities (H are ) c given in addition to the observed heterozy- Measurement of genetic variation gosities (H because the latter values can ) a be biased as a consequence of their depen- dency upon the underlying gene frequen- variation was measured by Intrapopulational cies. For H the given multilocus mean is , a of the observed and the conditional het- means arthmetic; for H it is equal to the ratio of the c (Gregorius et al, 1986), erozygosities ) c ,, a (H the number of alleles and of genotypes per lo- summed H values to the summed maxi- a cus, and the genic (allelic) diversities (Gregori- mum attainable heterozygosities. us, 1987). Interpopulational variation was quanti- The H values showed substantial varia- a fied by genetic distance (Gregorius, 1974) and tion among the gene loci. Loci reflecting population differentiation (Gregorius and Ro-
tion in most of the samples. The multilocus AP-A, DIA-A, large heterozygosities were: PER-B and PGM-A. Variation mean values showed little deviation among IDH-A, among the samples was particularly indi- the samples. The overall mean values of cated by the gene loci PER-B and SKDH- the species were nearly identical (56.6% A. The mean H value for the 2 oak spe- and 56.7%, respectively). a cies were nearly the same: 21.3% for Q ro- Generally, pronounced species-specific bur and 21.9% for Q petraea. The average effects were lacking and the sampling values of the multipopulation samples mode (multipopulation samples vs locally were slightly smaller than the species separated ones) did not seem to affect the mean (21.1 % for Q robur) or identical to it. heterozygosities considerably. The ob- Deviating trends in the H values (see served heterozygosities did not deviate c table III) as compared to the H values much from those reported for other decidu- a consequence of differences in the ous tree species, such as Fagus sylcatica: were a corresponding values (2-yr old plants from gene frequencies among samples and among loci within samples. For instance, 5 multipopulation samples genotyped at 12 AP-A, PER-A and PGM-A reflected great a 22.2% and H gene loci) were; H c = = H values but small H values. This differ- 52.4% (Müller-Starck and Ziehe, 1991). a c ence leads the H values to appear large a but to be small in reality, if the potential to form heterozygotes is taken into considera- Genetic multiplicity tion. The opposite trend was revealed, for instance, by the loci GOT-B and 6PGDH- Genetic multiplicity in terms of average B: large H values demonstrated that the c number of alleles or genotypes per locus is extraordinarily small H values could not a summarized in table IV. There is no allele have been much larger due the underlying allele frenquecies (1 frequent and a few which occurs in all samples of one species very rare alleles). Loci AP-A, DIA-A, IDH- but not in any sample of the other species. A, PER-B and PGM-A have 2 or 3 alleles. Alleles which are represented in some of The variation with respect to MDH-B and the samples of only one species are so rare MDH-C was primarily a consequence of (≈ 1 %) that sample size may account for the gene frenquency distributions close to fixa- non-representation in the other species.
The largest mean numbers of alleles genotypes per locus. In contrast to the revealed by the gene loci PER-B number of alleles per locus, the cor- mean were responding values for genotypes were (5.3) and AP-A (4.7), the smallest ones by smaller in Q robur than in Q petraea, ie, MDH-B (1.5) and MDH-C (1.8). The multi- 5.1 vs 5.4 genotypes per locus (for Q ro- locus means of the samples from multi- bur, this is equivalent to 76% of the maxi- populations compared to single popula- tions mum attainable mean number of geno- (3.4 vs 2.9 for Q robur, 3.3 vs 2.8 for Q petraea. The overall means of both spe- types; for Q petraea to 85%). For the time cies are nearly indentical (3.2 and 3.1 al- being it is suggested that characteristics of the reproductive system of the respective leles locus). parental populations may contribute to The mean number of genotypes per lo- these phenomena. varied more among the samples than cus the gene number did. This finding is not Genic only a consequence of the sample sizes: (allelic) diversity the sizes of samples 5,7 and 10 are ex- traordinarily small (between 72 and 96 in- The multilocus values (see table V) were calculated as harmonic means. The hypo- dividuals) (see table I) but these samples did not reveal the smallest number of thetical gametic multilocus diversity (HGMD)
of Quercus smaller in the isequal to the number of genetically differ- siderably case gametic types which hypothetically can petraea (164.9 193.9). ent vs be produced by individuals of each sample on the basis of their 13-locus genotypes lnterpopulational variation in Quercus robur and Q petraea Genetic distances where v is the diversity at locusl) (Gregori- l In table VI, results are given for 2 out of 13 al, 1986). This measure quantifies us et loci: AP-A represents those loci which re- the potential for creating genetic variation veal in both species, at least for some pairs in the next generation and is therefore an of samples, considerably large genic (alle- important determinant of the adaptability of lic) distances; SKDH-A, showed on the av- forest tree populations. erage, large values for Quercus petraea but In most cases, the single locus genic di- remarkably small values for Q robur. In this versities reflected trends similar to those sense, SKDH-A reveals greater deviations observed in the genic multiplicities (eg, among the 2 species than any other locus. largest values for PER-B and AP-A). This From the results of a statistical analysis was not true in cases of deviating distribu- between samples (log likelihood ratio test of tions of allele frenquencies; for instance, homogeneity in contingency tables), it can the locus DIA-A reflected on the average be stated that genic distance values greater smaller numbers of alleles but larger genic than 0.1 will reveal significant deviations in diversities than the GOT-C locus (DIA-A: most cases (at least at a significance level 3.4 vs 2.1; GOT-C: 4.0 vs 1.5). The reason of 0.05). Genic distances larger than 0.2 for this difference is the greater deviation can be expected to indicate substantial in- of the allele frequencies from the state of terpopulational genetic variation. For many eveness in the case of GOT-C compared pairs of samples, these values are exceed- to that of DIA-A. ed, eg, in case of populations 8 and 9 (AP- A). Each of these populations can be dis- The variation among the multilocus criminated easily from the other populations of the samples was smaller than means but not from each other. The similarity be- that among the mean number of alleles per tween populations 8 and 9 is a surprising locus (see table IV). In both species, there observation because these 2 samples origi- was not more deviation between the val- nate from single populations and should re- ues from multipopulation samples and lect more specific genetic information than those from single populations. mixed samples. As can be seen from the The hypothetical gametic multilocus di- same table, this trend was not confirmed by versities revealed a large variation among SKDH-A. The highly specific monitoring ef- the samples. The sample size did not fect of adaptive loci may account for this seem to affect these values substantially, phenomenon. because 2 of the smallest samples (5, 7) show quite large multilocus values. In the case of Quercus robur, the values of the Genetic differentiation single population samples did not deviate Genetic differentiation was quantified by much from those of the mixed samples (on means of the genetic distances between the average 136.9 vs 137.4), but are con-
specific information. In the combined pres- an the remaining ones which sample one entation (see graph ’gene pool’), loci- combined in order to form the re- were spective complement population. In this specific effects can be compensated. way genetic variation was measured as a The single locus graph show that the whole and not only in pairs. average level of differentiation varied con- 1 and 2 illustrate genetic diffe- siderably among the loci and among the Figures rentiation for 5 out of 13 gene loci (for a species. For both species, a large level of summary of numerical multilocus values, differentiation was found for instance, for PER-B. Deviations between the 2 species see Müller-Starck and Ziehe, 1991). For were particularly evident in the case of each species, the graphs refer to the allele SKDH-A and to a certain extent also those frequencies. In each graph, the radius of of AP-A and GOT-C. The gene pool graph the dotted line is equal to the average lev- el of differentiation at that particular locus. indicates greater differentiation among the samples of Quercus petraea than among The given scale measures the average those of Q robur. Within the first species, proportion of genes by which any sample differs from the remainder. For each samples 4 and 5 were differentiated above graph, the radii of the sample-specific sec- average, although the differences among tors are equal to the proportion of genes the samples were small. In the case of by which one sample differs from the Quercus petraea, all samples except no 6 were differentiated above average. The pooled remainder (the angles of the sec- to the sample size). The samples which originated from seed collec- tors correspond the sector radii approach the center, tions in single populations (no 4 and 5, and more the more representative of the remainder 8, 9 and 10) tended to be more differentiat- is the genetic information of this sample, ed than the remaining samples which de- and is equivalent to reduction of sample- scend from population mixtures. This ob-
servation similarities among populations in a very means that single population samples share a smaller proportion of the specific way. entire genetic information than the mixed The results presented herein suggest ones. that forest tree breeding and silviculture of Quercus robur and Q petraea need to take into account large genetic multiplicities. CONCLUSIONS Such genetic heterogeneity seems to cor- respond to the tremendous environmental heterogeneity to which long-lived oak pop- Enzyme gene marker reveals a substantial ulations are exposed. Particularly in oak genetic similarity between Quercus robur species, it appears that large genetic varia- and Q petraea. None of the genes was tion should be incorporated in productive represented exclusively in only one of the populations in order to maintain the poten- species. Within each species, heterozy- tial of these populations to adapt to and to gosities appear to be smaller than in sever- survive in complex environmental situa- al other tree species (for comparison of pa- tions. rameters and references, see Müller- Starck, 1991).Because information about heterozygosities in adult stands is still lack- ACKNOWLEDGMENTS ing, it cannot be excluded that relatively low heterozygosities might characterize The technical assistance of G Dinkel is greatly juvenile Quercus populations but not the appreciated. This study was financially support- succeeding life stages. The genetic multi- ed by the Commission of the European Commu- plicity was very large in all samples investi- nities, GD XII, Brussels, Belgium. gated including single-population samples. This finding concurs with results of a re- cent study in 32 European populations of REFERENCES Quercus petraea (Kremer et al, 1991). Great intrapopulational variation could indi- Anonymous (1972) Erste Verordnung zur cate a successful strategy of adaptation Durchführung des Gesetzes über forstliches and survival of species which are extreme- Saat-und Pflanzgut (1 FSaat V). Bundesge- setzblatt I, 92, 1561-1586 ly long-lived and exposed to extraordinarily heterogeneous environments. Relatively Gregorius HR (1974) Genetischer Abstand zwis- chen Populationen. 1. Zur Konzeption der small genetic diversities indicate minor genetischen Abstandsmessung. Silvae Gen- polymorphisms, ie, a constellation with 1 et 23, 22-27 predominant and a few rare alleles. The Gregorius HR (1978) The concept of genetic di- genetic distances between pairs of sam- versity and its formal relationship to heterozy- ples indicate remarkable differences in gosity and genetic distance. Math Biosci 41, many cases. These values tend to be high- 253-271 than those of Kremer et al (1991). The er Gregorius HR, Roberds JH (1986) Measure- illustrated genetic differentiation reveals ment of genetical differentiation among sub- that multipopulation samples tend to share populations. Theor Appl Genet 71, 826-834 a larger proportion of the entire gene pool Gregorius HR, Krauhausen J, Müller-Starck G than each of the single-population sam- (1986) Spatial and temporal genetic differen- ples. Gene loci reveal very different trends tiation among the seed in a stand of Fagus because each of the adaptive gene loci sylvatica L. Heredity 57, 255-262 may be subject to different selective forces Kremer A, Petit R, Zanetto A, Fougere V, Du- and thus monitor genetic similarities or dis- cousso A, Wagner D, Chauvin C (1991) Nu-
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