Báo cáo khoa học: "Hybridization and mating system in a mixed stand of sessile and pedunculate oak"

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article Original and mating system in a mixed stand Hybridization of sessile and pedunculate oak G Roussel R Bacilieri A Ducousso 1 Station de recherche forestière de Bordeaux, INRA Pierroton, BP 45, 33611 Gazinet, France 2 di Selvicoltura, Facoltà di Agraria, Via San Bonaventura 13, 50145 Florence, Italy Istituio Patterns of hybridization and of the mating system of Quercus petraea and Quercus Summary — robur have been inferred from examination of allozyme variation in 2 cohorts (adults and progeny) of a stand comprised of both species. Differences in allelic frequencies were found in each species be- tween the pollen pool and the adult trees, but the pattern of hybridization was apparently asymmetri- cal. Q petraea and Q robur are almost exclusively allogamic, the multilocus outcrossing rate being 0.96 for both species. allozymes / hybridization / mating system / pollen pool / Quercus robur / Quercus petraea Résumé — Hybridation et système de reproduction dans une forêt mixte de chêne sessile et chêne pédonculé. Les modalités d’hybridation et du système de reproduction de Quercus petraea et Quercus robur ont été étudiées à partir des variations allozymiques dans 2 cohortes (les adultes et leurs descendants) d’une forêt mixte composée des 2 espèces. Pour chaque espèce, des diffé- rences dans les fréquences alléliques entre le pool pollinique et les arbres adultes ont été trouvées, mais le sens de l’hybridation semble asymétrique. Q petraea et Q robur sont presque exclusivement allogames, le taux d’allofécondation multiloci étant de 0,96 pour chacune des 2 espèces. allozymes/ hybridation / système de reproduction / pool pollinique /Quercus robur / Quercus petraea
INTRODUCTION MATERIALS AND METHODS The population studied is a mixed adult stand of Quercus petraea (Matt) Liebl and Quercus Q petraea and Q robur located in the Petite robur L have a largely sympatric distribu- Charnie forest, in north-western France (Le tion in Europe and it is suspected that they Mans). The trees are about 120 years old. The hybridize in nature. The species are ane- study area was square (220 X 220 m), with a mophilous; a survey of phenology in the uniform slope. In this area, a good correlation same mixed stand, described below, did was observed between hydromorphic layer depth and frequency of the 2 species. Q robur not show any differences in flowering time prefers more humid sites than Q petraea. between the 2 species (Expert, 1990). Dif- For genetic analysis, all plants of both spe- ferences in habitat preference can form a cies form the adult cohort. The young cohort barrier to gene flow, but in the intermedi- was made up of the progenies of these adults ate habitats the species are in contact and (fig 1),taking a maximum of 6 open-pollinated it is there that one can find the greatest seeds per family for sessile oak (160 individuals, number of intermediate forms (Grandjean 28 families) and 10 open-pollinated seeds per and Sigaud, 1987). Nevertheless, in natu- family for pedunculate oak (133 individuals, 16 ral populations, adult trees with intermedi- families). This protocol was used to avoid bias due to local heterogeneity of the pollen pool. ate features seem to be quite rare, less than 5% of the total population (Dupouey, The taxonomic status of the adults was deter- mined using factorial correspondence analysis 1983; Dupouet and Badeau, 1993). (FCA). The morphological characters used The possibility of hybridization between were: pubescence, number of intercalary and sessile and pedunculate oaks was proven lobe veins, auricle form and embossing of the by interspecific controlled crosses (Rush- lobe. ton, 1977). The success rate of artificial hybridization is higher when Q robur is fer- tilized with the pollen of Q petreae than vice versa ( Aas, 1991; Steinhoff, 1993). A few authors (Kremer et al, 1991; Müller-Starck et al, 1993) have investigat- ed interspecific differentiation on a genetic basis using biochemical markers, but so far no conclusions have been drawn as to hybridization in nature. At present, the strongest evidence concerning active ex- change of genes between pedunculate and sessile oaks can be deduced from the pattern of chloroplast gene diversity (Kremer and Petit, 1993). The major questions are: 1) what is the real extent of hybridization? 2) how can the 2 species be mantained? In this paper patterns of hybridization and of the mating system of Q petraea and Q robur have been inferred from examination of allo- zyme variation in 2 cohorts of a stand comprised of both species.
Allozymes extracted from buds of the adults loci in other studies (table I). As (Kremer et and roots of the seedlings were electrophor- al, 1991; Müller-Starck et al, this volume), esed. Seeds were collected directly from adult did not find any species-specific alleles. we trees during the autumn of 1989, and germinat- There significant differences in ed in an incubator. Technical procedures and were genetic interpretations are described in detail in gene frequencies between the pollen pool Kremer et al (1991) and Zanetto et al (1993). and the adult trees (table I). In spite of the We stained and then scored 8 enzyme systems pollen environment, which is composed of encoded by 8 putative loci: acid phosphatase similar proportions of conspecific versus (ACP), glutamate-oxalacetate transaminase foreign plants of the 2 species (mother (GOT), isocitrate dehydrogenase (IDH), menadi- trees are encircled by 32 and 37% of trees one reductase (MR), phosphoglucose isome- rase (PGI), phosphoglucomutase (PGM), leu- of the other species, for Q robur and Q pe- cine aminopeptidase (LAP) and alanine- traea, respectively), the gene frequencies aminopeptidase (AAP). in the seeds of both species showed an Allelic frequencies in the pollen pool and mul- asymmetrical shift towards more pro- tilocus outcrossing rates (t) were estimated with nounced Q petraea genetic characters. For Ritland’s computer program (1990), based on Q robur, this shift was significant for 4 loci the mixed-mating model. To obtain the best esti- (ACP, PGM, LAP and MR) out of the 7 mate of t, we used only the largest families, from 12 sessile oaks (332 individuals) and 10 pedun- with interspecific differences; AAP showed culate oaks (236 individuals). the pattern, but the difference same was Differences in allelic frequencies at each lo- significant only at the 0.10 level. For Q pe- cus between adults and pollen pool were as- traea, gene frequencies in the pollen pool sessed by a G-test. The differences between were significantly different from those of adults and pollen pools over all loci were evalu- the adults for 2 loci (MR and PGI). ated by a sign test (Sokal and Rohlf, 1981) that enables detection of directionality in changes of The sign test for all the loci showed that allele frequencies. For each of the 2 most fre- the directionality of changes was signifi- quent alleles at each locus, we assigned a posi- cant for both species, at the 0.011 propa- tive sign if its frequency in the pollen pool was bility level for Q petraea and 0.038 for Q similar to that of the adults of the other species, robur. Progenies of Q robur are therefore and a negative sign if the opposite was the genetically closer to the genetic pool of Q case. We then tested the hypothesis that the 2 signs were present in equal proportions; such petraea. sampling should exhibit a binomial distribution. Since incorrect taxonomic determina- The sign test is an exact test and does not re- tion can be a source of error in allele fre- quire calculation of degrees of freedom. quency estimates, we repeatedly calculat- ed gene frenquencies in adult groups by restricting the sample size of the parent RESULTS trees. Those with intermediate morphologi- cal characters were progressively exclud- Morphological analysis (performed by ed from the estimation of allele frequen- FCA, not shown here), failed to identify the cies. However, taxonomic status of 2% of the trees. Trees in significant changes no that did not produce seeds in 1989 were gene frequencies were found in these new excluded from subsequent analysis. The groups. adult cohorts were then made up of 186 Estimates of multilocus outcrossing sessile oaks and 212 pedunculate oaks. rates were 0.96 (± 0.08) and 0.96 (± 0.05) In adult trees, significant differences in for Q petraea and Q robur respectively. allelic frequencies were found between Neither of these estimations is significantly sessile and pedunculate oaks in 7 out of 8 different from one.
DISCUSSION (Aas, 1991; Steinhoff, 1993) showing a preferential pollen gene flow from Q pe- traea to Q robur, while the in re- success In the Petite Charnie forest, the frequency is close to A similar ciprocal crosses zero. of intermediate individuals at the adult unidirectional introgression has been de- stage, as deduced from FCA on morpho- scribed in Populus (Keim et al, 1989) and logical characters, was low, in spite of the in Eucalyptus (Potts and Reid, 1988). apparent lack of spatial or phenological If the pattern of unidirectional hybridiza- barriers to hybridization. tion occurs in the future, which needs to be Differences in allele frequencies be- confirmed, the gene pool of the next gener- tween adult populations of the 2 species ation of the Petite Charnie oak stand would were large and, within each species, were comprise a greater number of Q petraea stable over morphological classes. These genes. results are in agreement with the findings of other authors (Dupouey, 1983; Grandje- an and Sigaud, 1987; Dupouey and Ba- REFERENCES deau, 1993). The observed shift in gene frequencies Aas G (1991) Kreuzungversuche mit Stiel - und of Q robur progeny could be explained by Traubeneiche (Quercus robur L und Q pe- the fertilization of a portion of female flow- traea (Matt) Lieb). Allg Forst Jagdztg 162, 141-145 ers with pollen of Q petraea; on the con- JL trary, the causes of the shift in frequencies (1983) Analyse multivariable Dupouey de quelques caractères morphologiques de po- of Q petraea progeny difficult to are more pulations de chênes du Hurepoix. Ann Sci understand. For 40, 251-264 Different pre- and postzygotic mecha- Dupouey JL, Badeau V (1993) Morphological nisms may explain this asymmetry. For the variability of oaks (Quercus robur L, Quercus moment, we can only exclude the effects petraea (Matt) Liebl, Quercus pubescens of differential proportion of selfing. On the Willd) in northeastern France. Ann Sci For 50 (suppl 1), 35s-40s contrary, we cannot exclude that, in 1989, male flowering of Q petraea was heavier or Expert F (1990) Phénologie comparée du chêne et du chêne sessile. Internal pédunculé more effective than that of Q robur, contrib- re- port INRA, Bordeaux, France uting in that way to the largest part of the Grandjean G, Sigaud P (1987) Contribution à la fertilization of both species. Indeed, strong taxonomie et à l’écologie des chênes du Ber- temporal and spatial differences in the ge- ry. Ann Sci For 44, 35-66 netic composition of the pollen pool have Keim P, Paige KN, Thomas GW, Lark KG been found in other species, such as Fa- (1989) Genetic analysis of an interspecific gus sylvatica (Merzeau et al, 1989) and Pi- hybrid swarm of Populus: occurrence of uni- cea mariana (O’Reilly et al, 1982). directional introgression. Genetics 123, 557- 565 Moreover, large differences can be ob- served between loci. This shift may then Kremer A, Petit R (1993) Gene diversity in natu- ral population of oak species. Ann Sci For 50 result not only from asymmetric hydridiza- (suppl 1),186s-202s tion but also from various differentiating Kremer A, Petit R, Zanetto A, Fougère V, Du- forces. A, Wagner D (1991) Nuclear and or- cousso Nevertheless, the hypothesis of asym- ganelle gene diversity in Q robur and Q pe- metric gene flow is confirmed by the re- traea. In: Genetic Variation of Forest Tree sults of interspecific controlled crosses in Populations Europe (Müller-Starck G,
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