Báo cáo khoa học: "Mating system in a clonal Douglas fir (Pseudotsuga menziesii (Mirb) Franco) seed orchard. II. Effective pollen dispersal"

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article Original Mating system in a clonal Douglas fir (Pseudotsuga menziesii (Mirb) Franco) seed orchard. II. Effective pollen dispersal D Prat Laboratoire INRA-ENGREF de sciences forestières, 14, Girardet, 54042 France Nancy cedex, rue (Received 15 February 1994; accepted 4 July 1994) Summary — Genetic variation of effective pollen received by individual ramets in a Douglas fir clonal seed orchard was not representative of the genetic variation of this clonal seed orchard. This deviation was not due to selfing rate. Effective pollen dispersal might be responsible for this deviation. It was then assessed around 3 individual ramets by comparing genotypes of effective pollen with those of pollinator and surrounding trees located in a circle or in an ellipse focused on the receptor tree. The best prediction of genetic diversity of effective pollen was obtained considering a maximum pollination distance of about 20-30 m and an elliptical pollen dispersal. Effective pollen dispersal took place preferentially along rows of trees and probably depended on the wind. As the flowering period was short and particularly synchronous in the year of seed collection, reproductive phenology probably did not favour some peculiar crosses. Matings between a few neighbour trees was probably the major cause of the homozy- gosity level in seed crop. isozyme / mating system / pollen dispersal / Pseudotsuga menziesii= Douglas fir / surrounding trees Résumé — Régime de reproduction dans un verger à graines de Douglas (Pseudotsuga men- ziesii (Mirb) Franco. II. Dispersion du pollen. La diversité génétique du pollen reçu par des ramets individuels d’un verger clonal à graines de Douglas n’était pas représentative de la diversité géné- tique de ce verger. Ceci ne résultait pas du taux d’autofécondation, comme l’ont montré des études anté- rieures. Une dispersion réduite du pollen pouvait en être la cause. Aussi, la dissémination du pollen a été étudiée autour de 3 ramets particuliers en comparant la diversité génétique du pollen efficace qu’ils ont reçu à la diversité génétique des arbres proches situés dans un cercle ou une ellipse. La meilleure prédiction de la composition du nuage pollinique efficace a été obtenue en considérant les arbres pollinisateurs situés à une distance maximale de 20-30 m et en pondérant leurs flux polli- niques à l’aide d’un modèle de dispersion du pollen en ellipse. La dissémination du pollen a eu lieu prin- cipalement le long des lignes de plantation mais les vents dominants paraissent aussi avoir une * Present address: INRA, station d’amélioration des arbres forestiers, Ardon, 45160 Olivet, France
influence. Lesdécalages phénologiques entre arbres ne semblent pas responsables de croisements car, l’année de la récolte, la période de floraison a été particulièrement brève et syn- préférentiels chrone. Les croisements, qui ont lieu principalement entre les quelques arbres voisins, sont vraisem- principale cause des excès d’homozygotie observés dans la semence commerciale. blablement la dispersion du pollen / effet de voisinage / isoenzyme / Pseudotsuga menziesii Douglas / = régime de reproduction INTRODUCTION position. A study of pollen dispersal was required to explain these unexpected pat- terns of paternity. In a clonal seed orchard, the genetic quality The aim of the present study was to of the crop is mainly affected by the genetic describe pollen dispersal patterns in a clonal value of clones and by the mating system, seed orchard in order to explain the genetic especially selfing (Sorensen and White, structure of the seed crop. The analysis of 1988). Various analyses of mating systems pollen dispersal included a study of pollina- in seed orchard have thus been carried out tion distance. Previous studies in Douglas fir in diverse tree species (Barrett et al, 1987; showed that little pollen was dispersed El-Kassaby et al, 1988; Paule et al, 1993). beyond about 30 m (Erickson and Adams, The mating system was studied in 1 of 1989), but this could be influenced by the the first Douglas fir clonal seed orchards to plantation density. Pollination distance was produce seed for afforestation in France. In also estimated in Bout seed orchard. a this Bout seed orchard (located near In anemophilous species like Douglas an Moulins), the selfing rate was estimated by fir, pollen could diffuse around pollinator tree using isozymes as genetic markers, and all with a possible preferential direction due to clones were genetically characterised (Prat wind. Circle and elliptical areas of pollen dis- and Caquelard, 1995). The selfing rates persal were tested to identify the actual num- were low enough (about 4%) not to alter the ber of pollinator trees of a given receptor genetic quality of seed but the level of tree. Seeds of several ramets in the seed homozygosity was higher than that expected orchard were analysed with isozymes in according to the selfing rate. A lack of het- order to determine the position of their erozygosity can result from a limitation of respective pollinator trees and consequently gene flow in the orchard as produced by the pollen dispersal. At the seed level, only relatedness, floral phenology or pollination effective pollen (resulting in viable seed pro- distance. In the Bout seed orchard, no rela- duction after fecundation) could be detected. tionships between trees in the orchard were In the case of competition and selection as expected since it consists of an orchard suggested by Apsit et al (1989) the com- planted with clones selected from different plete pollen flow could not be assessed. stands. Phenology often influences mating systems (El-Kassaby and Ritland, 1986; El- Kassaby et al, 1988; Erickson and Adams, MATERIALS AND METHODS 1989). However in the year of study of mat- ing system in Bout seed orchard, all clones flowered simultaneously (Prat and Caque- Plant material lard, 1995). Moreover, genetic variation of male gametes received by a single ramet was not representative of that expected Pollen flow was studied in a Douglas fir clonal according to the seed orchard genetic com- seed orchard, the Bout seed orchard, located in
central France. It consists of 60 clones. The dis- Circle tance between ramets (all grafted) was 5 m in all direction at plantation establishment. Each of the As first step, the pollen dispersal was presumed a 20 blocks in the orchard contained 1 ramet of to be without preferential direction around each each clone. The survival rate of trees was about pollinator surrounding the receptor tree. Various 50% the year of this study. All 60 clones were pollination distances, up to 40 m, were tested. In identified by unique multilocus genotypes at 9 the simulations, only male-flowering ramets were enzyme loci (Prat and Caquelard, 1995). Seeds considered. Their contribution was weighted were collected in 1987, the first year of large seed according to their distance to the receptor tree production in this orchard, which was planted in and to their flowering abundance (noted in 3 1966. classes: low, intermediate, high; respective weight: Three ramets from 2 clones were chosen for 1,2,3). the study: clone 64 (ramet 1 and ramet 2) and clone 95 (ramet 1) because they bore rare alleles Ellipse (Got-1 and G6pdh in clone 64, Mdh-3 in clone 1 6 1 95) and most of their gametes were identifiable If the allelic frequencies observed in the pollen according to their multilocus genotype (all gametes remained significantly different from the expected from clone 95 were identifiable). The selfing rates one whatever the pollination distance, pollen is of these ramets were already estimated for the presumed to be dispersed in a preferential direc- same year of seed collection (Prat and Caque- tion. Such preferential directions of pollination lard, 1995). Pollen genotypes of trees surrounding have already been studied by Baradat et al (1984) studied ramets can be thus determined. and Erickson and Adams (1989). In the presence of a single factor (such as dominant wind) induc- ing a preferential direction of pollen dispersal, Pollen dispersal pollen should be distributed within a upwind/down- wind stretched ellipse instead of a circle; the pol- linator tree is at the focus of this ellipse. The pol- The flowering period was very short the year of lination thus occurred preferentially on one side of seed collection, because of a late frost period. the pollinator tree. In this condition, the relative Consequently, floral phenology was not taken pollen flow can be assessed in each direction by into account in the present study although it can the distance from the focus to the point of the be one of the major components of the mating ellipse circumference in the corresponding direc- system in Douglas fir seed orchard when flower- tion (segment OA’ in fig 1). The relative length of ing stretches over a longer period (El-Kassaby segment OA’ depends on the orientation (&thetas;- ϕ) of et al, 1988; Erickson and Adams, 1989). the point A’ to the long axis of the ellipse and on The pollination distance and the pollen flow its flatness a (width/length) (fig 1). The pollen dis- direction were assessed from the position of trees persal was determined by 3 parameters: the ori- pollinating the studied ramets. The genotypes of entation ϕ of the ellipse, its flatness a and the male gametes providing embryos in the collected maximal distance D of pollination. When the same seeds were used to determine fatherhood and force (direction and intensity) was supposed to clone location. The ramets analysed were suffi- act on the pollen of each tree, the pollen flow ciently distant (more than 40 m) from any other received by a tree was dependent on the same 3 ramet of the same clone to avoid noticeable pol- parameters, as if the receptor tree was located lination between ramets of same genotype. This at the downwind focus of the ellipse. The pollen minimal distance between 2 ramets was chosen flow was proportional to the distance from the according to the results of Erickson and Adams receptor tree to the point of the ellipse circumfer- (1989) so as to be higher than the pollination dis- ence in the considered direction. The orientation tance that these authors observed in a Douglas fir ϕ and the flatness a of the ellipse were tested to seed orchard in the United States. Different pollen find the best relationship between the expected dispersal simulations were tried, which led to an and observed allelic frequencies in pollen. Trees allelic composition of pollen received by a recep- surrounding the receptor and located within the tor tree. The pollen dispersal model with the low- considered ellipse or located under a maximal est and non-significant difference between distance D from the receptor were taken into observed and expected allelic frequencies in account; their distance was also tested for weight- pollen (2 test) was presumed the best. χ ing.
Pollen allelic frequencies were deduced from tilocus. The observed allelic frequencies megagametophyte and embryo genotypes of the (selfing excluded) in the pollen for the 3 ram- same seed at several enzyme loci (G6pdh, ets were significantly different from those Mdh-1, Mdh-2, Mdh-3, α-Est, Lap-1 and Lap-2) of the seed orchard and from those around according to Prat and Caquelard (1995). More the ramet used as female whatever the pol- than 400 seeds were analysed for each studied lination distance considered up to 35 m. The ramet. Only outcrossed seeds were considered best concordance was observed when pol- in the present study. The pollen genotypes at the most polymorphic locus (G6pdh) or at all loci anal- linator trees were located not more than 25 ysed (multilocus analysis) were taken into account. m from the receptor as for instance in ramet 2 of clone 64 (table I). The same situation was observed for ramet 1 of clone 64, but RESULTS allele G6pdh was observed in the pollen 1 received by this ramet at the frequency 0.011, and no tree up to 45 m bore this No preferential direction allele. of pollen dispersal (by d -1 the distance d Weighting by or ) -2 d between receptor and pollinator trees A single locus analysis (G6pdh locus) was did not improve the expected frequencies, carried out for the 2 ramets of clone 64. For while the male-flowering intensity did a little. the ramet 1 of clone 95, the analysis was The observed and expected allelic fre- either single locus (α-Est or G6pdh) or mul- quencies in the pollen were different in all
the tested conditions. The distance of effi- pollen were observed according to ϕ, α or L. cient pollination was about 15-30 m. Variation of these parameters induced changes (presence or absence) of the pol- linators taken into account and consequently Preferential direction of pollen dispersal in their genetic diversity. The orientation ϕ of the ellipse appeared as a major factor. The expected allelic frequencies of pollen Although intensely male-flowering, certain remained significantly different from the ramets close to the tested female fertilized observed ones. relatively few seeds. In contrast some low male-flowering and more distant ramets fer- Pollinator trees were then only consid- tilized many seeds (fig 2). The pollen dis- ered when their distance to the receptor tree persal could not be considered as isotropic was less than the maximal pollination dis- around the trees. Pollen dispersal accord- tance D. The relative pollen flow from each ing to an elliptical area was then consid- pollinator was weighted according to its ori- ered. entation &thetas;- ϕ. A weighting by the distance d (d or d between receptor and polli- -1 -2 ) When only pollinator trees located inside ellipse (length of long axis, L, corre- nator trees, or by the male-flowering inten- an sponding to a maximal pollination distance, sity did not improve the expected allelic fre- D) were taken into account, dramatic varia- quencies in pollen. The orientation ϕ of the tions of the expected allelic frequencies in ellipse greatly influenced the expected fre-
quencies especially when the maximal dis- analysed, the In the 3 ramets over- tance of pollination was reduced to 15 m from the same represented pollen came (table II). Pollen dispersal occurred along direction and could be attributed to west- rows of trees. This preferential direction was erly dominant wind. Wind might be more the same whatever the considered distance important for pollen dispersal than the tree of pollination up to 35 m (fig 3a). The flat- distance. ness α of the ellipse also significantly influ- enced the expected frequencies up to a pol- lination distance of 25 m (fig 3b). The best DISCUSSION agreement between expected and observed frequencies was obtained with a flatness of about 0.4 and a distance of pollination of Pollination distance 15-20 m (ramet 2 of clone 64 and ramet 1 of clone 95) and 30 m (ramet 1 of clone 64). Erikson and Adams (1989) have shown Allelic frequencies expected from the ellipse that very little pollen was effectively dis- model were close to those observed for persed beyond 30 m in a Douglas fir seed each allele (table II). orchard in Washington State. In a Pinus In these ramets received conditions, pinaster seed orchard the dispersal of from a small number of pollen essentially pollen was restricted to about 10 m (Bara- pollinators (less than 10 trees: 5-7 accord- dat et al, 1984). In the present study at ing to the ramet). The number of live trees least 10% of pollen came from more than was higher around ramet 1 of clone 64 (101 25 m; most of the pollen was dispersed trees at less than 37 m) than around the within 20-30 m. ramet 1 of clone 95 (74 trees at less than Because of its dilution in the pollen pool, 37 m). A higher density of trees did not seem the pollen dispersal was not detected to reduce the pollination distance. beyond some tens of meters. The pollen The multilocus analysis applied for produced by a tree might partly diffuse ramet 1 of clone 95 allowed a more pre- around this tree (where it could be detected) cise analysis because of the possible iden- and partly diffuse very far, after suspension tification of some clones according to their in atmosphere (and become undetectable gametes. In fact, only 10% of gametes because of dilution). Moreover its viability allowed identification of unique parental after a long migration was probably clone. None of the gametes from these reduced. parental clones could be identified and trees with a large pollinic contribution could not be located. Several trees on the same Elliptical dispersal of pollen side did not produce any progeny in spite of their large production of pollen. About 10% Baradat et al (1984) previously used an of the identified pollen came from clones ellipse for representation of pollen disper- more than 25 m away. A larger part (20%) sal. However in their case, trees were not of the pollen might not be produced by the considered at a focus but at the centre of clones according to their assessed geno- the ellipse. This could result from the orchard types. It probably came from flowering root- stocks (in 7 clones out of 21, the ramets design because the distance between the of the same presumed clone did not show trees was not the same between and within one single genotype; Prat and Caquelard, the rows. In such a design, pollen disper- 1995). sal was not the same between and within
pollen grain arriving on the ovule was the presence of because of the physical rows fertilising one. This does not seems to trees. In their model, Baradat et al (1984) always be the observed situation (Prat, analysed the possible origin of pollen from unpublished results). According to the first- different sectors of an elliptical area in order on first-in hypothesis of Webber and Yeh to recognize some preferential directions of (1987), no gametic selection occurred and pollen flow. The number of parameters the non-random effects (deviation from the required for the complete description of expected probability of gamete association) pollen dispersal was higher than in the pre- resulted from heterogeneous pollen flow. sent study. Baradat et al (1984) showed that Preferential crosses observed between pollen flow occurred mainly along the tree distant trees in the Bout seed orchard more rows. might result from non-random mating. The As found from the 3 ramets of the Bout general orientation of the pollen flow is suf- seed orchard, consideration of a preferential ficient in the Bout orchard to explain the direction of pollen flow improved the assess- observations. ment of allelic frequencies. In ramet 2 of The small distance of pollination and the clone 64, considering the ellipse model of highly oriented pollen flow resulted in a small pollen dispersal, no significant difference number of major pollinators (less than 10) between observed and expected allelic fre- per tree. This reduced genetic mixing of quencies was noticed for 1 set of parameter pollen and might alter the Hardy-Weinberg values. This was not the case considering a equilibrium. The preferential crosses circular pollen dispersal. In ramet 1 of clone between neighbour trees affected the mat- 64, when allele G6pdh was not taken into 1 ing system in the orchard. This might explain account (no source of this allele up to 45 the lack of heterozygosity observed in the m), similar results were obtained. This allele orchard and not due to selfing. may belong to the 10-20% of pollen coming from more than 25 m as observed in ramet 1 of clone 95, or it may come from mis- Seed orchard design labelled trees since such trees exist in the seed orchard (Prat and Caquelard, 1995). receptor and polli- The direction of pollination was the main The distance between not a major factor in the effect on the pollen flow in the Bout seed nator trees was pollen dispersal up to about 25-30 m. The orchard. Even in a regular design, the ellip- minimal distance of ramets belonging to the tical area of the pollen dispersal appeared clone in a seed orchard should thus suitable. As in the general situation, phe- same be of the same amount to avoid intra-clonal nology is a major component of the mating crosses. When the distance between and system of the seed orchard and considera- within rows is the same, square blocks with tions of the elliptical dispersal of pollen and a minimal size of 30-35 m would be the phenology (as taken into account by Erick- best. son and Adams, 1989) can be combined. In the Bout seed orchard, mating sys- tem was mainly affected by crosses Preferential crosses between neighbour trees. Since the ram- ets were distributed randomly in the seed orchard, the surrounding ramets of a clone Apsit et al (1989) suggested that a selec- varied from one block to another and each tion took place to explain the distortion clone could be statistically pollinated by all observed in controlled crosses. But Web- other clones. Nevertheless none of the ber and Yeh (1987) observed that the first
neighbour effects were suppressed by ran- REFERENCES dom distribution of clones and an increase of the homozygosity level in seed crop was Apsit VJ, Nakamura RR, Wheeler NC (1989) Differential observed. An improved orchard design male reproductive success in Douglas fir. Theor Appl Genet 77, 681-684 should include all combinations of clone Baradat P, Marpeau A, Bernard-Dagan C (1984) Les neighbourhood in order to maximise pan- terpènes du pin maritime : aspects biologiques. VI. suggested by Vanclay (1986, mixia, as Estimation du taux moyen d’autofécondation et mise 1991 ). evidence d’écarts à la panmixie dans un verger à en graines de semis. Ann Sci For 41 , 107-134 Barrett JW, Knowles P, Cheliak WM (1987) The mating system in a black spruce clonal seed orchard. Can J CONCLUSION For Res 17, 379-382 El-Kassaby YA, Ritland K (1986) The relation of out- crossing and contamination to reproductive phenol- The model of pollen dispersal according ogy and supplemental mass pollination in a Douglas to an elliptical area allows for the detec- fir seed orchard. Silvae Genet 35, 240-244 tion of the orientation of dispersal and the El-Kassaby YA, Ritland K, Fashler AMK, Devitt WJB number of major pollinating trees. This (1988) The role of reproductive phenology upon the mating system of a Douglas fir seed orchard. Silvae model is more efficient than circular pollen Genet 37, 76-82 dispersal in prediction of pollen allelic fre- success in Erickson VJ, Adams WT (1989) Mating a quencies. Trees were pollinated by few coastal Douglas fir seed orchard as affected by dis- surrounding trees with a large influence of tance and floral phenology. Can J For Res 19, 1248- wind direction. The result is a lack of pollen 1255 flow between each clone and the occur- Lindgren D, Yazdani R (1993) Allozyme fre- Paule L, quencies, outcrossing rate and pollen contamina- rence of preferential crosses, and conse- tion in Picea abies seed orchard. Scand J For Res 8, quently a lack of heterozygosity in the crop, 8-17 in spite of the random design of clones in Caquelard T (1995) Mating system in a clonal Prat D, every block. Douglas fir (Pseudotsuga menziesii (Mirb) Franco) seed orchard. 1. Gene diversity and structure. Ann Sci For 52, 201-211 Sorensen FC, White TL (1988) Effect of natural inbreed- ACKNOWLEDGMENTS ing on variance structure in tests of wind-pollination Douglas fir progenies. For Sci 34, 102-118 I would like to thank JC Bastien, B Roman-Amat JK (1986) Design for a gene recombination Vanclay orchard. Silvae Genet 35, 1-3 and E Teissier du Cros for critical reading of the manuscript. This research was supported by the Vanclay JK (1991) Seed orchard designs by computer. Groupement d’intérêt scientifique - création, éva- Silvae Genet 40, 89-91 luation et diffusion de variétés forestières Webber JE, Yeh FCH (1987) Test of the first-on, first-in améliorées, grant No 87 G 0315 from ministère de pollination hypothesis in coastal Douglas fir. Can J la Recherche et de l’Enseignement supérieur. For Res 17, 63-68