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2Finnish Forest Research Institute, Suonenjoki Research Station, Juntintie 40, FIN-77600 Suonenjoki, Finland; 3Finnish Forest Research Institute, Haapastensyrjä Breeding Centre, Karkkilantie 247, FIN-12600 Läyliäinen, Finland; and 4Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35, FIN-40531 Jyväskylä, Finland
Received for publication April 18, 2000. Accepted for publication June 27, 2000.
ABSTRACT
The relationship between pollen and progeny performance has been a subject of many studies but the evidence for pollen-tube growth rate as an indicator of progeny fitness is equivocal. We used an anemophilous tree, Betula pendula, to examine the relationship between pollen-tube growth rate and seed and seedling performance. We crossed nine maternal plants with pollen from six pollen donors in a clonal B. pendula seed orchard, measured the pollen-tube growth rates for every cross, and analyzed the performance of the resulting seeds and seedlings. The only significant positive correlation was found between pollen-tube growth rate and seed mass when we controlled for seed number per inflorescence. Using seed mass as a covariate, we found that only maternal parent had a significant effect on the number of seeds per inflorescence, the percentage of germinable and embryonic seeds, and early seedling growth. Both maternal and paternal parents had significant effects on seedling height after 85 d of growth. These results are in concordance with the general view that maternal effects are usually most apparent in seed characters and during early plant growth. This study does not provide strong evidence for the theory of pollen-tube growth rate as an indicator of progeny quality.
Key Words: Betulaceae • Betula pendula • maternal effects • paternal effects • pollen-tube growth rate • progeny performance • seed abortion
During the last three decades many studies have been carried out on the relationship between pollen and progeny performance, but the evidence for pollen-tube growth rate as an indicator of progeny performance remains equivocal. The idea of a positive relationship between pollen and progeny performance is based on an overlap in gene expression between gametophytic and sporophytic stages of life cycle (Mulcahy, 1979
). The effects of pollen-tube growth rate on progeny quality have usually been studied by varying the intensity of pollen competition by applying different amounts of pollen to the stigmas (Winsor, Davis, and Stephenson, 1987
; Bertin, 1990
; Snow, 1990
; Richardson and Stephenson, 1992
; Janse and Verhaegh, 1993
; Quesada, Winsor, and Stephenson, 1993, 1996a
; Palmer and Zimmerman, 1994
; Johannsson and Stephenson, 1997
; Mitchell, 1997
) or by varying the distance pollen tubes must travel to the ovules (Mulcahy and Mulcahy, 1975
; Ter-Avanesian, 1978
). It has been assumed that under conditions of intense pollen competition only the fastest-growing pollen tubes achieve fertilization (Mulcahy, 1979
).
Studies employing small and large pollen loads have been criticized due to the fact that results from these studies are not always easy to interpret (Schlichting, Stephenson, and Small, 1990
; Mitchell, 1997
). For example, high pollen loads alone may stimulate the maternal parent to allocate more resources to seeds and fruits and thus better quality offspring may be a result of the pollen load itself, rather than competition among pollen tubes (Charlesworth, 1988
). Differences in sporophyte quality could also be due to undetected selective abortion of the embryos or the seeds (Walsh and Charlesworth, 1992
). Furthermore, it has been suggested that low pollen loads might not have been low enough to exclude all possibilities for pollen competition (Mitchell, 1997
). For these reasons, it would be informative to employ direct pollen-tube growth rate measurements and the analysis of progeny quality from different maternal plants pollinated by the same pollen donors.
In previous studies we have found evidence for the positive relationship between pollen-tube growth rate and seed siring success of the pollen donors in a self-incompatible, deciduous tree, Betula pendula Roth (Pasonen et al., 1999
). The aim of the present study was to investigate the maternal and paternal effects on seed and seedling performance, with particular attention to the relationship between pollen-tube growth rate and progeny performance. We also attempted to find out whether selective abortions of the seeds were likely to occur in Betula pendula. The clonal seed orchards of B. pendula established in plastic greenhouses provided an excellent controlled environment for studying evolutionarily important processes that might have been confounded by environmental variation in natural pollination conditions. No previous studies on the relationship between pollen and progeny performance have been performed in this commercially important tree species. To our knowledge, this study is also the first one in which direct pollen-tube growth rate measurements have been employed to examine the relationship between pollen and progeny performance.
MATERIALS AND METHODS
Study species and study site
Betula pendula Roth is a common, anemophilous, monoecious, and self-incompatible tree ranging throughout most of Europe from Norway to Sicilia (Tutin et al., 1964
). It has 200–300 male flowers in each catkin (Dahl and Fredrikson, 1996
) and 
600 female flowers in each pistillate inflorescence (H.-L. P., personal observation). Each female flower consists of a single two-locular ovary with two linear, dry stigmas and two ovules of which only one develops into a mature seed (Sulkinoja and Valanne, 1980
; Dahl and Fredrikson, 1996
).
The study was carried out in a plastic house seed orchard at Haapastensyrjä Forest Tree Breeding Centre in Läyliäinen (60° N, 24° E), Finland. The seed orchard consists of 38 B. pendula clones originating from 11 closely situated populations. The clones were originally selected for the seed orchard on the basis of the results from the field trials (Raulo and Koski, 1977
). These field trials were done to select genotypes with superior heritable growth characters (e.g., straight stem). A plastic house provides favorable conditions for flowering and seed development and isolates seed orchard clones from outside pollen sources.
Hand-pollinations and pollen-tube growth rate measurements
To study the pollen-tube growth rates of different pollen donors on several maternal plants, a single-donor hand-pollination experiment was conducted. Nine maternal plants (different B. pendula clones) were selected from a group of seed orchard clones and were later pollinated with pollen from six paternal clones. Maternal clones were selected on the basis of the number of female inflorescences (only the clones with large amounts of female inflorescences were used), and paternal clones were selected on the basis of the germination ability of pollen. Before any pollen was shed and before female inflorescences were receptive, twelve adjacent branches (two for each pollen donor) from each maternal plant were isolated with paper bags to prevent uncontrolled pollinations. Each branch was isolated with one paper bag, and each bag contained three (for pollen-tube growth rate measurements) or ten (for analyzing the progeny performance) female inflorescences. All of the male inflorescences in the bagged branches were removed to prevent self-pollinations. Adjacent branches were isolated to minimize the differences among the branches. When female inflorescences became receptive, an equal volume of pollen was applied to each pollination bag by using a pollination syringe. The amount of pollen applied into the pollination bags exceeded the number of ovules in the bags. Each maternal plant had a total of 12 hand-pollinations (six for pollen-tube growth rate measurements and six for the analysis of progeny performance). The single-donor hand-pollinations for pollen-tube growth rate measurements and for the analysis of progeny performance were performed in the plastic house seed orchard at the same time during a period of 3 d (maternal plants became receptive at different times) in the beginning of May 1997.
Pollen stored at -20°C was used in hand-pollinations. There was not enough time to collect fresh pollen before hand-pollination experiments because the flowering of male and female flowers occurs almost synchronously in B. pendula. Germination percentages of stored pollen from several donors were determined in vitro on agar medium (containing 1% agar, 0.01% boric acid, and 0.5 mol/L sucrose) (Käpylä, 1991
). In previous experiments we found that in vitro pollen tube growth of stored pollen correlates positively with pollen tube growth in vivo (Pasonen et al., 1999
). We have also found that if B. pendula pollen is properly collected (in dry conditions) and dried at room temperature (24°C) for 24 h before storage at -20°C, germination ability of the pollen samples remains fairly unchanged for several years. Finally, six paternal clones with as equal germination percentages as possible were chosen for the experiment. Germination percentages of the selected pollen donors varied between 60 and 70%. Paternal clones with equal germination percentages were selected because germination tests in vitro have revealed that high germination percentages can also enhance the pollen tube growth rate (Pasonen, Käpylä, and Pulkkinen, 1997
).
Twelve hours after pollination, three pollinated female inflorescences per cross were collected. After 12 h the fastest pollen tubes had almost reached the base of the elongate stigma, but not entered the funiculus. In B. pendula there is a resting state in pollen tube growth after which all the pollen tubes have a "fair start" to the ovules regardless of their original positions and arrival times on the stigma (Dahl and Fredrikson, 1996
). We measured the pollen tubes before the resting state had begun. In our previous studies we have found that there is a significant positive correlation between pollen tube length after 12 h of germination and seed siring success (Pasonen et al., 1999
). The female inflorescences were detached and immediately stored in glacial acetic acid and 60% ethanol (1:9). The inflorescences were stored in a refrigerator at 4°C until they were examined. The single flowers were detached from the inflorescence with a scalpel and stained with a solution of 0.1% aniline blue in aqueous K3PO4 (0.3 mol/L). After staining, pollen tube callose became fluorescent and distinguishable in the darker stylar tissue when examined with UV fluorescence microscopy. Between 80 and 120 pollen tubes (from three inflorescences and several flowers) were measured per cross so that a total of 162 (54 crosses x 3 inflorescences) female inflorescences was examined. Pollen tubes were measured by focusing the scale of the ocular on the pollen tube and counting the intervals of the scale from the base to the tip of the pollen tube. If the pollen tube was not straight, the scale was moved along the pollen tube while counting the intervals so that a reliable measure of pollen tube length was obtained. Pollen tubes were measured only in styles that had no more than 1–2 pollen tubes because it is known that the density of pollen tubes in the stylar tissue may influence pollen tube growth (Cruzan, 1990
; Holm, 1994
).
Analyzing the progeny performance
Seeds from the pollinated inflorescences were collected in July 1997. Total seed mass per cross, mass of a random sample of 100 seeds, and percentage of germinable and embryonic seeds (= seeds with fully developed embryos) were determined in the following autumn. A microfilm reading device was used to determine whether a seed contained a fully developed embryo. A fully developed embryo could be seen to fill a large part of a seed while seeds without embryos were fairly transparent. The number of seeds per cross was estimated by dividing the total seed mass per cross by the average mass of a single seed of the same cross. The number of seeds per inflorescence in each cross was estimated by dividing the total number of seeds per cross by the number of pollinated inflorescences. Seeds were stored in 4°C until all of the analyses were carried out. To determine the seed germination percentages, moist pieces of paper were placed on petri dishes, and a random sample of 100 seeds per cross were counted on each piece of paper. The petri dishes were placed in a germination chamber on a 12 h light/12 h dark cycle at room temperature (23°C). The number of germinated seeds was counted every day during a period of 12 d. A seed was considered germinated if it had a clearly visible primordial cotyledon whose length was at least 1 mm. The germination percentage on the 12th d of germination was used in the statistical analyses.
On the 1 June 1998, seeds from every cross were sown on peat substrata in a greenhouse at Haapastensyrjä Forest Tree Breeding Centre to study the seedling growth rate. Twenty to 25 seedlings per cross were raised as two replicates of 10–15 seedlings in natural light conditions (2 x 10–15 seedlings per cross). The replicates were scattered randomly throughout the greenhouse to minimize the effects of possible microclimatic differences in the greenhouse. Temperature in the greenhouse was not controlled, although it was higher than outdoors on sunny days. One month later the seedlings were transferred outdoors. The height of the seedlings was measured twice, in the beginning of July (growing time 30 d) and in the end of August (growing time 85 d) 1998.
Estimation of seed abortion frequency
Several (10–20) female inflorescences were collected from different parts of four randomly chosen maternal plants that were also used in the hand-pollination experiment. The number of female flowers in each inflorescence was counted to estimate: (1) whether the number of flowers per inflorescence varied within a tree and (2) the average number of flowers per inflorescence per tree. The number of female flowers within an inflorescence did not differ in different parts of the tree among three out of four maternal plants. Among these three maternal plants, it could be estimated whether the maternal plants aborted differentially seeds sired by different pollen donors (see Table 7). The frequency for seed abortion was estimated by comparing the average number of female flowers per inflorescence with the average number of seeds per inflorescence in each cross. Because excess pollen was applied to the stigmas it was assumed that all the female flowers in each inflorescence were pollinated and fertilized. However, this could not be verified, and thus all the values for the frequency of seed abortion should be considered as rough estimates of the number of aborted or undeveloped seeds.
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Partial correlations between PTL and seed and seedling performance were calculated using seed mass as a controlling factor due to the fact that seed mass was found to correlate with progeny performance. To assess the relationship between PTL and seed mass, the number of seeds per inflorescence (total number of seeds per cross divided by the number of pollinated inflorescences) was used as a controlling factor. To determine whether the rankings of the pollen donors in the form of percentage of germinable and embryonic seeds, seed mass, and seedling height changed depending on the maternal plant, a Kendall's coefficient of concordance (W) was calculated (Sokal and Rohlf, 1981
, p. 609).
Two-way (random-effects) ANOVAs using seed mass or seed number per inflorescence as covariates were performed to test the effect of maternal and paternal parent on seed number per inflorescence, percentage of germinable and embryonic seeds, and mean seed mass. Number of seeds per inflorescence was used as a covariate when the effect of parental clones on seed mass was tested. Percentages of germinable and embryonic seeds were arcsine square-root transformed to normalize the data. Two-way (random-effects) ANOVA of the effects of parental clones on seedling height was performed using the original heights of each measured seedling (not the means of the seedlings' heights for each cross). Seed mass could not be used as a covariate in this analysis due to the fact that only one value for seed mass was available in each cross.
One-way ANOVAs were performed to study whether the number of female flowers per inflorescence varied within and between the maternal plants. A Kendall's coefficient of concordance was calculated to study whether the ranking orders of the pollen donors differed in the number of aborted or undeveloped seeds across the maternal plants. In addition, a Spearman correlation coefficient between PTL and the number of aborted or undeveloped seeds was separately calculated for the three maternal plants.
RESULTS
Seed quality
A positive relationship between pollen tube length and seed quality expressed as seed mass and percentage of germinable and embryonic seeds was detected on most maternal plants. However, the significance of the correlation coefficients varied across the maternal plants and was mostly nonsignificant (Table 1). The only significant positive Spearman correlation calculated on the basis of the means of the nine maternal plants for each pollen donor (see Table 2a) was found between pollen tube length and percentage of germinable seeds (Table 1). Means of the six pollen donors on nine maternal plants have been summarized in Table 2b.
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Two-way ANOVA using seed mass as a covariate revealed significant maternal effects on seed number per inflorescence and percentage of germinable and embryonic seeds. Paternal clone had no effect on any of these traits. Neither maternal nor paternal clone had significant effect on seed mass (Table 5).
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Kendall's coefficients of concordance for six pollen donors on nine maternal plants were 0.12 (P > 0.05) for seedling height after 30 d of growth and 0.43 (0.001 < P < 0.01) for seedling height after 85 d, indicating concordance in the rankings of the pollen donors in later seedling growth. Two-way ANOVA of the effects of parental clones on seedling height revealed that only the maternal parent had significant effects on early seedling growth (growing time = 30 d) but both maternal and paternal parents had significant effects on later seedling growth (growing time = 85 d) (Table 6).
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DISCUSSION
Relationship between pollen-tube growth rate and progeny performance
In the present study, direct pollen-tube growth rate measurements were employed to study the relationship between pollen-tube growth rate and seed and seedling performance. A positive correlation was found between pollen-tube growth rate and seed mass when we controlled for the number of seeds per inflorescence. No significant correlations between pollen-tube growth rate and percentage of germinable and embryonic seeds and between pollen-tube growth rate and seedling height were detected. Previously, the effect of pollen-tube growth rate on progeny performance had been studied indirectly by varying the intensity of pollen competition. Many previous studies revealed no effect of the intensity of pollen competition on seed mass (Bertin, 1990
; Snow, 1990
; Björkman, 1995
; Johannsson and Stephenson, 1997
; Quesada, Winsor, and Stephenson, 1996a, b
) or on percentage of germinable seeds (Mulcahy and Mulcahy, 1975
; Snow, 1990
; Johannsson and Stephenson, 1997
). There are some studies that document a positive effect of the intensity of pollen competition on percentage of germinable seeds (Davis, Stephenson, and Winsor, 1987
; Bertin, 1990
; Palmer and Zimmerman, 1994
), but evidence for positive effects on seed mass was not found. The effects of the intensity of pollen competition on seedling growth rate have rarely been studied, and the results from the existing studies reveal no significant trend (Ter-Avanesian, 1978
; Bertin, 1990
; Janse and Verhaegh, 1993
; Palmer and Zimmerman, 1994
).
Delph, Weinig, and Sullivan (1998)
have hypothesized that it is the order of fertilization rather than speed of pollen tube growth that affects the vigor of the resulting progeny because the ovules fertilized early are better provisioned by the maternal plant than later fertilized ovules. Many studies show that faster growing pollen tubes fertilize ovules in different regions in the ovary than slower growing pollen tubes (Marshall and Ellstrand, 1988
; Stephenson, Winsor, and Schlichting, 1988
; Rocha and Stephenson, 1991
), and it has been suggested that different regions in the ovary might have nutritional advantages, which, in turn, could lead to increased vigor in progeny (Stephenson, Winsor, and Schlichting, 1988
). The eliminating of maternal effects by comparing the progeny resulting from small and large pollen loads in different regions of the ovary, as was done by Quesada, Winsor, and Stephenson (1993)
, does not rule out the possibility that high pollen loads alone may stimulate the maternal parent to allocate more resources to seeds and fruits.
In our study, the differences in progeny performance cannot be due to greater maternal allocation to higher pollen loads because the amount of pollen did not vary between pollinations. Furthermore, the differences in progeny performance cannot be due to differential maternal allocation to different regions of the ovary because there is only one ovule in the ovary that develops into mature seed in Betula pendula. It is, however, not known whether flowers in different parts of the inflorescence are differentially provisioned by the maternal plant. In this study, a random sample of pollen tubes and seeds from several flowers in different parts of the inflorescences were analyzed so that the results should not be biased by the location of the flowers in inflorescences.
In addition to differential maternal allocation, another widely discussed problem (Charlesworth, 1988
; Stephenson et al., 1988
; Walsh and Charlesworth, 1992
) linked with studies of pollen competition is the relationship between seed number and seed size. In many plants, average seed mass decreases as the number of seeds within a fruit increases (Stanton, 1984
), and differences in seed size, in turn, are known to affect many seedling fitness characters (Schaal, 1980
; Weis, 1982
; Zimmerman and Weis, 1982
). Seed size has sometimes been matched with the pollination treatments by comparing only the groups of same sized-seeds from two treatments (Winsor, Davis, and Stephenson, 1987
) or comparing the fruits containing the same number of seeds (Mulcahy and Mulcahy, 1975
). We found that the average number of seeds within an inflorescence varied between the crosses and that there was a negative relationship between seed number per inflorescence and seed mass as expected. Furthermore, a substantial positive correlation between seed mass and percentage of germinable and embryonic seeds was detected. Instead, a significant negative correlation between seed mass and later seedling growth was found. Effects of seed mass on seedling performance can, however, be transient, and a longer period for recording seedling growth in Betula pendula is needed. In Campanula americana, for example, seed size correlated positively with early seedling performance but not with any of the later vegetative measures or reproductive output (Richardson and Stephenson, 1992
). Furthermore, due to the fact that B. pendula seeds contain very little endosperm for early seedling growth, there might not be as clear a relationship between seed mass and seedling performance as observed in many other species.
An interesting finding in this study was the negative relationship between pollen-tube growth rate and early seedling growth when seed mass was used as a controlling factor. Many authors have addressed the question of the mechanisms that might maintain the variation in pollen performance if pollen tube growth rate is an important component of male fitness (Snow and Mazer, 1988
; Walsh and Charlesworth, 1992
). In our previous study, we concluded that pollen-tube growth rate is an important factor controlling the paternity of the seeds in B. pendula seed orchards (Pasonen et al., 1999
). If this is the case also in natural populations, there have to be mechanisms that maintain the variation in pollen competitive ability. It has been suggested that mechanisms like gene flow, mutations, genotype–environment interactions, pollen–pollen interactions, and negative genetic correlations between gametophytic and sporophytic stages of life cycle could explain the maintenance of genetic variation in pollen-tube growth rate (see Schlichting, Stephenson, and Small, 1990
; Walsh and Charlesworth, 1992
; Mulcahy, Sari-Gorla, and Mulcahy, 1996
). Pollen–pollen interactions (Pasonen and Käpylä, 1998
) and genotype–environment interactions in pollen-tube growth rate (Pasonen, Käpylä, and Pulkkinen, 2000)
have already been documented in Betula pendula. The negative relationship observed in this study between pollen-tube growth rate and early seedling height can be due to negative epistatic effects and have an evolutionary significance in maintaining the variation in pollen performance. Although the relationship was not statistically significant, even minor effects on the competitive ability of the seedlings can be biologically important during a vulnerable period of very early growth.
Parental effects on progeny performance
The results of the present study reveal substantial maternal effects on the number of seeds per inflorescence, percent germinable and embryonic seeds, and seedling height. Paternal effects were detected only for seedling height after 85 d of growth. These results are in concordance with the general view that paternal effects are rarely found in plants (Roach and Wulff, 1987
), and they may not easily be recognized in the presence of maternal effects because they are usually confounded with them (Schmid and Dolt, 1994
). Maternal effects are usually most apparent in seed characters and during early plant growth, due to the intimate dependence of the developing seed on the maternal plant (Roach and Wulff, 1987
). Seed traits are also more influenced by genes expressed in the maternal parent than by genes expressed in the embryo (Thiede, 1998
). Also paternal effects on seed development are known, though they are of lesser magnitude than maternal influences (Nakamura and Stanton, 1989
; Richardson and Stephenson, 1991
). The lack of paternal effects on seed performance and early seedling growth in this study can partly be explained by the fact that the experiment was not primarily designed to test for small paternal and maternal effects.
Maternal effects on seed development and on later progeny performance can be either environmental or genetic (or both). Because maternal plants were raised in uniform conditions in the plastic house, maternal effects observed in this study are more likely to be due to genetic than to environmental effects. Genetic maternal effects generally decline through the life cycle (Schmid and Dolt, 1994
), and studying later vegetative characters of the seedlings can reveal decreased maternal and more pronounced paternal effects. Recall that in the present study only the maternal parent had significant effect on early seedling growth but both maternal and paternal parents had significant effects on later seedling growth.
Seed abortion and maternal provisioning
It is possible that maternal plants can influence the quality of their offspring by aborting seeds nonrandomly with respect to paternal genotypes (Lee, 1984
; Marshall and Ellstrand, 1988
). Such maternal effects have rarely been demonstrated because pollen competition is always a potentially confounding effect (Marshall and Ellstrand, 1988
). We found that maternal plants differed significantly in the number of aborted or undeveloped seeds in Betula pendula. Because the number of flowers per inflorescence can only be counted by destroying the inflorescence, the initial number of flowers of pollinated inflorescences is not known, and thus, it is not known whether the variation in the number of seeds per inflorescence is due to the original differences in the number of flowers within an inflorescence or due to the abortion of the seeds sired by different pollen donors. Among three maternal plants, no differences in the number of female flowers within an inflorescence was detected in different parts of the tree but the number of seeds sired by different pollen donors varied remarkably. This may indicate that selective abortions occur. However, because it could not be verified whether all the flowers in inflorescences were fertilized or not, the number of aborted seeds should be considered as a rough estimate of the number of aborted or undeveloped seeds. No correlation between pollen-tube growth rate and the number of aborted or undeveloped seeds was found. The significant concordance in the rankings of the pollen donors in terms of aborted or undeveloped seeds on different maternal plants may indicate that all three maternal plants favor seeds sired by the same pollen donors. It is also possible that maternal plants allocate resources in different ways to seeds sired by different pollen donors during seed development. We found no evidence for the idea that maternal plants should allocate more resources to certain pollen donors because no concordance among pollen donors was detected in terms of seed mass and percentage of germinable and embryonic seeds on different maternal plants.
Conclusions
Due to the effects of seed mass on seed and seedling performance, the most reliable insight into the effects of pollen-tube growth rate on progeny performance is provided by the partial correlations in which seed mass has been used as a controlling factor. Although a positive relationship between pollen-tube growth rate and seed quality was found, the only significant positive correlation was detected between pollen-tube growth rate and seed mass. No correlation between pollen-tube growth rate and seedling height was detected. Because the positive effects of pollen-tube growth rate on seed quality did not translate into faster growing seedlings, no strong evidence for the theory of pollen-tube growth rate as an indicator of progeny quality is provided by this study. The slight negative relationship between pollen-tube growth rate and early seedling growth can be due to negative epistatic effects and have an evolutionary significance in maintaining the variation in pollen-tube growth rates. The result of substantial maternal and largely undetectable paternal effects on seed and seedling performance is in concordance with the general view that paternal effects are rarely found in plants (Roach and Wulff, 1987
), and they may not easily be recognized in the presence of maternal effects (Schmid and Dolt, 1994
). In conclusion, the answer to the question raised in the title of this paper is that pollen-tube growth rate is not a good predictor of progeny performance in Betula pendula.
FOOTNOTES
1 The authors thank Prof. A. G. Stephenson from the Pennsylvania State University, Dr. A. E. Faivre from the Ohio State University, the anonymous reviewers, and colleagues from the University of Jyväskylä for helpful comments on the manuscript, Sirkku Pöykkö and Jari Laukkanen at the Haapastensyrjä Forest Tree Breeding Centre for valuable help in field work, Airi Virtanen and Ritva Linnamäki for planting the seeds and raising the seedlings, and Laimi Myllylä at Patama Central Nursery for analyzing the quality of the seeds. This study was supported by Foundation for Forest Tree Breeding, the Academy of Finland, and by a grant from the University of Jyväskylä to H.-L. P. 
5 Author for correspondence (Hanna.Pasonen{at}metla.fi
).
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