Journal of Heredity Advance Access originally published online on September 8, 2005
Journal of Heredity 2005 96(6):635-643; doi:10.1093/jhered/esi104
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Mating Patterns of a Subdivided Population of the Andean Oak (Quercus humboldtii Bonpl., Fagaceae)
From the Laboratoire d'Ecologie, Systématique et Evolution, Bât 360, Université Paris Sud XI, 91405 Orsay Cedex, France (Fernández); and Department of Ecology and Evolutionary Biology, University of California–Los Angeles, Los Angeles, CA 90095-1606 (Sork)
Address correspondence to Juan F. Fernández-M. at the address above, or e-mail: juan.fernandez{at}ese.u-psud.fr.
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Abstract |
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Mating patterns play a critical role in the maintenance of genetic variation. We analyzed the mating system in a recently fragmented population of the Andean oak (Quercus humboldtii) using four microsatellite loci. Five fragments in northeastern Colombia, South America, were selected consisting of 30.4 trees on average. We sampled about 30 seeds from three target trees in each fragment and genotyped them with four microsatellite loci with a total of 40 alleles across loci. Progenies were analyzed under the mixed mating system model (MLTR program) and the TwoGener pollen pool structure analyses. The number of unshared pollen donors per family (Nu) was estimated using gametotypic counts with the program HAPLOTYPES. Low selfing (3%) is occurring at the population and fragment level. Biparental inbreeding is present (4.9%), but reduced, in the largest fragment. The average pollen neighborhood size (Nep = 5.1 to 6.1) appears comparable to other oak species in sparse landscapes. In contrast, Nu consists of 2 to 4 main donors, although up to 12 are possible, with the mode positively correlated with fragment size. The Andean oak appears to be a resilient species capable of tolerating population subdivision, provided landscapes include large fragments.
Habitat fragmentation can erode genetic diversity and increase population differentiation in plant populations by impeding gene flow and reducing local population size, which could in turn increase random genetic drift and inbreeding (Ledig et al. 2001). The size of the fragments and their degree of isolation will determine the extent to which remnant populations experience reduced gene flow among habitat remnants after fragmentation. For example, the detection of isolation by distance pattern in Tillandsia achyrostachys in a heavily logged area (González-Astorga et al. 2004) is interpreted as evidence of fragmentation effects, and hence calls attention to the need for conservation measures. In contrast, many studies point to the opposite. In wind-pollinated Acer saccharum, the genetic diversity of seedlings in fragments seems higher than the adults established under closed forest conditions (Young et al. 1993), suggesting that gene flow is higher in fragments. In wind-pollinated Pinus echinata, pollen movement was greater among trees in clearings than among those in closed forests, even though pine density was the same (Dyer and Sork, in preparation), possibly due to enhanced pollen movement where vegetation structure was reduced. An increase in pollen movement distance was also observed in fragment populations of insect-pollinated Swietenia humilis (White et al. 2002) and in several other studies of insect-pollinated tropical tree species (e.g., Apsit et al. 2001; Nason and Hamrick 1997). Thus it appears that for many species a paucity of local individuals results in a higher proportion of pollen coming from outside the fragment, which mitigates the potential detrimental effects of isolation.
The challenge of studying the genetic effects of fragmentation is that many different processes are occurring simultaneously. Within each fragment, the local population may experience increased self-pollination or an increased chance of mating with individuals sharing a recent common ancestry (Barrett et al. 1993; Raijmann et al. 1994; Young et al. 1996). Doligez and Joly (1997) found a significant reduction in outcrossing rates from unlogged to logged plots in Carapa procera, a large insect-pollinated lowland rainforest tree. Meanwhile, its congener, Carapa guianense, exhibited increased mating among relatives in logged populations, possibly because of the high density of adults who flower synchronously (Hall et al. 1994). In contrast, Dryobalanops aromatica, another large, insect-pollinated rainforest tree, showed no differences between logged and unlogged forests (Kitamura et al. 1994). Studies of wind-pollinated species show similar heterogeneity of results. In Pinus strobus (Rajora et al. 2002), the rates of seedling inbreeding were positively correlated with the average distance to the five nearest neighboring trees in small, isolated populations in eastern Canada. Thus the impact of fragmentation on the local mating system depends on complex interactions of phenology, local population size, and the extent to which the pollen vector overcomes the effects of landscape change.
A third potential genetic consequence of fragmentation in plants is a reduction in the effective number of pollen donors, which is an important element of effective population size (Wright 1969). In Wright's classical modeling, the size of the neighborhood is directly proportional to the variance in dispersal and to the density of potential parents. Postfragmentation populations may remain connected by gene flow, but the effective number of pollen donors could decrease if local adults contribute proportionately more to the pollen pool than distant pollen donors (Smouse and Sork 2004). Estimates of the effective number of pollen donors can be achieved indirectly with the correlated mating model based on paternity correlation of offspring arrays (Ritland 1989, 2002), by evaluating the pollen pool genetic structure (Austerlitz and Smouse 2001; Smouse et al. 2001), or directly by identifying the number of gametotypes in family arrays that can be produced by shared and unshared parents (DeWoody et al. 2000a, b; Fiumera et al. 2001). It is quite possible for outcrossing rates to vary little among populations, but the effective number of pollen donors may vary significantly with local population size (e.g., Barrett et al. 1993; Hoebee and Young 2001; but see El-Kassaby and Jaquish 1996). In fact, the structure of the pollen pool and the effective number of pollen donors may be a more sensitive measure of the impact of fragmentation than the mating system, especially for species with relatively high outcrossing rates.
The central goal of this study is to evaluate the extent to which different components of plant mating system are affected by habitat subdivision of the Andean oak (Quercus humboldtii). This species colonized the northern Andes of South America about 350,000 years BP (before the present) and was able to create extensive oak-dominated forests at mid to high elevations. This age is significantly less than the estimate of 20 million years BP given for the modern oaks in California (Raven and Axelrod 1974). These oak populations, like many high elevation plant populations in the Andes, have been subjected to several processes of population expansion and isolation during the late periods of glaciation (Hooghiemstra and Sarmiento 1991). Deforestation in the last 50–100 years has dramatically reduced the oak's population size to a fraction of its original extent. At the present time, oak forests are found in a mixture of pastures, crops, commercial timber, and in some cases, mixed Andean forests. Specific questions we address here are (1) what are the population outcrossing rates, biparental inbreeding, and effective number of pollen donors in a subdivided landscape of the Andean oak? (2) Do these parameters vary predictably with fragment size and isolation?
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Methods |
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Study Species
Quercus humboldtii Bonpl. is the southernmost species of oak in the western hemisphere and belongs to the group of red oaks of the subgenus Erythrobalanus (Nixon 1993). It is a medium-size to large tree endemic to the northern Andes, found primarily in moist forests between 1500 m and 3300 m, forming stands of almost a monospecific canopy. Like other oaks, it is wind pollinated, monoecious, and produces acorns dispersed by gravity, large frugivorous birds, and rodents.
Study Site
Samples were collected in the northeastern Andes of Colombia (73° 30' 13'' W, 5° 43' 14'' N) at 2400 m. This site contains multiple fragments that range in size from a few scattered trees within pastures to areas of more than 4000 ha. Many of the trees in the fragments have resprouted and smaller fragments are usually composed of a few large trees surrounded by smaller ones, possibly originating from seeds from these remnant trees. As far as we could observe, the chosen fragments (see below) were composed of larger trees possibly belonging to prefragmentation conditions and smaller trees probably from local seed dispersal. No resprouts were apparent in the sampled sites. Aerial photographs and topographic maps done in the 1960s show that present-day fragments have remained virtually the same for at least 40 years, but their age remains unknown [Agustín Codazzi Institute, Bogotá, Colombia, map C-2416 (1963)].
Sampling
Five fragments were sampled in 1999–2000 (Figure 1). These fragments were separated by at least 200 m from the nearest oak tree, with a mean intersite separation of 598 m (SD = 340 m, range 240–1200 m). Except for the site with the largest number of adults that had about 20 small trees in a fence nearby, we sampled all adults in the fragments, resulting in sample sizes of 9, 15, 25, 42, and 59 individuals from sites F1–F5, respectively. In fragments F1 and F5, we collected seedlings with acorns still attached under clear tree shadows of three focal trees. In sites F2, F3, and F4, we sampled 30 progeny from three focal trees in each fragment, yielding 90 progeny for each of the fragments. Focal trees were chosen by the availability of acorns to ensure sufficient sample sizes, because seed production was not great during the study period. We were unable to find acorns within neighboring forests that would have provided a valuable comparison. However, to ensure that the adults in the fragment were not an atypical sample, we compared the genetic structure of the adult population in our fragments with three subpopulations (50 adults each) within a large forest tract (approximately 4000 ha), sampled in a similar spatial array as the fragments we describe above (Fernández and Sork, submitted). These plots were located approximately 11 km away, spanning 1.4 km, with an average plot separation of 951 m (SD = 482 m).
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Molecular Methods
Molecular techniques for DNA extraction, polymerase chain reaction (PCR) conditions, and scoring follow Fernández et al. (2000). Four microsatellite loci (QpZAG58, QpZAG9, QpZAG46, and QpZAG110) were chosen based on the previous evaluation of 21 loci (Fernández et al. 2000) from the sequences of Quercus petrea (Steinkellner et al. 1997). Alternate primers were designed to reduce the presence of null genotypes for locus QpZAG58, but were partially successful, and the one used here is thus called QpZAG58f, being 22 bp shorter than QpZAG58. The allele size was determined by referencing the 30-330 10 bp DNA standards from Gibco BRL with Kodak Digital Science version 2.0.3 software on silver-stained polyacrylamide gels (6%, 7 M urea). Alleles were finally coded in terms of absolute repeat numbers, considering the smallest repeat observed for each locus as allele 1, and estimating the relative number of steps of 2 bp for the rest of alleles.
Data Analysis
Estimations of the mating system parameters for the global population and the fragments were obtained using the mixed mating model as implemented in the software MLTR 2.2 (Ritland 2002). This model simultaneously finds maximum-likelihood solutions for multilocus outcrossing rates tm, average minimum variance single locus outcrossing rates ts, biparental inbreeding (reflected in the difference tm – ts), allele frequencies, adult inbreeding (Wright's F), and paternity correlation (rp). The program was seeded with outcrossing rate t = 0.9, paternal inbreeding F = 0, correlation of outcrossing rt = 0.1, and correlation of paternity rp = 0.1. Results were subject to 1000 bootstraps using families as the resampling unit. The significance of selfing rates (i.e., tm < 1) was assessed using 95% confidence intervals (CIs) by sorting individual bootstrap values of tm and comparing the 976th percentile to unity. We ran the program using progeny from the five fragments, but assigning progeny from each fragment to a separate subpopulation. This approach yielded population and subpopulation estimates of the mating system parameters.
As mentioned, the loci QpZAG58 showed null genotypes in a global frequency of about 19% that were not significantly reduced with alternate primers. These null genotypes were concentrated in fragments 1 and 3, and less frequently elsewhere. Thus a Monte Carlo procedure was based on the approach of Doligez and Joly (1997) for mating system analysis with null alleles. We estimated pollen pool allele frequencies from the global population of adults and offspring combined. Next, alleles were chosen in relation to the global probability and the null genotype was replaced along with one of the maternal alleles drawn at random. All null genotypes were replaced as outcrossed to avoid inflating selfing rates; however, we acknowledge this approach could slightly underestimate true selfing. Allele frequencies from pollen pool estimation of MLTR were compared in various initial trial runs between the original data output and the simulated file using a Wilcoxon rank test (Sokal and Rohlf 1995) and no significant differences were observed. Thus all subsequent analyses were performed with the null allele corrected file.
MLTR provides an estimate of the effective number of pollen donors from the correlation of paternity based on the following equation: 
Because we obtained inconsistent results, as rp was either too close to one or too close to zero in the different iterations, possibly due to the low frequency of the many rare alleles found at one locus, we grouped together all alleles found at the most variable locus, QpZAG58f, whose frequency was less than 0.05 in the total population. Then we held fixed the other parameters (t, rt, and F) and estimated rp. In addition, we used a second approach to estimate rp, the TwoGener model (Austerlitz and Smouse 2001; Smouse et al. 2001), which uses the pollen pool structure sampled by maternal trees (
FT) to estimate the effective number of pollen donors Nep, based on the relationship Nep = (2FT)–1. Analyses were performed using the software TwoGener version 1.0, obtained from F. Austerlitz.
To estimate the actual number of pollen donors in each individual maternal tree, we estimated the number of parents contributing to each progeny array based on the count of haplotypes originated from unshared parents (DeWoody et al. 2000a,b; Fiumera et al. 2001). Briefly, the minimum number of unshared parents that contribute to a half-sib brood is one-half the number of different gametophytes inherited from the unshared parents. The estimation of the number of parents and the number of potential haplotypes can be inferred by using the half-sib data to reconstruct a parental population by creating offspring arrays from the reconstructed adult population and by examining how many different haplotypes can be produced by one, two, three, or more parents in several iterations of the data. Hence, by resampling, the most likely number of unshared parents that produced an observed number of haplotypes in a half-sib brood can be estimated. These methods are implemented in the BROOD, COUNTS, and HAPLOTYPES programs developed by DeWoody et al. (2000a,b) (available at http://www.agriculture.purdue.edu/fnr/html/faculty/DeWoody/DeWoodyweb/parentage.html).
The first program, BROOD, determines the minimum number of offspring needed to detect a specific number of parents given a set of multilocus allele frequency data. In our case, we used the allele frequencies of all the offspring from all the 15 maternal trees, and we chose to evaluate sample sizes for four parents with equal reproductive success (i.e., four effective pollen donors) as a guide of our marker resolution. For all simulations, the size of the re-created adult population was set to 1000 and the size of the simulated broods was set to 30. The resampling of each brood was performed 1000 times. This first simulation of BROOD yields the statistic n, which is the number of offspring needed per clutch to detect all marker-unique gametes from unshared parents, and n*, which is the number of offspring per clutch needed to observe all true haplotypes from unshared parents. Next, each offspring array was evaluated with the program COUNTS to estimate the number of unshared haplotypes that we call here Ch. Finally, the most probable number of unshared parents, Nu, was estimated with the program HAPLOTYPES, which calculates the mean and the mode of the estimated number of pollen donors given the counts of unshared gametes Ch. The mode of Ch is considered as the most accurate estimate of the unshared numbers of parents (Fiumera et al. 2001).
For examining the effects of fragment size and isolation on mating patterns, we regressed the mating system parameters for the fragments against local fragment adult population size and against mean isolation to the next 5 and 10 closest large tracts of oak forest (see Table 1 for isolation data). To compare the genetic structure of the adult populations in the fragments with existing adult populations in the control forest, we estimated the genetic diversity (number of alleles, effective number of alleles, observed and expected heterozygosity, and inbreeding), and the population structure parameter FST using the software GeneticStudio (available from R. Dyer, Virginia Commonwealth University, Richmond, VA).
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Results |
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Pollen Allele Frequencies
The four chosen microsatellite markers exhibited 40 alleles in the offspring representing 25, 4, 8, and 3 alleles for loci QpZAG58f, QpZAG9, QpZAG110, and QpZAG46, respectively (Table 2). Observation of global allele frequencies shows that variation consists of many rare alleles, especially for locus QpZAG58f, and that standard errors for those rare alleles sometimes have the same magnitude as the frequency itself. The rest of the loci with fewer alleles did not have any allele closer to fixation and exhibited standard errors smaller than those of the most variable locus.
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Mating System
Quercus humboldtii exhibited a multilocus population outcrossing rate of tm = 0.972 (SE = 0.009), which is significantly less than 100% (Table 3). The average single locus outcrossing rate was ts = 0.939 (SE = 0.020), and hence the population biparental inbreeding rate was (tm – ts) = 0.033 (SE = 0.016). The multilocus outcrossing rates (tm) per fragment were all significantly less than unity (tm = 0.966 to 0.971; Table 4). However, they are not related to either fragment size or mean isolation distance (results not shown). We observed that ts was higher for the smaller fragments (r2 = 0.426, ns) and biparental inbreeding (tm – ts) was close to zero for the largest fragment (r2 = 0.425, ns) (Figure 2). These trends are mainly a result of the values obtained for the largest fragment, as the other four present more or less equal values. These results suggest that in the largest fragment, trees are able to sample pollen from a greater local source that is probably formed of almost unrelated individuals, as indicated by the closeness to zero of the biparental inbreeding. Finally, none of the parameters evaluated (outcrossing rates, biparental inbreeding, number of pollen donors, etc.) showed any tendency when regressed against isolation estimates of the fragments to the nearest 5 or 10 closest tracts of forest (results not shown).
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At the population level, the MLTR program indicates a correlation of paternity of rp = 0.185 corresponding to Nep = 5.405. Similarly the TwoGener method indicates a pollen structuring of
FT = 0.082, that translates into Nep = 6.097 (Table 3). At the family level, the results of the program BROOD indicate that the sample size for identifying all marker-unique gametes per clutch from at least four equally contributing males is n = 13.1 (SD = 4.3) offspring. The number of offspring required for identifying all true gametotypes from unshared parents is n* = 16.1 (SD = 4.1). Hence about 20 offspring per clutch are necessary to detect four effective males. In our case, approximately 28 seedlings on average per maternal tree were collected and genotyped, so our sampling scheme is sufficient to detect at least the presence of one to four main (effective) contributing parents. The fragments exhibited variation in the number of unshared gametotypes (Ch) in the range of 9 to 14 according to the results of the COUNTS program. These numbers of distinct gametotypes correspond to a variation in the number of unshared parents Nu in the range of two to four pollen donors, according to the results of HAPLOTYPES (Table 5). The upper limit of the count of unshared parents is in the range of 4 (two smallest fragments) to 12 donors (largest fragment), suggesting that the actual census number may be higher, but comprising individuals that sire small fractions of the family arrays. The mode of the number of unshared parents per family appears to be related to some extent to fragment size (F = 5.04, r2 = 0.63, P < .11) (Figure 3), suggesting that larger fragments may have more local pollen donors than smaller ones. The average of the upper limit of the number of unshared parents (Numax = 6.3) is about the same as the effective value obtained for the population with TwoGener, suggesting that the mean results of HAPLOTYPES may underestimate the total census number of pollen donors.
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The genetic structure of the adult trees in the fragmented populations and those in the currently intact forest tract indicate that the fragments do not represent a biased sample of the historical populations. All the parameters for genetic diversity are similar, although the large tract of forest appears to have more variability, as judged by larger standard errors (Table 6). Values of inbreeding, fIS, are similar in the two populations and are not different from zero, which suggests that Q. humboldtii has been a historically outcrossed species. Similarly the FST values for the two sets of tree populations (fragments and plots) is not different from zero, suggesting that the trees in the fragmented area have not differentiated from those in the more continuous tract of forest.
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Discussion |
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Our findings indicate that, in general, mating patterns and pollen movement within and among fragments are showing only a modest impact of fragmentation. The remnant populations all showed marginal selfing and reduced levels of mating among relatives that was nonexistent in the largest fragment. The populations also exhibited a positive relationship between the number of pollen donors per maternal tree and fragment size, which illustrates a slight impact of fragmentation for the size of fragments sampled in this study. Oaks usually are regarded as complete outcrossers (Bacilieri et al. 1996; Schwarzmann and Gerhold 1991), but here we find selfing rates within the 3% boundary (tm = 0.97). In fact, evidence of selfing with the same magnitude (tm = 0.96) has been found for the California Valley oak (Quercus lobata), which grows in open savannas (Sork et al. 2002a). Thus some selfing in sparse populations of oaks is marginally feasible, and probably in human dissected habitats as well. Considering that both the control forest estimate of inbreeding and the parental inbreeding deduced from the MLTR procedures suggest no adult inbreeding, Q. humboldtii may undergo selection against any inbred offspring, preventing them from becoming adults. Thus even the small amount of selfing observed in present-day cohorts, which may have been induced by fragmentation of the landscape, could disappear in the adult population through selection against inbred progeny.
Although we see a slight increase in biparental inbreeding with a decrease in fragment size for these populations of Q. humboldtii, we want to emphasize that this current level should not be of great concern. First, the observed level of biparental inbreeding (3%) is small and is similar to levels in large, natural populations of oaks: for example, (tm – ts) = 0.051 in mixed stands of hybridizing Quercus robur and Q. petrea in France (Bacilieri et al. 1996), and (tm – ts) = 0.022 for Quercus velutina (also subgenus Erythrobalanus) in the Missouri Ozarks (Fernández et al., in preparation). These minimal levels of biparental inbreeding contrast with those from the insect-pollinated Caryocar brasiliensis, an endangered tropical tree (Collevatti et al. 2001), where (tm – ts) 
13–23% across four populations. The fact that we observe some degree of increased mating with relatives in the smaller fragments may reflect the smaller number of adults that contribute the majority of local pollen pool structure and the increased likelihood that relatives will be among those adults. However, the mating system of these Andean oak populations appears quite resilient to reductions in population size. This resilience could be due to a reproductive system that restricts inbreeding through selfing or mating with relatives, or a pollination system that allows sufficient local and long-distant gene flow to reduce inbreeding. Indeed, these fragments are receiving at least 32% of external pollen flow (Fernández 2002), which explains in part the resilience of the species. Thus it is quite possible that only extremely small or isolated population fragments are vulnerable to fragmentation effects.
Our expectation for this study was that the effective number of pollen donors would be more sensitive to the impact of fragmentation than the mating system. However, the population-wide estimate of the effective number of fathers Nep of between 5.405 and 6.097 is similar to that of other oaks. For example, Quercus alba in a Missouri (USA) forest exhibits values of Nep = 8.2 (Smouse et al. 2001) and Q. lobata in a savanna landscape exhibited a range of Nep = 4 to 8 donors per tree depending on year (Dutech et al. 2005; Sork et al. 2002a,b). For Q. velutina in a continuous mixed oak-hickory forest in Missouri, where this tree is about third in density, we found a value of Nep = 6.4, quite similar to the Andean oak (Fernández et al., in preparation) Thus when we examine the remnants collectively, the effective number of pollen donors is typical for oaks in general. However, when we examine the effective number of donors on an individual basis per fragment, we see a significant relationship between effective number of pollen donors and fragment size. Individual maternal trees may sample from 2 to 4 and may be up to 12 pollen donors, suggesting high variability around the mean population value. Thus it appears that if the process of fragmentation creates multiple small remnant populations that are also isolated from each other, local population size may be sufficiently reduced that processes associated with genetic drift and inbreeding may become more problematic.
Methodological Concerns
We will now briefly discuss three concerns for this study. First, we encountered null alleles in some of our remnant subpopulations. As was the case for Doligez and Joly (1997), we observed some inflation in estimates of selfing rates (both multilocus and average single locus). This bias is expected, as null genotypes are basically interpreted as missing data by the MLTR software, reducing the variability of offspring cohorts and thus increasing values for selfing. For the estimation of biparental inbreeding (tm – ts), one would expect that if null alleles are replaced from a global pool with a probability equal to their frequency in the total population, unrelated alleles will mask kinship in related individuals, thus reducing the values of biparental inbreeding. In our case, biparental inbreeding is greater than zero for the global population and variable for different population sizes, so we can conclude that the biparental signal was not completely erased by controlling for null alleles, but probably was reduced. Hence we conclude that the correction for null alleles did not significantly bias the results of the analysis.
Second, we were concerned that our study was unable to compare the mating patterns of these five remnants with intact forest. In fact, the observation that acorns were unavailable within intact forest indicates that some degree of stand opening in the fragments may promote reproduction. At this time we cannot address this possibility. However, we can state that adult populations in the two landscape arrays had similar genetic structure (Table 5). All values of genetic diversity are high, and similar, suggesting that no large-scale drift effects have occurred for the fragments. Hence we can assume that the genetic composition of the potential pollen pool available for the fragments does not represent a biased pollen source that will in turn produce offspring with atypical genotypes. In addition, the values of fIS not different from zero indicate that the populations of Andean oak have probably had 100% historical outcrossing rates, but only a mating system study within the large tracts of forest can assert that unambiguously.
The third issue is whether we sampled sufficiently for the DeWoody (2000a,b) method of estimating family levels of pollen donors. The individual family array analysis of the number of distinct unshared gametotypes indicates that the number of probable pollen donors (of both local and external origin) Nu is between two and four parents that contribute mainly to a given family array. One of the properties of the estimation is that Nu usually reflects the number of unshared parents that contribute the most and in equal proportion to the progeny array (DeWoody et al. 2000a). Hence, in practice, Nu reflects more of the effective number of breeders than the actual census numbers, unless family sizes are extremely large, and thus the number of alleles in all loci. In our case, we have a conservative measure, as 100 seeds or more are necessary to estimate 12 or more parental donors (DeWoody et al. 2000b). Nevertheless, our sampling is still robust enough to detect up to four equally contributing parents, and we found values equal to or less than that. In other words, we were able to detect those families with few pollen donors, but those that exceed five or more may be underestimated, and appear to be reflected in the upper limit of the bootstrap results (Table 4). Hence it is not unrealistic to consider that at least for the families that presented low values of Nu (10 of 15), about two to three adults performed most of the siring, even though the pollen contains contributions from other trees.
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Conclusion |
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The Andean oak appears to have a highly effective outcrossing mating system that prevents extensive selfing or consanguineous mating, even with a reduced local population size. Inbred individuals are probably selected against, as evidenced by the lack of adult inbreeding in both the fragmented and the reference population. These remnant Andean oak subpopulations also showed a tendency for low numbers of effective pollen donors, indicating that small fragments could be at risk for reduced genetic diversity through future genetic drift or inbreeding. Thus if future remnants experience loss of connectivity, genetic bottlenecks, or reproductive isolation, the future of Andean oaks may be at risk. However, at this time, the current landscape array of small and medium-size fragments mixed with larger forest tracts is not a threat to the Andean oak.
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Acknowledgments |
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The authors wish to thank R. Dyer for his input in the development of this project, and two anonymous reviewers for detailed and qualified input. J. F. Fernández-M. was supported by the Graduate School at the University of Missouri–St. Louis, the Humboldt Institute in Colombia, and the Biotechnology Research Unit at Centro Internacional de Agricultura Tropical (CIAT). Special thanks for J. D. Palacio and M. E. Rodriguez for crucial laboratory aid. J. F. Fernández-M. would like to thank especially the Fulbright Commission for financial support of his doctoral studies. V. L. Sork received support from the National Science Foundation (DEB-0089445 and DEB-0242422).
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Footnotes |
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Corresponding Editor: James Hamrick
Received June 24, 2004
Accepted June 24, 2005
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References |
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