The Breeding Structure of Tropical Tree Populations

Abstract Several approaches were used to study the breeding structure of tropical tree populations located on Barro Colorado Island, Republic of Panama. Allozyme analyses of 16 woody species indicated that the distribution of genetic diversity among collection sites was closely associated with the species'breeding system and seed dispersal mechanism. Low levels of diversity observed among collection sites separated by 1–2 km indicate that intersite gene flow is high. Estimates of the proportion of outcrossing obtained for seven species indicated that the majority (6) were highly outcrossed. A significant amount of tree to tree heterogeneity in pollen allele frequencies also existed for each outcrossing species. Degree of heterogencity in pollen allele frequencies received by maternal trees was negatively associated with flowering tree density. In the mixed mating species, Cavanillesia plantanifolia , the proportion of selfing was closely related to flowering tree density. A significant proportion of the pollen received by individuals of these eight species came from relative few individuals. Pollen movement within populations was estimated for two canopy species. Long distance pollen movement (>750m) was extensive (>20%) in both species. The breeding structure of these tropical species appears to be a mixture of near-neighbor (30%–50%) and long-distance pollen movement (10%–25%). If this pattern is typical of tropical tree species, it has important implications for the genetic structure of populations as well as for the design of conservation areas.     

Reproductive biology of the dry forest tree Enterolobium cyclocarpum (guanacaste) in Costa Rica: a comparison between trees left in pastures and trees in continuous forest

We compared the rate of pollen deposition, the likelihood of fruit production, the number of seeds per fruit, the outcrossing rate, and the progeny vigor of the tropical dry forest tree Enterolobium cyclocarpum for individuals in pastures vs. individuals in continuous forest. We found that flowers from trees growing in continuous forests were more likely to have pollen deposited on their stigmas than flowers from trees in pastures (52.1 vs. 32.3%, respectively). We also found that trees from continuous forests were almost six times more likely to set fruits and produce more seeds per fruit than trees in pastures. Morever, progeny from trees in continuous forests were, on average, more vigorous than the progeny from trees in pastures, as indicated by 12 of 16 indicators of plant vigor. However, there was no significant difference in the multilocus estimate of the outcrossing rate between the two groups of trees (tm = 1.00 and 0.99 for trees from continuous forest and trees from pastures, respectively). But there are differences in the correlation of paternity between the progeny of the two groups, where the progeny from trees in pastures showed a lower correlation of paternity than progeny of trees from continuous forests (r(p) = 0.104 and r(p) = 0.189, respectively). We argue that the mechanisms that regulate progeny vigor are disrupted in trees from pastures. We discuss the implications of these findings for the conservation of E. cyclocarpum.     

Impact of selective logging on inbreeding and gene dispersal in an Amazonian tree population of Carapa guianensis Aubl

Selective logging may impact patterns of genetic diversity within populations of harvested forest tree species by increasing distances separating conspecific trees, and modifying physical and biotic features of the forest habitat. We measured levels of gene diversity, inbreeding, pollen dispersal and spatial genetic structure (SGS) of an Amazonian insect-pollinated Carapa guianensis population before and after commercial selective logging. Similar levels of gene diversity and allelic richness were found before and after logging in both the adult and the seed generations. Pre- and post-harvest outcrossing rates were high, and not significantly different from one another. We found no significant levels of biparental inbreeding either before or after logging. Low levels of pollen pool differentiation were found, and the pre- vs. post-harvest difference was not significant. Pollen dispersal distance estimates averaged between 75 m and 265 m before logging, and between 76 m and 268 m after logging, depending on the value of tree density and the dispersal model used. There were weak and similar levels of differentiation of allele frequencies in the adults and in the pollen pool, before and after logging occurred, as well as weak and similar pre- and post-harvest levels of SGS among adult trees. The large neighbourhood sizes estimated suggest high historical levels of gene flow. Overall our results indicate that there is no clear short-term genetic impact of selective logging on this population of C. guianensis.     

Mating Systems of Three Tropical Dry Forest Tree Species1

Polymorphic allozyme loci were used to estimate outcrossing rates for three tree species from a disturbed dry forest in southern Costa Rica. Estimates of the multilocus outcrossing rates of Cedrela odorata and Jacaranda copaia were 0.969 and 0.982, respectively, and suggest that these species may be self-incompatible. The subcanopy tree Stemmadenia donnell-smithii also demonstrated little self-fertilization based on an estimated outcrossing rate of 0.896. Significant heterogeneity in pollen allele frequencies among maternal trees was detected for at least two enzyme loci for each species. A test of correlated mating between progeny of S. donnell-smithii revealed that all seeds within a fruit were singly sired. In addition, the low estimates of biparental inbreeding and significant differences in pollen and ovule allele frequencies for this species suggest that gene flow into the sampled forest fragment may occur. The implications of deforestation on the mating systems of these tropical tree taxa are discussed.     

Gene flow among established Puerto Rican populations of the exotic tree species, Albizia lebbeck

We estimate gene flow and patterns of genetic diversity in Albizia lebbeck, an invasive leguminous tree in the dry forest of southwestern Puerto Rico. Genetic diversity estimates calculated for 10 populations of 24 trees each indicated that these populations may have been formed from multiple introductions. The presence of unique genotypes in the northernmost populations suggests that novel genotypes are still immigrating into the area. This combination of individuals from disparate locations led to high estimates of genetic diversity (He = 0.266, P = 0.67). Indirect estimates of gene flow indicate that only 0.69 migrants per generation move between populations, suggesting that genetic diversity within populations should decrease due to genetic drift. Since migration-drift equilibrium was not found, however, this estimate needs to be viewed with caution. The regular production of pods in this outcrossing species (tm = 0.979) indicates that sufficient outcross pollen is received to insure successful reproduction. Direct estimates of gene flow indicate that between 44 and 100% of pollen received by trees in four small stands of trees (n < 11) was foreign. The role of gene flow in facilitating the spread of this invasive plant species is discussed.     

Low Inbreeding and High Pollen Dispersal Distances in Populations of Two Amazonian Forest Tree Species

Recent studies suggest that tropical tree species exhibit low inbreeding and high gene dispersal levels despite the typically low density of conspecifics in tropical forests. To examine this, we undertook a study of pollen gene dispersal and mating system of two Amazonian tree species. We analyzed 341 seeds from 33 trees at four microsatellite loci in a Carapa guianensis population from Brazil, and 212 seeds from 22 trees at four microsatellite loci in a Sextonia rubra population from French Guiana. Differentiation of allele frequencies among the pollen pool of individual trees was Φ_FT= 0.053 (95% CI: 0.027–0.074) for C. guianensis and Φ_FT= 0.064 (95% CI: 0.017–0.088) for S. rubra. The mean pollen dispersal distances were estimated at 69–355 m for C. guianensis , and 86–303 m for S. rubra , depending on the pollen dispersal model and the estimate of reproductive tree density used. The multi-locus outcrossing rate was estimated at 0.918 and 0.945, and the correlation of paternity at 0.089 and 0.096, for C. guianensis and S. rubra , respectively, while no significant levels of biparental inbreeding were detected. Comparing trees with high and low local density of conspecifics, we found no evidence for differences in inbreeding levels. The results are discussed within the framework of the emerging picture of the reproductive biology of tropical forest trees.     

Complex patterns of mating revealed in a Eucalyptus regnans seed orchard using allozyme markers and the neighbourhood model

The neighbourhood model apportions offspring of individual mother plants to self-fertilization, outcrossing to males within a circumscribed area around the mother plant (the neighbourhood), and outcrossing to males outside the neighbourhood. Formerly the model was applied only to haploid pollen gametes in the offspring of conifers, but is extended so that it can be used with genotypic data from diploid offspring of both angiosperms and gymnosperms. In addition, it is shown that the mating parameters can be estimated without independent estimates of allele frequencies in the pollen pools outside the neighbourhood; thus the model might be applied effectively to natural populations exposed to unknown external pollen sources. Parameters of the neighbourhood mating model were estimated for a 10-year-old seed orchard population of the insect-pollinated tree, Eucalyptus regnans, in southeast Australia, which contained a mixture of two geographical provenances (Victoria and Tasmania). The mating patterns revealed were complex. Crosses between trees of the same provenance occurred three times more often than crosses between trees of different provenances. Levels of self-fertilization and patterns of mating within neighbourhoods were influenced by provenance origin, crop fecundity and orchard position (central vs. edge) of mother trees. Gene dispersal, however, was extensive, with approximately 50% of effective pollen gametes coming from males more than 40 m away from mother trees (average distance between neighbouring trees was 7.4 m). Thus, insect pollinators are efficient promoters of cross-fertilization in this orchard, with the result that the effective number of males mating with each female is large.     

Hierarchical patterns of paternity within crowns of Albizia julibrissin (Fabaceae)

The floral architecture and phenology of the tree species Albizia julibrissin (Fabaceae) offer the potential for flowers within inflorescences to share common pollen donors. Patterns of paternity within individual tree crowns may differ among isolated individuals and those in populations due to differences in pollinator foraging behavior. To determine how genetic diversity is partitioned within individual seed pools and whether these patterns differ among isolated and population trees, we obtained all fruits from three inflorescences from four clusters from three isolated trees and from three population trees in Athens, Georgia. We assayed 14 polymorphic allozymes to genotype all progeny within singly sired fruits to determine the multilocus genotype of each fruit's pollen donor. Inflorescences had multiple pollen donors, but simulation analyses revealed that redundancy of pollen donors tended to be more likely within inflorescences than randomly across the crown. Analysis of genetic and genotypic diversity indicated that individual maternal trees received pollen from many donors in uneven frequencies. Results suggest that isolated trees receive pollen from slightly fewer pollen donors and experience more within-plant pollinator movement than trees in populations. However, isolated trees receive qualitatively similar pollen from many sources, suggesting that these trees are not effectively isolated and that pollen moves long distances in this species.     

A multiyear estimate of the effective pollen donor pool for Albizia julibrissin

Studies of pollen movement in plant populations are often limited to a single reproductive event, despite concerns about the adequacy of single-year measures for perennial organisms. In this study, we estimate the effective number of pollen donors per tree from a multiyear study of Albizia julibrissin Durazz (mimosa, Fabaceae), an outcrossing, insect-pollinated tree. We determined 40 seedling genotypes for each of 15 seed trees during 4 successive years. A molecular analysis of variance of the pollen gametes fertilizing the sampled seeds was used to partition variation in pollen pools among seed trees, among years, and within single tree-year collections. Using these variance components, we demonstrate significant male gametic variability among years for individual trees. However, results indicate that yearly variation in the 'global pollen pool', averaged over all 15 seed trees for these 4 years, is effectively zero. We estimate the effective number of pollen donors for a single mimosa tree (N(ep)) to be 2.87. Single season analyses yield N(ep) approximately 2.05, which is 40% less than the value of N(ep) estimated from 4 years of data. We discuss optimal sampling for future studies designed to estimate N(ep). Studies should include more trees, each sampled over at least a few years, with fewer seeds per tree per year than are needed for a traditional parentage study.     

Effect of gene flow on spatial genetic structure in the riparian canopy tree Cercidiphyllum japonicum revealed by microsatellite analysis

Few studies have analyzed pollen and seed movements at local scale, and genetic differentiation among populations covering the geographic distribution range of a species. We carried out such a study on Cercidiphyllum japonicum; a dioecious broad-leaved tree of cool-temperate riparian forest in Japan. We made direct measurement of pollen and seed movements in a site, genetic structure at the local scale, and genetic differentiation between populations covering the Japanese Archipelago. Parentage analysis of seedlings within a 20-ha study site indicated that at least 28.8% of seedlings were fertilized by pollen from trees outside the study site. The average pollination distance within the study site was 129 m, with a maximum of 666 m. The genotypes of 30% of seedlings were incompatible with those of the nearest female tree, and the maximum seed dispersal distance within the study site was over 300 m. Thus, long-distance gene dispersal is common in this species. The correlation between genetic relatedness and spatial distance among adult trees within the population was not significant, indicating an absence of fine-scale genetic structure perhaps caused by high levels of pollen flow and overlapping seed shadows. Six populations sampled throughout the distribution of C. japonicum in Japan showed significant isolation-by-distance but low levels of genetic differentiation (F(ST) = 0.043), also indicating long-distance gene flow in C. japonicum. Long-distance gene flow had a strong influence on the genetic structure at different spatial scales, and contributes to the maintenance of genetic diversity in C. japonicum.     

Fine-scale estimation of outcrossing in western redcedar with microsatellite assay of bulked DNA

Western redcedar (Thuja plicata, Cupressaceae) is a self-fertile conifer with a mixed mating system and significant variation for outcrossing among populations. In this paper, we conducted a fine-scale study of mating system variation to identify correlates of outcrossing in natural populations. We examined variation for outcrossing within and among individual trees, and describe a new method to estimate outcrossing using bulked DNA samples. Bulking (assaying DNA tissues from several individuals simultaneously) increases the experimental power without increasing the experimental effort. We sampled 80 trees from four natural populations in southwestern British Columbia. From each tree, we sampled from up to six crown positions (three heights and inner vs outer branches). From each position, two samples of three seedlings each were bulked before DNA extractions. Using four microsatellite loci, we obtained outcrossing rates for each tree and for each of the six crown positions. We found individual tree selfing rates to increase with tree height in all four populations, but selfing rates did not differ among crown positions. The higher selfing rate of larger trees is probably due to their greater proportional contribution to local pollen clouds. Individual tree outcrossing rates ranged from 22 to 100% and the population outcrossing rates from 66 to 78%. Missed alleles due to bulking and the estimation method used both cause a downward bias in outcrossing rates, so that these estimates are probably lower than the actual outcrossing rates. Nevertheless, the trends we observed are not affected by systematic biases of estimation.     

Genetic and reproductive consequences of forest fragmentation for populations of <Emphasis Type="Italic">Magnolia obovata</Emphasis>

In order to evaluate the consequences of forest fragmentation on populations of Magnolia obovata, we compared genetic diversity and reproductive characteristics at two nearby sites, one conserved and one fragmented. The genetic diversity between adults trees of the different sites was not significantly different. However, saplings in the conserved site showed a significantly higher genetic diversity than both adult trees in the conserved site and saplings in the fragmented sites; this was found to be the result of the larger gene flow into the conserved site. The density of the adult trees was significantly related to all of the reproductive traits analyzed (fertilization of ovules, insect attack to seeds, ovules that developed into seeds and outcrossing at the stage of seeds) at both sites. At both sites, fertilization of ovules and insect attack on seeds were positively correlated to adult tree density while outcrossing rate was negatively correlated to adult tree density. The fertilization of ovules and outcrossing were more dependent on adult tree density in the fragmented site than in the conserved site. The probability of ovules developing into outcrossed seeds showed a negative correlation with adult tree density at both sites, indicating the advantage of low density for this species and possibly implying a resilience to habitat fragmentation. A two-generation-analysis did not identify significant differences between sites in terms of the structure of the pollen pool and the number of pollen donors. Although fragmentation affected reproductive characteristics, the effect on seedling establishment and subsequent survival remains to be determined. Proposals for future studies that will assist in the development of management strategies for forests suffering fragmentation are made.     

Impact of monoecy in the genetic structure of a predominately dioecious conifer species, Araucaria angustifolia (Bert.) O. Kuntze

Understanding the mating system of a tree species is important in genetic conservation and tree breeding strategies because it affects the inbreeding and genetic diversity of the descendant populations. Araucaria angustifolia (Bert.) O. Kuntze is a mainly dioecious species that reproduces through outcrossing. However, some monoecious trees have been identified and they may reproduce through self-fertilization. The objective of this study was to confirm the expected relatedness of full-sibs in outcrossed hand-pollinated progenies of female seed trees, self-sibs of hand self-pollinated monoecious seed trees, and to investigate the mating system of open-pollinated progenies of female and monoecious A. angustifolia trees. To do this, eight microsatellite loci were used to genotype hand- and open-pollinated progenies. Our results show that the relatedness of outcrossed hand-pollinated progenies are true full-sibs and progenies from a selfed monoecious seed tree are self-sibs, which confirms the hand-pollination method used. Open-pollinated female seed trees reproduced only by outcrossing, generating progenies with a mixture of full- and half-sibs. Monoecious seed trees reproduced mainly by xenogamy, generating progenies with mixtures of self-sibs, full-sibs, half-sibs and self-half-sibs. We also found that an increase in the effective number of pollen donors ( $ N_{\text{ep}} $ ) would lead to an increase in the total number of alleles ( $ K $ ) within progenies. Our results also suggest that monecious trees have limited potential to modify the genetic structure through selfed seed production due to the very low estimated selfing rate in these trees and the rare occurrence of these trees in natural populations.     

Multiscale spatial genetic structure within and between populations of wild cherry trees in nuclear genotypes and chloroplast haplotypes

Spatial genetic structure (SGS) of plants mainly depends on the effective population size and gene dispersal. Maternally inherited loci are expected to have higher genetic differentiation between populations and more intensive SGS within populations than biparentally inherited loci because of smaller effective population sizes and fewer opportunities of gene dispersal in the maternally inherited loci. We investigated biparentally inherited nuclear genotypes and maternally inherited chloroplast haplotypes of microsatellites in 17 tree populations of three wild cherry species under different conditions of tree distribution and seed dispersal. As expected, interpopulation genetic differentiation was 6–9 times higher in chloroplast haplotypes than in nuclear genotypes. This difference indicated that pollen flow 4–7 times exceeded seed flow between populations. However, no difference between nuclear and chloroplast loci was detected in within‐population SGS intensity due to their substantial variation among the populations. The SGS intensity tended to increase as trees became more aggregated, suggesting that tree aggregation biased pollen and seed dispersal distances toward shorter. The loss of effective seed dispersers, Asian black bears, did not affect the SGS intensity probably because of mitigation of the bear loss by other vertebrate dispersers and too few tree generations after the bear loss to alter SGS. The findings suggest that SGS is more variable in smaller spatial scales due to various ecological factors in local populations.     

Estimation of gene flow into fragmented populations of Bursera simaruba (Burseraceae) in the dry-forest life zone of Puerto Rico

We examined the impact of habitat fragmentation on gene flow in populations of the neotropical tree Bursera simaruba. In particular, we compared the effectiveness of three common techniques to estimate gene flow in the context of a highly disturbed system. Paternity analysis on emerging seedlings from eight small (N = 3 to 9) stands of trees showed that between 45% and 100% of seedlings were sired from outside their stand, indicating pollen moved readily over the isolation distances examined. Based on six populations of 21-24 trees each, estimates of allozyme genetic diversity (P(s) = 73.3%; H(e) = 0.244) were higher than those reported for species with similar life history traits. Indirect, F(ST)-based gene flow estimates for these six populations yielded an estimate of 3.57 migrants per generation, although possible violations of model assumptions limit the reliability of the estimate. A twogener analysis showed pollen moved either 320 m or 361 m and that there were only 2.46 effective pollen donors per maternal tree. Despite the potential for long-distance pollen movement, seed abortion was high, especially in stands with fewer than four trees. Population size, rather than isolation distance, appears to limit reproduction in the populations examined.     

Mating system and pollen gene flow in Mediterranean maritime pine

Mating systems define the mode of gene transmission across generations, helping to determine the amount and distribution of genetic variation within and among populations of plant species. A hierarchical analysis of Mediterranean maritime pine mating system (61 mother trees from 24 plots, clustered in three populations) was used to identify factors affecting mating patterns and to fit pollen dispersal kernels. Levels of ovule and seed abortion, multi- and single-locus outcrossing rates and correlated paternity were estimated from progeny arrays and correlated with ecological stand variables and biometric tree measures. Pollen dispersal kernels were fitted using TwoGener and KinDist indirect methods and simulations were carried out to identify relevant factors affecting correlated paternity. Maritime pine showed high outcrossing rates (t(m) and t(s) approximately 0.96) and relatively low levels of correlated paternity [an r(p) of 0.018 (Ritland's estimate) or 0.048 (Hardy's estimate)], although higher than in other anemophilous tree species. Mating system parameters had high variation at the single-tree level (99-100%) but no stand or population effect was detected. At the single-tree level, outcrossing rates were correlated with tree (diameter and height) and crown size. In addition, correlated paternity showed a significant negative correlation with tree height, height to crown base and height to the largest crown width, probably reflecting the importance of the trees' 'ecological neighborhoods'. Indirectly estimated pollen dispersal kernels were very leptokurtic (exponential-power distributions with beta<0.5), with mean dispersal distances from 78.4 to 174.4 m. Fitted dispersal kernels will be useful in building explicit simulation models that include dispersal functions, and which will contribute to current conservation and management programs for maritime pine. Nevertheless, the numerical simulations showed that restricted dispersal, male fertility and phenological overlap could only partially explain the observed levels of correlated paternity; so other factors may also be relevant for the management of this valuable forest tree species.     

Mating system,outcrossing distance effects and pollen availability in the wind-pollinated treeline species Polylepis australis BITT. (Rosaceae)

Fragmentation may negatively affect plant fitness through pollen limitation and increased levels of inbreeding. Effects of fragmentation may vary with regard to life form and breeding system, and few studies exist for wind-pollinated trees. We examined the effects of hand-selfing, varying outcrossing distances and pollen addition on seed mass and germination rate of Polylepis australis BITT. (Rosaceae), a wind-pollinated treeline species endemic to Argentina. We also investigated pollen germination on the stigma and pollen tube growth to determine compatibility resulting from selfing and outcrossing. Selfing reduced seed germination rates with significant differences between open pollination and outcrosses at 30 km. In addition, we found a tendency for pollen germination and pollen tube growth to decrease following selfing. Between-fragment crosses resulted in a trend of higher reproductive output than within-fragment crosses, whereas values were similar between open pollination and between-fragment crosses. Pollen addition did not increase reproductive success neither in small nor in larger fragments. Our results suggest that highly isolated P. australis forests have a potential for inbreeding depression through selfing and within-fragment crosses. However, the results also indicate that pollen flow between P. australis forest fragments is still effective at the current fragmentation level, counteracting negative effects on seed quality resulting from reproductive isolation.     

Long‐distance pollen and seed dispersal and inbreeding depression in Hymenaea stigonocarpa (Fabaceae: Caesalpinioideae) in the Brazilian savannah

Hymenaea stigonocarpa is a neotropical tree that is economically important due to its high‐quality wood; however, because it has been exploited extensively, it is currently considered threatened. Microsatellite loci were used to investigate the pollen and seed dispersal, mating patterns, spatial genetic structure (SGS), genetic diversity, and inbreeding depression in H. stigonocarpa adults, juveniles, and open‐pollinated seeds, which were sampled from isolated trees in a pasture and trees within a forest fragment in the Brazilian savannah. We found that the species presented a mixed mating system, with population and individual variations in the outcrossing rate (0.53–1.0). The studied populations were not genetically isolated due to pollen and seed flow between the studied populations and between the populations and individuals located outside of the study area. Pollen and seed dispersal occurred over long distances (>8 km); however, the dispersal patterns were isolated by distance, with a high frequency of mating occurring between near‐neighbor trees and seeds dispersed near the parent trees. The correlated mating for individual seed trees was higher within than among fruits, indicating that fruits present a high proportion of full‐sibs. Genetic diversity and SGS were similar among the populations, but offspring showed evidence of inbreeding, mainly originating from mating among related trees, which suggests inbreeding depression between the seed and adult stages. Selfing resulted in a higher inbreeding depression than mating among relatives, as assessed through survival and height. As the populations are not genetically isolated, both are important targets for in situ conservation to maintain their genetic diversity; for ex situ conservation, seeds can be collected from at least 78 trees in both populations separated by at least 250 m.     

Regional population dynamics define the local genetic structure in Sorbus torminalis

Recent changes in sylvicultural practices in Central Europe have created forests with closed canopies, and tree species preferring open and sunny forests have declined in area and abundance. This led to increased isolation of populations of many rare insect-pollinated, fleshy-fruited species with a naturally scattered distribution. To gain insight into the regional population dynamics of such species, we investigated the consequences of spatial isolation, population size and density on the genetic structure of Sorbus torminalis and simultaneously considered the relationship between fecundity and habitat quality. Genotype data for biparentally (ISSRs) and maternally inherited (cpDNA PCR-RFLPs) molecular markers were generated for 26 Swiss populations of S. torminalis. We applied analyses of molecular variance (amova) to both marker types and separated the relative contributions of pollen and seed dispersal to historical gene flow. amova detected significant differentiation among populations (Phi(ST ISSR) = 0.107; Phi(ST cpDNA) = 0.370) in both marker types. The relative rate of pollen to seed gene flow was low (r = 2.919) and significantly different from equality. Isolation by distance was weak within Eastern and Western Switzerland, although populations were substantially differentiated. Within-population molecular variance was not explained by population size, whereas habitat quality (openness) positively influenced the percentage of fruiting trees and the degree of fruiting per tree, indicating that more open forests enhance sexual reproduction. Our findings of significant genetic differentiation in the absence of clear geographical structuring can be explained by the distinct ecology of S. torminalis and nondirectional colonization events in metapopulation-like dynamics.     

Comparative Effects of Pollen and Seed Migration on the Cytonuclear Structure of Plant Populations. I. Maternal Cytoplasmic Inheritance

We explicitly solve and analyze a series of deterministic continent-island models to delimit the effects of pollen and seed migration on cytonuclear frequencies and disequilibria in random-mating, mixed-mating and self-fertilized populations. Given the critical assumption of maternal cytoplasmic inheritance, five major findings are (i) nonzero cytonuclear disequilibria will be maintained in the island population if and only if at least some migration occurs each generation through seeds with nonrandom cytonuclear associations; (ii) immigrant seeds with no cytonuclear disequilibria can strongly affect the genetic structure of the island population by generating significant and long-lasting transient associations; (iii) with all else being equal, substantially greater admixture disequilibria are generally found with higher rates of seed migration into, or higher levels of self-fertilization within, the island population (with the possible exception of the heterozygote disequilibrium); (iv) pollen migration can either enhance or reduce the cytonuclear disequilibria caused by seed migration, or that due to mixed-mating in the absence of seed migration, but the effect is usually small and appears primarily to make a noticeable difference in predominantly outcrossing populations; and (v) pollen migration alone cannot generate even transient disequilibria de novo in populations with completely random associations. This same basic behavior is exhibited as long as there is some random outcrossing in the island population. Self-fertilized populations represent a special case, however, in that they are necessarily closed to pollen migration, and nonzero disequilibria can be maintained even in the absence of seed migration. All of these general results hold whether the population is censused as adults or as seeds, but the ability to detect nonrandom cytonuclear associations can depend strongly on the life stage censused in populations with a significant level of random outcrossing. We suggest how these models might be used for the estimation of seed and pollen migration.