Pollen Allele Frequencies Research Articles

Male Reproductive Success at Three Early Life Stages in the Tropical TreePlatypodium elegans

We conducted a paternity analysis within a population of the Neotropical tree Platypodium elegans using microsatellite markers to determine rates of gene flow and to investigate whether postpollination selection affects male reproductive success. Paternity was assigned for three early life stages (aborted fruit, mature seeds, and seedlings) of five adult trees. The rate of pollen immigration into the population was high (>40%). Significant heterogeneity in pollen allele frequencies was discovered among families but subsequently declined across life stages. Comparisons of parentage among the three life stages showed no significant differences in the number of progeny resulting from immigrant pollen, and, similarly, the average distance between mating pairs did not differ over time. Only self‐fertilized seeds exhibited a significant decline across life stages, indicating inbreeding depression due to self‐pollination. Estimates of total and effective numbers of pollen donors were high, and we did not detect a selective advantage among paternal trees. High initial levels of pollen heterogeneity and the lack of evidence for selection among outcrossed seeds of pollen parents suggest that differences in male fitness at early life stages other than self‐fertilization are due to initial pollination success rather than the differential survival of progeny among fathers.

Sistema de reprodução em populações de Eschweilera ovata (Cambess.) Miers

O sistema de reprodução de duas populações de Eschweilera ovata foi quantificado por análise de isoenzimas em estrutura de progênies usando os modelos misto de reprodução e cruzamentos correlacionados. Desvios do modelo misto de reprodução foram evidenciados entre as freqüências alélicas dos óvulos e do pólen e pela heterogeneidade nas freqüências alélicas do pólen que fecundou as diferentes árvores. A taxa de cruzamento multilocos foi alta em ambas populações Camarugipe (t m =0,999±0,004) e Itaparica (t m=0,985±0,023). A alta variação na taxa de cruzamento individual (t variando de 0,320 a 1,000) indicou que a espécie não é auto-incompatível. Diferenças positivas e significativamente diferentes de zero foram detectadas entre a taxa de cruzamento multiloco e uniloco, indicando cruzamentos endogâmicos em ambas populações Camarugipe (t m-t s =0,066±0,014) e Itaparica (t m-t s =0,073±0,016) e possível estruturação genética espacial. Valores altos de cruzamentos biparentais foram detectados nas populações (Camarugipe, r p=0,577±0,088; Itaparica r p =0,423±0,070), demonstrando que as progênies são constituídas principalmente por misturas de meios-irmãos e irmãos-completos. O coeficiente de coancestria nas progênies de ambas as populações (Camarugipe, tetaxy=0,211; Itaparica tetaxy =0,191) foi superior ao esperado em progênies de meios irmãos (0,125). Os resultados foram discutidos sob a ótica de amostragens para melhoramento, conservação genética e coleta de sementes para recuperação ambiental.

Mating system of a population of Myracrodruon urundeuva F.F. & M.F. Allemão using the fAFLP molecular marker

The mating system and genetic diversity were studied in a natural population of Myracrodruon urundeuva originating from 30 open-pollinated trees at the Paulo de Faria Ecological Station, SP, Brazil. The progenies were planted on the Teaching and Research Farm of the Ilha Solteira Engineering School, UNESP. Using the fAFLP molecular marker, eleven loci were selected to study the mating system. The mating system was analyzed using the multilocus mixed-mating model. The estimates of genetic divergence between pollen and ovule allele frequencies were significant for eight loci, suggesting nonrandom outcrossing. The estimates of the multilocus outcrossing rate revealed that M. urundeuva possesses a mating system with a predominance of outcrossing events (q = 0.940 ± 0.086). The estimates of coancestry among plants within progenies (q = 0.185) was higher than that expected for half-sib progenies (0.125) and the indirect estimate of the correlation of outcrossed paternity within progeny arrays (rp) was 0.403, suggesting that progenies have a high proportion of full-sibs. Result analysis suggests the need for the application of biometric models that take into account deviations from random outcrossing in the estimations of genetic parameters for quantitative traits and the need for retaining large sample sizes in order to preserve genetic variability.

Mating Systems of Three Tropical Dry Forest Tree Species1

ABSTRACTPolymorphic 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.

Genetic diversity and spatial structure within a natural stand of a tropical forest tree species, Carapa procera (Meliaceae), in French Guiana

Isozyme markers were used to characterize levels of diversity, genotypic structure and spatial genetic structure for the low-density tree species, Carapa procera (five adults per ha), within 300 ha of a continuous tropical rain forest stand. Both seed and adult stages were investigated, a high level of genetic diversity being found in both. Fixation indices showed excess homozygosity in seeds, and excess heterozygosity in adults, which might be caused by selection in favour of heterozygotes. Autocorrelation analysis of the spatial distribution of genotypes revealed no significant pattern in adults or in seeds before dispersal, and there was a high variability in correlogram shapes among alleles. This suggests that gene flow is extensive in C. procera, probably mainly through long-distance pollen dispersal, as seed dispersal is expected to be rather limited in this species (maximum distances of about 50 m). No clear-cut spatial pattern was observed in pollen allele frequencies, which supports the hypothesis of extensive pollen flow. This overall lack of structure is consistent with the data already available on the mating system of this predominantly outcrossing species.

Genetic diversity and spatial structure within a natural stand of a tropical forest tree species, Carapa procera (Meliaceae), in French Guiana

Isozyme markers were used to characterize levels of diversity, genotypic structure and spatial genetic structure for the low-density tree species, Carapa procera (five adults per ha), within 300 ha of a continuous tropical rain forest stand. Both seed and adult stages were investigated, a high level of genetic diversity being found in both. Fixation indices showed excess homozygosity in seeds, and excess heterozygosity in adults, which might be caused by selection in favour of heterozygotes. Autocorrelation analysis of the spatial distribution of genotypes revealed no significant pattern in adults or in seeds before dispersal, and there was a high variability in correlogram shapes among alleles. This suggests that gene flow is extensive in C. procera, probably mainly through long-distance pollen dispersal, as seed dispersal is expected to be rather limited in this species (maximum distances of about 50 m). No clear-cut spatial pattern was observed in pollen allele frequencies, which supports the hypothesis of extensive pollen flow. This overall lack of structure is consistent with the data already available on the mating system of this predominantly outcrossing species.

Estimation of mating system parameters in two Italian natural populations of Fagussylvatica

Mating system parameters were estimated in two Italian natural populations of European beech (Fagussylvatica L.) using a mixed mating model and considering seven allozyme loci (Idh-A, Lap-A, Mdh-B, Pgd-A, Pgd-B, Pgd-C, Skd-A). High values of multilocus estimates of the outcrossing rate were found in both populations, ranging from 94 to 98%. Comparison of single- and multi-locus estimates of outcrossing rates seems to indicate the presence of consanguineous matings, probably because the populations are substructured. This hypothesis seems to be confirmed by the presence of a heterogeneity of the pollen allele frequencies across female parent trees and by the significant coefficient of the regression of pollen allele frequencies on ovule genotype. Variation in the fixation indices in different life-cycle phases was observed, indicating possible presence of selective processes between seed set and sexual maturity. Possible explanations of these results are presented.

Mating system dynamics of Ocotea tenera (Lauraceae), a gynodioecious tropical tree

Progeny arrays of Ocotea tenera (Lauraceae), a gynodioecious tree endemic to Costa Rica, were electrophoretically surveyed for allozyme variation to estimate the outcrossing rate in the overall population and to test for differences in outcrossing rates between hermaphroditic and female trees. Multilocus outcrossing rate estimates across 3 yr indicated O. tenera predominantly outcrosses. However, significant heterogeneity in single‐locus outcrossing rates was found among loci. Two loci (Fe1, Fe2) gave high outcrossing estimates, and a third locus (Gdh) gave much lower outcrossing estimates. Heterogeneity in Gdh pollen allele frequencies, consanguineous matings, and selection against homozygous zygote genotypes at the Fe1 and Fe2 loci are factors contributing to the discrepancy in outcrossing rate estimates among loci. There were no differences in the mating systems of hermaphroditic and female trees, which suggests that factors beyond prevention of self‐fertilization may have also promoted the evolution of gynodioecy in O. tenera.

Mating System Dynamics of Ocotea tenera (Lauraceae), a Gynodioecious Tropical Tree

Progeny arrays of Ocotea tenera (Lauraceae), a gynodioecious tree endemic to Costa Rica, were electrophoretically surveyed for allozyme variation to estimate the outcrossing rate in the overall population and to test for differences in outcrossing rates between hermaphroditic and female trees. Multilocus outcrossing rate estimates across 3 yr indicated O. tenera predominantly outcrosses. However, significant heterogeneity in single-locus outcrossing rates was found among loci. Two loci (Fe1, Fe2) gave high outcrossing estimates, and a third locus (Gdh) gave much lower outcrossing estimates. Heterogeneity in Gdh pollen allele frequencies, consanguineous matings, and selection against homozygous zygote genotypes at the Fe1 and Fe2 loci are factors contributing to the discrepancy in outcrossing rate estimates among loci. There were no differences in the mating systems of hermaphroditic and female trees, which suggests that factors beyond prevention of self-fertilization may have also promoted the evolution of gynodioecy in O. tenera.

Homogeneity of pollen allele frequencies of single seed trees in Picea abies (L.) Karst. plantations

Allelic frequency distributions of genetically effective pollen clouds, which pollinated a total of 21 single seed trees in three populations of Norway spruce (Picea abies (L.) Karst.), were determined at seven polymorphic isozyme marker loci and compared with genetic structures of population samples. Differentiation among populations and among pollen allele frequencies of single seed trees within populations is low. In general, genetic distances between pollen and population gene pools are small. Both population and single tree outcrossing rates are high. Deviations from random mating are statistically significant at several marker loci for single seed trees. Results indicate extensive gene flow by pollen both within and among populations, which may counteract adaptive and nonadaptive processes of genetic differentiation.

Mating system parameters and population genetic structure in pioneer populations of Prosopis velutina (Leguminosae)

This study describes the genetic structure and provides estimates of mating system parameters in three natural populations of Prosopis velutina Woot. in southeastern Arizona. F statistics derived from isozyme data revealed the presence of both interpopulation and intrapopulation genetic differentiation. This population structure is discussed in relation to the ecological history of these populations that invaded the grasslands sites from adjacent riparian areas within the last 90 years, and possible modes of seed dispersal. The multilocus estimation program MLT of Ritland (Journal of Heredity 8: 235–237, 1990) was used to provide estimates of ts, tm, and the fixation index (F) of the maternal parents. Average estimates of ts ranged from 0.591 to 0.912. Estimates of tm ranged from 0.609 to 1.004, and averaged 0.758. The difference between tm and ts, which provides a measure of biparental inbreeding, averaged 0.058. This last result, together with significant heterogeneity found in pollen allele frequencies, suggests that a family structure has developed in the populations. A negative F value for one of the populations suggests selection against homozygotes between the seedling stage and maturity.

Effects of stand density on outcrossing rate in two Norway spruce (Picea abies) populations

The mating system was investigated in two neighbouring Norway spruce (Picea abies (L.) Karst.) populations with markedly different stand densities. The amount of outcrossing was estimated using open-pollinated array data at four enzyme loci. Multilocus estimates of the proportion of viable progeny owing to outcrossing were 0.956 and 0.955 for the low- and high-density stand, respectively, indicating that the large reduction in stand density had no impact on outcrossing rates. The multilocus estimate was higher than the mean single-locus estimate only for the low-density stand. The lowering of the mean estimate in the low-density stand is probably a consequence of the clustering of related individuals. The regression of pollen allele frequencies on ovule genotype, which is a direct measure of the effective selfing caused by consanguineous matings, had a significant coefficient for the low-density stand and a nonsignificant one for the high-density stand. This obvservation confirms that the major effect of low-stand density in Norway spruce is the occurrence of consanguineous matings. Key words: mating system, outcrossing rate, inbreeding, Norway spruce, stand density.

The effect of the density of flowering individuals on the mating systems of nine tropical tree species

Mating systems were examined through allozyme analysis for nine tropical tree species that occur on Barro Colorado Island, Republic of Panama. Estimates of outcrossing rates, using a mixed mating model, ranged from 0.35 to 1.08. Most species were highly outcrossed but two species that occurred at low densities had outcrossing rates below 0.7. Heterogeneity among trees in pollen allele frequencies contributing to progeny arrays was significant. The degree of heterogeneity in pollen allele frequencies was measured by Wright's FST, treating progeny arrays as populations. Mean FST values were negatively correlated with flowering tree density, which indicates that trees at low density sample pollen from fewer individuals than trees in denser populations. Among-year heterogeneity in pollen allele frequencies for two species was also related to the density of flowering individuals; years with lower flowering tree density had greater heterogeneity and/or more selfing. These results indicate that several population parameters, such as inbreeding, genetic relatedness of progeny arrays, gene flow and effective population size are associated with the density of flowering individuals and thus vary from year to year and among locations.

HETEROGENEITY IN POLLEN ALLELE FREQUENCIES AMONG CONES, WHORLS, AND TREES OF TABLE MOUNTAIN PINE (PINUS PUNGENS)

Pinus pungens Lambert (Table Mountain pine) is characterized by serotinous cones that are borne several (two to five) per whorl. Sporophytic and gametophytic tissues from whorls collected in different canopy zones of eight trees were electrophoresed and surveyed for six allozyme loci. Pollen allele frequencies received by individual trees, whorls on trees, and cones in whorls were estimated and used to estimate outcrossing rates and the distribution of genetic diversity within and among each level. Significant differences in pollen allele frequencies were detected at all levels, indicating that the genetic composition of the pollen pool available to different female strobili is heterogeneous. The fine‐scale genetic structure in the seed pool is a record of past reproductive events and is the precursor to genetic structure in the next generation.

Genetic relatedness in open-pollinated families of two leguminous tree species, Robinia pseudoacacia L. and Gleditsia triacanthos L.

When conducting tree breeding experiments, geneticists often assume that individuals from open-pollinated families are halfsibs. The reliability of this assumption was tested using data from enzyme electrophoresis to estimate the genetic relatedness among progeny within 22 open-pollinated families of Robinia pseudoacacia L. (black locust) and 34 open-pollinated families of Gleditsia triacanthos L. (honey locust) from natural stands. An algorithm employing population estimates of fixation indices, pollen allele frequencies, and selfing rates was used to calculate the mean expected number of alleles in common across loci under assumptions of either full-sib (i.e., a single pollen parent) or half-sib (i.e., random mating) relationships. For each open-pollinated family, the average coefficient of relationship among progeny was calculated by linear interpolation from the observed number of alleles in common. For most families of both species, coefficients were significantly higher than 0.25 (half-sib relation), but were significantly lower than 0.50 (full-sib relation). These results suggest that the assumption of a half-sib relationship among progeny of open-pollinated families is violated for these tree species. More critical to the estimation of heritabilities and the prediction of genetic gains was the observation that estimates of the coefficient of relationship varied widely among open-pollinated families (for R. pseudoacacia r 0=0.20-0.43, mean=0.34; for G. triacanthos r 0=0.29-0.55, mean=0.36).

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.