Short-distance pollen dispersal in a protogynous Annonaceae tree species from the Brazilian Cerrado

Annona crassiflora (Annonaceae) is a protogynous beetle-pollinated savannah tree species, widely distributed in the savannahs of the Cerrado biome. Studies on the mating system and pollen dispersal of protogynous species are very scarce. Here, we used six microsatellite loci to assess the mating system and pollen dispersal of A. crassiflora in a savannah remnant in Central Brazil. We mapped and sampled leaves of 112 adult trees and collected 74 fruits from 20 mother trees (1–4 fruits per plant) to obtain the seeds used (460) for mating system and parentage analyses. Annona crassiflora has predominantly allogamous mating systems, with a high multilocus outcrossing rate (t_m?=?0.974, SE?=?0.011) that did not differ among mother trees (F?=?1.32, p?=?0.165). However, t_m–t_s was variable among seed trees, indicating that some seeds were produced by mating among relatives. Our results also showed multiple paternity within fruits. Multilocus correlation of outcrossed paternity was high (r_p?=?0.302, SE?=?0.045), indicating that for each mother tree, the probability that the same pollen donor sired two random sibs was 30.2%, and the mean number of pollen donors per mother tree was high (6.3). We detected a maximum pollen dispersal distance of 360.7 m and an average of 124.3 m (SD?=?80 m), but most pollination events (73%) occurred at shorter distances (<?160 m), indicating short-distance pollen dispersal, most likely due to the pollinator behaviour.     

Flexible mating system in a logged population of <Emphasis Type="Italic">Swietenia macrophylla</Emphasis> King (Meliaceae): implications for the management of a threatened neotropical tree species

Microsatellites were used to evaluate the mating system of the remaining trees in a logged population of Swietenia macrophylla, a highly valuable and threatened hardwood species, in the Brazilian Amazon. A total of 25 open pollinated progeny arrays of 16 individuals, with their mother trees, were genotyped using eight highly polymorphic microsatellite loci. Genotypic data analysis from the progeny arrays showed that 373 out of the 400 seedlings (93.25%) were unambiguously the result of outcrossed matings and that the remaining 6.75% had genotypes consistent with self-fertilisation. Apomixis could be ruled out, since none of the 400 seedlings analysed had a multi-locus genotype identical to its mother tree. The high estimate of the multi-locus outcrossing rate (t _m = 0.938 ± 0.009) using the mixed mating model also indicated that the population in this remnant stand of S. macrophylla was predominantly allogamous. The relatively large difference between the multi-locus and single-locus outcrossing estimates (t _m −t _s = 0.117 ± 0.011) provides evidence that, in spite of the high outcrossing rate, a considerable degree of biparental inbreeding has contributed to the genetic structure of this population. Levels of outcrossing were not evenly distributed among maternal trees (t _m ranging from 0.39 to 1.00), suggesting the occurrence of a variable degree of self-incompatibility and/or dichogamy among individual trees of this monoecious species. Due to its generalist pollination system and some level of tolerance for selfing, S. macrophylla seems to be resilient to environmental disturbances such as those caused by logging, since it sets fruits with predominantly outcrossed seeds even at low stand densities. Therefore, the remaining individuals in logged areas or in relict fragments may be very important for long-term population recovery and genetic conservation programmes.     

Wild sorghum from different eco-geographic regions of Kenya display a mixed mating system

Knowledge of mating systems is required in order to understand the genetic composition and evolutionary potential of plant populations. Outcrossing in a population may co-vary with the ecological and historical factors influencing it. However, literature on the outcrossing rate is limited in terms of wild sorghum species coverage and eco-geographic reference. This study investigated the outcrossing rates in wild sorghum populations from different ecological conditions of Kenya. Twelve wild sorghum populations were collected in four sorghum growing regions. Twenty-four individuals per population were genotyped using six polymorphic simple sequence repeat (SSR) markers to compute their indirect equilibrium estimates of outcrossing rate as well as population structure. In addition, the 12 populations were planted in a field in a randomised block design with five replications. Their progeny (250 individuals per population) were genotyped with the six SSR markers to estimate multi-locus outcrossing rates. Equilibrium estimates of outcrossing rates ranged from 7.0 to 75.0%, while multi-locus outcrossing rates (t _m) ranged from 8.9 to 70.0% with a mean of 49.7%, indicating that wild sorghum exhibits a mixed mating system. The wide range of estimated outcrossing rates in wild sorghum populations indicate that environmental conditions may exist under which fitness is favoured by outcrossing and others under which selfing is more advantageous. The genetic structure of the populations studied is concordant with that expected for a species displaying mixed mating system.     

Mating system parameters at hierarchical levels of fruits, individuals and populations in the Brazilian insect-pollinated tropical tree, Tabebuia roseo-alba (Bignoniaceae)

For many tree species, mating system analyses have indicated potential variations in the selfing rate and paternity correlation among fruits within individuals, among individuals within populations, among populations, and from one flowering event to another. In this study, we used eight microsatellite markers to investigate mating systems at two hierarchical levels (fruits within individuals and individuals within populations) for the insect pollinated Neotropical tree Tabebuia roseo-alba. We found that T. roseo-alba has a mixed mating system with predominantly outcrossed mating. The outcrossing rates at the population level were similar across two T. roseo-alba populations; however, the rates varied considerably among individuals within populations. The correlated paternity results at different hierarchical levels showed that there is a high probability of shared paternal parentage when comparing seeds within fruits and among fruits within plants and full-sibs occur in much higher proportion within fruits than among fruits. Significant levels of fixation index were found in both populations and biparental inbreeding is believed to be the main cause of the observed inbreeding. The number of pollen donors contributing to mating was low. Furthermore, open-pollinated seeds varied according to relatedness, including half-sibs, full-sibs, self-sibs and self-half-sibs. In both populations, the effective population size within a family (seed-tree and its offspring) was lower than expected for panmictic populations. Thus, seeds for ex situ conservation genetics, progeny tests and reforestation must be collected from a large number of seed-trees to guarantee an adequate effective population in the sample.     

Individual and Temporal Variation in Outcrossing Rates and Pollen Flow Patterns in Ceiba pentandra (Malvaceae: Bombacoidea)

Ceiba pentandra is a tropical tree with high rates of selfing in some populations. In mixed‐mating species, variation in selfing is due to changes in adult density or variability of incompatibility systems. The effect of spatial isolation and phenology on selfing rates and pollen flow distances was analyzed using microsatellites in a fragmented population of Ceiba pentandra, in southern Costa Rica. Adult trees within a heterogeneous landscape were classified as grouped or isolated. We compared selfing rates at the individual level, between isolation conditions and 2 yr (2007, 2009), which differed in the number of flowering individuals. Mixed mating was estimated in both years (t_m = 0.624–0.759). Trees mated predominantly by outcrossing, while only a few trees reproduced through selfing. Spatial isolation did not significantly affect outcrossing rates. The progeny of grouped trees was mostly sired by near‐neighbors (<1 km) and by long‐distance pollen flow events in isolated trees. A reduction in the number of flowering individuals in 2009 reduced near‐neighbor matings, increased selfing in grouped trees, and decreased the number of unsampled sires in the progeny. Comparing selfing rates on individuals that flowered in both reproductive periods suggests a flexible mating system. Variation in self‐fertilization rates in this population appears to depend on variation of individual traits, such as genetic variability in self‐incompatibility genes, but it is independent of landscape heterogeneity. In contrast, pollen flow distances depend on local tree density as bats concentrate their foraging between near individuals to maximize energy efficiency.     

Understanding the genetic diversity, spatial genetic structure and mating system at the hierarchical levels of fruits and individuals of a continuous Theobroma cacao population from the Brazilian Amazon

Understanding the mating patterns of populations of tree species is a key component of ex situ genetic conservation. In this study, we analysed the genetic diversity, spatial genetic structure (SGS) and mating system at the hierarchical levels of fruits and individuals as well as pollen dispersal patterns in a continuous population of Theobroma cacao in Pará State, Brazil. A total of 156 individuals in a 0.56 ha plot were mapped and genotyped for nine microsatellite loci. For the mating system analyses, 50 seeds were collected from nine seed trees by sampling five fruits per tree (10 seeds per fruit). Among the 156 individuals, 127 had unique multilocus genotypes, and the remaining were clones. The population was spatially aggregated; it demonstrated a significant SGS up to 15 m that could be attributed primarily to the presence of clones. However, the short seed dispersal distance also contributed to this pattern. Population matings occurred mainly via outcrossing, but selfing was observed in some seed trees, which indicated the presence of individual variation for self-incompatibility. The matings were also correlated, especially within ($\widehat{r}$_p(m)=0.607) rather than among the fruits ($\widehat{r}$_p(m)=0.099), which suggested that a small number of pollen donors fertilised each fruit. The paternity analysis suggested a high proportion of pollen migration (61.3%), although within the plot, most of the pollen dispersal encompassed short distances (28 m). The determination of these novel parameters provides the fundamental information required to establish long-term ex situ conservation strategies for this important tropical species.     

Short-distance pollen dispersal for an outcrossed, wind-pollinated southern beech (Nothofagus nervosa (Phil.) Dim. et Mil.)

The mating system (outcrossing, selfing, and biparental inbreeding) and the extent of pollen flow are two of the most important genetic features that determine the genetic structure of plant populations, and both are crucial for the design of conservation strategies. The objectives here were to estimate mating system parameters and to fit the pollen dispersal kernel for the southern beech, Nothofagus nervosa. We sampled 25 mothers and 372 progeny from two stands in the Tromen Lake region of Argentina. We registered spatial positions of the maternal trees, and genotyped mothers and offspring for five simple sequence repeat markers. We estimated single-locus (t _s?=?0.95) and multilocus (t _m?=?0.99) outcrossing rates and biparental inbreeding (t _m-t _s?=?0.04). The species is strongly outcrossing, but correlated paternity within maternal sibships (r _p?=?0.10) indicates that each maternal parent is sampling a different and restricted array of pollen donors. We used two protocols (twogener and kindist) to fit an exponential power dispersal kernel to the structure of pollen clouds sampled by different mothers. The estimated effective number of pollen donors contributing to a single mother was N _ep?=?9.9. The twogener and kindist analyses yielded slightly different estimates, but both indicated short average distances for pollen dispersal (<35?m), indicating that the dispersal kernel was strongly leptokurtic (???=?0.36). While short-distance pollen dispersal predominates, there remains a nontrivial probability of long-distance dispersal. The results are discussed in the context of ongoing conservation and management programs.     

A novel mating system analysis for modes of self-oriented mating applied to diploid and polyploid arctic Easter daisies (Townsendia hookeri)

We have developed a new model for mating system analysis, which attempts to distinguish among alternative modes of self-oriented mating within populations. This model jointly estimates the rates of outcrossing, selfing, automixis and apomixis, through the use of information in the family structure given by dominant genetic marker data. The method is presented, its statistical properties evaluated, and is applied to three arctic Easter daisy populations, one consisting of diploids, the other two of tetraploids. The tetraploids are predominantly male sterile and reported to be apomictic while the diploids are male fertile. In each Easter daisy population, 10 maternal arrays of six progeny were assayed for amplified fragment length polymorphism markers. Estimates, confirmed with likelihood ratio tests of mating hypotheses, showed apomixis to be predominant in all populations (ca. 70%), but selfing or automixis was moderate (ca. 25%) in tetraploids. It was difficult to distinguish selfing from automixis, and simulations confirm that with even very large sample sizes, the estimates have a very strong negative statistical correlation, for example, they are not independent. No selfing or automixis was apparent in the diploid population, instead, moderate levels of outcrossing were detected (23%). Low but significant levels of outcrossing (2-4%) seemed to occur in the male-sterile tetraploid populations; this may be due to genotyping error of this level. Overall, this study shows apomixis can be partial, and provides evidence for higher levels of inbreeding in polyploids compared to diploids and for significant levels of apomixis in a diploid plant population.     

Population genetics of Cordia alliodora (Boraginaceae), a neotropical tree. 2. Mating system

A multilocus mixed mating model was used to evaluate the mating system of a natural population of Cordia alliodora (Boraginaceae), a neotropical tree. The population was highly outcrossed (t_m = 0.966 ± 0.027), in agreement with results from controlled crosses. Departures from the mixed mating model were evident, suggesting some nonrandom, correlated mating. Pollen pool heterogeneity and variation in estimates of individual outcrossing rates indicated that the population may be genetically substructured. Individual outcrossing rates obtained for the samples taken from within different parts of the same tree indicated reduced levels of outcrossing due to limited sampling of the pollen pool. The incompatibility mechanism in C. alliodora, combined with variation in flowering and stand density, appears to lead to both temporal and spatial substructuring of the population.     

A study of the mating system in Dolichandra cynanchoides (Bignoniaceae): an Argentinian Chaco woodlands liane with a late-acting self-incompatibility

We estimated the outcrossing rate in a population of 14 individuals of Dolichandra cynanchoides (Bignoniaceae), a species with late-acting self-incompatibility (LSI), at a site in Chaco woodland in Santa Fe province, Argentina. A subsample of five arbitrarily chosen loci from a total of 16 allozyme loci gave mating system parameters of t_m=0.881 (SD 0.039) and t_s=0.749 (SD 0.048), thus indicating that although predominantly outcrossing, D. cynanchoides has a mixed mating system. We draw attention to the fact that mixed mating in species with LSI is a very likely scenario, given that previous studies with diverse LSI taxa have shown that mixed cross-self pollen loads on the stigma, which is probably a common occurrence with natural pollinators, result in fruits with a proportion of selfed seeds.     

Inbreeding depression from selfing and mating between relatives in the Neotropical tree <Emphasis Type="Italic">Cariniana legalis</Emphasis> Mart. Kuntze

Selfing or mating between related individuals in self-compatible hermaphroditic tree species may lead to inbreeding depression (ID) due to homozygosis in recessive, identical by descent alleles. In general, studies of ID in tree species have been based on comparisons of selfed individuals (produced by controlled pollination) with outcrossed individuals for quantitative traits in progeny tests. However, this approach requires a long time to quantify the extent of ID. Thus, we used an approach based on genetic markers to estimate coancestry coefficients between assigned parents from paternity analysis in two populations of the Neotropical tree Cariniana legalis. Using this method, we were able to determine which seedlings in a nursery trial originated from; (i) outcrossing between un-related trees, (ii) mating between related trees and (iii) selfing. We detected a low selfing rate (<10 %), but a substantial quantity of seedlings from mating between related parents (minimum of 35.7 %). In general, the outcrossed seedlings from unrelated parents exhibited significantly greater genetic diversity than those resulting from selfing and mating among relatives. The extent of ID varied among traits and populations. Outcrossed seedlings originating from unrelated trees generally showed greater survival than seedlings originating from selfing and related parents. Inbreeding depression was greater in the selfed seedlings than in those from mating among related parents. The results are discussed in terms of implications for genetic conservation, breeding and environmental restoration using the species.     

The mating system and genetic significance of polycarpy in the neem tree (Azadirachta indica)

The objectives of this study were to estimate the outcrossing rate and to explain genetic consequences of the development of seed in the endocarp in a natural population of neem in Bangladesh. Cotyledons of germinated open-pollinated seeds of individual trees were analyzed by starch-gel electrophoresis to examine allozymes. Three loci with clear Mendelian segregation were used to estimate outcrossing rate. A multilocus mixed mating model was used to evaluate the mating system. The population exhibited high outcrossing rates both for multilocus (t_m=0.90±0.024) and mean single-locus (t_s=0.92±0.020) estimates. The difference between these two parameters (t_m–t_s=0±SE 0.038) was insignificant, indicating that there was no ’biparental inbreeding’ in the population. The degree of variance of the estimates of multilocus outcrossing rates decreased when two or more loci were included. In order to elucidate the significance of polycarpy a total of 471 seeds were counted out of 440 endocarps. This mechanism appears to be a possible way of avoiding inbreeding. The results indicated that the studied neem population was predominantly allogamous. Received: 10 January 1999 / Accepted: 10 April 1999     

Comparison of mating system parameters and genetic structure in three natural scenarios of Acacia visco (Leguminosae, Mimosoideae)

Acacia visco is a native South American tree species that has been extensively used for ornamental purposes and in carpentry, bodywork and parquet due to the hardness and durability of its wood. Little is known about genetic diversity and mating system of A. visco. The main aims of this study were to (1) estimate outcrossing rates in natural Argentinean populations using AFLP markers, (2) test for any difference in mating patterns among a large a patchy and relict population, and (3) compare the mating system of A. visco with other Acacia species. The three primer pairs used in the AFLP analysis revealed a total of 569 variable loci. Most genetic variation was observed among individuals within families (61.2 %). The estimate of multilocus outcrossing rate (t _m) was high (≥0.971) in all populations. Average pairwise coancestry between progenies within families for each population ranged from 0.082 to 0.105 or from 0.125 to 0.136, depending on the method θ was estimated. In the three populations studied, the progenies of open pollination were constituted mainly for half-sibs (94.3 %). This work shows a similar mating system in all populations of A. visco in spite of their size differences, hypothesizing that the entire species has a similar mating system of outcrossing preferential. Considering the results obtained here where a high percentage of individuals were half-sibs, sampling large numbers of pods from individual trees for ex situ conservation will result in a genetically diverse sample as a consequence of high outcrossing rates.     

Mating Systems of Psychotria tenuinervis (Rubiaceae): Distance from Anthropogenic and Natural Edges of Atlantic Forest Fragment

The aim of this study was to determine and compare the mating systems among Psychotria tenuinervis populations at anthropogenic edges, natural edges, and the forest interior using allozyme electrophoresis of naturally pollinated progeny arrays. P. tenuinervis showed low outcrossing rates, varying from 37% to 50% of the mating attributable to outcrossing and 50% to 63% attributable to self-fertilization, in the three habitats. The forest interior had the highest outcrossing rate (t _m = 0.50 and t _s = 0.43) among the three habitats. However, there were no differences in either multilocus or single-locus rates among the three habitats, indicating that the contribution of biparental inbreeding to the apparent selfing rate in these populations was very low. The multilocus (t _m) and single-locus (t _s) outcrossing rates for the P. tenuinervis in the sample plots within each habitat showed great heterogeneity. In conclusion, edge creation seems not to influence its mating systems. Additionally, although P. tenuinervis is a distylous species, the population’s inbreeding can be attributed almost entirely to self-fertilization.     

Composition of the pollinator community,pollination and the mating system for a shrub in fragments of species rich kwongan in south-west Western Australia

In this study we investigate the composition of the potential honeyeater pollinator community, patterns of honeyeater visitation, pollination and the mating system in a range of population fragments for the bird-pollinated mixed mating system shrub Calothamnus quadrifidus R.Br. Specifically, we aimed to answer the following questions. For smaller and more isolated population fragments are honeyeater species lost from the pollinator community, patterns of visitation different, levels of pollination lower and rates of selfing, biparental inbreeding and correlated paternity higher. The composition of the honeyeater community was similar across population fragments and there was no relationship between the abundance of birds and population fragment size. Honeyeaters were most commonly observed visiting numerous inflorescences within single plants in all populations, but as population fragments became larger movements between plants were more commonly observed. Our observations of honeyeater visitation were generally consistent with our measurements of pollination and patterns in the mating system across population fragments. We found no significant relationship between population fragment size and levels of pollination. Mating system studies showed outcrossing rates (t _m) comparable to those found in other bird-pollinated Myrtaceae, and ranged from 0.54 to 0.90 across populations. Outcrossing rates were not significantly correlated with log population size, but correlations of outcrossed paternity indicate a clear trend from low correlated paternity in larger populations to significantly higher correlated paternities in smaller populations. As a consequence mating in small populations will occur between much smaller groups of plants, and this may affect population fitness in subsequent generations.     

The genetic structure and mating system of Acrocomia aculeata (Arecaceae)

Acrocomia aculeata is a perennial, fruit-producing palm tree, native to tropical forests. Its fruits have spurred interest because of their significant potential for use in the cosmetic industry and as feedstock for biofuel. In the present study, the genetic structure and mating system in Acrocomia aculeata were analyzed, using eight nuclear micro-satellite loci and samples from São Paulo and Minas Gerais states, Brazil. By means of Bayesian analysis, these populations were clustered into two or three groups. A high multilocus outcrossing rate suggests that outcrosses were predominant, although a certain degree of biparental inbreeding also occurred. Thus, although monoecious and self-compatible, there is every indication that A. aculeata bears a mixed reproductive system, with a predominance of outcrossing. Given the genetic structure revealed hereby, future conservation strategies and germplasm collecting should be focussed on sampling and preserving individuals from different clusters.     

Mating systems of two Bombacaceous trees of a neotropical moist forest

Summary A multilocus mixed mating model was used to analyze the mating systems of two tropical canopy trees in the Bombacaceae that differ in successional status and overall abundance. One population of each species was studied on Barro Colorado Island, Republic of Panama. Population outcrossing estimates of 0.57 and 0.35 from two years indicate a mixed-mating system with intermediate outcrossing levels for the gap-specialist Cavanillesia platanifolia, a relatively rare component of the island flora. Population and individual outcrossing estimates were associated with flowering tree density or degree of spatial isolation. Trees within clusters of flowering individuals have a higher degree of outcrossing than isolated trees. Annual estimates of individual tree outcrossing rates varied greatly as a function of flowering in its nearest neighbors. In contrast to C. platanifolia, Quararibea asterolepis was completely outcrossed and may be self incompatible. Maternal trees of both species had significantly heterogeneous pollen pools indicating non-random outcrossing. Apomixis by sporophytic agamospermy was rejected in C. platanifolia as contributing to the apparent selfing rate.     

Mixed mating in androdioecious Mercurialis annua inferred using progeny arrays and diploid-acting microsatellite loci in a hexaploid background

Background and Aims

The frequency at which males can be maintained with hermaphrodites in androdioecious populations is predicted to depend on the selfing rate, because self-fertilization by hermaphrodites reduces prospective siring opportunities for males. In particular, high selfing rates by hermaphrodites are expected to exclude males from a population. Here, the first estimates are provided of the mating system from two wild hexaploid populations of the androdioecious European wind-pollinated plant M. annua with contrasting male frequencies.

Methods

Four diploid microsatellite loci were used to genotype 19–20 progeny arrays from two populations of M. annua, one with males and one without. Mating-system parameters were estimated using the program MLTR.

Key Results

Both populations had similar, intermediate outcrossing rates (t_m = 0·64 and 0·52 for the population with and without males, respectively). The population without males showed a lower level of correlated paternity and biparental inbreeding and higher allelic richness and gene diversity than the population with males.

Conclusions

The results demonstrate the utility of new diploid microsatellite loci for mating system analysis in a hexaploid plant. It would appear that androdioecious M. annua has a mixed-mating system in the wild, an uncommon finding for wind-pollinated species. This study sets a foundation for future research to assess the relative importance of the sexual system, plant-density variation and stochastic processes for the regulation of male frequencies in M. annua over space and time.     

Variation in the mating system of Sitka spruce (Picea sitchensis): evidence for partial assortative mating

Five allozyme polymorphisms were used to analyze the mating system in a Sitka spruce seed orchard in Saanichton, British Columbia. Allelic frequencies differed between the pollen and maternal pools at three of the five loci, with alleles rare in the maternal pool being even rarer in the effective pollen pool. Minor differences in pollen allelic frequencies were observed in the upper vs. lower crown. The multilocus outcrossing rate of the upper crown (t_m = 0.909) exceeded that of the lower crown (t_m = 0.764). Single-locus estimates of the outcrossing rate were significantly heterogeneous, with the lowest estimate of outcrossing, t = 0.773, observed for PGM-2 locus. Analyses of the mating system for the three maternal PGM-2 genotypes revealed heterogeneous pollen allelic frequencies and heterogeneous outcrossing rates, possibly due to assortative mating at this locus.     

Genetic structure and mating system of Stemonoporus oblongifolius (Dipterocarpaceae) in Sri Lanka

The genetic structure and mating system of an endemic Sri Lankan dipterocarp, Stemonoporus oblongifolius, was examined using allozyme analyses. Considering the small geographic distribution of the species, which is confined to the Peak Wilderness Sanctuary range, the species and population level genetic diversity are exceptionally high (H_e = 0.342 and 0.282, respectively). Although the populations sampled are separated by a maximum distance of only 10 kilometers, a fair amount of genetic structuring was detected (G_ST = 0.163). The mating system could contribute to the maintenance of a high level of genetic diversity, but in itself is insufficient to explain the high values for this study. The multilocus outcrossing rate, quantified for one population, was high (84%), with evidence for apomixis in one tree. Other factors potentially contributing to the high observed allozyme diversity in this species would be large effective population sizes, introgression of genes from sympatric species, and retention of ancient allozyme polymorphism. Gene flow among populations, estimated to be high, would have to be through pollen dispersal, since the large, resinous seeds typically fall and germinate beneath the maternal tree and are not known to be dispersed by any animal.