Impact of population structure on the apparent outcrossing rate of grain sorghum (Sorghum bicolor)

Pollen-mediated transfer of herbicide resistance between johnsongrass (Sorghum halepense) biotypes

Johnsongrass (Sorghum halepense) is a troublesome weed in row crop production in the United States. Herbicide resistance is a growing concern in this species, with resistance to ACCase-, ALS-, and EPSPS-inhibitors already reported. Pollen-mediated gene flow (PMGF) is capable of spreading herbicide resistance, but the extent of PMGF has not yet been studied in johnsongrass. Field experiments were conducted in a Nelder-wheel design to quantify the distance and frequency of PMGF from ALS-inhibitor-resistant (AR) to -susceptible (AS) johnsongrass across three environments (summer 2018, fall 2018, and fall 2019). The AR biotype (pollen donor) was established at the center of the wheel (5-m diameter), and a naturally occurring johnsongrass (AS) infestation was utilized as the pollen recipient, in eight directions and at nine distances (5, 10, 15, 20, 25, 35, 40, 45, and 50 m) within each direction. Seeds collected from the AS plants in each distance and direction were screened for survival to the ALS-inhibitor herbicide nicosulfuron (Accent Q) at 95 g ai ha⁻¹ under greenhouse conditions. The survivors (i.e. hybrids) were further confirmed based on the presence of the Trp₅₇₄Leu mutation. At the closest distance of 5 m, PMGF was 9.6–16.2% across the directions and environments, which progressively declined to 0.8–1.2% at 50 m. The exponential decay model predicted 50% reduction in PMGF at 2.2 m and 90% reduction at 5.8 m from the pollen donor block. Results demonstrate that herbicide resistance can spread between adjacent field populations of johnsongrass through PMGF, which necessitates sound monitoring and management.

Comparative evolutionary genetics of deleterious load in sorghum and maize

Sorghum and maize share a close evolutionary history that can be explored through comparative genomics^1,2. To perform a large-scale comparison of the genomic variation between these two species, we analysed ~13 million variants identified from whole-genome resequencing of 499 sorghum lines together with 25 million variants previously identified in 1,218 maize lines. Deleterious mutations in both species were prevalent in pericentromeric regions, enriched in non-syntenic genes and present at low allele frequencies. A comparison of deleterious burden between sorghum and maize revealed that sorghum, in contrast to maize, departed from the domestication-cost hypothesis that predicts a higher deleterious burden among domesticates compared with wild lines. Additionally, sorghum and maize population genetic summary statistics were used to predict a gene deleterious index with an accuracy greater than 0.5. This research represents a key step towards understanding the evolutionary dynamics of deleterious variants in sorghum and provides a comparative genomics framework to start prioritizing these variants for removal through genome editing and breeding.

Transcriptional trajectories of anther development provide candidates for engineering male fertility in sorghum

Sorghum is a self-pollinated crop with multiple economic uses as cereal, forage, and biofuel feedstock. Hybrid breeding is a cornerstone for sorghum improvement strategies that currently relies on cytoplasmic male sterile lines. To engineer genic male sterility, it is imperative to examine the genetic components regulating anther/pollen development in sorghum. To this end, we have performed transcriptomic analysis from three temporal stages of developing anthers that correspond to meiotic, microspore and mature pollen stages. A total of 5286 genes were differentially regulated among the three anther stages with 890 of them exhibiting anther-preferential expression. Differentially expressed genes could be clubbed into seven distinct developmental trajectories using K-means clustering. Pathway mapping revealed that genes involved in cell cycle, DNA repair, regulation of transcription, brassinosteroid and auxin biosynthesis/signalling exhibit peak expression in meiotic anthers, while those regulating abiotic stress, carbohydrate metabolism, and transport were enriched in microspore stage. Conversely, genes associated with protein degradation, post-translational modifications, cell wall biosynthesis/modifications, abscisic acid, ethylene, cytokinin and jasmonic acid biosynthesis/signalling were highly expressed in mature pollen stage. High concurrence in transcriptional dynamics and cis-regulatory elements of differentially expressed genes in rice and sorghum confirmed conserved developmental pathways regulating anther development across species. Comprehensive literature survey in conjunction with orthology analysis and anther-preferential accumulation enabled shortlisting of 21 prospective candidates for in-depth characterization and engineering male fertility in sorghum.

Genetic and genomic resources of sorghum to connect genotype with phenotype in contrasting environments

With the recent development of genomic resources and high-throughput phenotyping platforms, the 21st century is primed for major breakthroughs in the discovery, understanding and utilization of plant genetic variation. Significant advances in agriculture remain at the forefront to increase crop production and quality to satisfy the global food demand in a changing climate all while reducing the environmental impacts of the world's food production. Sorghum, a resilient C₄ grain and grass important for food and energy production, is being extensively dissected genetically and phenomically to help connect the relationship between genetic and phenotypic variation. Unlike genetically modified crops such as corn or soybean, sorghum improvement has relied heavily on public research; thus, many of the genetic resources serve a dual purpose for both academic and commercial pursuits. Genetic and genomic resources not only provide the foundation to identify and understand the genes underlying variation, but also serve as novel sources of genetic and phenotypic diversity in plant breeding programs. To better disseminate the collective information of this community, we discuss: (i) the genomic resources of sorghum that are at the disposal of the research community; (ii) the suite of sorghum traits as potential targets for increasing productivity in contrasting environments; and (iii) the prospective approaches and technologies that will help to dissect the genotype-phenotype relationship as well as those that will apply foundational knowledge for sorghum improvement.

Characterization of the mating system of a native perennial tetraploid herb, Silene stellata

PREMISE OF THE STUDY

Nursery pollination systems can range from obligate to facultative. In a system where generalists provide substantial pollination service, an important question is whether the cost of seed predation outweighs the benefit provided by the nursery pollinator to cause the plant to evolve toward more generalized pollination. Using a facultative system native to North America, we tested whether nursery pollinator vs. strictly mutualistic generalists affect mating-system parameters of the host plant and explored the implications for long-term coevolution.

METHODS

We used paternity analyses with 11 microsatellite markers to characterize the mating system of Silene stellata when pollination service is primarily through the nursery pollinator Hadena ectypa and generalist moths.

KEY RESULTS

Our experimental population of S. stellata was predominantly outcrossing (average outcrossing rate t = 0.83), and mating-system parameters were similar between pollinator groups. We detected significant correlations in both selfing and outcrossed paternity at the fruit and maternal family level, corresponding to limited pollen dispersal (mean = 3.9 m). Among individuals, variation in anther-stigma separation was positively associated with outcrossing rate, which suggests the importance of herkogamy in preventing selfing.

CONCLUSIONS

Correlated paternity suggests that seeds from the same fruit and/or plants are sired by a limited number of pollen donors, resulting from low pollen dispersal and potential male-male competition. The similar mating-system parameters of the two pollinator groups suggest that selection for higher outcrossing in S. stellata is likely to be through floral design rather than through increased pollinator specialization with H. ectypa.

The Mating System of the Wild-to-Domesticated Complex of Gossypium hirsutum L. Is Mixed

The domestication syndrome of many plants includes changes in their mating systems. The evolution of the latter is shaped by ecological and genetic factors that are particular to an area. Thus, the reproductive biology of wild relatives must be studied in their natural distribution to understand the mating system of a crop species as a whole. Gossypium hirsutum (upland cotton) includes both domesticated varieties and wild populations of the same species. Most studies on mating systems describe cultivated cotton as self-pollinated, while studies on pollen dispersal report outcrossing; however, the mating system of upland cotton has not been described as mixed and little is known about its wild relatives. In this study we selected two wild metapopulations for comparison with domesticated plants and one metapopulation with evidence of recent gene flow between wild relatives and the crop to evaluate the mating system of cotton's wild-to-domesticated complex. Using classic reproductive biology methods, our data demonstrate that upland cotton presents a mixed mating system throughout the complex. Given cotton's capacity for outcrossing, differences caused by the domestication process in cultivated individuals can have consequences for its wild relatives. This characterization of the diversity of the wild relatives in their natural distribution, as well as their interactions with the crop, will be useful to design and implement adequate strategies for conservation and biosecurity.

THE EVOLUTION OF SELF-FERTILIZATION AND INBREEDING DEPRESSION IN PLANTS. II. EMPIRICAL OBSERVATIONS

A bimodal distribution of outcrossing rates was observed for natural plant populations, with more primarily selfing and primarily outcrossing species, and fewer species with intermediate outcrossing rate than expected by chance. We suggest that this distribution results from selection for the maintenance of outcrossing in historically large, outcrossing populations with substantial inbreeding depression, and from selection for selfing when increased inbreeding, due to pollinator failure or population bottlenecks, reduces the level of inbreeding depression. Few species or populations are fixed at complete selfing or complete outcrossing. A low level of selfing in primarily outcrossing species is unlikely to be selectively advantageous, but will not reduce inbreeding depression to the level where selfing is selectively favored, particularly if accompanied by reproductive compensation. Similarly, occasional outcrossing in primarily selfing species is unlikely to regularly provide sufficient heterosis to maintain selection for outcrossing through individual selection. Genetic, morphological and ecological constraints may limit the potential for outcrossing rates in selfers to be reduced below some minimum level.

POLLEN DISCOUNTING AND THE SPREAD OF A SELFING VARIANT IN TRISTYLOUS EICHHORNIA PANICULATA: EVIDENCE FROM EXPERIMENTAL POPULATIONS

Floral traits that increase self-fertilization are expected to spread unless countered by the effects of inbreeding depression, pollen discounting (reduced outcross pollen success by individuals with increased rates of self-fertilization), or both. Few studies have attempted to measure pollen discounting because to do so requires estimating the male outcrossing success of plants that differ in selfing rate. In natural populations of tristylous Eichhornia paniculata, selfing variants of the mid-styled morph are usually absent from populations containing all three style morphs but often predominate in nontrimorphic populations. We used experimental garden populations of genetically marked plants to investigate whether the effects of population morph structure on relative gamete transmission by unmodified (M) and selfing variants (M') of the mid-styled morph could explain their observed distribution. Transmission through ovules and self and outcross pollen by plants of the M and M' morphs were compared under trimorphic, dimorphic (S morph absent), and monomorphic (L and S morphs absent) population structures. Neither population structure nor floral morphology affected female reproductive success, but both had strong effects on the relative transmission of male gametes. The frequency of self-fertilization in the M' morph was consistently higher than that of the M morph under all morph structures, and the frequency of self-fertilization by both morphs increased as morph diversity of experimental populations declined. In trimorphic populations, total transmission by the M and M' morphs did not differ. The small, nonsignificant increase in selfing by the M' relative to the M morph was balanced by decreased outcross siring success, particularly on the S morph. In populations lacking the S morph, male gamete transmission by the M' morph was approximately 1.5 times greater than that by the M morph because of both increased selfing and increased success through outcross pollen donation. Therefore, gamete transmission strongly favored the M' morph only in the absence of the S morph, a result consistent with the distribution of the M' morph in nature. This study indicates that floral traits that alter the selfing rate can have large and context-dependent influences on outcross pollen donation.

EFFECTS OF POPULATION STRUCTURE AND CONE PRODUCTION ON OUTCROSSING RATES IN ENGELMANN SPRUCE AND SUBALPINE FIR

Variation in the mating systems of Engelmann spruce and subalpine fir was examined in two adjacent subpopulations in the Colorado Front Range. Multilocus outcrossing rates were estimated from analysis of allozyme variation in embryo and megagametophyte tissue. The overall multilocus outcrossing rate of 0.93 for spruce was significantly higher than the outcrossing rate of 0.89 for fir. Outcrossing rates varied from 0.73 to 0.97 for spruce and from 0.65 to 0.94 for fir when trees of each species were grouped according to age, size, spatial distribution, maternal heterozygosity, cone production, and year of sampling. In both spruce and fir, trees with higher levels of male-cone production and/or more clumped spatial distribution had lower outcrossing rates. Temporal variation in outcrossing rates within subpopulations, examined only in spruce, was less than variation between subpopulations. Male-cone production was significantly correlated with tree size, and regression analyses revealed that the highest outcrossing rates are expected on trees that are medium to large in size, rather than the largest trees in the forest. The results show that, while there is some temporal variation, patterns of male cone production and spatial distribution of individuals are the most important factors influencing outcrossing levels in these wind-pollinated forest trees.

Morphological Characterization of a New and Easily Recognizable Nuclear Male Sterile Mutant of Sorghum (Sorghum bicolor)

Sorghum (Sorghum bicolor L. Moench) is one of the most important grain crops in the world. The nuclear male sterility (NMS) trait, which is caused by mutations on the nuclear gene, is valuable for hybrid breeding and genetic studies. Several NMS mutants have been reported previously, but none of them were well characterized. Here, we present our detailed morphological characterization of a new and easily recognizable NMS sorghum mutant male sterile 8 (ms8) isolated from an elite inbred BTx623 mutagenized by ethyl methane sulfonate (EMS). Our results show that the ms8 mutant phenotype was caused by a mutation on a single recessive nuclear gene that is different from all available NMS loci reported in sorghum. In fertile sorghum plants, yellow anthers appeared first during anthesis, while in the ms8 mutant, white hairy stigma emerged first and only small white anthers were observed, making ms8 plants easily recognizable when flowering. The ovary development and seed production after manual pollination are normal in the ms8 mutant, indicating it is female fertile and male sterile only. We found that ms8 anthers did not produce pollen grains. Further analysis revealed that ms8 anthers were defective in tapetum development, which led to the arrest of pollen formation. As a stable male sterile mutant across different environments, greenhouses, and fields in different locations, the ms8 mutant could be a useful breeding tool. Moreover, ms8 might be an important for elucidating male gametophyte development in sorghum and other plants.

Use of multicopy transposons bearing unfitness genes in weed control: four example scenarios

We speculate that multicopy transposons, carrying both fitness and unfitness genes, can provide new positive and negative selection options to intractable weed problems. Multicopy transposons rapidly disseminate through populations, appearing in approximately 100% of progeny, unlike nuclear transgenes, which appear in a proportion of segregating populations. Different unfitness transgenes and modes of propagation will be appropriate for different cases: (1) outcrossing Amaranthus spp. (that evolved resistances to major herbicides); (2) Lolium spp., important pasture grasses, yet herbicide-resistant weeds in crops; (3) rice (Oryza sativa), often infested with feral weedy rice, which interbreeds with the crop; and (4) self-compatible sorghum (Sorghum bicolor), which readily crosses with conspecific shattercane and with allotetraploid johnsongrass (Sorghum halepense). The speculated outcome of these scenarios is to generate weed populations that contain the unfitness gene and thus are easily controllable. Unfitness genes can be under chemically or environmentally inducible promoters, activated after gene dissemination, or under constitutive promoters where the gene function is utilized only at special times (e.g. sensitivity to an herbicide). The transposons can be vectored to the weeds by introgression from the crop (in rice, sorghum, and Lolium spp.) or from planted engineered weed (Amaranthus spp.) using a gene conferring the degradation of a no longer widely used herbicide, especially in tandem with an herbicide-resistant gene that kills all nonhybrids, facilitating the rapid dissemination of the multicopy transposons in a weedy population.

Estimation of in situ mating systems in wild sorghum (Sorghum bicolor (L.) Moench) in Ethiopia using SSR-based progeny array data: implications for the spread of crop genes into the wild

Because transgenic sorghum (Sorghum bicolor L.) is being developed for Africa, we investigated the potential for transgenes to spread to conspecific wild/weedy sorghum populations in Ethiopia, which is considered the centre of origin of cultivated sorghum. In the current study, the extent of outcrossing, and uniparental and biparental inbreeding were investigated in seven wild/weedy sorghum populations collected at elevations ranging from 631 to 1709 m. Based on allele frequency data of 1120 progenies and 140 maternal plants from five polymorphic microsatellite markers, outcrossing rates were estimated using standard procedures. The average multilocus outcrossing rate was 0.51, with a range of 0.31-0.65 among populations, and the family outcrossing rate was in the extreme range of 0 to 100%. The highest outcrossing (t(m) = 0.65) was recorded in a weedy population that was intermixed with an improved crop variety in Abuare (Wello region). It was also observed that the inbreeding coefficient of the progenies (F(p)) tend to be more than the inbreeding coefficient of both their maternal parents (F(m)) and the level of inbreeding expected at equilibrium (F(eq)), which is a characteristic of predominantly outbreeding species. Biparental inbreeding was evident in all populations and averaged 0.24 (range = 0.10-0.33). The high outcrossing rates of wild/weedy sorghum populations in Ethiopia indicate a high potential for crop genes (including transgenes) to spread within the wild pool. Therefore, effective risk management strategies may be needed if the introgression of transgenes or other crop genes from improved cultivars into wild or weedy populations is deemed to be undesirable.

Estimation of outcrossing rates at small-scale flowering sites of the dwarf bamboo species, Sasa cernua

We estimated the outcrossing rates at small-scale flowering sites of an endemic dwarf bamboo species, Sasa cernua. The multi-locus estimation of the outcrossing rate of the dwarf bamboo population was 0.148 (SD 0.118). Two culms with the highest outcrossing rates had heterozygous genotypes at one locus, but other culms in the locus were homozygotes. Five culms with high outcrossing rates bore 2-17% seeds with homozygous genotypes. Due to predominant selfing, the overall inbreeding coefficient of seeds was high, although it declined in seedlings. This suggests that selection against inbred progenies began early in the establishment process in the natural habitat.

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.

Genetic diversity in sorghum (Sorghum bicolor (L.) Moench) accessions of Zambia as revealed by simple sequence repeats (SSR)

Twenty seven accessions of sorghum conserved in the national gene bank of Zambia, representing two of the three agroecological regions of the country, were investigated using simple sequence repeats (SSR) markers in order to determine the extent and distribution of its genetic diversity. We used 10 microsatellite primer-pairs, which generated 2-9 alleles per locus and a total of 44 alleles across the 27 accessions. The observed heterozygosity (Ho(P) ) among the accessions ranged from 0 to 0.19 with an average of 0.04 whereas the average expected heterozygosity (He(P) ) among accessions was 0.07 in line with the fact that sorghum is predominately inbreeder. The analysis of molecular variance (AMOVA) revealed that 82% of the total genetic variation was attributable to the genetic variation among accessions (F(ST) = 0.824; p < 0.001) whereas the genetic variation within accessions accounted for 18% of the total genetic variation. AMOVA on sorghum accessions grouped based on four ethnic groups (Soli, Chikunda, Lozi and Tonga) associated with collection sites revealed a highly significant variation among groups (23%; p < 0.001). Although cluster analysis grouped most accessions according to their sites of collection, some accessions that originated from the same site were placed under different clusters. In addition to the extent and pattern of genetic diversity, consideration should also be given to other factors such as ecogeographic and ethnic differences when sampling sorghum genetic resources for rational and efficient conservation and utilization in the breeding program.

Growth and fitness components of wild x cultivated Sorghum bicolor (Poaceae) hybrids in Nebraska

PREMISE OF THE STUDY

Gene flow from crops to wild relatives has received considerable attention since the advent of genetically modified crops. Numerous researchers have found wild-crop hybrids to be nearly as fit as their wild parents, which suggests that crop genes may persist in wild populations. Components of the ecological fitness of cultivated sorghum, its wild relative, shattercane, and their hybrids have not been studied. •

METHODS

To assess the potential for gene introgression into shattercane, we crossed cultivated sorghum to a single inbred shattercane line to produce F(1) hybrids and measured growth and several components of ecological fitness in relation to both parents in Nebraska, USA. •

KEY RESULTS

Germination of F(1) seeds was similar to that of its shattercane parent except at high temperatures, where it was as sensitive as the sorghum parent. The F(1) grew taller and produced more biomass than either parent, but the F(1) leaf area index was intermediate. Fecundity of the F(1) plant was similar to that of shattercane and much greater than that of cultivated sorghum. •

CONCLUSIONS

Considering all data, the ecological fitness of shattercane × cultivated sorghum F(1) hybrids may be equivalent to the wild shattercane parent, which suggests that crop genes that are either neutral or beneficial to shattercane would persist in populations within agroecosystems.

High outcrossing rates in fields with mixed sorghum landraces: how are landraces maintained?

The effect of mating system on genetic diversity is a major theme in plant evolutionary genetics, because gene flow plays a large role in structuring the genetic variability within and among populations. Understanding crop mating systems and their consequences for gene flow can aid in managing agricultural systems and conserving genetic resources. We evaluated the extent of pollen flow, its links with farming practices and its impact on the dynamics of diversity of sorghum in fields of Duupa farmers in Cameroon. Duupa farmers grow numerous landraces mixed in a field, a practice that favours extensive pollen flow. We estimated parameters of the mating system of five landraces representative of the genetic diversity cultivated in the study site, using a direct method based on progeny array. The multilocus outcrossing rate calculated from all progenies was 18% and ranged from 0 to 73% among progenies. Outcrossing rates varied greatly among landraces, from 5 to 40%. Our results also showed that individual maternal plants were usually pollinated by more than eight pollen donors, except for one landrace (three pollen donors). Although the biological traits of sorghum (inflorescence morphology, floral traits, phenology) and the spatial planting practices of Duupa farmers led to extensive pollen flow among landraces, selection exerted by farmers appears to be a key parameter affecting the fate of new genetic combinations from outcrossing events. Because both natural and human-mediated factors shape evolution in crop populations, understanding evolutionary processes and designing in situ conservation measures requires that biologists and anthropologists work together.

Local genetic diversity of sorghum in a village in northern Cameroon: structure and dynamics of landraces

We present the first study of patterns of genetic diversity of sorghum landraces at the local scale. Understanding landrace diversity aids in deciphering evolutionary forces under domestication, and has applications in the conservation of genetic resources and their use in breeding programs. Duupa farmers in a village in Northern Cameroon distinguished 59 named sorghum taxa, representing 46 landraces. In each field, seeds are sown as a mixture of landraces (mean of 12 landraces per field), giving the potential for extensive gene flow. What level of genetic diversity underlies the great morphological diversity observed among landraces? Given the potential for gene flow, how well defined genetically is each landrace? To answer these questions, we recorded spatial patterns of planting and farmers' perceptions of landraces, and characterized 21 landraces using SSR markers. Analysis using distance and clustering methods grouped the 21 landraces studied into four clusters. These clusters correspond to functionally and ecologically distinct groups of landraces. Within-landrace genetic variation accounted for 30% of total variation. The average F(is) over landraces was 0.68, suggesting high inbreeding within landraces. Differentiation among landraces was substantial and significant (F(st) = 0.36). Historical factors, variation in breeding systems, and farmers' practices all affected patterns of genetic variation. Farmers' practices are key to the maintenance, despite gene flow, of landraces with different combinations of agronomically and ecologically pertinent traits. They must be taken into account in strategies of conservation and use of genetic resources.

Mating system and reproductive fitness traits of eastern white pine (Pinus strobus) in large, central versus small, isolated, marginal populations

Multilocus (tm) and single-locus (ts) outcrossing and actual inbreeding rates and seed traits were determined for eastern white pine (Pinus strobus L.) in six small, remnant, and marginal populations from two regions (East and West) in Newfoundland and in three large populations from the center of the species' geographic range in Ontario to examine the effects of small population size and fragmentation on mating system parameters and reproductive fitness. The population tm ranged from 0.867 to 0.991, with a mean of 0.924 over all nine populations. The mean ts ranged from 0.672 to 0.908, with a mean of 0.797 over the nine populations. The Ontario populations, on average, showed higher but statistically similar outcrossing rates (tm = 0.947, ts = 0.848) to the Newfoundland populations (tm = 0.912, ts = 0.772). The Newfoundland West populations, on average, showed the lowest outcrossing rates (tm = 0.889, ts = 0.716). Individual family outcrossing rates, although slightly higher, were similar to their respective population outcrossing rates, and no significant differences were observed among families within populations. The mean ts were significantly lower than their corresponding tm, and the differences were significantly and positively correlated with the number of loci showing significant regression of pollen allele frequency on ovule genotype, suggesting possible occurrence of consanguineous mating. The Ontario populations showed the highest and the Newfoundland West populations the lowest reproductive fitness, with Newfoundland East populations ranking higher than Newfoundland West but significantly lower than Ontario populations. Actual inbreeding rates, determined by combining allozyme-based estimates of selfing in the filled seed component with estimates of inbreeding from the proportions of empty seeds, ranged from 7.4 to 31.6%, with an average of 22% for all populations and 11.1% for the Ontario, 24.7% for the Newfoundland East, and 30.1% for the Newfoundland West populations. Multilocus outcrossing rates were significantly correlated (i) negatively with the average distance to the five nearest neighboring trees (a surrogate measure for within-stand densities of reproductively mature trees) and (ii) positively with the proportion of filled seeds per cone. The filial seed progeny fixation index was positively correlated with both (i) average nearest-neighbor distances and (ii) proportion of empty seeds per cone. Thus, we detected strong interrelationships between the within-stand density of reproductively mature trees and both outcrossing rates and filled seed production. Interestingly, there was no relationship between the fixation index of the mature parent stands and their density. The genetic status or integrity of the extant parental populations may have been largely unaffected by the large-scale population decline experienced by eastern white pine early in the 20th century, a decline that showed an adverse effect on reproductive fitness of these populations.Key words: outcrossing and inbreeding, small population size, conservation, genetic and reproductive fitness, actual inbreeding rates, seed traits.

Indicators of population viability in red spruce, Picea rubens. II. Genetic diversity, population structure, and mating behavior

Red spruce (Picea rubens Sarg.) has become increasingly rare across large portions of its range in eastern North America as a result of a general and widespread decline over the past century. Genetic diversity, population genetic structure, outcrossing rates in the filled seeds, and actual inbreeding levels were characterized in five small, isolated, remnant red spruce populations from the disjunct northwestern limits of its range in Ontario and five populations from the larger, more extensive Maritime populations of Nova Scotia and New Brunswick to determine genetic and reproductive status, to provide some benchmarks for monitoring genetic changes resulting from isolation and restricted population sizes, and to assist the development of restoration and conservation strategies. Thirty-seven allozyme loci coding for 15 enzymes were used for genetic diversity assessments, and six of the most polymorphic loci were used for mating system determination. On average, 29.1% (95% criterion) of the loci were polymorphic, the number of alleles per locus was 1.60, and the observed and expected heterozygosities were 0.097 and 0.100, respectively. The Ontario populations were comparable to or slightly less genetically variable than those from the Maritimes. Only 4.7% of the detected genetic variation was among stands; the remainder was among individuals within stands. The Maritime populations were genetically less differentiated from each other than those in Ontario. With the exception of three Maritime populations clustering tightly in one group, there was no clear separation of Ontario red spruce populations from Maritime red spruce populations based on genetic distance as well as canonical discriminant analyses. The average multilocus (tm) and single-locus (ts) population outcrossing rates were 0.595 and 0.558, respectively, indicating a comparatively high tolerance for inbreeding up to the filled seed stage of development in red spruce. The Ontario populations, on average, showed higher outcrossing rates (tm = 0.654, ts = 0.641) than the Maritime populations (tm = 0.535, ts = 0.475). Individual family outcrossing rates were similar to their respective population outcrossing rates and no significant differences were observed among families within populations for the multilocus estimates. When such high levels of inbreeding in filled seeds were combined with the proportions of empty (post-pollination-aborted) seeds, it appears that actual inbreeding levels may vary from 48 to 86%. The highest inbreeding levels occurred in the smallest, most isolated Ontario populations and in those populations most likely to have been affected by poorer pollination conditions. Allozyme variation indicates that in the short term, extant remnants of Ontario red spruce have maintained their genetic diversity and integrity. For artificial restoration of red spruce in Ontario, local seed sources could be used without undue concern over losses of genetic diversity. However, over the longer term, genetic drift and inbreeding may be expected to result in further losses of genetic diversity and (or) reproductive fitness if population sizes, numbers, and distribution continue to decline.Key words: Picea rubens, allozymes, gene conservation, restoration, genetic diversity, population structure, outcrossing rates, inbreeding.

Pollen pool heterogeneity in jack pine (Pinus banksiana Lamb.): a problem for estimating outcrossing rates?

Pollen pool heterogeneity, which violates an assumption of the mixed-mating model, is a major potential problem in measuring plant mating systems. In this study, isozyme markers were used to examine pollen pool heterogeneity in two natural populations of jack pine, Pinus banksiana Lamb., in northwestern Ontario, Canada. Population multilocus estimates of outcrossing rate ranged from 0.83 to 0.95 and differed significantly between populations. Single-tree multilocus outcrossing rates were found to be homogeneous among trees in both populations. Computer simulation studies indicated that a consanguineous pollen pool (pollen gametes related to the mother tree) was capable of biasing population outcrossing estimates downward. Random pollen pool heterogeneity (uncorrelated with maternal genotypes) did not appear to affect population outcrossing estimates in the simulations. Heterogeneity G-tests and Spearman rank tests showed that pollen pool heterogeneity existed in the two natural populations examined; however, it did not have a major effect on population outcrossing estimates, since the consanguineous pollen pool detected was probably a relatively minor component of the outcross pollen pool in both populations. In addition, heterogeneity G-tests were found to be not sensitive in detecting pollen pool heterogeneity caused by consanguineous pollen pool.

Genetics of Brazil nut (Bertholletia excelsa Humb. & Bonpl.: Lecythidaceae) : 2. Mating system

Here we report the results of a mating system analysis of an Amazonian population of Bertholletia excelsa, a tropical rain forest canopy tree species. Using progeny data from 29 seed parents, two highly polymorphic isozymes were analyzed to derive single locus and multilocus estimates of outcrossing, based on a mixed mating model. The two single locus estimates were very similar, and both were somewhat smaller than the multilocus estimate, indicating the possibility that the populations are genetically structured. The multilocus outcrossing estimate (tm=0.85±0.03) reveals that outcrossing is prevalent, but that a significantly low level of inbreeding may be occurring. The high outcrossing rate indicates that even though dispersion of individuals is very low within populations of this tropical rain forest tree, pollen dispersal mechanisms appear to be adequate to enable crosses with a relatively large number of potential mates.

Mating system and multilocus associations in a natural population of Pseudotsuga menziesii (Mirb.) Franco

Arrays of open-pollinated seeds were assayed for allozyme polymorphisms at ten loci (Aat2, Est1, G6pd, Idh, Mdh2, Mdh3, Pgm, Sod, 6Pgd1, 6Pgd2) to obtain estimates of the outcrossing rate and assess multilocus association in a natural population of coastal Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco. The allele frequencies in the samples of adult trees and pollen-gamete pool were similar. Maximum-likelihood estimators of the outcrossing rate for individual loci and two multilocus models were derived using counting methods. The single-locus maximum likelihood estimates (MLEs) of the outcrossing rate were significantly heterogeneous; they varied over a more than two-fold range from 0.404 to 0.935, with an average MLE of 0.741. Both multilocus MLEs of the outcrossing rate were 0.887. The sample of trees was in random mating equilibrium when assessed on a pairwise-locus basis using Burrows' composite measure of gametic disequilibrium, with one exception (Mdh2 Sod) that was attributable to a rare "gametic" class. In the sample of pollen gametes, 5 of the 45 pairwise-locus associations were nominally significant at the 0.05 level: Idh Est1, Mdh2 Sod, Aat2 Est1, Aat2 Mdh3, and Est1 Mdh3. These apparent associations were attributable in most cases to the relative excess of uncommon or rare paternal gametes of discernibly outcrossed embryos. An additional two-locus association was identified for Mdh2 Pgm which was marginally significant for the major partition of the contingency table that excluded paternal gametes with the rare allele Mdh2 (2) .

The nature of inbreeding in a seed orchard of Douglas fir as shown by an efficient multilocus model

The amounts of self-fertilization versus consanguineous matings (as measured by effective selfing) was estimated in a seed orchard of Douglas-fir, using progeny array data at six allozyme loci. The orchard is family structured, consisting several grafts (clones) and/or open-pollinated (o-p) progeny from each of several 'plus-trees'. Population-wide selfing rates were found to be 7% for the o-p trees and 2% for the cloned trees. Estimates of mating system parameters for individual trees showed this difference for average outcrossing rate t (1) still largely remained when outcrossingpollen gene frequency p was not allowed to vary among trees and (2) disappeared when p was allowed to vary among trees. Under this joint t and p estimation, o-p trees showed both significant variation of t (based upon a one-way ANOVA grouped by common plus-tree) and significant regressions of p on ovule genotype (indicative of consanguineous matings); cloned trees showed neither. This higher rate of consanguineous mating for o-p trees might be explained by the larger and more variable size of o-p families in the orchard. Estimates of outcrossing rate t and outcrossingpollen gene frequency p were based upon a multilocus model which makes full use of the information in the data. The increased information it gives over 'observed outcross' models is equivalent to adding 30-50% more loci, and it gives enough degrees of freedom to jointly estimate t and p for individual trees (individual progeny arrays) under certain conditions. In addition, inclusion of megagametophyte data nearly doubles the information about the mating system of individual trees.

Genetic neighbourhood structure in a population of Picea abies L

Natural populations are currently the basic material for studying forest tree breeding, but little is known on the genotypic spatial structures in these stands. The use of gene markers, such as isozymes, leads to the determination of part of the allelic constitution of individuals. A method is presented here to estimate the degree of genetic relationship between any pair of genotypes. A French Picea abies population is analyzed by these means and a slight but significant correlation between estimated genetic relationship and topographic distance is found.