Open-pollinated Seeds Research Articles

Genetic Diversity, Mating System, and Seed Viability Reveal a Trade-Off between Outcrossing and Inbreeding in Pinus yunnanensis var. tenuifolia, an Ecologically Important Conifer Species Growing in a Hot-Dry River Basin Habitat in Southwest China

Revealing the relationship between the mating system (i.e., the outcrossing/inbreeding degree) and the fitness of seeds in tree species under wild conditions is essential for understanding the ecological adaptability and evolutionary stability of the species. This study collected open-pollinated seeds from seven wild populations of Pinus yunnanensis var. tenuifolia that exhibited fragmentation in the Nanpan–Hongshui River basin, an ecologically fragile area in China. The seeds and sprouts (germinated seeds) from 20 families were genotyped (24 seeds and 24 sprouts per family) using twelve microsatellite loci to reveal the genetic diversity, mating status, and effect of inbreeding on the three seed quality indicators (thousand-seed weight, germination rate, and germination potential). The three seed quality indicators differed significantly between families (p < 0.001). Higher values of genetic diversity (except the observed heterozygosity) were observed in the sprout group than those in the seed group. Families from different populations showed a notable genetic differentiation (Φst = 0.12), and a large part of families from the common populations had a high degree of coancestry, which signified that the current habitat fragmentation is limiting gene flow between populations. High levels of outcrossing rates (tm) were observed in both the seed group (tm = 0.974) and the sprout group (tm = 0.978), indicating that a low proportion of seeds were self-fertilized. Although there was a slightly higher single outcrossing rate (ts = 0.888) and a lower proportion of biparental inbreeding (tm − ts = 0.077) in sprouts compared to the seeds (ts = 0.871, tm − ts = 0.091), indicating that a part of inbred seeds were purged during the germination stage, curve fitting between the outcrossing rate and seed quality indicators showed that a certain degree of biparental inbreeding (ts between 0.89 and 0.91 and tm − ts between 0.09 and 0.11) did have a positive effect on seed germination ability. This highlights that excessive inbreeding or outbreeding seems to be unfavorable to seed viability. The peculiar relationship between seed viability and the mating system in P. yunnanensis var. tenuifolia was likely an evolutionary consequence of a trade-off between the nature of mixed mating and its specific ecological niche.

Cultivar and Maternal Plant Environment Influence Cold Stratification Requirements and Germination Rates of Vitis Species

Open-pollinated seeds from grapevines in Parlier and Davis (in California) and Geneva (in New York) were collected in 2016, 2017, and 2018. Seeds were subjected to a series of cold stratification treatments of varying lengths and germinated in incubators to compare germination rates. Two V. vinifera cultivars (Chardonnay and Cabernet Sauvignon) and three other cultivars (V. labrusca hybrids) with a similar genetic background were compared across three locations to test for maternal environmental effects on germination rates under different cold stratification durations. Two interspecific hybrids (‘Salamander’ and ‘Sovereign Rose’) and three genotypes each from two species, V. riparia and V. cinerea, were evaluated to compare germination rate variability at different cold stratification durations among and within species and hybrids. Large variability in germination rates was evident among and within grape species, with some accessions requiring little to no cold stratification, and others requiring 10 to 12 weeks. These differences could be useful for breeding grapevines with high or low dormancy requirements. The maternal plant environment impacted the seed weight and total seed germination across years and locations.

Standardization of an In Vitro Seed Germination Protocol Compared to Acid Scarification and Cold Stratification Methods for Different Raspberry Genotypes

In raspberry (Rubus idaeus L.) breeding programs, seed double dormancy causes delayed and low germination rates. In this study, an in vitro germination protocol was developed to increase raspberry seed germination rate and decrease germination time. This protocol was optimized on the cultivars “Polka” and “Tulamagic”, specifically, their open-pollinated seeds, by applying two different cutting types (transverse, or longitudinal), followed by two different treatments (direct in vitro culture at 24 °C, or 2 weeks at 4 °C followed by in vitro culture at 24 °C). The best results for both cultivars were obtained when seeds were cut transversally and directly cultured in vitro at 24 °C (germination rate 73–78%, starting germination time 4 days after treatment). The optimized in vitro protocol was applied to 14 different combination crosses, observing a germination frequency around 87%, and speed in line with “Polka” and “Tulamagic”. These results were compared to those from the 14 cross combinations having undergone acid scarification (95% H2SO4 for 30 min) and cold stratification (3 months), which, in comparison, only yielded a 25% average germination rate. The optimized in vitro protocol favors high germination efficiency for all raspberry genotypes tested, and presents advantages for the standardization of germination time and reduction in the parental genotypes’ impact on germination rate.

Impact of inbreeding on growth and development of young open-pollinated progeny of Eucalyptus globulus

The use of open-pollinated seeds from seed orchards is a common strategy for the deployment of genetically improved eucalypts, including Eucalyptus globulus, an important pulpwood tree in many temperate climate areas. However, seed quality can be affected by the rate of selfing and to a lesser extent by contamination from pollen outside the orchard. Inbreeding between related parents and especially from self-crosses is known to cause diminished growth and developmental abnormalities in the resulting progeny. This study looks at the magnitude and variation in selfing and the impact in inbreeding depression across several E. globulus families collected over the years in a seed orchard. The effects on growth and development of outcrossed and selfed progeny were studied across five progeny trials, after pedigree reconstruction of the open pollinated progeny based on SSR genotyping. An additive genetic mixed linear model was fitted to the data to evaluate the impact of inbreeding on height growth. The results showed a significant inbreeding depression, with a height growth reduction of 15% in selfed progeny, when compared with crosses from unrelated parents. These inbreeding depression values varied among families, ranging between 7% and 24%, evidencing the importance of genetic background. Contamination rates were on average 10% suggesting long distance pollen dispersal was present. A small number of abnormal phenotypes (less than 10%) was observed in the field. This was associated with specific, unrelated, crosses and not to high inbreeding rates such as found among selfed progeny. The relevance of these results for orchard management and parent selection is discussed.

Genetic variation and QTL analysis of saturated fatty acids in two doubled haploid populations of oilseed rape (Brassica napus L.)

Consumption of foodstuff with low contents of saturated fatty acids is considered beneficial for human health. Reducing saturated fatty acid content in oilseed rape (canola) and other oil and protein crops is a relevant breeding aim. The objective of this work was to study the genetic variation and inheritance of saturated fatty acids in two DH populations of oilseed rape, to map QTL and to identify candidate genes. In addition, the correlation to other seed quality traits was studied. To this end, two half-sib DH populations were tested in up to five field environments in north-western Europe and seeds harvested from open-pollinated seeds were analyzed. Genotyping was performed using Illumina Brassica 15 K SNP chip. In both populations, significant effects for the genotypes and for the environments were detected, and heritability ranged from 68 to 89% for the predominant palmitic acid and stearic acid content. Up to 48 QTL for different fatty acids, oil and acid detergent lignin (ADL) content were mapped in the two populations. Co-locating QTL for palmitic acid, stearic acid, the C16/18 fatty acid ratio, the FATB/A ratio, oil and ADL content were identified on different chromosomes. A large number of candidate genes were identified within the vicinity of QTL flanking markers. Identification of several co-locating QTL positions, of associated candidate genes and SNP markers should facilitate oilseed rape breeding for low saturated fatty acid content.

Application of somatic embryogenesis for development of emerald ash borer-resistant white ash and green ash varietals.

Emerald ash borer (Agrilus planipennis; EAB) has devastated populations of ash (Fraxinus spp.) trees in dozens of U.S. states and Canada over the past few decades. The continued survival of scattered ash trees known as “lingering ash” in heavily infested natural stands, however, offers evidence of genetic resistance or tolerance to EAB. These surviving or “lingering” ash individuals may form the basis for reforestation programs in EAB-impacted areas, and clonal mass-propagation of these genotypes can help accelerate these efforts. Between 2013 and 2018, we initiated embryogenic cultures by culturing immature zygotic embryos from open-pollinated (OP) seeds collected from several surviving white ash and green ash trees in Michigan and Pennsylvania. In addition, in 2018, we initiated cultures from crosses made between lingering green ash parents from the USDA Forest Service ash breeding program in Ohio. Somatic embryos were produced by growing cultures in liquid suspension, followed by fractionation and plating on semisolid medium to produce developmentally synchronous populations of somatic embryos. Somatic embryo germination and conversion were enhanced by a combination of pre-germination cold treatment and inclusion of activated charcoal and gibberellic acid in the germination medium. Ash somatic seedlings derived from OP explants grew rapidly following transfer to potting mix and somatic seedlings representing nine ash clones were acclimatized, grown in the greenhouse and planted in a preliminary field test, along with EAB-resistant Manchurian ash (F. mandshurica) and EAB-susceptible control seedlings. Somatic seedlings have now been produced from cultures that originated from seeds derived from the progeny of lingering green ash parents and an ex vitro germination protocol has shown some promise for accelerating early somatic seedling growth. Results of this research could provide the basis for scaled-up production of EAB-resistant ash varieties for seed orchard production for forest restoration and cultivar development for urban tree restoration.

Extended Pedigrees of Apple Cultivars from the University of Minnesota Breeding Program Elucidated Using SNP Array Markers

Apple (Malus ×domestica Borkh.) breeding at the University of Minnesota (UMN) has been ongoing continuously since 1908 when staff originally planted thousands of seedlings from open-pollinated (OP) seeds collected from regional orchards. The first cultivar from the program, ‘Minnehaha’, was introduced in 1920 and several others from these OP seeds followed over the next 3 decades. Controlled crosses were initiated in 1916, and until the time of this publication, 28 cultivars have been introduced. Historical records of parentage, as recorded by staff in notebooks and in 20th-century publications, have been used to inform breeding decisions but might be incorrect as indicated by earlier explorations of parentage using simple sequence repeat (SSR) markers. Our objective was to elucidate parentage and extended pedigrees of all available cultivars introduced from the UMN apple breeding program using evaluations of Mendelian errors and shared haplotype length information based on data from single nucleotide polymorphism (SNP) arrays. Sixteen of the 21 cultivars introduced before ‘Honeycrisp’ (1991) had incorrect or incomplete pedigrees that are now at least partially elucidated. These include the two most important regional cultivars in the 20th century: ‘Haralson’ (parents: ‘Malinda’ and ‘Wealthy’) and ‘Fireside’ (parents: ‘Wealthy’ and ‘Northwest Greening’). ‘Wealthy’, a widely grown cultivar in the United States in the late 19th and early 20th centuries, was a frequent parent of older UMN cultivars. ‘Malinda’ was a less frequent parent than indicated by breeding records. ‘Duchess of Oldenburg’ (synonym ‘Borowitsky’) was revealed as an ancestor of overwhelming importance in the UMN breeding program. It was an ancestor of 27 of the 28 UMN cultivars, including as a parent of two cultivars, and a grandparent of 15 cultivars, including ‘Honeycrisp’.

Determining phenological and genetic variation in genotypes obtained from open-pollinated seeds of ‘Maraş 12’ walnut (Juglans regia L.) cultivar

Determining phenological and genetic variation in genotypes obtained from open-pollinated seeds of ‘Maraş 12’ walnut (Juglans regia L.) cultivar

Descriptions of Chestnut Cultivars for Nut Production in the Eastern and Midwestern United States

The Chinese chestnut (Castanea mollissima Blume) and other Castanea species (Castanea spp. Mill.) have been imported and circulated among growers and scientists in the United States for more than a century. Initially, importations of C. mollissima after 1914 were motivated by efforts to restore the American chestnut [Castanea dentata (Marsh.) Borkh.], with interests in timber-type characters and chestnut blight resistance. Chestnut for orchard nut production spun off from these early works. Starting in the early 20th century, open-pollinated seeds from seedlings of Chinese chestnut and other Castanea species were distributed widely to interested growers throughout much of the eastern United States to plant and evaluate. Germplasm curation and sharing increased quite robustly through grower networks over the 20th century and continues today. More than 100 cultivars have been named in the United States, although a smaller subset remains relevant for commercial production and breeding. The University of Missouri Center for Agroforestry curates and maintains a repository of more than 60 cultivars, and open-pollinated seed from this collection has been provided to growers since 2008. Currently, more than 1000 farms cultivate seedlings or grafted trees of the cultivars in this collection, and interest in participatory on-farm research is high. Here, we report descriptions of 57 of the collection’s cultivars as a comprehensive, readily accessible resource to support continued participatory research.

Seed Source Effects on Germination, Growth, and Yield of Carrots Under Natural Farming

Abstract Natural farming is a type of low-input sustainable agricultural system that is characterized by the avoidance of the use of any animal materials. Production of self-produced seeds by farmers is recommended under natural farming environments, with an expectation of improved crop growth and yield. However, there have been no scientific studies that investigated the performance of self-produced seeds under natural farming. This study investigated the germination rate, growth, and yield of carrots (Daucus carota L.) from different seed sources under natural farming. The main experimental factor was the cultivation method: CT – control; CF – conventional farming; NF1 – natural farming with tillage and compost input; and NF2 – natural farming without tillage and compost, and the subfactor was the seed source: OPN – open-pollinated seeds self-produced under natural farming; OPC – open-pollinated seeds produced under conventional farming; and HB – hybrid seeds. The germination rate of OPN seeds was higher than that of other seeds. In addition, root fresh weight and other root parameters of OPN were as high as for HB seeds. This suggests that using self-produced seeds by farmers improves crop performance in a natural farming system.

Seed production and storage for endangered Morus boninensis using an ex-situ living collection.

Meaningful conservation techniques for the endangered Morus boninensis require seed production and storage, since insitu purebred seed production is infrequent in Ogasawara Islands (World Natural Heritage Site) in Japan. Ex situ living M. boninensis specimens produced seeds by open- and artificial-pollination in a greenhouse. Seed desiccation tolerance and storage characteristics were examined. Open-pollinated seeds were dried at 75, 33 and 8% relative humidity (RH) for 5days then cryopreserved at -170°C. Artificially pollinated seeds were dried at 8% RH then stored cryogenically at -170°C and in freezers at -80 or -20°C for 6months. Germination rates were compared among seeds before and after drying and storage. Germination rates of undried seeds were 83% for open-pollinated seeds and 65% for artificially pollinated seeds. M. boninensis seeds exhibited the characteristic high desiccation tolerance of orthodox seeds and maintained a high germination rate after drying to a moisture content of approximately 4%. Cryopreservation of open-pollinated dry seeds with moisture contents of ca. 12, 7 and 4% maintained germination rates, while cryopreservation of undried seeds (moisture content ca. 44%) resulted in no germinability. Cryogenic and freezer storage of artificially pollinated dry seeds for 6months resulted in maintenance of germination rates comparable to undried seeds before storage. Seed production was enabled by ex situ M. boninensis collection. The present study also developed a method for low temperature seed storage. Ex situ living tree collection represents a feasible seed resource for reintroduction and restoration programmes for M. boninensis.

Advances in Hazelnut (Corylus avellana L.) Rootstocks Worldwide

Studies on hazelnut (Corylus avellana L.) rootstocks have been limited to date. However, the use of vigorous, non-suckering rootstocks for this species could increase the cost-effectiveness of orchards by reducing the annual need to prune suckers, thus facilitating mechanical harvesting, and reducing orchard management costs and environmental impact. Seedlings of the non-suckering Turkish tree hazel (C. colurna L.) have been used traditionally in Serbia. In the 1970s, the United States Department of Agriculture in Corvallis, Oregon (USA) released the first two non-suckering clonal rootstocks—‘Dundee’ and ‘Newberg’—from open-pollinated seeds of C. colurna. Moreover, selection of C. avellana cvs. with few suckers is continuing. Trials carried out in different countries with own-rooted and grafted plants have shown good performance of grafted hazelnuts. Currently, some nurseries in several countries are propagating hazelnut rootstocks and grafting trees for planting commercial orchards. Interest in these cultivar/rootstock combinations is increasing, and more new orchards of grafted trees are expected to be planted in the coming years.

Xenia and Deficit Nitrogen Influence the Iron and Zinc Concentration in the Grains of Hybrid Maize

Xenia is the immediate effect of pollen on seed development after pollination. This study was conducted to investigate xenia’s effects on Fe and Zn concentration in self- and open-pollinated maize seeds. Eighteen maize hybrids derived from parents with varying concentrations of Fe and Zn were planted at Potchefstroom, Cedara and Vaalharts in South Africa for two seasons, 2017 and 2018. Open- and self-pollinated seeds were compared. Self-pollination of five selected healthy maize hybrids of uniform height from each plot was performed. Fe, Zn and phytic acid (Pa) analysis was conducted on self -and open-pollinated maize seeds, and the molar ratios of Fe and Zn to Pa were also calculated to assess the bioavailability of Fe and Zn. The ranges of Fe, Zn and Pa, and the molar ratios of Fe and Zn to Pa in self-pollinated maize seeds were 15.93–21.36 mg kg−1, 18.50–24.34 mg kg−1, 4.63–5.84 mg g−1, 21.97–31.22 and 23.53–30.16 under high N, and 12.76–19.29 mg kg−1, 16.78–23.50 mg kg−1, 5.05–6.48 mg g−1, 26.12–44.54 and 25.44–35.94 under low N conditions, respectively. The ranges of Fe, Zn and Pa, and the molar ratios of Fe and Zn to Pa in open-pollinated seeds were 14.34–19.12 mg kg−1, 17.27–23.27 mg kg−1, 4.96–5.89 mg g−1, 20.90–28.22 and 22.92–29.37 under high N, and 12.54–18.39 mg kg−1, 16.94–20.93 mg kg−1, 5.30–6.17 mg g−1, 27.90–38.75 and 27.95–36.34 under low N conditions, respectively. The bioavailability of Fe and Zn was high in self-pollinated seeds under high N conditions, while Fe was higher in self-pollinated seeds, and Zn was higher in open-pollinated seeds under low N conditions. Combined over seasons, there were no significant differences between the values of Fe and Zn in self- and open-pollinated seeds under both high N and low N conditions. This indicated that unknown males had no influence on Fe and Zn concentration in maize. Therefore, either self- or open-pollinated seeds can be used for Fe and Zn determination in maize crosses.

Genetic Variability and Elite Line Selection for High Essential Oil and Nepetalactone Content in Catmint (<i>Nepeta cataria</i> L.)

Nepeta cataria L., commonly known as catmint or catnip, belongs to the family “Lamiaceae” and is indigenous to Europe and Asia. The essential oil of this species is known for the richness and diversity of nepetalactones (NPL) which are used as mosquito/insect repellents in perfumery and cosmetic industries. Reports on Indian catmint germplasm are very meager and warrants exploration of its commercial potential as a natural, non-toxic source of insect repellents. With this objective, commercial open-pollinated seeds of catmint collected from its native, temperate habitat in Himalayas were introduced in the tropical plains. Subsequent to adaptation to a new zone we were able to isolate nineteen individual plants based on plant growth. Hydrodistillation of the fresh herb yielded essential oil in the range of 0.01% to 0.2%. Gas Chromatography (GC) and GC-Mass Spectrometry (GC-MS) analyses of the oil revealed the dominance of monoterpene hydrocarbon, namely, 4aα,7α,7aα NPL (1) isomer (84%). The other two isomers of nepetalactone, 4aα,7α,7aβ NPL (2) and 4aα,7β,7aα NPL (3) were also present, although in very less amounts (1.0% and 1.6%, respectively). Sesquiterpenes identified were α-humulene (traces), (E)-caryophyllene (0.6%) and caryophyllene oxide (1.7%). We compared the identified Indian catmint chemotype with the other oils from temperate, sub-tropical and tropical locations based on literature search. The Indian chemotype was found to be similar to the oils from Burundi, France, Turkey, UK and USA, mainly due to more accumulation of 4aα,7α,7aα NPL (1) isomer. These oils grouped together in Principal Component Analysis. Breeding lines are presently being developed to improve yield related traits in this plant. Multidisciplinary R&D efforts along with setting up industry related guidelines are required to successfully commercialize catmint cultivation. Several species of Nepeta genus have high nepetalactone content too and their potential as a commercial source of these isomers still needs to be explored.

COVID‐19 and Food Security in Fiji: The Reinforcement of Subsistence Farming Practices in Rural and Urban Areas

<scp>COVID</scp>‐19 and Food Security in Fiji: The Reinforcement of Subsistence Farming Practices in Rural and Urban Areas

Learning from the Past in the Transition to Open-Pollinated Varieties

In Nepal, hybrid seed introduction caused major yield gains in agricultural production, but at high environmental costs. The development of high-yielding open-pollinated varieties has spurred hope for more sustainable production systems. Nepal’s government is interested in boosting their use. This research aimed to identify farmer perceptions on the factors behind the past adoption of hybrid seeds in order to propose guidelines to support the diffusion of open-pollinated varieties. Using in-depth interviews, a focus group and participant observation we explored how the process of hybrid seed diffusion has taken place in Panchkhal valley, a representative case study. Social influencers such as change agents, peers, neighbours and seed sellers, as well as economic gains emerged as major reasons for hybrid seed adoption. We learnt that the role of external agents, on which most of the governmental strategies rely, changed over time as peer-based strategies became essential after the diffusion process started. To boost the adoption of open-pollinated seeds, efforts should concentrate in developing high-yielding varieties, engaging early-adopters among influential caste members and seed sellers, distributing seeds to both disadvantaged and wealthy farmers, and using different instruments, from institutional agencies to NGOs, to deliver training on sustainable farming techniques and their economic and environmental advantages.

Yield Performance of Corn Under Heat Stress: A Comparison of Hybrid and Open-Pollinated Seeds During a Period of Technological Transformation, 1933-1955

Starting in the 1930s, commercial hybrid corn seeds rapidly replaced the once predominant open-pollinated varieties planted by farmers. By the mid-1950s almost all corn grown in the United States was of hybrid varieties. Observers have argued that the drought tolerant qualities of these hybrids were a major factor driving farmers’ decisions regarding hybrid adoption, but there is little statistical evidence to substantiate this assertion. Hybrid seeds exhibited other attractive qualities, such as improved performance during prime weather conditions, resistance to wind damage, and increased suitability toward mechanized harvesting. Using historical evidence from Zvi Griliches’s archival records, we reconstruct data on hybrid corn adoption and yields at a more disaggregated geographic level than previously available. We match these data with historical weather records to measure the extent to which hybrid seeds mediated the adverse effects of extreme heat. Our findings suggest that hybrid corns grown in Iowa from 1928 to 1942 did exhibit heat tolerance relative to open-pollinated varieties. This result is unique to Iowa as this reduced temperature sensitivity does not appear when comparing hybrid and open-pollinated grown in other states.

The effect of conditioning Acacia mearnsii De Wild. hedge-plants under shadenetting on rooting, elemental uptake and productivity of cuttings

Acacia mearnsii is a commercial forestry tree in South Africa, largely grown from open-pollinated seeds. Clonal deployment of superior selections of the species is restricted due to its poor rooting ability. The effects of conditioning parent hedge-plants under three levels of reduced light (80% black; 60% red and 40% black/ white shadenet) were investigated. Hedge-plant nutrition, carbohydrate content, productivity and rooting of cuttings were measured. Conditioning hedge-plants under 80% black shadenet for five weeks gave the highest rooting percentages in both seasons (39.3% and 22.1%) (p < 0.05), despite having a lower productivity of 42.5 cuttings per hedge-plant (p > 0.05). As the shading increased to 80% black, so the rooting increased, and stem/leaf total non-structural carbohydrate levels decreased (r = −0.61 to −0.97). Conditioning under shadenet reduced leaf and increased stem total macronutrient percentages but increased both leaf and stem total micronutrient concentrations. Total macro- and micronutrient levels were greatest after conditioning under 80% black shadenet, except for stem-nitrogen, stem-carbon and leaf-manganese, which had the lowest values (p > 0.05). In conclusion, hedge-plant conditioning under 80% black shadenet can significantly improve rooting of A. mearnsii cuttings, which could facilitate the commercial clonal propagation of the species.

Pollen contamination and mating patterns in a Prosopis alba clonal orchard: impact on seed orchards establishment

Abstract: Prosopis alba (Leguminosae) is an important species from ecologic and economical points of view in arid and semi-arid regions of Argentina. In several open-pollinated species, pollen contamination from off-orchard parents and selfing have been proven to reduce orchard seed quality. In 2002, the first clonal orchard of Prosopis alba was established in Fernandez (Santiago del Estero, Argentina) with 12 trees phenotypically selected from a progeny trial, based on height, pod production per year and pod sweetness. The aim of this study was to evaluate the mating patterns and pollen contamination rate in the orchard using ten SSR markers and paternity analysis. All the clones together with the progeny of a single clone (open-pollinated seeds) were genotyped. Data was processed by two different methods based on likelihood and Bayesian approaches, respectively. A high consistency (89%) of results was observed between the two methods, and pollen contamination rate was estimated between 27% and 37%. The minimum number of different pollen donors per mother plant varied from three to five and selfing occurrence was low (<1.6%). Based on the estimated status number (Ns = 4.4), the expected coancestry in the seed crop is equal to a Mendelian population with an effective size of 4-5 individuals. Genetic analyses are encouraged during the establishment and monitoring of trials in forest breeding and management programmes. It is strongly recommended to establish seed orchards in isolated areas and to guarantee equal representation of parental genotypes in the orchards.

Life cycle expression of inbreeding depression in Eucalyptus regnans and inter-generational stability of its mixed mating system.

Many plants exhibit a mixed mating system. Published models suggest that this might be an evolutionarily stable rather than a transitional state despite the presence of inbreeding depression, but there is little empirical evidence. Through field experimentation, we studied the role of inbreeding depression in eliminating inbred progeny from the reproductive cohort of the forest tree Eucalyptus regnans, and demonstrate a stable mixed primary mating system over two successive generations. Two field experiments were conducted using seed from natural populations. We sowed open-pollinated seeds to simulate a natural regeneration event and determined isozyme genotypes of dominant and suppressed individuals over 10 years. We also planted a mixture of open-pollinated, outcross and selfed families with common maternal parentage; monitored survival of cross types over 29 years; and determined the percentage of outcrosses in open-pollinated seed from a sample of reproductively mature trees using microsatellite analysis. Both experiments demonstrated progressive competitive elimination of inbred plants. By 29 years, the reproductive cohort in the planted experiment consisted only of outcrosses which produced seed which averaged 66 % outcrosses, similar to the estimate for the parental natural population (74 %). Selective elimination of inbred genotypes during the intense intra-specific competition characteristic of the pre-reproductive phase of the life cycle of E. regnans results in a fully outcrossed reproductive population, in which self-fertility is comparable with that of its parental generation. The mixed mating system may be viewed as an unavoidable consequence of the species' reproductive ecology, which includes the demonstrated effects of inbreeding depression, rather than a strategy which is actively favoured by natural selection.