An integrated genetic linkage map for eucalypts using RFLP, RAPD and isozyme markers

Phenotypic and Genetic Characterization of Botrytis cinerea Population from Kiwifruit in Sichuan Province, China

Botrytis cinerea (anamorph of Botryotinia fuckeliana) causes gray mold on numerous plants, including kiwifruit. The primary aim of this study was to investigate the phenotypic and genetic characteristics of the Botrytis cinerea population from kiwifruit in Sichuan Province, China. In all, 176 isolates were collected from kiwifruit orchards from eight geographic regions in Sichuan. All isolates were identified as B. cinerea sensu stricto based on the combined datasets, including morphological criteria, determination of the Bc-hch allele, and phylogenetic analysis of the genes RPB2, G3PDH, and HSP60. Three colony types (i.e., sclerotial, mycelial, and conidial) were observed on potato dextrose agar after 2 weeks, with sclerotial isolates, the predominant category, accounting for 40.91%. No obvious differences in microscopic characteristics were observed among the three types. Three genotypes of transposable elements were identified in the B. cinerea population: boty, flipper, and transposa types. The most prevalent genotype from different geographic populations of B. cinerea was transposa; in contrast, the flipper genotype accounted for only 3.98% of the total population, whereas the vacuma genotype was absent. According to MAT locus amplification, 87 and 89 isolates are MAT1-1 and MAT1-2 type, respectively, and the two mating types were found to be balanced overall in the population. Forty-eight representative isolates were all able to cause gray mold to some extent, and disease severities were significantly different between the cultivars Hongyang and Hort16A (P < 0.01). Disease severity was significantly greater on young leaves than on mature leaves (P < 0.01). No significant relationship was found between pathogenicity and geographical region, colony type, or transposon distribution. The results obtained in the present study suggest a relatively uniform species diversity of Botrytis but rich phenotypic and genetic differentiation within the B. cinerea population on kiwifruit in China. Utilizing resistant cultivars and rain-shelter cultivation instead of fungicides may be an effective approach to delaying pathogen variability.

Genetic linkage map and QTL identification for adventitious rooting traits in red gum eucalypts

The eucalypt species, Eucalyptus tereticornis and Eucalyptus camaldulensis, show tolerance to drought and salinity conditions, respectively, and are widely cultivated in arid and semiarid regions of tropical countries. In this study, genetic linkage map was developed for interspecific cross E. tereticornis × E. camaldulensis using pseudo-testcross strategy with simple sequence repeats (SSRs), intersimple sequence repeats (ISSRs), and sequence-related amplified polymorphism (SRAP) markers. The consensus genetic map comprised totally 283 markers with 84 SSRs, 94 ISSRs, and 105 SRAP markers on 11 linkage groups spanning 1163.4 cM genetic distance. Blasting the SSR sequences against E. grandis sequences allowed an alignment of 64% and the average ratio of genetic-to-physical distance was 1.7 Mbp/cM, which strengths the evidence that high amount of synteny and colinearity exists among eucalypts genome. Blast searches also revealed that 37% of SSRs had homologies with genes, which could potentially be used in the variety of downstream applications including candidate gene polymorphism. Quantitative trait loci (QTL) analysis for adventitious rooting traits revealed six QTL for rooting percent and root length on five chromosomes with interval and composite interval mapping. All the QTL explained 12.0-14.7% of the phenotypic variance, showing the involvement of major effect QTL on adventitious rooting traits. Increasing the density of markers would facilitate the detection of more number of small-effect QTL and also underpinning the genes involved in rooting process.

RAPD markers for screening shoot gall maker (Betousa stylophora Swinhoe) tolerant genotypes of amla (Phyllanthus emblica L.)

Phyllanthus emblica Linn. is the most important medicinally useful tree crop in Asian Subcontinent and is severely infested by Betousa stylophora Swinhoe, known as shoot gall maker (SGM). This pest tunnels the shoots of seedlings and actively growing branches of trees and develops gall, leading to stunted growth, unusual branching and death of actively growing shoots. Our study revealed that trees possessing smooth bark were free from the attack of this pest than those with rough bark surface. Unfortunately, this character is not detectable either at seedling stage or during early growth of trees in the orchard. RAPD genetic fingerprinting of trees possessing smooth and rough bark revealed distinguishable and highly reproducible DNA banding pattern between the two genotypes. Of the 20 RAPD primers tested, five of them produced distinguishable RAPD bands between rough and smooth barked genotypes of P. emblica. Trees with smooth bark produced five unique RAPD bands with molecular weight ranging from 350 bp to 1500 bp and those with rough bark produced six RAPD bands (350 bp–650 bp) to utilize these DNA bands as potential DNA marker for screening tolerant genotypes of this crop against SGM. The utility of this finding in genetic improvement of this tree crop against SGM is discussed.

Genomic characterization of DArT markers based on high-density linkage analysis and physical mapping to the Eucalyptus genome

Diversity Arrays Technology (DArT) provides a robust, high throughput, cost-effective method to query thousands of sequence polymorphisms in a single assay. Despite the extensive use of this genotyping platform for numerous plant species, little is known regarding the sequence attributes and genome-wide distribution of DArT markers. We investigated the genomic properties of the 7,680 DArT marker probes of a Eucalyptus array, by sequencing them, constructing a high density linkage map and carrying out detailed physical mapping analyses to the Eucalyptus grandis reference genome. A consensus linkage map with 2,274 DArT markers anchored to 210 microsatellites and a framework map, with improved support for ordering, displayed extensive collinearity with the genome sequence. Only 1.4 Mbp of the 75 Mbp of still unplaced scaffold sequence was captured by 45 linkage mapped but physically unaligned markers to the 11 main Eucalyptus pseudochromosomes, providing compelling evidence for the quality and completeness of the current Eucalyptus genome assembly. A highly significant correspondence was found between the locations of DArT markers and predicted gene models, while most of the 89 DArT probes unaligned to the genome correspond to sequences likely absent in E. grandis, consistent with the pan-genomic feature of this multi-Eucalyptus species DArT array. These comprehensive linkage-to-physical mapping analyses provide novel data regarding the genomic attributes of DArT markers in plant genomes in general and for Eucalyptus in particular. DArT markers preferentially target the gene space and display a largely homogeneous distribution across the genome, thereby providing superb coverage for mapping and genome-wide applications in breeding and diversity studies. Data reported on these ubiquitous properties of DArT markers will be particularly valuable to researchers working on less-studied crop species who already count on DArT genotyping arrays but for which no reference genome is yet available to allow such detailed characterization.

A reference linkage map for Eucalyptus

Background

Genetic linkage maps are invaluable resources in plant research. They provide a key tool for many genetic applications including: mapping quantitative trait loci (QTL); comparative mapping; identifying unlinked (i.e. independent) DNA markers for fingerprinting, population genetics and phylogenetics; assisting genome sequence assembly; relating physical and recombination distances along the genome and map-based cloning of genes. Eucalypts are the dominant tree species in most Australian ecosystems and of economic importance globally as plantation trees. The genome sequence of E. grandis has recently been released providing unprecedented opportunities for genetic and genomic research in the genus. A robust reference linkage map containing sequence-based molecular markers is needed to capitalise on this resource. Several high density linkage maps have recently been constructed for the main commercial forestry species in the genus (E. grandis, E. urophylla and E. globulus) using sequenced Diversity Arrays Technology (DArT) and microsatellite markers. To provide a single reference linkage map for eucalypts a composite map was produced through the integration of data from seven independent mapping experiments (1950 individuals) using a marker-merging method.

Results

The composite map totalled 1107 cM and contained 4101 markers; comprising 3880 DArT, 213 microsatellite and eight candidate genes. Eighty-one DArT markers were mapped to two or more linkage groups, resulting in the 4101 markers being mapped to 4191 map positions. Approximately 13% of DArT markers mapped to identical map positions, thus the composite map contained 3634 unique loci at an average interval of 0.31 cM.

Conclusion

The composite map represents the most saturated linkage map yet produced in Eucalyptus. As the majority of DArT markers contained on the map have been sequenced, the map provides a direct link to the E. grandis genome sequence and will serve as an important reference for progressing eucalypt research.

High-throughput SNP genotyping in the highly heterozygous genome of Eucalyptus: assay success, polymorphism and transferability across species

BACKGROUND

High-throughput SNP genotyping has become an essential requirement for molecular breeding and population genomics studies in plant species. Large scale SNP developments have been reported for several mainstream crops. A growing interest now exists to expand the speed and resolution of genetic analysis to outbred species with highly heterozygous genomes. When nucleotide diversity is high, a refined diagnosis of the target SNP sequence context is needed to convert queried SNPs into high-quality genotypes using the Golden Gate Genotyping Technology (GGGT). This issue becomes exacerbated when attempting to transfer SNPs across species, a scarcely explored topic in plants, and likely to become significant for population genomics and inter specific breeding applications in less domesticated and less funded plant genera.

RESULTS

We have successfully developed the first set of 768 SNPs assayed by the GGGT for the highly heterozygous genome of Eucalyptus from a mixed Sanger/454 database with 1,164,695 ESTs and the preliminary 4.5X draft genome sequence for E. grandis. A systematic assessment of in silico SNP filtering requirements showed that stringent constraints on the SNP surrounding sequences have a significant impact on SNP genotyping performance and polymorphism. SNP assay success was high for the 288 SNPs selected with more rigorous in silico constraints; 93% of them provided high quality genotype calls and 71% of them were polymorphic in a diverse panel of 96 individuals of five different species.SNP reliability was high across nine Eucalyptus species belonging to three sections within subgenus Symphomyrtus and still satisfactory across species of two additional subgenera, although polymorphism declined as phylogenetic distance increased.

CONCLUSIONS

This study indicates that the GGGT performs well both within and across species of Eucalyptus notwithstanding its nucleotide diversity ≥ 2%. The development of a much larger array of informative SNPs across multiple Eucalyptus species is feasible, although strongly dependent on having a representative and sufficiently deep collection of sequences from many individuals of each target species. A higher density SNP platform will be instrumental to undertake genome-wide phylogenetic and population genomics studies and to implement molecular breeding by Genomic Selection in Eucalyptus.

A high-density Diversity Arrays Technology (DArT) microarray for genome-wide genotyping in Eucalyptus

BACKGROUND

A number of molecular marker technologies have allowed important advances in the understanding of the genetics and evolution of Eucalyptus, a genus that includes over 700 species, some of which are used worldwide in plantation forestry. Nevertheless, the average marker density achieved with current technologies remains at the level of a few hundred markers per population. Furthermore, the transferability of markers produced with most existing technology across species and pedigrees is usually very limited. High throughput, combined with wide genome coverage and high transferability are necessary to increase the resolution, speed and utility of molecular marker technology in eucalypts. We report the development of a high-density DArT genome profiling resource and demonstrate its potential for genome-wide diversity analysis and linkage mapping in several species of Eucalyptus.

FINDINGS

After testing several genome complexity reduction methods we identified the PstI/TaqI method as the most effective for Eucalyptus and developed 18 genomic libraries from PstI/TaqI representations of 64 different Eucalyptus species. A total of 23,808 cloned DNA fragments were screened and 13,300 (56%) were found to be polymorphic among 284 individuals. After a redundancy analysis, 6,528 markers were selected for the operational array and these were supplemented with 1,152 additional clones taken from a library made from the E. grandis tree whose genome has been sequenced. Performance validation for diversity studies revealed 4,752 polymorphic markers among 174 individuals. Additionally, 5,013 markers showed segregation when screened using six inter-specific mapping pedigrees, with an average of 2,211 polymorphic markers per pedigree and a minimum of 859 polymorphic markers that were shared between any two pedigrees.

CONCLUSIONS

This operational DArT array will deliver 1,000-2,000 polymorphic markers for linkage mapping in most eucalypt pedigrees and thus provide high genome coverage. This array will also provide a high-throughput platform for population genetics and phylogenetics in Eucalyptus. The transferability of DArT across species and pedigrees is particularly valuable for a large genus such as Eucalyptus and will facilitate the transfer of information between different studies. Furthermore, the DArT marker array will provide a high-resolution link between phenotypes in populations and the Eucalyptus reference genome, which will soon be completed.

Identification of a Cis-acting regulatory polymorphism in a Eucalypt COBRA-like gene affecting cellulose content

Populations with low linkage disequilibrium (LD) offer unique opportunities to study functional variants influencing quantitative traits. We exploited the low LD in forest trees to identify functional polymorphisms in a Eucalyptus nitens COBRA-like gene (EniCOBL4A), whose Arabidopsis homolog has been implicated in cellulose deposition. Linkage analysis in a full-sib family revealed that EniCOBL4A is the most strongly associated marker in a quantitative trait locus (QTL) region for cellulose content. Analysis of LD by genotyping 11 common single-nucleotide polymorphisms (SNPs) and a simple sequence repeat (SSR) in an association population revealed that LD declines within the length of the gene. Using association studies we fine mapped the effect of the gene to SNP7, a synonymous SNP in exon 5, which occurs between two small haplotype blocks. We observed patterns of allelic expression imbalance (AEI) and differential binding of nuclear proteins to the SNP7 region that indicate that SNP7 is a cis-acting regulatory polymorphism affecting allelic expression. We also observed AEI in SNP7 heterozygotes in a full-sib family that is linked to heritable allele-specific methylation near SNP7. This study demonstrates the potential to reveal functional polymorphisms underlying quantitative traits in low LD populations.

Eucalyptus applied genomics: from gene sequences to breeding tools

Eucalyptus is the most widely planted hardwood crop in the tropical and subtropical world because of its superior growth, broad adaptability and multipurpose wood properties. Plantation forestry of Eucalyptus supplies high-quality woody biomass for several industrial applications while reducing the pressure on tropical forests and associated biodiversity. This review links current eucalypt breeding practices with existing and emerging genomic tools. A brief discussion provides a background to modern eucalypt breeding together with some current applications of molecular markers in support of operational breeding. Quantitative trait locus (QTL) mapping and genetical genomics are reviewed and an in-depth perspective is provided on the power of association genetics to dissect quantitative variation in this highly diverse organism. Finally, some challenges and opportunities to integrate genomic information into directional selective breeding are discussed in light of the upcoming draft of the Eucalyptus grandis genome. Given the extraordinary genetic variation that exists in the genus Eucalyptus, the ingenuity of most breeders, and the powerful genomic tools that have become available, the prospects of applied genomics in Eucalyptus forest production are encouraging.

Segregation distortion and its effect on genetic mapping in plants

Segregation distortion, identified as a strong evolutionary force, is common in genetic mapping research. In plants, the percentage, degree, origin and genetic effects of segregation distortion vary significantly with species, population types, crosses and marker types. The exhibition, common features, causes, methods of mapping segregation distortion loci, effects of segregation distortion on map construction and corresponding mapping strategies in plants, mainly in crops, are reviewed in this paper.

Identification of quantitative trait loci influencing foliar concentrations of terpenes and formylated phloroglucinol compounds in Eucalyptus nitens

Leaves of eucalypt species contain a variety of plant secondary metabolites, including terpenoids and formylated phloroglucinol compounds (FPCs). Both terpene and FPC concentrations are quantitative traits that can show large variation within a population and have been shown to be heritable. The molecular genetic basis of this variation is currently unknown. Progeny from a field trial of a three-generation mapping pedigree of Eucalyptus nitens were assayed for terpenes and FPCs. Quantitative trait loci (QTL) analyses were conducted using a map constructed from 296 markers to locate regions of the genome influencing foliar concentrations of these plant secondary compounds. A large number of significant QTL for 14 traits were located across nine linkage groups, with significant clustering of QTL on linkage groups 7, 8 and 9. As expected, QTL for biosynthetically related compounds commonly colocated, but QTL for unrelated monterpenes and FPCs also mapped closely together. Colocation of these QTL with mapped candidate genes from the various biosynthetic pathways, and subsequent use of these genes in association mapping, will assist in determining the causes of variation in plant secondary metabolites in eucalypts.

A microsatellite-based consensus linkage map for species of Eucalyptus and a novel set of 230 microsatellite markers for the genus

BACKGROUND

Eucalypts are the most widely planted hardwood trees in the world occupying globally more than 18 million hectares as an important source of carbon neutral renewable energy and raw material for pulp, paper and solid wood. Quantitative Trait Loci (QTLs) in Eucalyptus have been localized on pedigree-specific RAPD or AFLP maps seriously limiting the value of such QTL mapping efforts for molecular breeding. The availability of a genus-wide genetic map with transferable microsatellite markers has become a must for the effective advancement of genomic undertakings. This report describes the development of a novel set of 230 EMBRA microsatellites, the construction of the first comprehensive microsatellite-based consensus linkage map for Eucalyptus and the consolidation of existing linkage information for other microsatellites and candidate genes mapped in other species of the genus.

RESULTS

The consensus map covers approximately 90% of the recombining genome of Eucalyptus, involves 234 mapped EMBRA loci on 11 linkage groups, an observed length of 1,568 cM and a mean distance between markers of 8.4 cM. A compilation of all microsatellite linkage information published in Eucalyptus allowed us to establish the homology among linkage groups between this consensus map and other maps published for E. globulus. Comparative mapping analyses also resulted in the linkage group assignment of other 41 microsatellites derived from other Eucalyptus species as well as candidate genes and QTLs for wood and flowering traits published in the literature. This report significantly increases the availability of microsatellite markers and mapping information for species of Eucalyptus and corroborates the high conservation of microsatellite flanking sequences and locus ordering between species of the genus.

CONCLUSION

This work represents an important step forward for Eucalyptus comparative genomics, opening stimulating perspectives for evolutionary studies and molecular breeding applications. The generalized use of an increasingly larger set of interspecific transferable markers and consensus mapping information, will allow faster and more detailed investigations of QTL synteny among species, validation of expression-QTL across variable genetic backgrounds and positioning of a growing number of candidate genes co-localized with QTLs, to be tested in association mapping experiments.

Genomic research in Eucalyptus

Eucalyptus L'Hérit. is a genus comprised of more than 700 species that is of vital importance ecologically to Australia and to the forestry industry world-wide, being grown in plantations for the production of solid wood products as well as pulp for paper. With the sequencing of the genomes of Arabidopsis thaliana and Oryza sativa and the recent completion of the first tree genome sequence, Populus trichocarpa, attention has turned to the current status of genomic research in Eucalyptus. For several eucalypt species, large segregating families have been established, high-resolution genetic maps constructed and large EST databases generated. Collaborative efforts have been initiated for the integration of diverse genomic projects and will provide the framework for future research including exploiting the sequence of the entire eucalypt genome which is currently being sequenced. This review summarises the current position of genomic research in Eucalyptus and discusses the direction of future research.

Linkage map of birch, Betula pendula Roth, based on microsatellites and amplified fragment length polymorphisms

The first genetic linkage map for silver birch, Betula pendula Roth, was constructed by using a pseudo-testcross mapping strategy and integration of linkage data from 3 unrelated 2-generation pedigrees. The map is based on the genetic inheritance and segregation of 82 amplified fragment length polyhmorphisms and 19 microsatellite markers, and was constructed by simultaneously comparing the performance of CRI-MAP and OUTMAP packages. The analysis revealed 16 linkage groups, and the total map coverage is 1561 cM (Kosambi units). Average map distance between adjacent markers is 15.5 cM. Linkage groups range between 6 and 18 loci and from 81.2 to 326.5 cM; the remaining 9 linkage groups consist of 2 or 3 loci ranging from 6.3 to 42.4 cM. The uncertainty of the map is illustrated with sensitivity analysis. This initial map can serve as a basis for developing a more detailed genetic map.

Development of a composite map in Vicia faba, breeding applications and future prospects

A composite map of the Vicia faba genome based on morphological markers, isozymes, RAPDs, seed protein genes and microsatellites was constructed. The map incorporates data from 11 F(2) families for a total of 654 individuals all sharing the common female parent Vf 6. The integrated map is arranged in 14 major linkage groups (five of which were located in specific chromosomes). These linkage groups include 192 loci and cover 1559 cM with an overall average marker interval of 8 cM. By joining data of a new F(2) population segregating for resistance to ascochyta, broomrape and others traits of agronomic interest, have been saturated new areas of the genome. The combination of trisomic segregation, linkage analysis among loci from different families with a recurrent parent, and the analysis of new physically located markers, has allowed the establishment of the present status of the V. faba map with a wide coverage. This map provides an efficient tool in breeding applications such as disease-resistance mapping, QTL analyses and marker-assisted selection.

A genetic linkage map for Tribolium confusum based on random amplified polymorphic DNAs and recombinant inbred lines

Tribolium beetles provide an excellent and easily manipulated model system for the study of genetics. However, despite significant increases in the availability of molecular markers for the study of genetics in recent years, a significant genetic linkage map for these beetles remains undeveloped. We present the first molecular genetic linkage map for Tribolium confusum using random amplified polymorphic DNA markers. The linkage map contains 137 loci mapped on to eight linkage groups totaling 968.5 cM.

Effects of native forest regeneration practices on genetic diversity in Eucalyptus consideniana

Impacts of forest harvesting and regeneration practices on genetic diversity in the Australian native forest species Eucalyptus consideniana Maiden (yertchuk) were examined using 29 Mendelian DNA markers (18 RFLPs and 11 microsatellites). Two replicate logging coupes were studied from each of the two most commonly employed silvicultural treatments: clear felling with aerial re-sowing and the seed tree system. For each coupe, genetic diversity measures were compared between a sample of the sapling regeneration and a corresponding control sample from bordering unharvested trees. When calculations were performed over all 29 loci, significant reductions of allelic richness (AR), effective number of alleles (AE) and/or expected heterozygosity (HE) were detected on one or both of seed tree coupes, but on neither of the clear falls. When calculations were performed over the 11 microsatellites alone, all three measures, AR, AE and HE, were significantly reduced on both of the seed replicates but on neither of the two clear falls. In contrast, when the RFLPs were examined separately, there were no significant reductions of diversity on either of the two seed tree coupes or on the two clear falls. These results suggest that genetic erosion is more likely under the seed tree system than under clear-felling with aerial re-sowing and that there is greater statistical power to detect it with microsatellites than with RFLPs. A Monte Carlo simulation to test the statistical significance of the number of apparently lost or gained alleles showed that significant losses of alleles above specified threshold frequencies occurred only in the two seed tree replicates. Three of the four control and regeneration population pairs were significantly differentiated, as indicated by exact tests or by pairwise FST estimates. Comparisons of CONTML dendrograms, constructed for the regeneration populations only versus the control populations only, indicated that genetic drift was significantly promoted under forest management. No significant decreases in observed heterozygosity, or increases in the panmictic index (f), were observed in any of the comparisons suggesting that inbreeding was not promoted by a single rotation of forest management.

Nuclear RFLP variation in Eucalyptus camaldulensis Dehnh. from northern Australia

Eucalyptus camaldulensis Dehnh. is the most widely planted eucalypt in the tropics. Natural populations are riparian and sampling strategies for breeding programmes have assumed that gene flow among drainage basins is limited. RFLP variation, within and among 31 populations from river systems across northern Australia, was analysed to test this hypothesis. To allow comparisons within and between river systems, trees were sampled from up to three populations per river system. Allele frequencies were correlated with longitude for more than half the 33 RFLP loci surveyed. Genetic identity was greatest between populations in closest geographic proximity, irrespective of river system, suggesting that sampling strategies for breeding programmes should be based on geographic distance rather than river system. The level of genetic variation was similar throughout the geographic range examined (mean H(E) = 0.49). However, there was evidence of a barrier to gene flow between populations in the east and west of the species range. The RFLP data support morphological evidence of hybridisation between E. camaldulensis and E. tereticornis Sm. in several populations in northeast Queensland and the genetic divergence of E. camaldulensis subsp. simulata Brooker and Kleinig.

Genetic differentiation between mallee and tree forms in the Eucalyptus loxophleba complex

The Eucalyptus loxophleba complex comprises two tree taxa (E. loxophleba ssp. loxophleba and ssp. supralaevis), two mallee taxa (E. loxophleba ssp. lissophloia and ssp. gratiae), and three rare mallee species, of which one, E. blaxellii, is included in this study. The genetic diversity and phylogenetic relationships between the taxa, particularly in relation to habit, were assessed using anonymous nuclear RFLP loci. The level of diversity in the taxa was high and similar to that detected in other eucalypt species. The populations showed low differentiation at both the subspecies and the species levels. Phylogenetic relationships showed some genetic separation between the tree and mallee habit but no separation of the two taxa within the tree habit or within the mallee habit. The genetic analysis does not support the recognition of E. gratiae as a separate species. The geographically restricted E. blaxellii showed similar levels of diversity to populations of the other widespread taxa of E. loxophleba.

Two Independent Loci Control Agamospermy (Apomixis) in the Triploid Flowering Plant Erigeron annuus

Asexual seed production (agamospermy) via gametophytic apomixis in flowering plants typically involves the formation of an unreduced megagametophyte (via apospory or diplospory) and the parthenogenetic development of the unreduced egg cell into an embryo. Agamospermy is almost exclusively restricted to polyploids. In this study, the genetic basis of agamospermy was investigated in a segregating population of 130 F1's from a cross between triploid (2n = 27) agamospermous Erigeron annuus and sexual diploid (2n = 18) E. strigosus. Correlations between markers and phenotypes and linkage analysis were performed on 387 segregating amplified fragment length polymorphisms (AFLPs). Results show that four closely linked markers with polysomic inheritance are significantly associated with parthenogenesis and that 11 cosegregating markers with univalent inheritance are completely associated with diplospory. This indicates that diplospory and parthenogenesis are unlinked and inherited independently. Further, the absence of agamospermy in diploid F1's appears to be best explained by a combination of recessive-lethal gametophytic selection against the parthenogenetic locus and univalent inheritance of the region bearing diplospory. These results may have major implications for attempts to manipulate agamospermy for agricultural purposes and for interpreting the evolution of the trait.

Forest tree biotechnology

The forest products industry has traditionally viewed trees as merely a raw, and more or less immutable, natural resource. However, unlike such inanimate resources as metallic ores, trees have the potential to be modified genetically, essentially transmuting lead into gold. Increasingly, modern alchemists are applying the tools of biotechnology in efforts to reduce the biological constraints on forest productivity. Several new methodologies being used to address problems in forest biology are described with respect to their potential impact on forest tree improvement. In addition to addressing problems inherent to the current use of trees for production of pulp and paper or solid wood products, genetic manipulation of trees brings with it the potential to create new industries based on the novel characteristics of transgenic trees, e.g. trees containing transgenes to detoxify specific pollutants could be used in the remediation of sites contaminated with hazardous wastes. Efforts to modify trees through biotechnology are in their infancy, and this review seeks to outline the underpinnings of what will undoubtedly be an area of increased emphasis in the next millennium.

Genetic mapping inEucalyptus urophyllaandEucalyptus grandisusing RAPD markers

Two single-tree linkage maps were constructed for Eucalyptus urophylla and Eucalyptus grandis, based on the segregation of 480 random amplified polymorphic DNA (RAPD) markers in a F1interspecific progeny. A mixture of three types of single-locus segregations were observed: 244 1:1 female, 211 1:1 male, and 25 markers common to both, segregating 3:1. Markers segregating in the 1:1 ratio (testcross loci) were used to establish separate maternal and paternal maps, while markers segregating in the 3:1 ratio were used to identify homology between linkage groups of the two species maps. An average of 2.8 polymorphic loci were mapped for each arbitrary decamer primer used in the polymerase chain reaction. The mean interval size beween framework markers on the maps was 14 cM. The maps comprised 269 markers covering 1331 cM and 236 markers covering 1415 cM, in 11 linkage groups, for E. urophylla (2n = 2x = 22) and E. grandis (2n = 2x = 22), respectively. A comparative mapping analysis with two other E. urophylla and E. grandis linkage maps showed that RAPDs could be reliable markers for establishing a consensus species map. RAPD markers were automatically and quantitatively scored with an imaging analyzer. They were classified into four categories based on their optical density. A fragment intensity threshold is proposed to optimize the selection of reliable RAPD markers for future mapping experiments. Key words : genetic linkage map, Eucalyptus urophylla, Eucalyptus grandis, random amplified polymorphic DNA, RAPD, automated data collection.

Estimation of heterozygosity in Ribes nigrum L. using RAPD markers

Heterozygosity in three cultivars of the blackcurrant (Ribes nigrum L.) was estimated. A selfed population of each cultivar was screened for Random Amplified Polymorphic DNA markers (RAPDs) and heterozygous loci were identified by band segregation in contrast with the non-segregation of homozygous loci. On average, 21% of the loci scored in each cultivar were heterozygous. The implications for mapping studies in Ribes nigrum are discussed.