A High-Resolution Linkage Map Construction and QTL Analysis for Morphological Traits in Anthurium (Anthurium andraeanum Linden)

Anthurium andraeanum Linden is a prominent ornamental plant belonging to the family Araceae and is cultivated worldwide. The morphology characteristics are crucial because they significantly impact ornamental values, commercial properties, and the efficiency of space utilization in production. However, only a few related investigations have been conducted in anthurium to date. In this study, an F1 genetic segregation population containing 160 progenies was generated through hybridization between potted and cut anthurium varieties. Fifteen morphological traits were assessed and revealed substantial levels of genetic variation and widespread positive correlation. Based on specific length amplified fragment (SLAF) sequencing technology, 8171 single nucleotide polymorphism (SNP) markers were developed, and the high-density linkage map of 2202.27 cM in length distributed on 15 linkage groups was constructed successfully, with an average distance of 0.30 cM. Using the inclusive composite interval mapping (ICIM) method, 59 QTLs related to 15 key morphological traits were successfully identified, which explained phenotypic variance (PVE) ranging from 6.21% to 17.74%. Thirty-three of those associated with 13 traits were designated as major QTLs with PVE > 10%. These findings offer valuable insights into the genetic basis of quantitative traits and are beneficial for molecular marker-assisted selection (MAS) in anthurium breeding.

Effectiveness of Single Nucleotide Polymorphism Markers in Genotyping Germplasm Collections of Coffea canephora Using KASP Assay

Accurate genotype identification is imperative for effective use of Coffea canephora L. germplasm to breed new varieties with tolerance or resistance to biotic and abiotic stresses (including moisture stress and pest and disease stresses such as coffee berry borer and rust) and for high yield and improved cup quality. The present study validated 192 published single nucleotide polymorphism (SNP) markers and selected a panel of 120 loci to examine parentage and labeling errors, genetic diversity, and population structure in 400 C. canephora accessions assembled from different coffee-producing countries and planted in a field gene bank in Ghana. Of the 400 genotypes analyzed, both synonymous (trees with same SNP profiles but different names, 12.8%) and homonymous (trees with same name but different SNP profiles, 5.8%) mislabeling were identified. Parentage analysis showed that 33.3% of the progenies derived from controlled crossing and 0% of the progenies derived from an open pollinated biclonal seed garden had parentage (both parents) corresponding to breeder records. The results suggest mislabeling of the mother trees used in seed gardens and pollen contamination from unwanted paternal parents. After removing the duplicated accessions, Bayesian clustering analysis partitioned the 270 unique genotypes into two main populations. Analysis of molecular variance (AMOVA) showed that the between-population variation accounts for 41% of the total molecular variation and the genetic divergence was highly significant (Fst = 0.256; P < 0.001). Taken together, our results demonstrate the effectiveness of using the selected SNP panel in gene bank management, varietal identification, seed garden management, nursery verification, and coffee bean authentication for C. canephora breeding programs.

Segregation distortion: high genetic load suggested by a Chinese shrimp family under high-intensity selection

Segregation distortion is a common phenomenon found in most genetic mapping studies and is an important resource to dissect the mechanism of action in gene loci that cause deviation. Marine animals possess high genetic diversity and genomic heterozygosity, they therefore are ideal model organisms to study segregation distortion induced by selection. In the present study, we constructed a full-sib family of Fenneropenaeus chinensis and exerted high-intensity selection on 10,000 incipient progenies. 2b-RAD method was employed in remaining 273 individuals to develop genome-wide SNPs for segregating analysis and 41,612 SNPs were developed. 50.77% of 32,229 high-quality representative markers deviated from the expected Mendelian ratio. Results showed that most of these distorted markers (91.57%) were influenced at zygotic level. Heterozygote excess (53.07%) and homozygous deletions (41.96%) may both play an important role, sum of which explained 95.03% of distortion after fertilization. However, further results identified highly probable linkage among deleterious alleles, which may account for a considerable portion of heterozygote excess rather than single locus with heterozygote advantage. Results of this study support a major role of deleterious alleles in genetic load, thus in favor of partial dominance hypothesis. It would also offer necessary recommendations for the formulation of breeding strategy in shrimps.

Genome-wide association studies for inflorescence type and remontancy in Hydrangea macrophylla

Inflorescence type and remontancy are two valuable traits in bigleaf hydrangea (Hydrangea macrophylla L.) and both are recessively inherited. Molecular marker-assisted selection (MAS) can greatly reduce the time necessary to breed cultivars with desired traits. In this study, a genome-wide association study (GWAS) using 5803 single-nucleotide polymorphisms (SNPs) was performed using a panel of 82 bigleaf hydrangea cultivars. One SNP locus (Hy_CAPS_Inflo) associated with inflorescence type was identified with general linear model (GLM) and mixed linear model (MLM) methods that explained 65.5% and 36.1% of the phenotypic variations, respectively. Twenty-three SNPs associated with remontancy were detected in GLM whereas no SNP was detected in MLM. The SNP locus (Hy_CAPS_Inflo) was converted to a cleaved amplified polymorphic sequence (CAPS) marker that showed absolute identification accuracy (100%) of inflorescence type in a validation panel consisting of eighteen H. macrophylla cultivars. The SNP was investigated in 341 F1 progenies using genotyping by sequencing (GBS) and co-segregated with inflorescence type (χ 2 = 0.12; P = 0.73). The SNP was subsequently used for breeding selection using kompetitive allele specific PCR (KASP) technology. Future directions for the use of genomics and MAS in hydrangea breeding improvement are discussed. The results presented in this study provide insights for further research on understanding genetic mechanisms behind inflorescence type and remontancy in H. macrophylla. The CAPS and KASP markers developed here will be immediately useful for applying MAS to accelerate breeding improvement in hydrangea.

Construction of a genetic linkage map in Pyropia yezoensis (Bangiales, Rhodophyta) and QTL analysis of several economic traits of blades

Pyropia yezoensis is an economically important seaweed but its molecular genetics is poorly understood. In the present study, we used a doubled haploid (DH) population that was established in our previous work to construct a genetic linkage map of P. yezoensis and analyze the quantitative trait loci (QTLs) of blades. The DH population was genotyped with fluorescent sequence-related amplified polymorphism (SRAP) markers. A chi-square test identified 301 loci with normal segregation (P ≥ 0.01) and 96 loci (24.18%) with low-level skewed segregation (0.001 ≤ P < 0.01). The genetic map was constructed after a total of 92 loci were assembled into three linkage groups (LGs). The map spanned 557.36 cM covering 93.71% of the estimated genome, with a mean interlocus space of 6.23 cM. Kolmogorov-Smirnov test (α = 5%) showed a uniform distribution of the markers along each LG. On the genetic map, 10 QTLs associated with five economic traits of blades were detected. One QTL was for length, one for width, two for fresh weight, two for specific growth rate of length and four for specific growth rate of fresh weight. These QTLs could explain 2.29–7.87% of the trait variations, indicating that their effects were all minor. The results may serve as a framework for future marker-assisted breeding in P. yezoensis.

BDBM 1.0: A Desktop Application for Efficient Retrieval and Processing of High-Quality Sequence Data and Application to the Identification of the Putative Coffea S-Locus

Nowadays, bioinformatics is one of the most important areas in modern biology and the creation of high-quality scientific software supporting this recent research area is one of the core activities of many researchers. In this context, high-quality sequence datasets are needed to perform inferences on the evolution of species, genes, and gene families, or to get evidence for adaptive amino acid evolution, among others. Nevertheless, sequence data are very often spread over several databases, many useful genomes and transcriptomes are non-annotated, the available annotation is not for the desired coding sequence isoform, and/or is unlikely to be accurate. Moreover, although the FASTA text-based format is quite simple and usable by most software applications, there are a number of issues that may be critical depending on the software used to analyse such files. Therefore, researchers without training in informatics often use a fraction of all available data. The above issues can be addressed using already available software applications, but there is no easy-to-use single piece of software that allows performing all these tasks within the same graphical interface, such as the one here presented, named BDBM (Blast DataBase Manager). BDBM can be used to efficiently get gene sequences from annotated and non-annotated genomes and transcriptomes. Moreover, it can be used to look for alternatives to existing annotations and to easily create reliable custom databases. Such databases are essential to prepare high-quality datasets. The analyses that we have performed on the Coffea canephora genome using BDBM aimed at the identification of the S-locus region (that harbours the genes involved in gametophytic self-incompatibility) led to the conclusion that there are two likely regions, one on chromosome 2 (around region 6600000-6650000), and another on chromosome 5 (around 15830000-15930000). Such findings are discussed in the context of the Rubiaceae gametophytic self-incompatibility evolution.

An EST-SSR based genetic linkage map and identification of QTLs for anthracnose disease resistance in water yam (Dioscorea alata L.)

Water yam (Dioscorea alata L.) is one of the most important food yams with wide geographical distribution in the tropics. One of the major constraints to water yam production is anthracnose disease caused by a fungus, Colletotrichum gloeosporioides (Penz.). There are no economically feasible solutions as chemical sprays or cultural practices, such as crop rotation are seldom convenient for smallholder farmers for sustainable control of the disease. Breeding for development of durable genetic resistant varieties is known to offer lasting solution to control endemic disease threats to crop production. However, breeding for resistance to anthracnose has been slow considering the biological constraints related to the heterozygous and vegetative propagation of the crop. The development of saturated linkage maps with high marker density, such as SSRs, followed by identification of QTLs can accelerate the speed and precision of resistance breeding in water yam. In a previous study, a total of 1,152 EST-SSRs were developed from >40,000 EST-sequences generated from two D. alata genotypes. A set of 380 EST-SSRs were validated as polymorphic when tested on two diverse parents targeted for anthracnose disease and were used to generate a saturated linkage map. Majority of the SSRs (60.2%) showed Mendelian segregation pattern and had no effect on the construction of linkage map. All 380 EST-SSRs were mapped into 20 linkage groups, and covered a total length of 3229.5 cM. Majority of the markers were mapped on linkage group 1 (LG 1) comprising of 97 EST-SSRs. This is the first genetic linkage map of water yam constructed using EST-SSRs. QTL localization was based on phenotypic data collected over a 3-year period of inoculating the mapping population with the most virulent strain of C. gloeosporioides from West Africa. Based on threshold LOD scores, one QTL was consistently observed on LG 14 in all the three years and average score data. This QTL was found at position interval of 71.1-84.8 cM explaining 68.5% of the total phenotypic variation in the average score data. The high marker density allowed identification of QTLs and association for anthracnose disease, which could be validated in other mapping populations and used in marker-assisted breeding in D. alata improvement programmes.

Identification of Putative Molecular Markers Associated with Root Traits in Coffea canephora Pierre ex Froehner

Coffea canephora exhibit poor root system and are very sensitive to drought stress that affects growth and production. Deeper root system has been largely empirical as better avoidance to soil water limitation in drought condition. The present study aimed to identify molecular markers linked to high root types in Coffea canephora using molecular markers. Contrasting parents, L1 valley with low root and S.3334 with high root type, were crossed, and 134 F1 individuals were phenotyped for root and associated physiological traits (29 traits) and genotyped with 41 of the 320 RAPD and 9 of the 55 SSR polymorphic primers. Single marker analysis was deployed for detecting the association of markers linked to root associated traits by SAS software. There were 13 putative RAPD markers associated with root traits such as root length, secondary roots, root dry weight, and root to shoot ratio, in which root length associated marker OPS1850 showed high phenotypic variance of 6.86%. Two microsatellite markers linked to root length (CPCM13400) and root to shoot ratio (CM211300). Besides, 25 markers were associated with more than one trait and few of the markers were associated with positively related physiological traits and can be used in marker assisted trait selection.

Linkage group correction using epistatic distorted markers in F2 and backcross populations

Epistasis has been frequently observed in all types of mapping populations. However, relatively little is known about the effect of epistatic distorted markers on linkage group construction. In this study, a new approach was proposed to correct the recombination fraction between epistatic distorted markers in backcross and F2 populations under the framework of fitness and liability models. The information for three or four markers flanking with an epistatic segregation distortion locus was used to estimate the recombination fraction by the maximum likelihood method, implemented via an expectation-maximisation algorithm. A set of Monte Carlo simulation experiments along with a real data analysis in rice was performed to validate the new method. The results showed that the estimates from the new method are unbiased. In addition, five statistical properties for the new method in a backcross were summarised and confirmed by theoretical, simulated and real data analyses.

Development of new genomic microsatellite markers from robusta coffee (Coffea canephora Pierre ex A. Froehner) showing broad cross-species transferability and utility in genetic studies

BACKGROUND

Species-specific microsatellite markers are desirable for genetic studies and to harness the potential of MAS-based breeding for genetic improvement. Limited availability of such markers for coffee, one of the most important beverage tree crops, warrants newer efforts to develop additional microsatellite markers that can be effectively deployed in genetic analysis and coffee improvement programs. The present study aimed to develop new coffee-specific SSR markers and validate their utility in analysis of genetic diversity, individualization, linkage mapping, and transferability for use in other related taxa.

RESULTS

A small-insert partial genomic library of Coffea canephora, was probed for various SSR motifs following conventional approach of Southern hybridisation. Characterization of repeat positive clones revealed a very high abundance of DNRs (1/15 Kb) over TNRs (1/406 kb). The relative frequencies of different DNRs were found as AT >> AG > AC, whereas among TNRs, AGC was the most abundant repeat. The SSR positive sequences were used to design 58 primer pairs of which 44 pairs could be validated as single locus markers using a panel of arabica and robusta genotypes. The analysis revealed an average of 3.3 and 3.78 alleles and 0.49 and 0.62 PIC per marker for the tested arabicas and robustas, respectively. It also revealed a high cumulative PI over all the markers using both sib-based (10-6 and 10-12 for arabicas and robustas respectively) and unbiased corrected estimates (10-20 and 10-43 for arabicas and robustas respectively). The markers were tested for Hardy-Weinberg equilibrium, linkage dis-equilibrium, and were successfully used to ascertain generic diversity/affinities in the tested germplasm (cultivated as well as species). Nine markers could be mapped on robusta linkage map. Importantly, the markers showed ~92% transferability across related species/genera of coffee.

CONCLUSION

The conventional approach of genomic library was successfully employed although with low efficiency to develop a set of 44 new genomic microsatellite markers of coffee. The characterization/validation of new markers demonstrated them to be highly informative, and useful for genetic studies namely, genetic diversity in coffee germplasm, individualization/bar-coding for germplasm protection, linkage mapping, taxonomic studies, and use as conserved orthologous sets across secondary genepool of coffee. Further, the relative frequency and distribution of different SSR motifs in coffee genome indicated coffee genome to be relatively poor in microsatellites compared to other plant species.

Comparison between a coffee single copy chromosomal region and Arabidopsis duplicated counterparts evidenced high level synteny between the coffee genome and the ancestral Arabidopsis genome

The Arabidopsis thaliana genome sequence provides a catalogue of reference genes that can be used for comparative analysis of other species thereby facilitating map-based cloning in economically important crops. We made use of a coffee bacterial artificial chromosome (BAC) contig linked to the S(H)3 leaf rust resistance gene to assess microsynteny between coffee (Coffea arabica L.) and Arabidopsis. Microsynteny was revealed and the matching counterparts to C. arabica contigs were seen to be scattered throughout four different syntenic segments of Arabidopsis on chromosomes (Ath) I, III, IV and V. Coffee BAC filter hybridizations were performed using coffee putative conserved orthologous sequences to Arabidopsis predicted genes located on the different Arabidopsis syntenic regions. The coffee BAC contig related to the S(H)3 region was successfully consolidated and later on validated by fingerprinting. Furthermore, the anchoring markers appeared in same order on the coffee BAC contigs and in all Arabidopsis segments with the exception of a single inversion on AtIII and AtIV Arabidopsis segments. However, the S(H)3 coffee region appears to be closer to the ancestral genome segment (before the divergence of Arabidopsis and coffee) than any of the duplicated counterparts in the present-day Arabidopsis genome. The genome duplication events at the origin of its Arabidopsis counterparts occurred most probably after the separation (i.e. 94 million years ago) of Euasterid (Coffee) and Eurosid (Arabidopsis).

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.

Linkage maps of the dwarf and Normal lake whitefish (Coregonus clupeaformis) species complex and their hybrids reveal the genetic architecture of population divergence

Elucidating the genetic architecture of population divergence may reveal the evolution of reproductive barriers and the genomic regions implicated in the process. We assembled genetic linkage maps for the dwarf and Normal lake whitefish species complex and their hybrids. A total of 877 AFLP loci and 30 microsatellites were positioned. The homology of mapped loci between families supported the existence of 34 linkage groups (of 40n expected) exhibiting 83% colinearity among linked loci between these two families. Classes of AFLP markers were not randomly distributed among linkage groups. Both AFLP and microsatellites exhibited deviations from Mendelian expectations, with 30.4% exhibiting significant segregation distortion across 28 linkage groups of the four linkage maps in both families (P < 0.00001). Eight loci distributed over seven homologous linkage groups were significantly distorted in both families and the level of distortion, when comparing homologous loci of the same phase between families, was correlated (Spearman R = 0.378, P = 0.0021). These results suggest that substantial divergence incurred during allopatric glacial separation and subsequent sympatric ecological specialization has resulted in several genomic regions that are no longer complementary between dwarf and Normal populations issued from different evolutionary glacial lineages.

Construction and characterization of a Coffea canephora BAC library to study the organization of sucrose biosynthesis genes

The first bacterial artificial chromosome (BAC) library of Robusta coffee (Coffea canephora) was constructed, with the aim of developing molecular resources to study the genome structure and evolution of this perennial crop. Clone 126, which is highly productive and confers good technological and organoleptic qualities of beverage, was chosen for development of this library. The BAC library contains 55,296 clones, with an average insert size of 135 Kb per plasmid, therefore representing theoretically nine haploid genome equivalents of C. canephora. Its validation was achieved with a set of 13 genetically anchored single-copy and 4 duplicated RFLP probes and yielded on average 9 BAC clones per probe. Screening of this BAC library was also carried out with partial cDNA probes coding for enzymes of sugar metabolism like invertases and sucrose synthase, with the aim of characterizing the organization and promoter structure of this important class of genes. It was shown that genes for both cell wall and vacuolar forms of invertases were probably unique in the Robusta genome whereas sucrose synthase was encoded by at least two genes. One of them (CcSUS1) was cloned and sequenced, showing that our BAC library is a valuable tool to rapidly identify genes of agronomic interest or linked to cup quality in C. canephora.

Inheritance of spontaneous mutant homostyles in Turnera subulata x krapovickasii and in autotetraploid T. scabra (Turneraceae)

To explore the genetic architecture of distyly in Turnera spp., we determined the inheritance and compatibility behaviour of two spontaneous homostyled mutants. A long-homostyled mutant shoot arose on an otherwise short-styled plant that was an artificial hybrid (Turnera subulata x T. krapovickasii) between two diploid distylous species. The mutation appears to be an allele, SH, of the distyly locus with the dominance relationships, S>SH>s, where S confers the short-styled phenotype, SH confers homostyly in SHSH and SHs genotypes, and ss genotypes are long-styled. Aberrant segregation ratios were observed among some crosses and might be the result of pollen competition. Compatibility relationships are consistent with the hypothesis that a gene complex determines distyly. Infrequently, revertant short-styled flowers have appeared on cuttings of the T. subulata x T. krapovickasii mutant and on occasion, short-styled progeny have appeared in crosses where none were expected. A second mutant homostyle was discovered in autotetraploid T. scabra. The mutation is inherited as above, however, tetrasomic inheritance occurs at the locus. This homostyled mutant carries two copies of the SH allele and has the duplex genotype SHSHss. Compatibility relationships were as observed above. The occurrence of homostyled mutants is consistent with the hypothesis that a linked gene complex underlies the inheritance of distyly in Turnera but we cannot discount the hypothesis that an allelic series is responsible.

Genome-wide analysis of the frequency and distribution of crossovers at male and female meiosis in Sinapis alba L. (white mustard)

We present the first genetic linkage maps of Sinapis alba (white mustard) and a rigorous analysis of sex effects on the frequency and distribution of crossovers at meiosis in this species. Sex-averaged maps representing recombination in two highly heterozygous parents were aligned to give a consensus map consisting of 382 loci defined by restriction fragment length polymorphisms and arranged in 12 linkage groups with no unlinked markers. The loci were distributed in a near-random manner across the genome, and there was little evidence of segregation distortion. From these dense maps, a subset of spaced informative markers was used to establish recombination frequencies assayed separately in male and female gametes and derived from two distinct genetic backgrounds. Analyses of 746 gametes indicated that recombination frequencies were greater in male gametes, with the greatest differences near the ends of linkage groups. Genetic background had a lesser effect on recombination frequencies, with no discernible pattern in the distribution of such differences. The possible causes of sex differences in recombination frequency and the implications for plant breeding are discussed.

Recombination difference between sexes: a role for haploid selection

Why the autosomal recombination rate differs between female and male meiosis in most species has been a genetic enigma since the early study of meiosis. Some hypotheses have been put forward to explain this widespread phenomenon and, up to now, only one fact has emerged clearly: In species in which meiosis is achiasmate in one sex, it is the heterogametic one. This pattern, known as the Haldane-Huxley rule, is thought to be a side effect, on autosomes, of the suppression of recombination between the sex chromosomes. However, this rule does not hold for heterochiasmate species (i.e., species in which recombination is present in both sexes but varies quantitatively between sexes) and does not apply to species lacking sex chromosomes, such as hermaphroditic plants. In this paper, we show that in plants, heterochiasmy is due to a male-female difference in gametic selection and is not influenced by the presence of heteromorphic sex chromosomes. This finding provides strong empirical support in favour of a population genetic explanation for the evolution of heterochiasmy and, more broadly, for the evolution of sex and recombination.

Factors influencing gene introgression into the allotetraploid Coffea arabica L. from its diploid relatives

Factors controlling gene introgression into cultivated arabica coffee (Coffea arabica L.) were investigated. Interspecific triploid hybrid plants between the tetraploid species C. arabica (2n = 44) and a diploid species (2n = 22), either Coffea canephora or Coffea eugenioides, were backcrossed to C. arabica (male parent). Flow cytometric analysis of the nuclear DNA content revealed that most of the BC1 individuals derived from triploid hybrids involving C. eugenioides were tetraploid or nearly tetraploid. Among the gametes produced by the interspecific triploid hybrids, those possessing approximately 22 chromosomes appeared strongly favored. The amount of introgression in BC1 individuals (21 and 43 for the BC1 progenies involving C. canephora and C. eugenioides, respectively) was estimated using species-specific microsatellite markers. A large number of introgressed markers was observed in all BC1 individuals. Nevertheless, while the frequency of introgressed markers seemed as expected, assuming random chromosome segregation and diploid gamete formation, in the BC1 derived from triploid hybrids involving C. canephora, this frequency appeared significantly lower in the BC1 derived from triploid hybrids involving C. eugenioides. Furthermore, the comparison of reciprocal progenies between C. arabica and triploid interspecific hybrids (C. arabica × C. canephora) used as male or female parent revealed a very strong effect of the backcross direction.Key words: irregular meiosis, coffee, reciprocal crosses, molecular marker, triploid hybrids.

A genetic linkage map of quinoa ( Chenopodium quinoa) based on AFLP, RAPD, and SSR markers

Quinoa ( Chenopodium quinoa Willd.) is an important seed crop for human consumption in the Andean region of South America. It is the primary staple in areas too arid or saline for the major cereal crops. The objective of this project was to build the first genetic linkage map of quinoa. Selection of the mapping population was based on a preliminary genetic similarity analysis of four potential mapping parents. Breeding lines 'Ku-2' and '0654', a Chilean lowland type and a Peruvian Altiplano type, respectively, showed a low similarity coefficient of 0.31 and were selected to form an F(2) mapping population. The genetic map is based on 80 F(2) individuals from this population and consists of 230 amplified length polymorphism (AFLP), 19 simple-sequence repeat (SSR), and six randomly amplified polymorphic DNA markers. The map spans 1,020 cM and contains 35 linkage groups with an average marker density of 4.0 cM per marker. Clustering of AFLP markers was not observed. Additionally, we report the primer sequences and map locations for 19 SSR markers that will be valuable tools for future quinoa genome analysis. This map provides a key starting point for genetic dissection of agronomically important characteristics of quinoa, including seed saponin content, grain yield, maturity, and resistance to disease, frost, and drought. Current efforts are geared towards the generation of more than 200 mapped SSR markers and the development of several recombinant-inbred mapping populations.

Gene introgression into Coffea arabica by way of triploid hybrids (C. arabica x C. canephora)

Interspecific triploid hybrid plants between the tetraploid species Coffea arabica L. and the diploid species C. canephora P. were backcrossed to C. arabica. Although characterised by a low production and an important fruit dropping, all attempted crosses (ie, 6) generated BC(1) progenies. Flow cytometric analysis of the nuclear DNA content revealed that most of the BC1 individuals were nearly tetraploid. Among the male gametes produced by the interspecific triploid hybrids, those presenting a high number of chromosomes appeared strongly favoured. Only pollen mother cells having nearly 22 chromosomes were effective, the others leading to deficient endosperm and fruit dropping. Molecular markers (ie, microsatellite and AFLP) combined with evaluations of morphological characteristics and resistance to leaf rust were applied to verify the occurrence of gene transfer from C. canephora into C. arabica, and to estimate the amount of introgression present in BC(1) individuals. The results reveal a strong deficiency in the C. canephroa alleles indicating a severe counter-selection against the introgression of genetic material from C. canephora into C. arabica by way of triploid hybrids. However, introgressants displaying desirable traits such as a high resistance to leaf rust were obtained. The low level of introgression could be an advantage by facilitating the recovery of the recurrent parent and possibly reducing the number of required backcrosses. On the other hand, this could be a limitation when attempting the transfer of a complex trait or several simply inherited traits.

A genetic linkage map of water yam ( Dioscorea alata L.) based on AFLP markers and QTL analysis for anthracnose resistance

A genetic linkage map of the tetraploid water yam ( Dioscorea alata L.) genome was constructed based on 469 co-dominantly scored amplified fragment length polymorphism (AFLP) markers segregating in an intraspecific F(1) cross. The F(1) was obtained by crossing two improved breeding lines, TDa 95/00328 as female parent and TDa 87/01091 as male parent. Since the mapping population was an F(1) cross between presumed heterozygous parents, marker segregation data from both parents were initially split into maternal and paternal data sets, and separate genetic linkage maps were constructed. Later, data analysis showed that this was not necessary and thus the combined markers from both parents were used to construct a genetic linkage map. The 469 markers were mapped on 20 linkage groups with a total map length of 1,233 cM and a mean marker spacing of 2.62 cM. The markers segregated like a diploid cross-pollinator population suggesting that the water yam genome is allo-tetraploid (2n = 4 x = 40). QTL mapping revealed one AFLP marker E-14/M52-307 located on linkage group 2 that was associated with anthracnose resistance, explaining 10% of the total phenotypic variance. This map covers 65% of the yam genome and is the first linkage map reported for D. alata. The map provides a tool for further genetic analysis of traits of agronomic importance and for using marker-assisted selection in D. alata breeding programmes. QTL mapping opens new avenues for accumulating anthracnose resistance genes in preferred D. alata cultivars.