Production of 27-, 28-, and 56-chromosome apomictic hybrid derivatives between pearl millet (2n=14) and Pennisetum squamulatum (2n=54)

Summary An interspecific hybridization program designed to transfer gene(s) controlling apomixis from Pennisetum squamulatum Fresen. (2n=6x=54) to induced tetraploid (2n=4x=28) cultivated pearl millet, Pennisetum americanum (L.) Leeke resulted in four offtype plants, two with 27 chromosomes and two with 28 chromosomes. These plants were found among 217 spaced plants established from open-pollinated seed of an apomictic 21-chromosome polyhaploid (2n=21) plant derived from an apomictic interspecific hybrid (2n=41) between tetraploid pearl millet and Pennisetum squamulatum. It appeared that a 21- (or 20-) chromosome unreduced egg from the apomictic polyhaploid united with a 7-chromosome pearl millet (2n=2x=14) gamete to produce a 28- (or 27-) chromosome offspring. Meiotic chromosome behavior was irregular averaging from 3.60 to 4.05 bivalents per microsporocyte in the 27- and 28-chromosome hybrids. The 27- or 28-chromosome hybrids, like the 21-chromosome female parent, shed no pollen, but set from 1.8 to 28 seed per panicle when allowed to outcross with pearl millet. Progeny of the 28-chromosome hybrids were uniform and identical to their respective female parents, indicating that apomixis had been effectively transferred through the egg. In addition, a 56-chromosome plant resulting from chromosome doubling of a 28-chromosome hybrid was identified. Pollen was 68 per cent stainable and the plant averaged 2.3 selfed seeds per panicle. Chromosomes of the 56-chromosome plant paired as bivalents (x=10.67) or associated in multivalents. Three to nine chromosomes remained unpaired at metaphase I. Multiple four-nucleate embryo sacs indicated the 56-chromosome hybrid was an obligate apomict. The production of 27-, 28-, and 56-chromosome hybrid derivatives were the results of interspecific hybridization, haploidization, fertilization of unreduced apomictic eggs, and spontaneous chromosome doubling. These mechanisms resulted in new unique genome combinations between x=7 and x=9 Pennisetum species.     

The production and behaviour of hexaploid hybrids between Hordeum vulgare and H. Bulbosum

Summary To produce hexaploid (or other polyploid) hybrids, diploid or tetraploid Hordeum vulgare was crossed with hexaploid or octoploid H. bulbosum, and perennial triploid hybrids between the two species were treated with colchicine. The crosses did not yield viable plants: seedset was low, the seed aborted and embryo culture was unsuccessful. The colchicine treatments geve rise to plants in which hexaploid chromosome numbers were observed. At the hexaploid level chromosomal instability occurred, resulting in chromosome elimination.The colchicine-treated triploid hybrids showed in the first years after the treatment better fertility after open flowering than untreated plants, but the level of fertility remained very low. The offspring consisted of haploid, diploid and approximately triploid plants like H. vulgare, tetraploid and approximately tetraploid plants like H. bulbosum, and plants with hybrid morphology and unstable chromosome number, which were highly sterile. Thus the crossing barrier between H. vulgare and H. bulbosum could not be broken down at higher ploidy level.     

A self-fertile trigeneric hybrid in the Triticeae involving Triticum,Hordeum, and Secale

Summary A self-fertile trigeneric hybrid in the Triticeae involving species from the Hordeum, Triticum and Secale genera has been produced. The trigeneric hybrid was obtained by crossing octoploid triticale (x Triticosecale Wittmack) with octoploid tritordeum (H. chilense × T. aestivum amphiploid). The trigeneric hybrid presented a genome constitution AABBDDRH^ch and 2n=8X=56 chromosomes. The cytogenetical analyses showed no chromosome instability nor homeologous pairing between Hordeum and Secale chromosomes. In the F₂ generation the chromosome number ranged from 42 to 52. Within this range, the plants with smaller numbers of chromosomes were more frequent. A preferential transmission of rye chromosomes could be inferred.     

Intergeneric hybrids and an amphiploid between Elymus canadensis and Psathyrostachys juncea

Summary Fifty four hybrid plants between Elymus canadensis and Psathyrostachys juncea were obtained by handpollination and embryo culture. The average cross compatibility between both species was 31.2 percent. One amphiploid plant was induced by colchicine treatment. The hybrid and amphiploid plants resembled P. juncea in appearance but showed a higher plant height and dry matter yield than the parents. The hybrids showed extremely low pollen stainability and were completely sterile. With the exception of one plant (2n=3x+1=22), all hybrid plants were allotriploids (SHN, 2n=3x=21). The amphiploid plant (SSHHNN, 2n=6x=42) showed 58.9% pollen stainability and 11.6% seed fertility.Mean chromosome associations of the hybrids and amphiploid at metaphase I were 0.02IV+0.06III+2.03II+16.91I and 0.07III+18.00II+5.85I, respectively. Lagging chromosomes, chromosome bridges, abnormal cytokinesis, and micronuclei were occasionally observed at the anaphase, telophase, or tetrad stage.     

Nearly Total Absence of Homoeologous Pairing in a Hybrid between Tetraploid Hordeum chilense and Triticum aestivum

A hybrid between an induced tetraploid of Hordeum chilense (2n = 28 = H^chH^chH^chH^ch) and Triticum aestivum var. ‘Chinese Spring’ (2n = 42 = AABBDD) has been produced to test gene effects of this wild barley on homoeologous pairing in wheat. Cytological investigations in metaphase I have shown that the hybrid, which is perennial like H. chilense but morphologically more similar to the wheat parent, possesses the expected genome composition H^chH^ch ABD and a stable euploid chromosome number of 2n = 35. Pairing among the homologous H. chilense chromosomes was almost complete. The level of non-homologous chromosome association proved to be lower than the range of pairing known from euhaploids of ‘Chinese Spring’.     

Variation in progenies of an Arachis hypogaea x diploid wild species hybrid

Summary Derivatives of a cross between cultivated peanuts, Arachis hypogaea L. (2n=40), and the wild species collection GKP 10017 (2n=20) were compared morphologically, for leafspot resistance and for yield. The objective of the study was to determine the effects of wild species germplasm on the A. hypogaea genome. The sterile F₁ hybrid which resulted from crossing the two species was treated with colchicine to restore fertility at the 6x ploidy level. The resulting hexaploid was cytologically unstable and progeny lost chromosomes until stability was regained at the 2n=40 chromosome level. Forty-seven characters were used to analyze the variation among plants in the tetraploid interspecific hybrid population. The plants were compared to four cultivated lines plus GKP 10017. Many hybrids were intermediate to the two parents in morphology. Individual traits such as growth habit, pod and seed size, elongation of the constricted area between pods, nodulation and leaflet size were altered by the presence of GKP 10017 germplasm in many of the hybrid plants. Cercospora arachidicola Hori and Cercosporidium personatum (Berk. & Curt.) Deighton resistances were evaluated for all plants. Several hybrids had few lesions due to either leafspot pathogen. In addition, 24 largeseeded interspecific hybrid selections were compared to the cultivated variety NC 5 for yield. Five selections were superior to both parents at p=0.01. Morphology, disease resistance and yields appeared to be greatly influenced by the wild species GKP 10017 germplasm in plants of the interspecific hybrid population. The potentials of using wild species for improvement of the cultivated peanut are discussed.Paper number 5948 of the journal series of the North Carolina Agricultural Research Service, Raleigh, NC 27650. The investigation was supported in part by ICRISAT and SEA-CR grant no. 701-15-51.     

Somatic hybrids between navel orange (Citrus sinensis) and grapefruit (C. paradisi) for seedless triploid breeding

Protoplasts from cell suspension cultures of ‘Bonnaza’ navel orange (Citrus sinensis (L.) Osbeck) were electrically fused with mesophyll protoplasts isolated from seedless ‘Red Blush’ grapefruit (Citrusparadisi). After 6 months of culture, a total of 20 plants were regenerated. Root tip chromosome counting revealed that 4 of them were tetraploids (2n = 4x = 36)and the rest were diploids (2n = 2x = 18) morphologically resembling the mesophyll parent. After 6 months of transplantation into the greenhouse, 4 of the diploidmesophyll regenerants unexpectedly flowered, but this phenomenon disappeared in the next year. This is the first report of precocious flowering in citrus via protoplast fusion. RAPD analysis further confirmed that the tetraploid regenerants were somatic hybrids while the diploid regenerants were mesophyll parent type. This somatic hybrid will be utilized as a possible pollen parent for improving the seedy pummelo cultivars in China by producing triploid seedless pummelo hybrid. The mechanism of early flowering was also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The transfer of triazine resistance from Brassica napus L. to B. oleracea L. III. First backcross to parental species

Summary Atrazine resistant Brassica napus × B. oleracea F₁ hybrids were backcrossed to both parental species. The backcrosses to B. napus produced seeds in both directions but results were much better when the F₁ hybrid was the pollen parent. Backcrosses to B. oleracea failed completely but BC₁s were rescued by embryo culture both from a tetraploid hybrid (2n = 4x = 37; A₁C₁CC) and sesquidiploid hybrids (2n = 3x = 8; A₁C₁C). Progeny of crosses between the tetraploid hybrid and B. oleracea had between 25 and 28 chromosomes. That of crosses between the sesquidiploid hybrid and B. oleracea had between 21 and 27. A few plants that had chromosome counts outside the expected range may have originated from either diploid parthenogenesis, unreduced gametes or spontaneous chromosome doubling during in vitro culture. Pollen stainability of the BC₁s ranged from 0% to 91.5%. All the BC₁s to B. oleracea were resistant to atrazine.     

Karyotype analysis and interspecific hybridisation in three perennial Sesbania species (Leguminosae)

Summary The somatic chromosome number in Sesbania sesban var. nubica, S. goetzei and S. keniensis (Leguminosae; Papilionoidae) was found to be 2n=12. These findings were in agreement with earlier reports on S. sesban and S. keniensis. The chromosome number 2n=12 is a new record for S. goetzei. Similarities in karyotypes were found in the three species. All species had one pair of long metacentric chromosomes; the second pair was submedian, followed by four smaller pairs of metacentric chromosomes. Nucleolar organiser regions in the form of satellites were found on the short arm of the fourth chromosome pair in S. sesban and S. keniensis. Interspecific crosses in all possible combinations were carried out, resulting in pod and viable seed formation for the crosses S. sesban x S. goetzei, S. sesban x S. keniensis, S. goetzei x S. sesban and S. goetzei x S. keniensis. The two crosses with S. keniensis as a female parent were unsuccessful. The hybrid plants established normally and produced viable seeds.     

An allotriploid derived from a amphidiploid × diploid

A fully fertile interspecific hybrid (Cucumis hytivus Chen and Kirkbride, 2n =4x =38) between Cucumis hystrix Chakr. (2n= 2x =24) and C. sativus L. (2n = 2x = 14) was previously produced by means of F₁ (2n = 19) embryo rescue and subsequent chromosome doubling. This amphidiploid, a new synthetic species, may serveas a genetic bridge in Cucumis, and thus is a source for broadening the genetic base of C. sativus. The identification and characterization of fertile progeny possessing lower ploidy levels would facilitate bridging among Cucumis species. Putative allotriploids (2n = 26) were recovered from C. hytivus × C. sativus matings by means of embryo culture, and experiments were designed to confirm their genetic constitution, describe their morphology, and establish an efficient protocol for their micropropagation. Apical and axillary buds of these putative allotriploid plants were used as explants to establish a micropropagation system for subsequent verification and characterization of ploidy. Of the array of micropropagation media tested, then ability to be most effective for the induction of adventitious buds (desginated Stage II) was a Murashige and Skoog (MS)growth media containing 13.3μM BA + 1.1μM NAA or containing10 μm BA only. The mean number of adventitious buds per explant in the two media was 6.8 and 6.5, respectively. Shoots resulting from adventitious buds produced roots (Stage III) in relative abundance (39 of 42, 92.8%) on half-strength MS medium containing 1.0 μm IBA. The survivorship of rooted plantlets after acclimatization as assessed by relative production of leaves in plantlets (designated Stage IV) was 91.4% (148 of 162). The chromosome number in putative allotriploid plants as determined in mitotic root tip figures in all plants was 2n = 26, the number expected for allotriploids derived from such a mating. An examination of pollen viability in five samples of each plant by cytochemical staining revealed stainability to be < %.Compared to their parents, the allotriploid genotypes possess a high degree of parthenocarpy (84.8%) as measured by setting fruit in pollen-free conditions. While allotriploid fruit are black-spined and similar to the maternal parent C. hytivus, the dark green leaves typical of allotriploid plants mirrors that of the paternal C. sativus parent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of morphological variation and cold resistance in interspecific somatic hybrids between potato (Solanum tuberosum L.) and S. brevidens Phil.

Summary Somatic hybrids between Solanum tuberosum L. cv. Gracia (2n=4x=48) and Solanum brevidens Phil. (2n=2x=24) were produced via fusion of mesophyll protoplasts. Selection of the protoplast derived putative hybrid calli was based on their vigorous growth. Additive isozyme patterns and chromosome numbers as well as the expression of parental morphological characters have proved the hybrid origin of the selected regenerants. Extensive chromosome loss during the regeneration process resulted in aneuploid hybrids with high frequency. Genomic instability could not be detected in these plants during the period of vegetative propagation. Regenerants from hybrid tissues exhibited wide morphological variation especially in tuber formation. The detailed morphological analysis based on the use of multivariate method (principal component analysis, PCA) enabled to identify morphological groups among the hybrid clones. The positioning of hybrid clones in the PCA space could not be correlated with chromosome numbers. The genomic ratio represented by the tetraploid and diploid parents influenced the morphology of somatic hybrid population according to the applied analytical system. Two selected hybrid clones have exhibited an intermediate degree of frost tolerance compared to the parents, based on the recovery of plants from lower buds after freezing of potted plants.     

Introgressive hexaploid oats from the Avena abyssinica (♀) × A. sativa hybrid: performance, grain lipids and proteins

A fertile hexaploid oat plant was obtained after several generations of selection for seed set and plant type from a colchicine-produced decaploid hybrid, Avena abyssinica(2n = 4x = 28, AABB) × A. sativa (2n = 6x = 42, AACCDD). The selected line proved to be stably fertile and in many characteristics equal or superior to the hexaploid parent. The grain protein fractions showed two qualitative differences from those of the pollen parent and several differences from the maternal parent. The fractionating extraction used was new for oats. The fatty acid composition of grains of the hybrid derivative was similar to that of the pollen parent, but different from that of the maternal parent. The maternal parent (A. abyssinica) had a relatively high 16:0 fatty acid content (ca. 20.5 mol%) compared with the level of the hexaploid parent and the hybrid derivation (ca. 17.5 mol% each) in field-grown grain. However, in grain produced in the greenhouse, the hexaploids had ca. 20.5 mol% of 16:0 fatty acid and a decrease in 18:1 fatty acid, whereas seed of the A. abyssinica parent showed only a slight increase (ca.21.5 mol%) in 16:0 fatty acid. These and other responses statistically significant may be due to adaptation to temperature conditions being wider in the hexaploids than in the East-African A. abyssinica. A new method of grain lipid extraction was introduced and showed good reproducibility. The derived hexaploid oat can be crossed with A. sativa for breeding purposes and due to its early maturity might also have direct use in northern or high-altitude cultivation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic analysis of resistance to barley scald (Rhynchosporium secalis) in the Ethiopian line `Abyssinian' (CI668)

A doubled haploid barley (Hordeum vulgare L.) population from a cross between the cultivar `Ingrid' and the Ethiopian landrace `Abyssinian' was mapped by AFLP, RFLP, SSR and STS markers and tested for resistance to isolates`4004', `2', `16-6', `17', `22' and `WRS 1872' of Rhynchosporium secalis (Oudem.) J.J. Davis, the causal agent of leaf scald. Resistance tests were conducted on parents, DH-lines, a near-isogenic line of `Abyssinian' (NIL) into `Ingrid', and an F₂ population descended from the same F₁ plants as the DHs. The DH population segregated for at least two major R. secalis resistance QTL. All isolates tested identified a major QTL on chromosome 3 (3H) associated with R. secalis resistance, in a 4 cM support interval between the co-segregating markers Bmac0209/Falc666 and MWG680. The QTL was linked with the markers Falc666 (2.3 cM), YLM/ylp (0.3 cM), MWG680 (1.7 cM), cttaca2 (2.5 cM) and agtc17 (9.8 cM). The second QTL was located on chromosome 1 (7H).However, this QTL was only detected by one isolate and was located in an interval of 16 cM in the distal part of the chromosome. At this QTL the allele for improved scald resistance originated from the parent `Ingrid'. There were a number of minor QTL on chromosomes 2 (2H), 4 (4H) and 6 (6H) that were not repeatable either across replications or analysis methods. The importance of checking QTL-models by cross-validation is stressed. This revised version was published online in August 2006 with corrections to the Cover Date.     

A synthetic hexaploid (2n = 42) oat from the cross of Avena strigosa (2n = 14) and domesticated A. magna (2n = 28)

A synthetic hexaploid oat was produced by chromosome doubling of a sterile triploid hybrid between cultivated Avena strigosa (2n = 14) cv. Saia and a domesticated form of A. magna (2n = 28). The synthetic hexaploid was intermediate between its parents in panicle shape and lemma color, similar to the tetraploid parent in spikelet structure, and to the diploid parent in having a single, albeit partially shriveled seed per spikelet, and low protein content. By the third generation, plants with yellowish lemmas, mostly two seeds per spikelet and better filled grains had been selected. Rust resistance of the diploid parent was retained in the synthetic hexaploid, but not tolerance to barley yellow dwarf virus disease (BYDV). Chromosome associations at meiosis in the triploid hybrid was low, with over 60% of them being univalents. Bivalent association was the rule in the synthetic hexaploid with an occasional one or two quadrivalents. Regular meiosis with 21 bivalents was observed in further generations. The preferential pairing of homologous chromosomes in the synthetic hexaploid was probably contributed by the A. strigosa genome which exhibits this tendency in artificial allopolyploid situations. Selection of yellow lemma color and two seeds per spikelet suggests that the genes controlling these traits are located on the chromosomes involved in quadrivalents in the synthetic hexaploid. The potential and limitations of utilizing the synthetic hexaploid in oat research and breeding are briefly discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genes for resistance in barley to Finnish isolates of Rhynchosporium secalis

Summary Twenty Finnish isolates of Rhynchosporium secalis (Oud.) J.J. Davis, the causal agent of scald, were taken from infected barley (Hordeum vulgare L.) plants and inoculated on to seedlings of a differential series of barley containing a range of major genes for resistance to the fungus, as well as on to six Nordic 6-row spring barleys and three winter ryes (Secale cereale L.). These fungal isolates derived from four sites and three host varieties. Disease development was monitored on two leaves of seedlings in the greenhouse employing a standard scale, and on adult plants in the field by assessing the diseased area on the three uppermost leaves. A comparison was also made between the pathogenicity and virulence of ten Finnish and ten Canadian R. secalis isolates. The Finnish isolates varied in virulence, but with the exception of Algerian (CI 1179) seedlings and adult La Mesita (CI 7565) all seedlings and adult plants of the entire differential series were resistant to all isolates. Canadian isolates were, on average, less virulent than Finnish isolates. All the Nordic checks were susceptible to all Finnish and seven Canadian isolates, but differences in the degree of susceptibility were evident. Isolates of R. secalis from barley were non-pathogenic on rye, isolates from Elymus repens L. were non-pathogenic on barley and rye, and isolates from rye were only pathogenic on rye. Finnish R. secalis isolates contain no redundant pathogenic diversity. The differential series represents a useful, but as yet untapped, source of resistance to R. secalis for Finnish barley breeders.     

Breeding for hard red winter wheat cultivars adapted to conventional-till and no-till systems in northern latitudes

Summary Monosomic alien addition lines combining individual F. drymeja chromosomes and the L. multiflorum complement were isolated from the cross between the triploid hybrid L. multiflorum (4x) × F. drymeja (2x) and diploid L. multiflorum (2x). Chromosome pairing in the addition lines was studied at metaphase 1 of meiosis and the relationship between single F. drymeja chromosomes and the corresponding homologous pair in L. multiflorum is discussed. Trivalent frequency in the addition lines was higher than expected from observations of chromosome pairing in the triploid hybrid and there were differences between lines in the number of trivalent associations formed. There is some evidence to suggest that trivalent frequency is not entirely dependent on chromosome length and that transmission of the alien chromosome in the female is dependent on the size of the added chromosome. Morphological studies were made to assess the phenotypic effects of the addition of single F. drymeja chromosomes to the L. multiflorum complement. Two plants (2n=14) with recombination between a L. multiflorum and a F. drymeja chromosome were identified.     

Interspecific hybridisation of soybeans and perennial Glycine species indigenous to Australia via embryo culture

Summary Five sterile hybrids (2n=59) between a synthetic amphiploid of Glycine tomentella (2n=38) and G. caneseens (2n=40) as female and soybean cultivars Lincoln and Hark as males have been produced by embryo or ovule culture using transplanted endosperm. The hybrid plants are twining perennials like the female parent but possess a number of morphological characters which reflect the presence of the soybean genome. Indophenol oxidase isozymes from leaf extracts also provide good evidence of the hybrid nature of the cultured plants. These hybrids open the way for the exploitation of the diverse germplasm resources of the perennial Glycine species in soybean breeding.     

A New Intergeneric Hybrid between Triticum aestivum L. and Agropyron fragile (Roth) Candargy: Variation in A. fragile for Suppression of the Wheat Ph-Locus Activity

New intergeneric hybrids were obtained between Triticum aestivum L. cv. Tukuho’ (2n = 6x = 42, AABBDD) and Agropyron fragile (Roth) Candargy PGR 8097 (2n = 4x = 28, PPPP) at a frequency of 1.06 %, through the use of direct embryo culture and in ovulo embryo culture. Such hybrids could be used to transfer barley yellow dwarf virus (BYDV) resistance and winterhardiness into bread wheat. The somatic chromosome number in all the hybrid plants was 2n = 5x = 35, as expected. Considerable variation in chromosome pairing was observed among the different hybrid plants. Average meiotic chromosome configuration at metaphase I was 17.29 Is + 6.57 rod Us + 1.97 ring Us + 0.18 III + 0.03 IV + 0.002 VI. The high level of chromosome pairing in some F₁ hybrids was attributed to Ph-suppressor gene(s) present in A. fragile. The hybrids could not be backcrossed to wheat, but amphiploid seeds have been obtained by colchicine treatment.     

Synthesis and preliminary characterization of a new species (amphidiploid) in Cucumis

A successful interspecific hybridization between cucumber (Cucumis sativus L., 2n = 14) and a wild it Cucumis species, C. hystrix Chakr. (2n = 24) was made via embryo rescue. Hybrid plants (2n = 19; 7 from cucumber and 12 from C. hystrix) were sterile, but morphologically uniform. Self-pollination and backcrossing of F₁ hybrid plants to either parent confirmed presence of both male- and female-sterility that were likely caused by lack of homology and improper pairing during meiosis. While the multiple-branching habit, densely brown hairs (on corolla and pistil), orange-yellow corolla, and ovate fruit of F₁ hybrid plants were similar to that of the C. hystrix parent, the appearance of the first pistillate flower was more similar to that of C. sativus parent. The diameter and internode length of the stem, and the shape and size of leaves and flowers were intermediate when compared to the parents. The chromosome number in the hybrid was doubled through somaclonal variation during embryo culture and regeneration process to restore the fertility. Pollen grains were released and fruits with viable seeds matured on fertile, synthetic amphidiploid plants. The results from flow cytometry indicated that, on average, 7.3% of the morphologically unique regenerants had the 4C DNA content of 2.35 pg relative to the 2C DNA content of the original F₁ hybrid at 1.17 pg and, therefore, were likely chromosome-doubled F₁ hybrids (2n = 38). Nutrition alanalysis indicated that the synthetic species had higher protein (0.78%)and mineral (0.35%) content compared to the normal pickling cucumber(0.62% and 0.27%, respectively), and could be considered a new Cucumis crop having a special place in the future agriculture. Preliminarily evaluation indicates that C. hystrix possesses a high level of root-knot nematode resistance, and that this resistance is partially expressed in the interspecific F₁ and chromosome-doubled F₁. This and the fact that the fruit morphology of the fertile amphidiploid differs during the growing season (e.g., short and long fruit) suggest that it could be useful in broadening the germplasm base of cucumber. This revised version was published online in July 2006 with corrections to the Cover Date.     

A skeletal linkage map of Hordeum bulbosum L. and comparative mapping with barley (H. vulgare L.)

A restriction fragment length polymorphism (RFLP) based linkage map of a cross between two diploid Hordeum bulbosum (2n = 2x = 14) clones, PB1 and PB11, was constructed from 46 recombinant progeny clones. Since both parents are heterozygous, separate and combined parental maps were constructed. All of the RFLP markers screened had previously been mapped in barley (H. vulgare L.) so that comparative maps could be produced. The PB1 linkage map consists of 20 RFLP marker loci assigned to four linkage groups covering 94.3 cM. The PB11 linkage map consists of 27 RFLP marker loci assigned to six linkage groups covering 149.1 cM. Thirteen markers polymorphic in both parents were used as ‘anchors’ to create a combined linkage map consisting of 38 loci assigned to six linkage groups and covering a genetic distance of 198 cM. Marker order was highly conserved in a comparison with the linkage map of H. vulgare (Laurie etal., 1995). However, in contrast, the genetic distances for the same markers were very different being 649 cM and 198 cM respectively, a genetic distance ratio of 1: 3.3. Thus although the map was short, it can be presumed to cover half the genome of H. bulbosum. This study provides further confirmation of the close relationship between the two species and gives a basis for the development of marker mediated introgression through interspecific hybridisation between the two species. This revised version was published online in July 2006 with corrections to the Cover Date.