Multi-environment QTL studies suggest a role for cysteine-rich protein kinase genes in quantitative resistance to blackleg disease in Brassica napus.

BACKGROUND: Resistance to the blackleg disease of Brassica napus (canola/oilseed rape), caused by the hemibiotrophic fungal pathogen Leptosphaeria maculans, is determined by both race-specific resistance (R) genes and quantitative resistance loci (QTL), or adult-plant resistance (APR). While the introgression of R genes into breeding material is relatively simple, QTL are often detected sporadically, making them harder to capture in breeding programs. For the effective deployment of APR in crop varieties, resistance QTL need to have a reliable influence on phenotype in multiple environments and be well defined genetically to enable marker-assisted selection (MAS). RESULTS: Doubled-haploid populations produced from the susceptible B. napus variety Topas and APR varieties AG-Castle and AV-Sapphire were analysed for resistance to blackleg in two locations over 3 and 4 years, respectively. Three stable QTL were detected in each population, with two loci appearing to be common to both APR varieties. Physical delineation of three QTL regions was sufficient to identify candidate defense-related genes, including a cluster of cysteine-rich receptor-like kinases contained within a 49 gene QTL interval on chromosome A01. Individual L. maculans isolates were used to define the physical intervals for the race-specific R genes Rlm3 and Rlm4 and to identify QTL common to both field studies and the cotyledon resistance response. CONCLUSION: Through multi-environment QTL analysis we have identified and delineated four significant and stable QTL suitable for MAS of quantitative blackleg resistance in B. napus, and identified candidate genes which potentially play a role in quantitative defense responses to L. maculans.

Co-localisation of the blackleg resistance genes Rlm2 and LepR3 on Brassica napus chromosome A10.

BACKGROUND: The protection of canola (Brassica napus) crops against blackleg disease, caused by the fungal pathogen Leptosphaeria maculans, is largely mediated by race-specific resistance genes (R-genes). While many R-genes effective against blackleg disease have been identified in Brassica species, information of the precise genomic locations of the genes is limited. RESULTS: In this study, the Rlm2 gene for resistance to blackleg, located on chromosome A10 of the B. napus cultivar 'Glacier', was targeted for fine mapping. Molecular markers tightly linked to the gene were developed for use in mapping the resistance locus and defining the physical interval in B. napus. Rlm2 was localised to a 5.8 cM interval corresponding to approximately 873 kb of the B. napus chromosome A10. CONCLUSION: The recently-cloned B. napus R-gene, LepR3, occupies the same region of A10 as Rlm2 and analysis of the putative B. napus and B. rapa genes in the homologous region identified several additional candidate defense-related genes that may control Rlm2 function.

Genetic control of immunity to Turnip mosaic virus (TuMV) pathotype 1 in Brassica rapa (Chinese cabbage).

Turnip mosaic virus (TuMV) is the major virus infecting crops of the genus Brassica worldwide. A dominant resistance gene, TuRB01b, that confers immunity to the virus isolate UK 1 (a representative pathotype 1 isolate of TuMV) on Brassica rapa was identified in the Chinese cabbage cultivar Tropical Delight. The TuRB01b locus was mapped to a 2.9-cM interval on B. rapa chromosome 6 (A6) that was flanked by RFLP markers pN101e1 and pW137e1. This mapping used a first backcross (B(1)) population segregating for the resistance gene at TuRB01b and sets of RFLP markers employed in previous mapping experiments in Brassica. Virus-plant interaction phenotypes were assayed in inbred progeny derived from B(1) individuals to allow different virus isolates to be tested. Comparative mapping confirmed that A6 of B. rapa was equivalent to chromosome 6 of Brassica napus (A6) and that the map position of TuRB01b in B. rapa could be identical to that of TuRB01 in B. napus. Detailed evaluation of plant-virus interactions showed that TuRB01 and TuRB01b had indistinguishable specificities to a range of TuMV isolates. The possibility that TuRB01 and TuRB01b represent similar or identical alleles at the same A genome resistance locus suggests that B. napus acquired TuRB01 from the B. rapa gene pool.

Genetic mapping of the Leptosphaeria maculans avirulence gene corresponding to the LepR1 resistance gene of Brassica napus.

AvrLepR1 of the fungal pathogen Leptosphaeria maculans is the avirulence gene that corresponds to Brassica LepR1, a plant gene controlling dominant, race-specific resistance to this pathogen. An in vitro cross between the virulent L. maculans isolate, 87-41, and the avirulent isolate, 99-56, was performed in order to map the AvrLepR1 gene. The disease reactions of the 94 of the resulting F(1) progenies were tested on the canola line ddm-12-6s-1, which carries LepR1. There were 44 avirulent progenies and 50 virulent progenies suggesting a 1:1 segregation ratio and that the avirulence of 99-56 on ddm-12-6s-1 is controlled by a single gene. Tetrad analysis also indicated a 1:1 segregation ratio. The AvrLepR1 gene was positioned on a genetic map of L. maculans relative to 259 sequence-related amplified polymorphism (SRAP) markers, two cloned avirulence genes (AvrLm1 and AvrLm4-7) and the mating type locus (MAT1). The genetic map consisted of 36 linkage groups, ranging in size from 13.1 to 163.7 cM, and spanned a total of 2,076.4 cM. The AvrLepR1 locus was mapped to linkage group 4, in the 13.1 cM interval flanked by the SRAP markers SBG49-110 and FT161-223. The AvrLm4-7 locus was also positioned on linkage group 4, close to but distinct from the AvrLepR1 locus, in the 5.4 cM interval flanked by FT161-223 and P1314-300. This work will make possible the further characterization and map-based cloning of AvrLepR1. A combination of genetic mapping and pathogenicity tests demonstrated that AvrLepR1 is different from each of the L. maculans avirulence genes that have been characterized previously.

Integration of linkage maps for the Amphidiploid Brassica napus and comparative mapping with Arabidopsis and Brassica rapa.

BACKGROUND: The large number of genetic linkage maps representing Brassica chromosomes constitute a potential platform for studying crop traits and genome evolution within Brassicaceae. However, the alignment of existing maps remains a major challenge. The integration of these genetic maps will enhance genetic resolution, and provide a means to navigate between sequence-tagged loci, and with contiguous genome sequences as these become available. RESULTS: We report the first genome-wide integration of Brassica maps based on an automated pipeline which involved collation of genome-wide genotype data for sequence-tagged markers scored on three extensively used amphidiploid Brassica napus (2n = 38) populations. Representative markers were selected from consolidated maps for each population, and skeleton bin maps were generated. The skeleton maps for the three populations were then combined to generate an integrated map for each LG, comparing two different approaches, one encapsulated in JoinMap and the other in MergeMap. The BnaWAIT_01_2010a integrated genetic map was generated using JoinMap, and includes 5,162 genetic markers mapped onto 2,196 loci, with a total genetic length of 1,792 cM. The map density of one locus every 0.82 cM, corresponding to 515 Kbp, increases by at least three-fold the locus and marker density within the original maps. Within the B. napus integrated map we identified 103 conserved collinearity blocks relative to Arabidopsis, including five previously unreported blocks. The BnaWAIT_01_2010a map was used to investigate the integrity and conservation of order proposed for genome sequence scaffolds generated from the constituent A genome of Brassica rapa. CONCLUSIONS: Our results provide a comprehensive genetic integration of the B. napus genome from a range of sources, which we anticipate will provide valuable information for rapeseed and Canola research.

The mosaic of ancestral karyotype blocks in the Sinapis alba L. genome.

The organisation of the Sinapis alba genome, comprising 12 linkage groups (n = 12), was compared with the Brassicaceae ancestral karyotype (AK) genomic blocks previously described in other crucifer species. Most of the S. alba genome falls into conserved triplicated genomic blocks that closely match the AK-defined genomic blocks found in other crucifer species including the A, B, and C genomes of closely related Brassica species. In one instance, an S. alba linkage group (S05) was completely collinear with one AK chromosome (AK1), the first time this has been observed in a member of the Brassiceae tribe. However, as observed for other members of the Brassiceae tribe, ancestral genomic blocks were fragmented in the S. alba genome, supporting previously reported comparative chromosome painting describing rearrangements of the AK karyotype prior to the divergence of the Brassiceae from other crucifers. The presented data also refute previous phylogenetic reports that suggest S. alba was more closely related to Brassica nigra (B genome) than to B. rapa (A genome) and B. oleracea (C genome). A comparison of the S. alba and Arabidopsis thaliana genomes revealed many regions of conserved gene order, which will facilitate access to the rich genomic resources available in the model species A. thaliana for genetic research in the less well-resourced crop species S. alba.

Effects of a coumarin derivative, 4-methylumbelliferone, on seed germination and seedling establishment in Arabidopsis.

The root system is central for plant adaptation to soil heterogeneity and is organized primarily by root branching. To search for compounds that regulate root branching, a forward chemical genetics screen was employed, and 4-methylumbelliferone (4-MU), a coumarin derivative, was found to be a potent regulator of lateral root formation. Exogenous application of 4-MU to Arabidopsis thaliana seeds affected germination and led to reduced primary root growth, the formation of bulbous root hairs, and irregular detached root caps accompanied by reorganization of the actin cytoskeleton in root tips before seedling establishment. Abundant lateral roots formed after exposure to 125 µM 4-MU for 22 days. Molecular, biochemical, and phytochemical approaches were used to determine the effect of 4-MU on root growth and root branching. Arabidopsis seedlings grown in the presence of 4-MU accumulated this compound only in roots, where it was partially transformed by UDP-glycosyltransferases (UGTs) into 4-methylumbelliferyl-ß-D-glucoside (4-MU-Glc). The presence of 4-MU-Glc in seedling roots was consistent with the upregulation of several genes that encode UGTs in the roots. This shows that UGTs play an integral role in the detoxification of 4-MU in plants. The increased expression of two auxin efflux facilitator genes (PIN2 and PIN3) in response to 4-MU and the lack of response of the auxin receptor TIR1 and the key auxin biosynthetic gene YUCCA1 suggest that auxin redistribution, rather than auxin biosynthesis, may directly or indirectly mediate 4-MU-induced root branching.

Segmental structure of the Brassica napus genome based on comparative analysis with Arabidopsis thaliana.

Over 1000 genetically linked RFLP loci in Brassica napus were mapped to homologous positions in the Arabidopsis genome on the basis of sequence similarity. Blocks of genetically linked loci in B. napus frequently corresponded to physically linked markers in Arabidopsis. This comparative analysis allowed the identification of a minimum of 21 conserved genomic units within the Arabidopsis genome, which can be duplicated and rearranged to generate the present-day B. napus genome. The conserved regions extended over lengths as great as 50 cM in the B. napus genetic map, equivalent to approximately 9 Mb of contiguous sequence in the Arabidopsis genome. There was also evidence for conservation of chromosome landmarks, particularly centromeric regions, between the two species. The observed segmental structure of the Brassica genome strongly suggests that the extant Brassica diploid species evolved from a hexaploid ancestor. The comparative map assists in exploiting the Arabidopsis genomic sequence for marker and candidate gene identification within the larger, intractable genomes of the Brassica polyploids.

Single R Gene Introgression Lines for Accurate Dissection of the Brassica - Leptosphaeria Pathosystem.

Seven blackleg resistance (R) genes (Rlm1, Rlm2, Rlm3, Rlm4, LepR1, LepR2 & LepR3) were each introgressed into a common susceptible B. napus doubled-haploid (DH) line through reciprocal back-crossing, producing single-R gene introgression lines (ILs) for use in the pathological and molecular study of Brassica-Leptosphaeria interactions. The genomic positions of the R genes were defined through molecular mapping and analysis with transgenic L. maculans isolates was used to confirm the identity of the introgressed genes where possible. Using L. maculans isolates of contrasting avirulence gene (Avr) profiles, we preformed extensive differential pathology for phenotypic comparison of the ILs to other B. napus varieties, demonstrating the ILs can provide for the accurate assessment of Avr-R gene interactions by avoiding non-Avr dependant alterations to resistance responses which can occur in some commonly used B. napus varieties. Whole-genome SNP-based assessment allowed us to define the donor parent introgressions in each IL and provide a strong basis for comparative molecular dissection of the pathosystem.

Detection and effects of a homeologous reciprocal transposition in Brassica napus.

A reciprocal chromosomal transposition was identified in several annual oilseed Brassica napus genotypes used as parents in crosses to biennial genotypes for genetic mapping studies. The transposition involved an exchange of interstitial homeologous regions on linkage groups N7 and N16, and its detection was made possible by the use of segregating populations of doubled haploid lines and codominant RFLP markers. RFLP probes detected pairs of homeologous loci on N7 and N16 for which the annual and biennial parents had identical alleles in regions expected to be homeologous. The existence of an interstitial reciprocal transposition was confirmed by cytological analysis of synaptonemal complexes of annual x biennial F1 hybrids. Although it included approximately one-third of the physical length of the N7 and N16 chromosomes, few recombination events within the region were recovered in the progenies of the hybrids. Significantly higher seed yields were associated with the parental configurations of the rearrangement in segregating progenies. These progenies contained complete complements of homeologous chromosomes from the diploid progenitors of B. napus, and thus their higher seed yields provide evidence for the selective advantage of allopolyploidy through the fixation of intergenomic heterozygosity.

SCARECROW-LIKE15 interacts with HISTONE DEACETYLASE19 and is essential for repressing the seed maturation programme.

Epigenetic regulation of gene expression is critical for controlling embryonic properties during the embryo-to-seedling phase transition. Here we report that a histone deacetylase19 (HDA19)-associated regulator, scarecrow-like15 (SCL15), is essential for repressing the seed maturation programme in vegetative tissues. SCL15 is expressed in and GFP-tagged SCL15 predominantly localizes to, the vascular bundles particularly in the phloem companion cells and neighbouring specialized cells. Mutation of SCL15 leads to a global shift in gene expression in seedlings to a profile resembling late embryogenesis in seeds. In scl15 seedlings, many genes involved in seed maturation are markedly derepressed with concomitant accumulation of seed 12S globulin; this is correlated with elevated levels of histone acetylation at a subset of seed-specific loci. SCL15 physically interacts with HDA19 and direct targets of HDA19-SCL15 association are identified. These studies reveal that SCL15 acts as an HDA19-associated regulator to repress embryonic traits in seedlings.

Repression of seed maturation genes by a trihelix transcriptional repressor in Arabidopsis seedlings.

The seed maturation program is repressed during germination and seedling development so that embryonic genes are not expressed in vegetative organs. Here, we describe a regulator that represses the expression of embryonic seed maturation genes in vegetative tissues. ASIL1 (for Arabidopsis 6b-interacting protein 1-like 1) was isolated by its interaction with the Arabidopsis thaliana 2S3 promoter. ASIL1 possesses domains conserved in the plant-specific trihelix family of DNA binding proteins and belongs to a subfamily of 6b-interacting protein 1-like factors. The seedlings of asil1 mutants exhibited a global shift in gene expression to a profile resembling late embryogenesis. LEAFY COTYLEDON1 and 2 were markedly derepressed during early germination, as was a large subset of seed maturation genes, such as those encoding seed storage proteins and oleosins, in seedlings of asil1 mutants. Consistent with this, asil1 seedlings accumulated 2S albumin and oil with a fatty acid composition similar to that of seed-derived lipid. Moreover, ASIL1 specifically recognized a GT element that overlaps the G-box and is in close proximity to the RY repeats of the 2S promoters. We suggest that ASIL1 targets GT-box-containing embryonic genes by competing with the binding of transcriptional activators to this promoter region.

Identification and mapping of a third blackleg resistance locus in Brassica napus derived from B. rapa subsp. sylvestris.

The spectrum of resistance to isolates of Leptosphaeria maculans and the map location of a new blackleg resistance gene found in the canola cultivar Brassica napus 'Surpass 400' are described. Two blackleg resistance genes, LepR1 and LepR2, from B. rapa subsp. sylvestris and introgressed in B. napus were identified previously. 'Surpass 400' also has blackleg resistance introgressed from B. rapa subsp. sylvestris. Using 31 diverse isolates of L. maculans, the disease reaction of 'Surpass 400' was compared with those of the resistant breeding lines AD9 (which contains LepR1), AD49 (which contains LepR2), and MC1-8 (which contains both LepR1 and LepR2). The disease reaction on 'Surpass 400' was different from those observed on AD9 and MC1-8, indicating that 'Surpass 400' carries neither LepR1 nor both LepR1 and LepR2 in combination. Disease reactions of 'Surpass 400' to most of the isolates tested were indistinguishable from those of AD49, which suggested 'Surpass 400' might contain LepR2 or a similar resistance gene. Classical genetic analysis of F1 and BC1 plants showed that a dominant allele conferred resistance to isolates of L. maculans in 'Surpass 400'. The resistance gene, which mapped to B. napus linkage group N10 in an interval of 2.9 cM flanked by microsatellite markers sR12281a and sN2428Rb and 11.7 cM below LepR2, was designated LepR3. A 9 cM region of the B. napus genome containing LepR3 was found to be syntenic with a segment of Arabidopsis chromosome 5.

Enhancing the carotenoid content of Brassica napus seeds by downregulating lycopene epsilon cyclase.

The accumulation of carotenoids in higher plants is regulated by the environment, tissue type and developmental stage. In Brassica napus leaves, beta-carotene and lutein were the main carotenoids present while petals primarily accumulated lutein and violaxanthin. Carotenoid accumulation in seeds was developmentally regulated with the highest levels detected at 35-40 days post anthesis. The carotenoid biosynthesis pathway branches after the formation of lycopene. One branch forms carotenoids with two beta rings such as beta-carotene, zeaxanthin and violaxanthin, while the other introduces both beta- and epsilon-rings in lycopene to form alpha-carotene and lutein. By reducing the expression of lycopene epsilon-cyclase (epsilon-CYC) using RNAi, we investigated altering carotenoid accumulation in seeds of B. napus. Transgenic seeds expressing this construct had increased levels of beta-carotene, zeaxanthin, violaxanthin and, unexpectedly, lutein. The higher total carotenoid content resulting from reduction of epsilon-CYC expression in seeds suggests that this gene is a rate-limiting step in the carotenoid biosynthesis pathway. epsilon-CYC activity and carotenoid production may also be related to fatty acid biosynthesis in seeds as transgenic seeds showed an overall decrease in total fatty acid content and minor changes in the proportions of various fatty acids.

Characterization of a beta-carotene hydroxylase of Adonis aestivalis and its expression in Arabidopsis thaliana.

Carotenoids are plant secondary metabolites that comprise two main groups: carotenes and xanthophylls. The latter group includes zeaxanthin which is synthesized by beta-carotene hydroxylase catalyzing the hydroxylation of the beta-rings of beta-carotene molecules. To develop tools to alter carotenoid biosynthesis in plants, we isolated a cDNA clone encoding a candidate beta-carotene hydroxylase, CrtH1, from the flower petals of Adonis aestivalis. CrtH1 protein has homology to beta-carotene hydroxylases from other organisms, and possesses the four histidine motifs conserved in this family of enzymes. Sequence analysis predicted the presence of a putative plastid transit peptide at the amino terminus and four transmembrane helical regions. Southern-blot analysis showed CrtH1 to be encoded by a multicopy gene family with at least three members in A. aestivalis. Analysis of CrtH1 transcript abundance by Northern blotting indicates it is highly expressed in flower petals, roots and stems, with relatively low expression in leaves and developing seeds. CrtH1 was able to catalyze the formation of zeaxanthin and its intermediate precursor beta-cryptoxanthin from beta-carotene in functional assays conducted in E. coli. Expression of CrtH1 in Arabidopsis thaliana wild type and a mutant deficient for endogenous beta-carotene hydroxylases enhanced the biosynthesis of violaxanthin in the seeds.

Isolation and characterization of a GCN5-interacting protein from Arabidopsis thaliana.

An Arabidopsis protein, AtEML, was isolated based on its interaction with the histone acetyltransferase AtGCN5 in a yeast two-hybrid screen. RNA blot and RT-PCR analysis showed that AtEML is expressed in flowers, leaves, stems and siliques. The promoter region of AtEML has several cis-acting elements associated with response to biotic and abiotic stress conditions, and the accumulation of the AtEML transcript was found to be regulated by cold and salt treatments. In vitro and in vivo protein-protein interaction assays indicated that AtEML interacts with AtGCN5 through the N-terminal region. Furthermore, AtEML was shown to activate expression of the lacZ reporter gene in yeast through recruitment of AtGCN5. Such recruitment was accompanied by an increase in histone H3 acetylation at the promoter driving lacZ expression, as determined by chromatin immunoprecipitation. A higher level of AtEML gene expression was detected in the Arabidopsis gcn5 knockout mutant as compared to wild type Arabidopsis, indicating that AtEML expression is regulated by AtGCN5. These results suggest that AtEML may be a transcription factor that co-ordinates the expression of target stress regulated genes through involvement in recruiting AtGCN5 to their promoters.

Genetic control of broad-spectrum resistance to turnip mosaic virus in Brassica rapa (Chinese cabbage).

The Brassica rapa line RLR22 was resistant to eight diverse turnip mosaic virus (TuMV) isolates. A B. rapa genetic map based on 213 marker loci segregating in 120 first back-cross (B(1)) individuals was established and aligned with the B. rapa genome reference map using some of the RFLP probes. B(1) individuals were self-pollinated to produce B(1)S(1) families. The existence of two loci controlling resistance to TuMV isolate CDN 1 was established from contrasting patterns of segregation for resistance and susceptibility in the B(1)S(1) families. The first gene, recessive TuMV resistance 01 (retr01), had a recessive allele for resistance, was located on the upper portion of chromosome R4 and was epistatic to the second gene. The second gene, Conditional TuMV resistance 01 (ConTR01), possessed a dominant allele for resistance and was located on the upper portion of chromosome R8. These genes also controlled resistance to TuMV isolate CZE 1 and might be sufficient to explain the broad-spectrum resistance of RLR22. The dominant resistance gene, ConTR01, was coincident with one of the three eukaryotic initiation factor 4E (eIF4E) loci of B. rapa and possibly one of the loci of eIF(iso)4E. The recessive resistance gene retr01 was apparently coincident with one of the three loci of eIF(iso)4E in the A genome of Brassica napus and therefore, by inference, in the B. rapa genome. This suggested a mode of action for the resistance that is based on denying the viral RNA access to the translation initiation complex of the plant host. The gene retr01 is the first reported example of a recessive resistance gene mapped in a Brassica species.

The reference genetic linkage map for the multinational Brassica rapa genome sequencing project.

We describe the construction of a reference genetic linkage map for the Brassica A genome, which will form the backbone for anchoring sequence contigs for the Multinational Brassica rapa Genome Sequencing Project. Seventy-eight doubled haploid lines derived from anther culture of the F(1) of a cross between two diverse Chinese cabbage (B. rapa ssp. pekinensis) inbred lines, 'Chiifu-401-42' (C) and 'Kenshin-402-43' (K) were used to construct the map. The map comprises a total of 556 markers, including 278 AFLP, 235 SSR, 25 RAPD and 18 ESTP, STS and CAPS markers. Ten linkage groups were identified and designated as R1-R10 through alignment and orientation using SSR markers in common with existing B. napus reference linkage maps. The total length of the linkage map was 1,182 cM with an average interval of 2.83 cM between adjacent loci. The length of linkage groups ranged from 81 to 161 cM for R04 and R06, respectively. The use of 235 SSR markers allowed us to align the A-genome chromosomes of B. napus with those of B. rapa ssp. pekinensis. The development of this map is vital to the integration of genome sequence and genetic information and will enable the international research community to share resources and data for the improvement of B. rapa and other cultivated Brassica species.

New evidence from Sinapis alba L. for ancestral triplication in a crucifer genome.

We present clear evidence of ancestral genome triplication in Sinapis alba, a close relative of the cultivated Brassica species. Exceptionally high levels of heterozygosity in the parents of an F1 intercross permitted the mapping of an estimated 87% of all detected restriction fragment length polymorphism (RFLP) loci, with each RFLP probe typically detecting 2 or 3 loci. These duplicated loci were arranged in 8 triplicated homologous linkage blocks and 2 small, duplicated, homologous linkage blocks covering the majority of the S. alba genome. Several large-scale inversions and translocations appear to have rearranged the order of loci within homologous blocks. The role of successive polyploidization events on the evolution of crucifer species is discussed.

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.