Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few d... more Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few days after harvest. This limits shelf life of the product and results in high wastage. Recombinant inbred lines (RILs) derived from a cross between lettuce cvs. Saladin and Iceberg were shown to be suitable for genetic analysis of postharvest discolouration traits in lettuce. An intra-specific linkage map based on this population was generated to enable genetic analysis. A total of 424 markers were assigned to 18 linkage groups covering all nine chromosomes. The linkage map has a total length of 1,040 cM with an average marker distance of 2.4 cM within the linkage groups and was anchored to the ultra-dense, transcript-based consensus map. Significant genetic variation in the postharvest traits ‘pinking’, ‘browning’ and ‘overall discolouration’ was detected among the RILs. Seven significant quantitative trait loci (QTL) were identified for postharvest discolouration traits providing markers linked to the QTL that can be used for marker-assisted selection. Phenotypic stability was confirmed for extreme lines possessing the corresponding QTL parental alleles and which had shown transgressive segregation. This study indicates that a desired phenotype with reduced levels of postharvest discolouration can be achieved by breeding using natural variation.
Seed and seedling traits related to germination and stand establishment are important in the prod... more Seed and seedling traits related to germination and stand establishment are important in the production of cultivated lettuce (Lactuca sativa L.). Six seed and seedling traits segregating in a L. sativa cv. Salinas x L. serriola recombinant inbred line population consist-ing of 103 F8 families revealed a total of 17 significant quantitative trait loci (QTL) resulting from three seed production environments. Significant QTL were identi-fied for germination in darkness, germination at 25 and 35C, median maximum temperature of germination, hypocotyl length at 72 h post-imbibition, and plant (seedling) quality. Some QTL for germination and early seedling growth characteristics were co-located, sugges-tive of pleiotropic loci regulating these traits. A single QTL (Htg6.1) described 25 and 23 % of the total phe-notypic variation for high temperature germination in California- and Netherlands-grown populations, respectively, and was significant between 33 and 37C. Additionally, Htg6.1 show...
Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when ... more Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when imbibed at warm temperatures. Chemical mutagenesis was employed to develop lettuce lines that exhibit germination thermotolerance. Two independent thermotolerant lettuce seed mutant lines, TG01 and TG10, were generated through ethyl methanesulfonate mutagenesis. Genetic and physiological analyses indicated that these two mutations were allelic and recessive. To identify the causal gene(s), we applied bulked segregant analysis by whole genome sequencing. For each mutant, bulked DNA samples of segregating thermotolerant (mutant) seeds were sequenced and analyzed for homozygous singlenucleotide polymorphisms. Two independent candidate mutations were identified at different physical positions in the zeaxanthin epoxidase gene (ABSCISIC ACID DEFICIENT 1/ZEAXANTHIN EPOXIDASE, or ABA1/ZEP) in TG01 and TG10. The mutation in TG01 caused an amino acid replacement, whereas the mutation in TG10 resulted in alternative mRNA splicing. Endogenous abscisic acid contents were reduced in both mutants, and expression of the ABA1 gene from wild-type lettuce under its own promoter fully complemented the TG01 mutant. Conventional genetic mapping confirmed that the causal mutations were Accepted Article This article is protected by copyright. All rights reserved. located near the ZEP/ABA1 gene, but the bulked segregant whole genome sequencing approach more efficiently identified the specific gene responsible for the phenotype.
Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide... more Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide. Breeding for resistance to this disease is a major priority for most lettuce breeding programs. Many genes and factors for resistance to B. lactucae have been reported by multiple researchers over the past ~50 years. Their nomenclature has not been coordinated, resulting in duplications and gaps in nominations. We have reviewed the available information and rationalized it into 51 resistance genes and factors and 15 quantitative trait loci along with supporting documentation as well as genetic and molecular information. This involved multiple rounds of consultation with many of the original authors. This paper provides the foundation for naming additional genes for resistance to B. lactucae in the future as well as for deploying genes to provide more durable resistance.
The Plant journal : for cell and molecular biology, Jan 13, 2016
Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when ... more Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when imbibed at warm temperatures. Chemical mutagenesis was employed to develop lettuce lines that exhibit germination thermotolerance. Two independent thermotolerant lettuce seed mutant lines, TG01 and TG10, were generated through ethyl methanesulfonate mutagenesis. Genetic and physiological analyses indicated that these two mutations were allelic and recessive. To identify the causal gene(s), we applied bulked segregant analysis by whole genome sequencing. For each mutant, bulked DNA samples of segregating thermotolerant (mutant) seeds were sequenced and analyzed for homozygous single-nucleotide polymorphisms. Two independent candidate mutations were identified at different physical positions in the zeaxanthin epoxidase gene (ABSCISIC ACID DEFICIENT 1/ZEAXANTHIN EPOXIDASE, or ABA1/ZEP) in TG01 and TG10. The mutation in TG01 caused an amino acid replacement, whereas the mutation in TG10 resu...
Twenty recombinant inbred line "RIL# populations of European two! row spring barley and their par... more Twenty recombinant inbred line "RIL# populations of European two! row spring barley and their parents were tested in six environments in the Netherlands to investigate the prediction of progeny yield level\ yield variance\ stability level and stability variance\ based on parent information[ Progeny yield level is positively correlated with midparent value for average yield[ Progeny yield variance is more di.cult to predict\ but there does appear to be a promising negative correlation between progeny yield variance and Habgood|s "0866# parental simi! larity measure[ To quantify yield stability\ three statistics were cal! culated] Finlay and Wilkinson|s "0852# regression coe.cient b i \ Shukla|s "0861# stability variance s i 1 and Eberhart and Russell|s "0855# mean squared deviation d i 1 [ The _rst stability statistic describes a di}erent aspect of the response pattern to change in environment from the last two[ Parents with high b i values appear to have a better average yield\ i[e[ they react more positively to an improvement in the environment than the other genotypes[ The average b i value of the progeny is posi! tively correlated with the midparent value\ indicating its heritable nature[ There are also indications that d i 1 and s i 1 are heritable but their repeatability is poor[ Therefore\ it is concluded that only prediction of b i is useful in practical plant breeding[ There is a positive correlation between progeny yield variance and progeny variance for b i but we conclude that the inaccuracy of the stability variance estimates is too high for good predictors for progeny stability variance to be found[
Genebanks serve as a rich source of diversity that can be exploited for crop improvement. However... more Genebanks serve as a rich source of diversity that can be exploited for crop improvement. However, large numbers of accessions usually have to be evaluated to find material with the characters of interest, and therefore, enhanced trait information can facilitate the more efficient selection of accessions by users. In this study, we report on the distribution of resistances to 28 races of downy mildew among 1223 genebank accessions of cultivated lettuce (Lactuca sativaL.) and 14 related wild species. Due to modern plant breeding, the overall level of resistance of cultivars released after 1950 appears to have increased two- to three-fold compared with varieties from earlier periods. Although fully resistant reactions could be observed among the accessions of cultivated lettuce for each of the 28 investigated races, the resistance probability was more than two-fold higher on average for accessions from the wild gene pool. In general, species of the primary gene pool appeared less resistant than those of the secondary or tertiary gene pool. Probabilities for examinedLactucaspecies ranged from 0.29 forL. serriolato 1.00 forL. perenniscompared with 0.19 for cultivated lettuce, with lower overall resistance probabilities observed only forL. altaica,L. dregeanaandL. tenerrima. ForL. serriola, the closest relative of cultivated lettuce and the wild species with the highest number of examined accessions, resistance probabilities to each of the investigated downy mildew races were relatively high for populations originating from Eastern Europe and Northern Asia.
Twenty recombinant inbred line (RIL) populations of European two-row spring barley and their pare... more Twenty recombinant inbred line (RIL) populations of European two-row spring barley and their parents were tested in six environments in the Netherlands to investigate the prediction of progeny yield level, yield variance, stability level and stability variance, based on parent information. Progeny yield level is positively correlated with midparent value for average yield. Progeny yield variance is more difficult to predict, but there does appear to be a promising negative correlation between progeny yield variance and Habgood's (1977) parental similarity measure. To quantify yield stability, three statistics were calculated: Finlay and Wilkinson's (1963) regression coefficient bi, Shukla's (1972) stability variance i2 and Eberhart and Russell's (1966) mean squared deviation di2. The first stability statistic describes a different aspect of the response pattern to change in environment from the last two. Parents with high bi values appear to have a better average...
Seed and seedling traits related to germination and stand establishment are important in the prod... more Seed and seedling traits related to germination and stand establishment are important in the production of cultivated lettuce (Lactuca sativa L.). Six seed and seedling traits segregating in a L. sativa cv. Salinas x L. serriola recombinant inbred line population consisting of 103 F8 families revealed a total of 17 significant quantitative trait loci (QTL) resulting from three seed production environments. Significant QTL were identified for germination in darkness, germination at 25 and 35°C, median maximum temperature of germination, hypocotyl length at 72 h post-imbibition, and plant (seedling) quality. Some QTL for germination and early seedling growth characteristics were co-located, suggestive of pleiotropic loci regulating these traits. A single QTL (Htg6.1) described 25 and 23% of the total phenotypic variation for high temperature germination in California-and Netherlands-grown populations, respectively, and was significant between 33 and 37°C. Additionally, Htg6.1 showed significant epistatic interactions with other Htg QTL and a consistent effect across all the three seed production environments. L. serriola alleles increased germination at these QTL. The estimate of narrow-sense heritability (h 2 ) of Htg6.1 was 0.84, indicating potential for L. serriola as a source of germination thermotolerance for lettuce introgression programs.
Seed and seedling traits related to germination and stand establishment are important in the prod... more Seed and seedling traits related to germination and stand establishment are important in the production of cultivated lettuce (Lactuca sativa L.). Six seed and seedling traits segregating in a L. sativa cv. Salinas x L. serriola recombinant inbred line population consisting of 103 F8 families revealed a total of 17 significant quantitative trait loci (QTL) resulting from three seed production environments. Significant QTL were identified for germination in darkness, germination at 25 and 35°C, median maximum temperature of germination, hypocotyl length at 72 h post-imbibition, and plant (seedling) quality. Some QTL for germination and early seedling growth characteristics were co-located, suggestive of pleiotropic loci regulating these traits. A single QTL (Htg6.1) described 25 and 23% of the total phenotypic variation for high temperature germination in California-and Netherlands-grown populations, respectively, and was significant between 33 and 37°C. Additionally, Htg6.1 showed significant epistatic interactions with other Htg QTL and a consistent effect across all the three seed production environments. L. serriola alleles increased germination at these QTL. The estimate of narrow-sense heritability (h 2 ) of Htg6.1 was 0.84, indicating potential for L. serriola as a source of germination thermotolerance for lettuce introgression programs.
Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few d... more Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few days after harvest. This limits shelf life of the product and results in high wastage. Recombinant inbred lines (RILs) derived from a cross between lettuce cvs. Saladin and Iceberg were shown to be suitable for genetic analysis of postharvest discolouration traits in lettuce. An intra-specific linkage map based on this population was generated to enable genetic analysis. A total of 424 markers were assigned to 18 linkage groups covering all nine chromosomes. The linkage map has a total length of 1,040 cM with an average marker distance of 2.4 cM within the linkage groups and was anchored to the ultra-dense, transcript-based consensus map. Significant genetic variation in the postharvest traits ‘pinking’, ‘browning’ and ‘overall discolouration’ was detected among the RILs. Seven significant quantitative trait loci (QTL) were identified for postharvest discolouration traits providing markers linked to the QTL that can be used for marker-assisted selection. Phenotypic stability was confirmed for extreme lines possessing the corresponding QTL parental alleles and which had shown transgressive segregation. This study indicates that a desired phenotype with reduced levels of postharvest discolouration can be achieved by breeding using natural variation.
Breeding lettuce (Lactuca sativa) for resistance to the downy mildew pathogen Bremia lactucae is ... more Breeding lettuce (Lactuca sativa) for resistance to the downy mildew pathogen Bremia lactucae is mainly achieved by introgression of dominant downy mildew resistance (Dm) genes. New Bremia races quickly render Dm genes ineffective, possibly by mutation of recognized host-translocated effectors or by suppression of effector-triggered immunity. We have previously identified 34 potential RXLR(-like) effector proteins of B. lactucae that were here tested for specific recognition within a collection of 129 B. lactucae-resistant Lactuca lines. Two effectors triggered a hypersensitive response: BLG01 in 52 lines, predominantly L. saligna, and BLG03 in two L. sativa lines containing Dm2 resistance. The N-terminal sequences of BLG01 and BLG03, containing the signal peptide and GKLR variant of the RXLR translocation motif, are not required for in planta recognition but function in effector delivery. The locus responsible for BLG01 recognition maps to the bottom of lettuce chromosome 9, whereas recognition of BLG03 maps in the RGC2 cluster on chromosome 2. Lactuca lines that recognize the BLG effectors are not resistant to Bremia isolate Bl:24 that expresses both BLG genes, suggesting that Bl:24 can suppress the triggered immune responses. In contrast, lettuce segregants displaying Dm2-mediated resistance to Bremia isolate Bl:5 are responsive to BLG03, suggesting that BLG03 is a candidate Avr2 protein.
Reverse breeding (RB) is a novel plant breeding technique designed to directly produce parental l... more Reverse breeding (RB) is a novel plant breeding technique designed to directly produce parental lines for any heterozygous plant, one of the most sought after goals in plant breeding. RB generates perfectly complementing homozygous parental lines through engineered meiosis. The method is based on reducing genetic recombination in the selected heterozygote by eliminating meiotic crossing over. Male or female spores obtained from such plants contain combinations of non-recombinant parental chromosomes which can be cultured in vitro to generate homozygous doubled haploid plants (DHs). From these DHs, complementary parents can be selected and used to reconstitute the heterozygote in perpetuity. Since the fixation of unknown heterozygous genotypes is impossible in traditional plant breeding, RB could fundamentally change future plant breeding. In this review, we discuss various other applications of RB, including breeding per chromosome. 11 98 111 138 377 Banana, watermelon, celery, fennel, common bean 12 138 157 195 532 Tomato, pepper, melon, rice, egg plant ª
Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few d... more Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few days after harvest. This limits shelf life of the product and results in high wastage. Recombinant inbred lines (RILs) derived from a cross between lettuce cvs. Saladin and Iceberg were shown to be suitable for genetic analysis of postharvest discolouration traits in lettuce. An intra-specific linkage map based on this population was generated to enable genetic analysis. A total of 424 markers were assigned to 18 linkage groups covering all nine chromosomes. The linkage map has a total length of 1,040 cM with an average marker distance of 2.4 cM within the linkage groups and was anchored to the ultra-dense, transcript-based consensus map. Significant genetic variation in the postharvest traits ‘pinking’, ‘browning’ and ‘overall discolouration’ was detected among the RILs. Seven significant quantitative trait loci (QTL) were identified for postharvest discolouration traits providing markers linked to the QTL that can be used for marker-assisted selection. Phenotypic stability was confirmed for extreme lines possessing the corresponding QTL parental alleles and which had shown transgressive segregation. This study indicates that a desired phenotype with reduced levels of postharvest discolouration can be achieved by breeding using natural variation.
Seed and seedling traits related to germination and stand establishment are important in the prod... more Seed and seedling traits related to germination and stand establishment are important in the production of cultivated lettuce (Lactuca sativa L.). Six seed and seedling traits segregating in a L. sativa cv. Salinas x L. serriola recombinant inbred line population consist-ing of 103 F8 families revealed a total of 17 significant quantitative trait loci (QTL) resulting from three seed production environments. Significant QTL were identi-fied for germination in darkness, germination at 25 and 35C, median maximum temperature of germination, hypocotyl length at 72 h post-imbibition, and plant (seedling) quality. Some QTL for germination and early seedling growth characteristics were co-located, sugges-tive of pleiotropic loci regulating these traits. A single QTL (Htg6.1) described 25 and 23 % of the total phe-notypic variation for high temperature germination in California- and Netherlands-grown populations, respectively, and was significant between 33 and 37C. Additionally, Htg6.1 show...
Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when ... more Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when imbibed at warm temperatures. Chemical mutagenesis was employed to develop lettuce lines that exhibit germination thermotolerance. Two independent thermotolerant lettuce seed mutant lines, TG01 and TG10, were generated through ethyl methanesulfonate mutagenesis. Genetic and physiological analyses indicated that these two mutations were allelic and recessive. To identify the causal gene(s), we applied bulked segregant analysis by whole genome sequencing. For each mutant, bulked DNA samples of segregating thermotolerant (mutant) seeds were sequenced and analyzed for homozygous singlenucleotide polymorphisms. Two independent candidate mutations were identified at different physical positions in the zeaxanthin epoxidase gene (ABSCISIC ACID DEFICIENT 1/ZEAXANTHIN EPOXIDASE, or ABA1/ZEP) in TG01 and TG10. The mutation in TG01 caused an amino acid replacement, whereas the mutation in TG10 resulted in alternative mRNA splicing. Endogenous abscisic acid contents were reduced in both mutants, and expression of the ABA1 gene from wild-type lettuce under its own promoter fully complemented the TG01 mutant. Conventional genetic mapping confirmed that the causal mutations were Accepted Article This article is protected by copyright. All rights reserved. located near the ZEP/ABA1 gene, but the bulked segregant whole genome sequencing approach more efficiently identified the specific gene responsible for the phenotype.
Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide... more Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide. Breeding for resistance to this disease is a major priority for most lettuce breeding programs. Many genes and factors for resistance to B. lactucae have been reported by multiple researchers over the past ~50 years. Their nomenclature has not been coordinated, resulting in duplications and gaps in nominations. We have reviewed the available information and rationalized it into 51 resistance genes and factors and 15 quantitative trait loci along with supporting documentation as well as genetic and molecular information. This involved multiple rounds of consultation with many of the original authors. This paper provides the foundation for naming additional genes for resistance to B. lactucae in the future as well as for deploying genes to provide more durable resistance.
The Plant journal : for cell and molecular biology, Jan 13, 2016
Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when ... more Lettuce (Lactuca sativa) seeds exhibit thermoinhibition, or failure to complete germination when imbibed at warm temperatures. Chemical mutagenesis was employed to develop lettuce lines that exhibit germination thermotolerance. Two independent thermotolerant lettuce seed mutant lines, TG01 and TG10, were generated through ethyl methanesulfonate mutagenesis. Genetic and physiological analyses indicated that these two mutations were allelic and recessive. To identify the causal gene(s), we applied bulked segregant analysis by whole genome sequencing. For each mutant, bulked DNA samples of segregating thermotolerant (mutant) seeds were sequenced and analyzed for homozygous single-nucleotide polymorphisms. Two independent candidate mutations were identified at different physical positions in the zeaxanthin epoxidase gene (ABSCISIC ACID DEFICIENT 1/ZEAXANTHIN EPOXIDASE, or ABA1/ZEP) in TG01 and TG10. The mutation in TG01 caused an amino acid replacement, whereas the mutation in TG10 resu...
Twenty recombinant inbred line "RIL# populations of European two! row spring barley and their par... more Twenty recombinant inbred line "RIL# populations of European two! row spring barley and their parents were tested in six environments in the Netherlands to investigate the prediction of progeny yield level\ yield variance\ stability level and stability variance\ based on parent information[ Progeny yield level is positively correlated with midparent value for average yield[ Progeny yield variance is more di.cult to predict\ but there does appear to be a promising negative correlation between progeny yield variance and Habgood|s "0866# parental simi! larity measure[ To quantify yield stability\ three statistics were cal! culated] Finlay and Wilkinson|s "0852# regression coe.cient b i \ Shukla|s "0861# stability variance s i 1 and Eberhart and Russell|s "0855# mean squared deviation d i 1 [ The _rst stability statistic describes a di}erent aspect of the response pattern to change in environment from the last two[ Parents with high b i values appear to have a better average yield\ i[e[ they react more positively to an improvement in the environment than the other genotypes[ The average b i value of the progeny is posi! tively correlated with the midparent value\ indicating its heritable nature[ There are also indications that d i 1 and s i 1 are heritable but their repeatability is poor[ Therefore\ it is concluded that only prediction of b i is useful in practical plant breeding[ There is a positive correlation between progeny yield variance and progeny variance for b i but we conclude that the inaccuracy of the stability variance estimates is too high for good predictors for progeny stability variance to be found[
Genebanks serve as a rich source of diversity that can be exploited for crop improvement. However... more Genebanks serve as a rich source of diversity that can be exploited for crop improvement. However, large numbers of accessions usually have to be evaluated to find material with the characters of interest, and therefore, enhanced trait information can facilitate the more efficient selection of accessions by users. In this study, we report on the distribution of resistances to 28 races of downy mildew among 1223 genebank accessions of cultivated lettuce (Lactuca sativaL.) and 14 related wild species. Due to modern plant breeding, the overall level of resistance of cultivars released after 1950 appears to have increased two- to three-fold compared with varieties from earlier periods. Although fully resistant reactions could be observed among the accessions of cultivated lettuce for each of the 28 investigated races, the resistance probability was more than two-fold higher on average for accessions from the wild gene pool. In general, species of the primary gene pool appeared less resistant than those of the secondary or tertiary gene pool. Probabilities for examinedLactucaspecies ranged from 0.29 forL. serriolato 1.00 forL. perenniscompared with 0.19 for cultivated lettuce, with lower overall resistance probabilities observed only forL. altaica,L. dregeanaandL. tenerrima. ForL. serriola, the closest relative of cultivated lettuce and the wild species with the highest number of examined accessions, resistance probabilities to each of the investigated downy mildew races were relatively high for populations originating from Eastern Europe and Northern Asia.
Twenty recombinant inbred line (RIL) populations of European two-row spring barley and their pare... more Twenty recombinant inbred line (RIL) populations of European two-row spring barley and their parents were tested in six environments in the Netherlands to investigate the prediction of progeny yield level, yield variance, stability level and stability variance, based on parent information. Progeny yield level is positively correlated with midparent value for average yield. Progeny yield variance is more difficult to predict, but there does appear to be a promising negative correlation between progeny yield variance and Habgood's (1977) parental similarity measure. To quantify yield stability, three statistics were calculated: Finlay and Wilkinson's (1963) regression coefficient bi, Shukla's (1972) stability variance i2 and Eberhart and Russell's (1966) mean squared deviation di2. The first stability statistic describes a different aspect of the response pattern to change in environment from the last two. Parents with high bi values appear to have a better average...
Seed and seedling traits related to germination and stand establishment are important in the prod... more Seed and seedling traits related to germination and stand establishment are important in the production of cultivated lettuce (Lactuca sativa L.). Six seed and seedling traits segregating in a L. sativa cv. Salinas x L. serriola recombinant inbred line population consisting of 103 F8 families revealed a total of 17 significant quantitative trait loci (QTL) resulting from three seed production environments. Significant QTL were identified for germination in darkness, germination at 25 and 35°C, median maximum temperature of germination, hypocotyl length at 72 h post-imbibition, and plant (seedling) quality. Some QTL for germination and early seedling growth characteristics were co-located, suggestive of pleiotropic loci regulating these traits. A single QTL (Htg6.1) described 25 and 23% of the total phenotypic variation for high temperature germination in California-and Netherlands-grown populations, respectively, and was significant between 33 and 37°C. Additionally, Htg6.1 showed significant epistatic interactions with other Htg QTL and a consistent effect across all the three seed production environments. L. serriola alleles increased germination at these QTL. The estimate of narrow-sense heritability (h 2 ) of Htg6.1 was 0.84, indicating potential for L. serriola as a source of germination thermotolerance for lettuce introgression programs.
Seed and seedling traits related to germination and stand establishment are important in the prod... more Seed and seedling traits related to germination and stand establishment are important in the production of cultivated lettuce (Lactuca sativa L.). Six seed and seedling traits segregating in a L. sativa cv. Salinas x L. serriola recombinant inbred line population consisting of 103 F8 families revealed a total of 17 significant quantitative trait loci (QTL) resulting from three seed production environments. Significant QTL were identified for germination in darkness, germination at 25 and 35°C, median maximum temperature of germination, hypocotyl length at 72 h post-imbibition, and plant (seedling) quality. Some QTL for germination and early seedling growth characteristics were co-located, suggestive of pleiotropic loci regulating these traits. A single QTL (Htg6.1) described 25 and 23% of the total phenotypic variation for high temperature germination in California-and Netherlands-grown populations, respectively, and was significant between 33 and 37°C. Additionally, Htg6.1 showed significant epistatic interactions with other Htg QTL and a consistent effect across all the three seed production environments. L. serriola alleles increased germination at these QTL. The estimate of narrow-sense heritability (h 2 ) of Htg6.1 was 0.84, indicating potential for L. serriola as a source of germination thermotolerance for lettuce introgression programs.
Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few d... more Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few days after harvest. This limits shelf life of the product and results in high wastage. Recombinant inbred lines (RILs) derived from a cross between lettuce cvs. Saladin and Iceberg were shown to be suitable for genetic analysis of postharvest discolouration traits in lettuce. An intra-specific linkage map based on this population was generated to enable genetic analysis. A total of 424 markers were assigned to 18 linkage groups covering all nine chromosomes. The linkage map has a total length of 1,040 cM with an average marker distance of 2.4 cM within the linkage groups and was anchored to the ultra-dense, transcript-based consensus map. Significant genetic variation in the postharvest traits ‘pinking’, ‘browning’ and ‘overall discolouration’ was detected among the RILs. Seven significant quantitative trait loci (QTL) were identified for postharvest discolouration traits providing markers linked to the QTL that can be used for marker-assisted selection. Phenotypic stability was confirmed for extreme lines possessing the corresponding QTL parental alleles and which had shown transgressive segregation. This study indicates that a desired phenotype with reduced levels of postharvest discolouration can be achieved by breeding using natural variation.
Breeding lettuce (Lactuca sativa) for resistance to the downy mildew pathogen Bremia lactucae is ... more Breeding lettuce (Lactuca sativa) for resistance to the downy mildew pathogen Bremia lactucae is mainly achieved by introgression of dominant downy mildew resistance (Dm) genes. New Bremia races quickly render Dm genes ineffective, possibly by mutation of recognized host-translocated effectors or by suppression of effector-triggered immunity. We have previously identified 34 potential RXLR(-like) effector proteins of B. lactucae that were here tested for specific recognition within a collection of 129 B. lactucae-resistant Lactuca lines. Two effectors triggered a hypersensitive response: BLG01 in 52 lines, predominantly L. saligna, and BLG03 in two L. sativa lines containing Dm2 resistance. The N-terminal sequences of BLG01 and BLG03, containing the signal peptide and GKLR variant of the RXLR translocation motif, are not required for in planta recognition but function in effector delivery. The locus responsible for BLG01 recognition maps to the bottom of lettuce chromosome 9, whereas recognition of BLG03 maps in the RGC2 cluster on chromosome 2. Lactuca lines that recognize the BLG effectors are not resistant to Bremia isolate Bl:24 that expresses both BLG genes, suggesting that Bl:24 can suppress the triggered immune responses. In contrast, lettuce segregants displaying Dm2-mediated resistance to Bremia isolate Bl:5 are responsive to BLG03, suggesting that BLG03 is a candidate Avr2 protein.
Reverse breeding (RB) is a novel plant breeding technique designed to directly produce parental l... more Reverse breeding (RB) is a novel plant breeding technique designed to directly produce parental lines for any heterozygous plant, one of the most sought after goals in plant breeding. RB generates perfectly complementing homozygous parental lines through engineered meiosis. The method is based on reducing genetic recombination in the selected heterozygote by eliminating meiotic crossing over. Male or female spores obtained from such plants contain combinations of non-recombinant parental chromosomes which can be cultured in vitro to generate homozygous doubled haploid plants (DHs). From these DHs, complementary parents can be selected and used to reconstitute the heterozygote in perpetuity. Since the fixation of unknown heterozygous genotypes is impossible in traditional plant breeding, RB could fundamentally change future plant breeding. In this review, we discuss various other applications of RB, including breeding per chromosome. 11 98 111 138 377 Banana, watermelon, celery, fennel, common bean 12 138 157 195 532 Tomato, pepper, melon, rice, egg plant ª
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