Evidence of epistatic suppression of repeat fruiting in cultivated strawberry.

BACKGROUND: Consumers purchase fresh strawberries all year long. Extending the fruiting season for new strawberry cultivars is a common breeding goal. Understanding the inheritance of repeat fruiting is key to improving breeding efficiency. Several independent research groups using multiple genotypes and analytic approaches have all identified a single genomic region in strawberry associated with repeat fruiting. Markers mapped to this region were used to evaluate breeding parents from the United States Department of Agriculture - Agricultural Research Service (USDA-ARS) strawberry breeding program at Beltsville, Maryland. RESULTS: Markers mapped to repeat fruiting identified once-fruiting genotypes but not repeat-fruiting genotypes. Eleven of twenty-three breeding parents with repeat-fruiting marker profiles were actually once fruiting, indicating at least one additional locus acting epistatically to suppress repeat fruiting. Family segregation ratios could not be predicted reliably by the combined use of parental phenotypes and marker profiles, when using a single-gene model. Expected segregation ratios were calculated for all phenotypic and marker-profile combinations possible from the mapped locus combined with a hypothetical dominant or recessive suppressor locus. Segregation ratios specific to an epistatic suppressor acting on the mapped locus were observed in four families. The segregation ratios for two families were best explained by a dominant suppressor acting on the mapped locus, and, for the other two, by a recessive suppressor. Not all of the observed ratios could be explained by one model or the other, and when multiple families with a common parent were compared, there was no predicted genotype for the common parent that would lead to all of the observed segregation ratios. CONCLUSIONS: Considering all lines of evidence in this study and others, repeat-fruiting in commercial strawberry is controlled primarily by a dominant allele at a single locus, previously mapped by multiple groups. At least two additional genes, one dominant and one recessive, exist that act epistatically to suppress repeat fruiting. Environmental effects and/or incomplete penetrance likely affect phenotype through the suppressor loci, rather than the primary mapped locus. One of the dominant suppressors acts only in the first year, the year the plant is germinated from seed, and not after the plant has experienced a winter.

GUS expression in blueberry (Vaccinium spp.): factors influencing Agrobacterium-mediated gene transfer efficiency.

Several factors were investigated for their influence on the transfer of an intron-containing ß-glucuronidase (GUS) gene into blueberry (Vaccinium spp.) leaf explants during the early stages of Agrobacterium-mediated gene transfer, including days of cocultivation, strain of Agrobacterium tumefaciens, explant age and genotype. The number of GUS-expressing leaf zones and calli were counted immediately and 2 weeks after cocultivation, respectively, to evaluate the gene transfer process. Agrobacterium tumefaciens strain EHA105 (pEHA105/p35SGUS-int) was significantly more effective for transformation than strain LBA4404 (pAL4404/p35SGUSint). Four days of cocultivation with A. tumefaciens strain EHA105 yielded about 50-fold more GUS-expressing zones than 2 days of cocultivation. Significant differences among cultivars were observed for both GUS-expressing leaf zones and calli. For some cultivars, explant age influenced the number of GUS-expressing leaf zones and calli. In most cases, the number of GUS-expressing calli was highest in those cultivars where GUS expression in the leaves was high.

Identification of dehydrin-like proteins responsive to chilling in floral buds of blueberry (Vaccinium, section Cyanococcus).

The level of three major polypeptides of 65, 60, and 14 kD increased in response to chilling unit accumulation in floral buds of a woody perennial, blueberry (Vaccinium, section Cynaococcus). The level of the polypeptides increased most dramatically within 300 h of chilling and decreased to the prechilling level with the initiation of budbreak. Cold-hardiness levels were assessed for dormant buds of Vaccinium corymbosum and Vaccinium ashei after different chilling treatments until the resumption of growth. These levels coincided with the level of the chilling-responsive polypeptides. Like some other previously described cold-induced proteins in annual plants, the level of the chilling-induced polypeptides also increased in leaves in response to cold treatment; the chilling-induced polypeptides were heat stable, resisting aggregation after incubation at 95 degrees C for 15 min. By fractionating bud proteins first by isoelectric point (pI) and then by molecular mass, the pI values of the 65- and 60-kD polypeptides were found to be 7.5 to 8.0 and the pI value of the 14-kD polypeptide was judged to be 8.5. Purification of the 65- and 60-kD polypeptides, followed by digestion with endoproteinase Lys-C and sequencing of selected fragments, revealed similarities in amino acid composition between the 65- and 60-kD polypeptides and dehydrins. Indeed, antiserum to the lysine-rich consensus sequence EKKGIMDKIKEKLPG of dehydrin proteins cross-reacted to all three of the major chilling-responsive polypeptides of blueberry, identifying these as dehydrins or dehydrin-like proteins.

RAPD-based genetic linkage map of blueberry derived from a cross between diploid species (Vaccinium darrowi and V. elliottii).

An initial genetic linkage map for blueberry has been constructed from over 70 random amplified polymorphic DNA (RAPD) markers that segregated 1∶1 in a testcross population of 38 plants. The mapping population was derived from a cross between two diploid blueberry plants: the F1 interspecific hybrid (Vaccinium darrowi Camp x V. elliottii Chapm.) and another V. darrowi plant. The map currently comprises 12 linkage groups (in agreement with the basic blueberry chromosome number) and covers a total genetic distance of over 950cM, with a range of 3-30cM between adjacent markers. The use of such a map for identifying molecular markers linked to genes controlling chilling requirement and cold hardiness is discussed.

Chromosome breakage undetectable in active Mu lines of maize.

We have used a set of Mutator-induced mutants of Bz1 to test whether members of the Mutator (Mu) family of maize transposable elements produce broken chromosomes. From our inability to demonstrate the simultaneous loss of two dominant endosperm markers distal to Mu insertions at Bz1 we conclude that either Mu, unlike many elements of the Ds family, does not induce such breaks, or it does so at a very low frequency.

Insertion of Mu into the Shrunken 1 gene of maize affects transcriptional and post-transcriptional regulation of Sh1 RNA.

Insertion of a Mu transposable element at the Shrunken 1 (Sh1) locus of maize has resulted in kernels with the typical collapsed appearance of sh mutants. Molecular analysis of the mutant gene has revealed the presence of a 1.4 kb insertion immediately upstream from the normal transcriptional start site. Mu insertion has brought about a series of changes in gene expression: the mRNA cap site has been shifted downstream so that it now lies inside the Mu element; transcription is reduced approximately sixfold, and the sh mRNA steady-state level is less than 4% of that found in the nonmutant. This disparity reflects a mutational defect in post-transcriptional regulation which is manifested as a decrease in Sh RNA abundance.

Anaerobic treatment of maize roots affects transcription of Adh1 and transcript stability.

Among the adaptations to stress exhibited by plants is the anaerobic response of roots, induced by submerging roots in water. The response consists of a programmed change in gene expression: proteins produced under aerobic conditions are no longer synthesized but are replaced by approximately 20 so-called anaerobic peptides (M. M. Sachs, M. Freeling, and R. Okimoto, Cell 20:761-767, 1980). The gene for maize alcohol dehydrogenase I (Adh1) is expressed at high levels under such conditions. We report here that changes in alcohol dehydrogenase I RNA levels in anaerobic roots are associated with changes in both transcription rate and transcript stability.

Insertion of an unstable element in an intervening sequence of maize Adh1 affects transcription but not processing.

Three independently isolated and unstable mutants of the maize alcohol dehydrogenase 1 gene (Adh1) have arisen by insertion of the Mu transposing element into the first intervening sequence of the progenitor Adh1 allele. The mutants have been selected for their decreased levels of alcohol dehydrogenase 1 activity. The original mutants were unstable, giving rise to both revertant alleles and altered mutant alleles. From one of the original mutants, two derivative mutants have been recovered and described. We analyzed the effect of Mu insertion in all five of these mutants by measuring relative levels of run-off transcripts from the progenitor and mutant alleles and by comparing levels of run-off transcripts corresponding to regions lying 5' and 3' to the insertion sites. In this paper we present evidence that early transcriptional events are affected, but that, in spite of the inclusion of a 1.4-kilobase transposing DNA element, processing of transcripts occurs normally.