Tomato: a crop species amenable to improvement by cellular and molecular methods

Tomato is a crop plant with a relatively small DNA content per haploid genome and a well developed genetics. Plant regeneration from explants and protoplasts is feasable which led to the development of efficient transformation procedures. In view of the current data, the isolation of useful mutants at the cellular level probably will be of limited value in the genetic improvement of tomato. Protoplast fusion may lead to novel combinations of organelle and nuclear DNA (cybrids), whereas this technique also provides a means of introducing genetic information from alien species into tomato. Important developments have come from molecular approaches. Following the construction of an RFLP map, these RFLP markers can be used in tomato to tag quantitative traits bred in from related species. Both RFLP's and transposons are in the process of being used to clone desired genes for which no gene products are known. Cloned genes can be introduced and potentially improve specific properties of tomato especially those controlled by single genes. Recent results suggest that, in principle, phenotypic mutants can be created for cloned and characterized genes and will prove their value in further improving the cultivated tomato.

Integrated genetic map and genetic analysis of a region associated with root traits on the short arm of rye chromosome 1 in bread wheat

A rye–wheat centric chromosome translocation 1RS.1BL has been widely used in wheat breeding programs around the world. Increased yield of translocation lines was probably a consequence of increased root biomass. In an effort to map loci-controlling root characteristics, homoeologous recombinants of 1RS with 1BS were used to generate a consensus genetic map comprised of 20 phenotypic and molecular markers, with an average spacing of 2.5 cM. Physically, all recombination events were located in the distal 40% of the arms. A total of 68 recombinants was used and recombination breakpoints were aligned and ordered over map intervals with all the markers, integrated together in a genetic map. This approach enabled dissection of genetic components of quantitative traits, such as root traits, present on 1S. To validate our hypothesis, phenotyping of 45-day-old wheat roots was performed in five lines including three recombinants representative of the entire short arm along with bread wheat parents ‘Pavon 76’ and Pavon 1RS.1BL. Individual root characteristics were ranked and the genotypic rank sums were subjected to Quade analysis to compare the overall rooting ability of the genotypes. It appears that the terminal 15% of the rye 1RS arm carries gene(s) for greater rooting ability in wheat

Nomad DNA — A model for movement and duplication of DNA sequences in plant genomes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43425/1/11103_2004_Article_BF00019160.pd

Inheritance of isozyme and RFLP markers in Brassica campestris and comparison with B. oleracea

Using primarily cDNA restriction fragment length polymorphism markers (RFLPs) previously located to Brassica oleracea (cabbage, 2n=18) chromosomes, we initiated a comparative RFLP map in an F 2 population of B. campestris (turnip x mock pak-choi, 2n=20). As with B. oleracea , the genome of B. campestris showed extensive gene duplication, and the majority of detected duplicated loci were unlinked. Only 6 of the 49 identified loci were represented as a single copy, and 3 of these 6 were clustered on a single linkage group showing a distorted segregation ratio. Comparison with B. Oleracea indicates this synteny is conserved between species. Two other linkage groups also appeared syntenic between B. oleracea and B. campestris . One single copy locus appears to have changed synteny between B. oleracea and B. campestris . These observations suggest that B. oleracea and B. campestris share a common ancestor, but that chromosome repatterning has occurred during or after speciation. Within B. campestris , 5 loci appeared duplicated in one parent or the other, and 2 of these were linked. Differentiation through subspecies-specific duplication or deletion events is suggested as one mechansim for the evolution of numerous morphotypes within each of these species.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46007/1/122_2004_Article_BF00227309.pd

A Single Molecule Scaffold for the Maize Genome

About 85% of the maize genome consists of highly repetitive sequences that are interspersed by low-copy, gene-coding sequences. The maize community has dealt with this genomic complexity by the construction of an integrated genetic and physical map (iMap), but this resource alone was not sufficient for ensuring the quality of the current sequence build. For this purpose, we constructed a genome-wide, high-resolution optical map of the maize inbred line B73 genome containing >91,000 restriction sites (averaging 1 site/∼23 kb) accrued from mapping genomic DNA molecules. Our optical map comprises 66 contigs, averaging 31.88 Mb in size and spanning 91.5% (2,103.93 Mb/∼2,300 Mb) of the maize genome. A new algorithm was created that considered both optical map and unfinished BAC sequence data for placing 60/66 (2,032.42 Mb) optical map contigs onto the maize iMap. The alignment of optical maps against numerous data sources yielded comprehensive results that proved revealing and productive. For example, gaps were uncovered and characterized within the iMap, the FPC (fingerprinted contigs) map, and the chromosome-wide pseudomolecules. Such alignments also suggested amended placements of FPC contigs on the maize genetic map and proactively guided the assembly of chromosome-wide pseudomolecules, especially within complex genomic regions. Lastly, we think that the full integration of B73 optical maps with the maize iMap would greatly facilitate maize sequence finishing efforts that would make it a valuable reference for comparative studies among cereals, or other maize inbred lines and cultivars

The maize Dwarf3 gene encodes a cytochrome P450-mediated early step in Gibberellin biosynthesis.

Gibberellins (GAs) are phytohormones required for normal growth and development in higher plants. The Dwarf3 (D3) gene of maize encodes an early step in the GA biosynthesis pathway. We transposon-tagged the D3 gene using Robertson's Mutator (Mu) and showed that the mutant allele d3.2::Mu8 is linked to a Mu8 element. The DNA flanking the Mu8 element was cloned and shown to be linked to the d3 locus by mapping in a high-resolution population developed by selecting for recombination between d3 and linked genetic markers. To establish unambiguously the identity of the cloned gene as D3, a second mutant allele of D3 (d3.4) was also cloned and characterized using the d3.2::Mu8 sequences as a probe. d3.4 was found to have a novel insertion element, named Sleepy, inserted into an exon. A third mutant allele, d3.1, which has the same size 3' restriction fragments as d3.4 but different 5' restriction fragments, was found to contain a Sleepy insertion at the same position as d3.4. On the basis of the pedigree, Sleepy insertion, and restriction map, d3.1 appears to represent a recombinational derivative of d3.4. The D3 gene encodes a predicted protein with significant sequence similarity to cytochrome P450 enzymes. Analysis of D3 mRNA showed that the D3 transcript is expressed in roots, developing leaves, the vegetative meristem, and suspension culture cells. We detected reduced D3 mRNA levels in the mutant allele d3.5