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.

Molecular characterization and genetic mapping of DNA sequences encoding the Type I chlorophyll a/b-binding polypeptide of photosystem I in Lycopersicon esculentum (tomato)

We report the isolation and characterization of a tomato nuclear gene encoding a chlorophyll a/b-binding (CAB) protein of photosystem I (PSI). The coding nucleotide sequence of the gene, designated Cab -6B, is different at eight positions from that of a previously isolated cDNA clone derived from the Cab -6A gene, but the two genes encode identical proteins. Sequence comparison with the cDNA clone revealed the presence of three short introns in Cab -6B. Genetic mapping experiments demonstrate that Cab -6A and Cab -6B are tightly linked and reside on chromosome 5, but the physical distance between the two genes is at least 7 kilobases. Cab -6A and Cab -6B have been designated Type I PSI CAB genes. They are the only two genes of this branch of the CAB gene family in the tomato genome, and they show substantial divergence to the genes encoding CAB polypeptides of photosystem II. The Type I PSI CAB genes, like the genes encoding PSII CAB proteins, are highly expressed in illuminated leaf tissue and to a lesser extent in other green organs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43459/1/11103_2004_Article_BF00166457.pd

A sex-chromosome mutation in Silene latifolia

Silene latifolia is dioecious, yet rare hermaphrodites have been found, and such natural mutants can provide valuable insight into genetic mechanisms. Here, we describe a hermaphrodite-inducing mutation that is almost certainly localized to the gynoecium-suppression region of the Y chromosome in S. latifolia. The mutant Y chromosome was passed through the megaspore, and the presence of two X chromosomes was not necessary for seed development in the parent. This result supports a lack of degeneration of the Y chromosome in S. latifolia, consistent with the relatively recent formation of the sex chromosomes in this species. When crossed to wild-type plants, hermaphrodites performed poorly as females, producing low seed numbers. When hermaphrodites were pollen donors, the sex ratio of offspring they produced through crosses was biased towards females. This suggests that hermaphroditic S. latifolia would fail to thrive and potentially explains the rarity of hermaphrodites in natural populations of S. latifolia. These results indicate that the Y chromosome in Silene latifolia remains very similar to the X, perhaps mostly differing in the primary sex determination regions

The tomato Cab -4 and Cab -5 genes encode a second type of CAB polypeptides localized in Photosystem II

The photosynthetic apparatus of plant chloroplasts contains two photosystems, termed Photosystem I (PSI) and Photosystem II (PSII). Both PSI and PSII contain several types of chlorophyll a/b-binding (CAB) polypeptides, at least some of which are structurally related. It has been previously shown that multiple genes encoding one type of PSII CAB polypeptides exist in the genome of many higher plants. In tomato, there are at least eight such genes, distributed in three independent loci. Genes encoding a second type of CAB polypeptides have been isolated from several plant species, but the precise location of the gene products has not been determined. Here we show that tomato has two unlinked genes encoding this second type and that this type of CAB polypeptide is also localized in PSII.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43458/1/11103_2004_Article_BF00015643.pd

Using Microsatellites to Understand the Physical Distribution of Recombination on Soybean Chromosomes

Soybean is a major crop that is an important source of oil and proteins. A number of genetic linkage maps have been developed in soybean. Specifically, hundreds of simple sequence repeat (SSR) markers have been developed and mapped. Recent sequencing of the soybean genome resulted in the generation of vast amounts of genetic information. The objectives of this investigation were to use SSR markers in developing a connection between genetic and physical maps and to determine the physical distribution of recombination on soybean chromosomes. A total of 2,188 SSRs were used for sequence-based physical localization on soybean chromosomes. Linkage information was used from different maps to create an integrated genetic map. Comparison of the integrated genetic linkage maps and sequence based physical maps revealed that the distal 25% of each chromosome was the most marker-dense, containing an average of 47.4% of the SSR markers and 50.2% of the genes. The proximal 25% of each chromosome contained only 7.4% of the markers and 6.7% of the genes. At the whole genome level, the marker density and gene density showed a high correlation (R2) of 0.64 and 0.83, respectively with the physical distance from the centromere. Recombination followed a similar pattern with comparisons indicating that recombination is high in telomeric regions, though the correlation between crossover frequency and distance from the centromeres is low (R2 = 0.21). Most of the centromeric regions were low in recombination. The crossover frequency for the entire soybean genome was 7.2%, with extremes much higher and lower than average. The number of recombination hotspots varied from 1 to 12 per chromosome. A high correlation of 0.83 between the distribution of SSR markers and genes suggested close association of SSRs with genes. The knowledge of distribution of recombination on chromosomes may be applied in characterizing and targeting genes

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

An Autotetraploid Linkage Map of Rose (Rosa hybrida) Validated Using the Strawberry (Fragaria vesca) Genome Sequence

Polyploidy is a pivotal process in plant evolution as it increase gene redundancy and morphological intricacy but due to the complexity of polysomic inheritance we have only few genetic maps of autopolyploid organisms. A robust mapping framework is particularly important in polyploid crop species, rose included (2n = 4x = 28), where the objective is to study multiallelic interactions that control traits of value for plant breeding. From a cross between the garden, peach red and fragrant cultivar Fragrant Cloud (FC) and a cut-rose yellow cultivar Golden Gate (GG), we generated an autotetraploid GGFC mapping population consisting of 132 individuals. For the map we used 128 sequence-based markers, 141 AFLP, 86 SSR and three morphological markers. Seven linkage groups were resolved for FC (Total 632 cM) and GG (616 cM) which were validated by markers that segregated in both parents as well as the diploid integrated consensus map