Conservation of gene repertoire but not gene order in pepper and tomato

Homologies of tomato and pepper genes have been compared, and genetic linkage maps have been constructed based on a common set of cDNA clones and selected single-copy genomic clones. We report here that the gene repertoire of these two species is highly conserved, yet the linear order of the genes on the chromosomes has been greatly modified. Although the two species share the same number of centromeres, the chromosomal regions around those centromeres have undergone extensive rearrangements. Accompanying the extensive chromosome rearrangement has been a change in locus number for approximately 12% of the loci detected by random cDNA clones. Duplicated loci within each genome are normally found on different chromosomes and are not confined to one species, thus ruling out gene duplication as an explanation for the 4-fold higher DNA content of pepper. At least one of the duplications occurred since the divergence of tomato and pepper from their last common ancestor.

A small asparagine-rich protein required for S-allele-specific pollen rejection in Nicotiana

Although S-locus RNases (S-RNases) determine the specificity of pollen rejection in self-incompatible (SI) solanaceous plants, they alone are not sufficient to cause S-allele-specific pollen rejection. To identify non-S-RNase sequences that are required for pollen rejection, a Nicotiana alata cDNA library was screened by differential hybridization. One clone, designated HT, hybridized strongly to RNA from N. alata styles but not to RNA from Nicotiana plumbaginifolia, a species known to lack one or more factors necessary for S-allele-specific pollen rejection. Sequence analysis revealed a 101-residue ORF including a putative secretion signal and an asparagine-rich domain near the C terminus. RNA blot analysis showed that the HT-transcript accumulates in the stigma and style before anthesis. The timing of HT-expression lags slightly behind S(C10)-RNase in SI N. alata S(C10)S(C10) and is well correlated with the onset of S-allele-specific pollen rejection in the style. An antisense-HT construct was prepared to test for a role in pollen rejection. Transformed (N. plumbaginifolia x SI N. alata S(C10)S(C10)) hybrids with reduced levels of HT-protein continued to express S(C10)-RNase but failed to reject S(C10)-pollen. Control hybrids expressing both S(C10)-RNase and HT-protein showed a normal S-allele-specific pollen rejection response. We conclude that HT-protein is directly implicated in pollen rejection.

Molecular mapping of resistance to blight in an interspecific cross in the genus castanea

ABSTRACT A three-generation American chestnut x Chinese chestnut pedigree was used to construct a genetic linkage map for chestnut and to investigate the control of resistance to Endothia parasitica (chestnut blight fungus). DNA genotypes for 241 polymorphic markers (eight isozymes, 17 restriction fragment length polymorphisms [RFLPs], and 216 random amplified polymorphic DNAs [RAPDs]) were assayed on an F(2) family consisting of 102 individuals. Of these markers, 196 were segregating as expected and, subsequently, used for primary linkage mapping. Two isozymes, 12 RFLPs, and 170 RAPDs were mapped to 12 linkage groups spanning a total genetic distance of 530.1 Kosambi centimorgans. F(2) plants were evaluated for a response to E. parasitica infection by directly inoculating them with two unique fungal isolates and measuring canker expansion over a period of 3.5 months. Results were compared with the marker genotype data, thereby identifying genomic regions significantly associated with a resistance response. Single-marker or nonsimultaneous analyses of variance identified seven genomic regions that appear to have an effect on host response. Multiple-marker or simultaneous models suggest that three of these regions have a significant effect on host response, together explaining as much as 42.2% of the total variation for canker size. At each of the three putative resistance loci, alleles derived from the Chinese chestnut grandparent were associated with smaller canker size, or higher levels of resistance.

S-related protein can be recombined with self-compatibility in interspecific derivatives of Lycopersicon

Stylar proteins involved in the self-incompatible (SI) response of Lycopersicon hirsutum have been identified and mapped to the locus that controls SI (S locus). L. esculentum, a self-compatible (SC) species of cultivated tomato, does not display these proteins. Hybrids between SC L. esculentum and SI L. hirsutum are self-sterile despite these individuals bearing pollen containing the S allele of L. esculentum. In progeny derived from backcrossing the hybrids to L. esculentum, there was a strong correlation between the presence of the S allele from L. hirsutum and self-infertility. However, this relationship was uncoupled in a number of backcross (BC) progeny. The SI response appeared to be nonexistent in two self-fertile BC individuals that were heterozygous for the S allele of L. hirsutum, based on Mendelian segregation of a tightly linked DNA marker, CD15, in selfed progeny. Among these progeny self-fertile individuals that were homozygous for the L. hirsutum allele of the linked marker were also determined to be homozygous for an S-related proteins are not sufficient to elicit a self-incompatible response in L. esculentum and that there is a mutation(s) in L. esculentum somewhere other than the S locus that leads to self-compatibility.

Genetic mapping and protein product diversity of the self-incompatibility locus in wild tomato (Lycopersicon peruvianum)

Phenotypic diversity of self-incompatibility (S) alleles within nine natural populations of Lycopersicon peruvianum was investigated. Only 7 incompatible responses were observed of a total of 276 unique combinations tested, on the basis of controlled pollinations, indicating the large number of alleles that exist within these populations. Molecular weight polymorphism for specific major stylar proteins observed on SDS-PAGE was also evident in two of the populations examined. Five proteins were shown to map to the S locus and to be associated with different S alleles through controlled pollinations and segregation of the proteins. Two of these S related proteins had been described previously in terms of spatial and temporal expression consistent with their involvement in self-incompatibility (Mau et al., Planta 169, 184-191, 1986). A mapping population derived from a fully compatible cross was used to establish linkage of the S locus to two DNA markers, CD15 and TG184, that lie on chromosome 1. The order of the markers and estimates of map distances are given.

Molecular diversity at the self-incompatibility locus is a salient feature in natural populations of wild tomato (Lycopersicon peruvianum)

A cDNA encoding a stylar protein was cloned from flowers of self-incompatible wild tomato (Lycopersicon peruvianum). The corresponding gene was mapped to the S locus, which is responsible for self-incompatibility. The nucleotide sequence was determined for this allele, and compared to other S-related sequences in the Solanaceae. The S allele was used to probe DNA from 92 plants comprising 10 natural populations of Lycopersicon peruvianum. Hybridization was conducted under moderate and permissive stringencies in order to detect homologous sequences. Few alleles were detected, even under permissive conditions, underscoring the great sequence diversity at this locus. Those alleles that were detected are highly homologous. Sequences could not be detected in self-incompatible Nicotiana alata, self-compatible L. esculentum (cultivated tomato) or self-compatible L. hirsutum. However, hybridization to an individual of self-incompatible L. hirsutum revealed a closely related sequence that maps to the S locus in this reproductively isolated species. This supports the finding that S locus polymorphism predates speciation. The extraordinarily high degree of sequence diversity present in the gametophytic self-incompatibility system is discussed in the context of other highly divergent systems representing several kingdoms.

Sequence variability of three alleles of the self-incompatibility gene of Nicotiana alata

Three alleles of the self-incompatibility gene of Nicotiana alata have been cloned and sequenced. A comparison of the sequences shows a surprisingly low level of homology (56%) and the presence of defined regions of homology and variability. The homologous regions include the N-terminal sequence, most of the cysteine residues and glycosylation sites, as well as other blocks throughout the sequence. We interpret these conserved regions as "framework" and nonconserved regions as "hypervariable," following the terminology used to describe analogous regions in the IgG supergene family. The low level of overall homology forms the basis of a general method for isolating S-allele-specific cDNAs. Allele-specific antibodies can be generated using synthetic peptides corresponding to one of the variable regions. When applied to sections of the pistil, these antibodies label the intercellular matrix in the stigma and transmitting tissue of the style and the cell walls in the epidermis of the placenta. HindIII digestion of genomic DNA generates a characteristic pattern of S-gene fragments for each genotype. These restriction fragment length polymorphisms can be used to assign S-genotype to progeny arising from breeding experiments.

A nuclear sequence associated with self-incompatibility in Nicotiana alata has homology with mitochondrial DNA

A 1.0-kb nuclear fragment located 5' to a coding sequence associated with self-incompatibility in N. alata shows homology with mitochondrial chromosomal DNA on Southern blots. This sequence is also present in the mitochondrial DNA of two species of tomato, L. esculentum and L. pennellii, but shows no homology to mtDNA of Zea mays. The homologous mitochondrial fragment from N. alata was cloned and sequenced. A short region of 56 bp matches the nuclear sequence in 53/56 bp. Other matched but misaligned segments flank the 3' end. The nuclear sequence is marked at the 5' end by two 8 bp direct repeats. The function of the nuclear sequence is not known although, it is located 397 bp upstream from the site of transcription of the self-incompatibility gene. The mitochondrial sequence contains only limited open reading frames and the nuclear sequence has none. There is evidence that additional segments of the mitochondrial clone hybridize to other nuclear sequences. The exchange of sequences between the mitochondrial and nuclear genomes of plants is discussed.

CR1 - a dispersed repeated element associated with the Cab-1 locus in tomato

Cab-1 is a complex genetic locus in tomato consisting of four clustered genes encoding chlorophyll a/b-binding polypeptide. Southern blot analysis of total tomato DNA with genomic clones corresponding to the Cab-1 locus has revealed the presence of a repetitive element in the 3 kb spacer regions between two of these genes. This repetitive element, named CR1, has been characterized via sequencing, genetic mapping and hybridization to related solanaceous species. Results indicate that there are as many as 30 copies of this element in the tomato genome and that most, if not all, are found at independent loci. Sites corresponding to 12 of the repeats have been located on different regions of chromosomes 2, 4, 5, 7, 10 and 11. A 1.6 kb PstI-EcoRI fragment from the Cab-1 locus containing the element was sequenced and found to be 75% AT-rich. No open reading frames larger than 150 bp were detected. Several imperfect inverted repeats flanked by direct repeats could be found at the ends of the element. This arrangement is reminiscent of known transposons. Southern hybridization analysis indicates that multiple copies of CR1 exist in all species of the genus Lycopersicon as well as in Solanum lycopersicoides and S. tuberosum (potato), but not in eggplant, pepper, petunia, Datura or tobacco. Melt-off experiments indicate that members of the CR1 family in the tomato genome are more closely related to one another than to homologous members in the genomes of S. lycopersicoides or S. tuberosum, suggesting some type of concerted evolution.

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.

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.

Genetics of actin-related sequences in tomato

The genomic distribution of actin-related sequences in tomato was investigated using a cloned actin gene from soybean. Ten actin loci account for most of the hybridizing fragments observed with Southern analysis. Single loci were found on chromosomes 1, 3 and 10 and two loci on chromosome 4. One locus is linked to an unmapped isozyme marker, Sod-1. The four remaining actin loci are independent of each other and of any of the other markers tested. The number of actin loci in tomato (10) is greater than that estimated for soybean (8). As soybean is apparently a tetraploid and tomato a diploid, these results suggest that the number of actin loci has not been stable during the evolution of dicots. A number of these mapped loci lie in regions of the genome previously devoid of molecular markers and thus may be useful in basic and applied genetic research.

Evidence for selection as a mechanism in the concerted evolution of Lycopersicon esculentum (tomato) genes encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase

The nuclear gene sequences encoding RBCS, the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) from several plants show extensive interspecific divergence but little intraspecific divergence, suggesting that these genes are evolving in concert within a genome. In this study, the nucleotide sequences of two tomato (Lycopersicon esculentum) RBCS genes and a cDNA clone containing the entire coding region of a third tomato RBCS gene were determined. The three genes, designated Rbcs-1, Rbcs-2A, and Rbcs-3A, each belong to a different one of the three RBCS loci in the tomato genome. The nucleotide sequence of Rbcs-1 differs from that of Rbcs-2A and Rbcs-3A by 13.9% and 13.1%, respectively. Rbcs-2A and Rbcs-3A differ from each other by 10.7%. A recently published RBCS gene sequence from tobacco (Nicotiana tabacum) [Mazur, B. J. & Chui, C.-F. (1985) Nucleic Acids Res. 13, 2373-2386] differs by 10.6% and 11.3% from Rbcs-2A and Rbcs-3A, respectively, and by 15.0% from Rbcs-1. Thus the tobacco gene seems to be phylogenetically as closely related to the tomato genes Rbcs-2A and Rbcs-3A as the latter two are to each other, and more closely related to them than Rbcs-1 is. However, the mature part of the polypeptide encoded by the tobacco RBCS gene differs by five and six amino acids from the corresponding region in the polypeptides encoded by Rbcs-2A and Rbcs-3A, respectively, while these two tomato RBCS polypeptides differ from each other in the mature part by a single amino acid. Rbcs-1, whose nucleotide sequence shows higher divergence from both the tobacco RBCS gene and Rbcs-2A and Rbcs-3A, encodes a polypeptide whose mature part differs by eight amino acids from the corresponding region in the tobacco polypeptide but only by three and four amino acids from the corresponding regions of Rbcs-2A- and Rbcs-3A-encoded polypeptides, respectively. Thus, it appears that in the tomato selection has maintained near uniformity of the coding information in the portion of the RBCS genes encoding the mature polypeptides.

TOWARD A SATURATED LINKAGE MAP IN TOMATO BASED ON ISOZYMES AND RANDOM cDNA SEQUENCES

A linkage map in tomato has been developed based on isozyme and random cDNA clones derived from mRNA. Interspecific backcross and F2 populations of Lycopersicon esculentum and L. pennellii were employed in the linkage analysis. Allelic differences in cDNA markers were based on restriction fragment length polymorphisms detected through Southern analysis. A total of 57 unique cDNA clones have been analyzed. The majority of cDNA markers correspond to single loci and are dispersed throughtout the genome. Of those clones that hybridize to two or more loci, most show genetic independence (ie., they are unlinked). The combination of isozyme, cDNA and previously mapped DNA markers total 112 loci. It is estimated that approximately 92% of the genome can be monitored during segregation with these markers. Molecular maps, such as the one being constructed in tomato, may allow genetic and breeding experiments that previously were not possible.

LOCALIZATION IN THE TOMATO GENOME OF DNA RESTRICTION FRAGMENTS CONTAINING SEQUENCES HOMOLOGOUS TO THE rRNA (45S), THE MAJOR CHLOROPHYLL a/b BINDING POLYPEPTIDE AND THE RIBULOSE BISPHOSPHATE CARBOXYLASE GENES

DNA restriction fragments containing sequences homologous to the ribosomal RNA (45s), the major chlorophyll a/b binding polypeptide (CAB) and the small subunit of ribulose bisphosphate carboxylase (RBCS) genes have been localized and mapped in the tomato nuclear genome by linkage analysis. Ribosomal RNA genes map to a single locus, R45s, which resides in a terminal position on the short arm of chromosome 2 and corresponds to the Nucleolar Organizer Region. The size of the 45s repeating unit is estimated to be approximately 9 kb in Lycopersicon esculentum and 11 kb in Lycopersicon pennellii. Five loci were found to contain CAB sequences. Two of the loci, Cab-1 (chromosome 2) and Cab-3 (chromosome 8), together accounted for more than 80% of the hybridization signal. These loci contain more than one CAB structural gene. The other three loci, Cab-2 (chromosome 8), Cab-4 (chromosome 7) and Cab-5 (chromosome 12), each account for <10% of the total signal and may contain only a single copy of the CAB structural sequence. Three loci were found to contain RBCS sequences. Rbcs-2 (chromosome 3) and Rbcs-3 (chromosome 2) were responsible for >80% of the signal, with the remainder being associated with Rbcs-1 (chromosome 2). Rbcs-2 and Rbcs-3 may contain more than one copy of the gene.

Molecular characterization and genetic mapping of two clusters of genes encoding chlorophyll a/b-binding proteins in Lycopersicon esculentum (tomato)

We have constructed a tomato genomic library in the gamma Charon 4 phage vector. The library was screened with a pea cDNA probe encoding a chlorophyll a/b-binding protein (CAB), and several recombinant phages containing tomato CAB genes were isolated and characterized by restriction mapping, heteroduplex analysis and nucleotide sequencing. Two phages with overlapping segments of the tomato genome contain a total of four CAB genes, all arranged in tandem. A third phase contains three CAB genes, two arranged in tandem and one in opposite orientation, and an additional, truncated CAB gene. Genetic mapping experiments showed that the four CAb genes on the first two phages belong to a locus, previously designated Cab-1, on chromosome 2. The CAB genes from the third phage belong to the Cab-3 locus on chromosome 3. Complete sequence determination of two CAB genes, one from each locus, and additional sequence determination of about 50% of each of the other five CAB genes showed that each gene within a CAB locus is more similar to other CAB genes in the same locus than it is to the CAB genes from the second locus. Furthermore, the polypeptides encoded by Cab-1 genes diverge significantly from those encoded by Cab-3 genes in the domains of transit peptide and the N terminus of the mature polypeptide but are essentially identical in the rest of the sequence.