Solanum Section Lycopersicon Research Articles

Differential Expression of Sugar Uniporter Genes of the SWEET Family in the Regulation of Qualitative Fruit Traits in Tomato Species (Solanum Section Lycopersicon)

Tomato (Solanum lycopersicum L.) is an important crop and, due to the existence of wild relatedspecies (Solanum section Lycopersicon), a model for studying the development of the fleshy fruit. In thestudy, expression analysis of sugar uniporter genes SWEET1а, 1b, 1e, 3, 7a, 10a, 12c, 14, and 15 in tomatospecies and cultivars was carried out. In cv. Heinz (S. lycopersicum), genes that are most active in roots(SWEET1e, 3, 10a, and 12c), leaves (SWEET1a, 1e, 3, 10a, and 12c) and flowers (SWEET1a, 1b, 7a, 10a,12s, 14, and 15) were revealed. The growth of the fruit is accompanied by an increase in the level of theSWEET 110a and 12c transcripts; maturation of the fruit is accompanied by an increase in the level of theSWEET 1a and 15 transcripts. Differential expression of the SWEET1a, 1b, 12c, and 15 genes in the ripe fruitof inbred lines obtained from crossing of S. lycopersicum cv. M82 × S. pennellii was demonstrated. qRT-PCRanalysis showed that the expression of the SWEET1a and 12c genes is common for ripe fruit of the analyzedtomato species, while the expression of the SWEET1b and 10a genes is common for S. pennellii, S. habrochaites,and S. cheesmaniae. It was determined that the fructose : glucose ratio is equimolar in the accessionsexcept for cv. Black Jack and White Beauty (fructose : glucose ≥ 1.10). Correlations between the level ofSWEET gene transcripts and the ratio of hexoses was not revealed.

Differential Expression of Sugar Uniporter Genes of the SWEET Family in the Regulation of Qualitative Fruit Traits in Tomato Species (Solanum Section Lycopersicon)

Differential Expression of Sugar Uniporter Genes of the SWEET Family in the Regulation of Qualitative Fruit Traits in Tomato Species (Solanum Section Lycopersicon)

Super-pangenome analyses highlight genomic diversity and structural variation across wild and cultivated tomato species

Effective utilization of wild relatives is key to overcoming challenges in genetic improvement of cultivated tomato, which has a narrow genetic basis; however, current efforts to decipher high-quality genomes for tomato wild species are insufficient. Here, we report chromosome-scale tomato genomes from nine wild species and two cultivated accessions, representative of Solanum section Lycopersicon, the tomato clade. Together with two previously released genomes, we elucidate the phylogeny of Lycopersicon and construct a section-wide gene repertoire. We reveal the landscape of structural variants and provide entry to the genomic diversity among tomato wild relatives, enabling the discovery of a wild tomato gene with the potential to increase yields of modern cultivated tomatoes. Construction of a graph-based genome enables structural-variant-based genome-wide association studies, identifying numerous signals associated with tomato flavor-related traits and fruit metabolites. The tomato super-pangenome resources will expedite biological studies and breeding of this globally important crop.

Characterization of 15-cis-ζ-Carotene Isomerase Z-ISO in Cultivated and Wild Tomato Species Differing in Ripe Fruit Pigmentation.

Isomerization of 9,15,9′-tri-cis-ζ-carotene mediated by 15-cis-ζ-carotene isomerase Z-ISO is a critical step in the biosynthesis of carotenoids, which define fruit color. The tomato clade (Solanum section Lycopersicon) comprises the cultivated tomato (Solanum lycopersicum) and 12 related wild species differing in fruit color and, thus, represents a good model for studying carotenogenesis in fleshy fruit. In this study, we identified homologous Z-ISO genes, including 5′-UTRs and promoter regions, in 12 S. lycopersicum cultivars and 5 wild tomato species (red-fruited Solanum pimpinellifolium, yellow-fruited Solanum cheesmaniae, and green-fruited Solanum chilense, Solanum habrochaites, and Solanum pennellii). Z-ISO homologs had a highly conserved structure, suggesting that Z-ISO performs a similar function in tomato species despite the difference in their fruit color. Z-ISO transcription levels positively correlated with the carotenoid content in ripe fruit of the tomatoes. An analysis of the Z-ISO promoter and 5′-UTR sequences revealed over 130 cis-regulatory elements involved in response to light, stresses, and hormones, and in the binding of transcription factors. Green- and red/yellow-fruited Solanum species differed in the number and position of cis-elements, indicating changes in the transcriptional regulation of Z-ISO expression during tomato evolution, which likely contribute to the difference in fruit color.

Reproductive Proteins Evolve Faster Than Non-reproductive Proteins Among Solanum Species

Elevated rates of evolution in reproductive proteins are commonly observed in animal species, and are thought to be driven by the action of sexual selection and sexual conflict acting specifically on reproductive traits. Whether similar patterns are broadly observed in other biological groups is equivocal. Here, we examine patterns of protein divergence among wild tomato species (Solanum section Lycopersicon), to understand forces shaping the evolution of reproductive genes in this diverse, rapidly evolving plant clade. By comparing rates of molecular evolution among loci expressed in reproductive and non-reproductive tissues, our aims were to test if: (a) reproductive-specific loci evolve more rapidly, on average, than non-reproductive loci; (b) ‘male’-specific loci evolve at different rates than ‘female’-specific loci; (c) genes expressed exclusively in gametophytic (haploid) tissue evolve differently from genes expressed in sporophytic (diploid) tissue or in both tissue types; and (d) mating system variation (a potential proxy for the expected strength of sexual selection and/or sexual conflict) affects patterns of protein evolution. We observed elevated evolutionary rates in reproductive proteins. However, this pattern was most evident for female- rather than male-specific loci, both broadly and for individual loci inferred to be positively selected. These elevated rates might be facilitated by greater tissue-specificity of reproductive proteins, as faster rates were also associated with more narrow expression domains. In contrast, we found little evidence that evolutionary rates are consistently different in loci experiencing haploid selection (gametophytic-exclusive loci), or in lineages with quantitatively different mating systems. Overall while reproductive protein evolution is generally elevated in this diverse plant group, some specific patterns of evolution are more complex than those reported in other (largely animal) systems, and include a more prominent role for female-specific loci among adaptively evolving genes.

Population Genomics of the "Arcanum" Species Group in Wild Tomatoes: Evidence for Separate Origins of Two Self-Compatible Lineages.

Given their diverse mating systems and recent divergence, wild tomatoes (Solanum section Lycopersicon) have become an attractive model system to study ecological divergence, the build-up of reproductive barriers, and the causes and consequences of the breakdown of self-incompatibility. Here we report on a lesser-studied group of species known as the “Arcanum” group, comprising the nominal species Solanum arcanum, Solanum chmielewskii, and Solanum neorickii. The latter two taxa are self-compatible but are thought to self-fertilize at different rates, given their distinct manifestations of the morphological “selfing syndrome.” Based on experimental crossings and transcriptome sequencing of a total of 39 different genotypes from as many accessions representing each species’ geographic range, we provide compelling evidence for deep genealogical divisions within S. arcanum; only the self-incompatible lineage known as “var. marañón” has close genealogical ties to the two self-compatible species. Moreover, there is evidence under multiple inference schemes for different geographic subsets of S. arcanum var. marañón being closest to S. chmielewskii and S. neorickii, respectively. To broadly characterize the population-genomic consequences of these recent mating-system transitions and their associated speciation events, we fit demographic models indicating strong reductions in effective population size, congruent with reduced nucleotide and S-locus diversity in the two independently derived self-compatible species.

Differential Regulation of Phytoene Synthase PSY1 During Fruit Carotenogenesis in Cultivated and Wild Tomato Species (Solanum section Lycopersicon).

In plants, carotenoids define fruit pigmentation and are involved in the processes of photo-oxidative stress defense and phytohormone production; a key enzyme responsible for carotene synthesis in fruit is phytoene synthase 1 (PSY1). Tomatoes (Solanum section Lycopersicon) comprise cultivated (Solanum lycopersicum) as well as wild species with different fruit color and are a good model to study carotenogenesis in fleshy fruit. In this study, we identified homologous PSY1 genes in five Solanum section Lycopersicon species, including domesticated red-fruited S. lycopersicum and wild yellow-fruited S. cheesmaniae and green-fruited S. chilense, S. habrochaites and S. pennellii. PSY1 homologs had a highly conserved structure, including key motifs in the active and catalytic sites, suggesting that PSY1 enzymatic function is similar in green-fruited wild tomato species and preserved in red-fruited S. lycopersicum. PSY1 mRNA expression directly correlated with carotenoid content in ripe fruit of the analyzed tomato species, indicating differential transcriptional regulation. Analysis of the PSY1 promoter and 5′-UTR sequence revealed over 30 regulatory elements involved in response to light, abiotic stresses, plant hormones, and parasites, suggesting that the regulation of PSY1 expression may affect the processes of fruit senescence, seed maturation and dormancy, and pathogen resistance. The revealed differences between green-fruited and red-fruited Solanum species in the structure of the PSY1 promoter/5′-UTR, such as the acquisition of ethylene-responsive element by S. lycopersicum, could reflect the effects of domestication on the transcriptional mechanisms regulating PSY1 expression, including induction of carotenogenesis during fruit ripening, which would contribute to red coloration in mature fruit.

Search for sources of genetic resistance to Meloidogyne enterolobii in tomato and in wild Solanum (section Lycopersicon) accessions

Meloidogyne enterolobii (syn. M. mayaguensis) is an emerging plant pathogen capable of inducing root galls and yield reduction in a wide range of host species. This pathogen has also been reported as a global threat for tomato (Solanum lycopersicum L.) crop production mainly due to its ability to overcome the Meloidogyne resistance meditated by the Mi-1 gene. Despite the potential importance of this nematode, sources of resistance to M. enterolobii are not yet available for breeding purposes. The main objective of the present work was to evaluate a large Solanum (section Lycopersicon) germplasm (comprising nine species and one botanic variety) aiming to identify useful sources of resistance to M. enterolobii. In the first screening assay, 101 accessions and the susceptible standard S. lycopersicum ‘Santa Cruz’ were inoculated and evaluated under controlled (greenhouse) conditions. Twenty-day-old seedlings were inoculated with a suspension of M. enterolobii eggs (710 eggs per plant) and maintained in expanded 128-cell polystyrene trays during the entire course of the experiment. The phenotypic criteria used for evaluation (done at 45 days after inoculation) were the number of root galls, gall index, number of eggs, and the reproduction factor. Twenty accessions with promising resistant reaction to M. enterolobii in the first screening test were selected and employed in a subsequent assay. Plants of the 20 selected accessions were cultivated in 400 cm3 pots filled with sterile soil. Inoculation procedures were identical to the first assay, but with higher inoculum pressure (3,300 eggs per plant). Three accessions with superior tolerance levels to M. enterolobii were identified viz. S. lycopersicum ‘Yoshimatsu’, S. lycopersicum ‘CNPH 1246’ and S. pimpinelifolium CGO 7650 (= CNPH 1195). These accessions were re-evaluated against a distinct M. enterolobii population as well as against two other root-knot nematode species (M. javanica and one M. incognita race 1). Under higher inoculum pressure, ‘Yoshimatsu’ was found to be resistant to M. javanica and M. incognita race 1, but susceptible to a M. enterolobii from guava. The other two sources displayed susceptibility to all three nematodes. The results indicated additional germplasm screening since no source of stable genetic resistance to M. enterolobii was found so far.

Differences in the sucrose synthase gene SUS1 expression pattern between Solanum lycopersicum and wild tomato species

Plant sucrose synthase (SUS) reversibly hydrolyzes sucrose, incorporating sucrose into different metabolic pathways. SUS1 homologous genes were characterized in 12 accessions of cultivated tomato Solanum lycopersicum and wild tomato species (Solanum section Lycopersicon). In total, 34 non-synonymous SNPs were found; among them, 17 caused radical amino acid substitutions, mostly in the wild species. The SUS1-based phylogenetic tree topology was represented by two clusters of red-fruited and green-fruited species, where S. pennellii and S. habrochaites showed the highest divergence. SUS1 transcription was detected in source and sink organs of one cultivated and four wild tomato species; it was the highest in buds and flowers, but lower in mature green fruit of the final size. In leaves and roots, the level of SUS1 expression corresponded to species evolutionary history, whereas no correlation was found between SUS1 levels and fruit size and sugar composition. Our findings suggest that SUS1-mediated reversible sucrose hydrolysis is important for maintaining the balance between sucrose and its monomers not only in fruits, but also in the other tomato organs.

Differences in Effective Ploidy Drive Genome-Wide Endosperm Expression Polarization and Seed Failure in Wild Tomato Hybrids.

Parental imbalances in the endosperm leading to impaired development and eventual hybrid seed failure are common causes of postzygotic isolation in flowering plants. Endosperm sensitivity to parental dosage is reflected by canonical phenotypes of "parental excess" in reciprocal interploid crosses. Moreover, parental-excess traits are also evident in many homoploid interspecific crosses, potentially reflecting among-lineage variation in "effective ploidy" driven by endosperm properties. However, the genetic basis of effective ploidy is unknown and genome-wide expression perturbations in parental-excess endosperms from homoploid crosses have yet to be reported. The tomato clade (Solanum section Lycopersicon), encompassing closely related diploids with partial-to-complete hybrid seed failure, provides outstanding opportunities to study these issues. Here, we compared replicated endosperm transcriptomes from six crosses within and among three wild tomato lineages. Strikingly, strongly inviable hybrid crosses displayed conspicuous, asymmetric expression perturbations that mirror previously characterized parental-excess phenotypes. Solanum peruvianum, the species inferred to have evolved higher effective ploidy than the other two, drove expression landscape polarization between maternal and paternal roles. This global expression divergence was mirrored in functionally important gene families such as MADS-box transcription factors and E3 ubiquitin ligases, and revealed differences in cell cycle tuning that match phenotypic differences in developing endosperm and mature seed size between reciprocal crosses. Our work starts to uncover the complex interactions between expression divergence, parental conflict, and hybrid seed failure that likely contributed to plant diversity.

Characteristics of INNER NO OUTER Homologous Genes in Wild Tomato Species

INNER NO OUTER belongs to the YABBY transcription factor family and determines the abaxial identity of the outer ovule integument. In this study, complete genomic INO sequences were identified and characterized in 12 accessions of 11 tomato species (Solanum section Lycopersicon). It was shown that the identified INO genes encode proteins with the conserved zinc finger and YABBY domains. The INO sequences of all analyzed tomato species contain 243 (12.9% of the aligned length) variable sites, 43 of which are exon-specific and include 22 nonsynonymous SNPs leading to amino acid residue substitutions in zinc finger domain (10), YABBY domain (2), and interdomain regions (10). A deletion of 10 aa at the C-terminal interdomain sequence is characteristic of the S. arcanum INO.

The expression pattern of the Pho1a genes encoding plastidic starch phosphorylase correlates with the degradation of starch during fruit ripening in green-fruited and red-fruited tomato species.

Genes encoding plastidic starch phosphorylase Pho1a were identified in 10 tomato species (Solanum section Lycopersicon). Pho1a genes showed higher variability in green-fruited than in red-fruited tomato species, but had an extremely low polymorphism level compared with other carbohydrate metabolism genes and an unusually low ratio of intron to exon single nucleotide polymorphisms (SNPs). In red-fruited species, Pho1a was expressed in all analysed tissues, including fruit at different developmental stages, with the highest level in mature green fruit, which is strong sink organ importing sucrose and accumulating starch. In green-fruited species Solanum peruvianum and Solanum arcanum, the Pho1a expression level was similar in mature green and ripe fruit, whereas in Solanum chmielewskii, it was higher in ripe fruit, and in Solanum habrochaites, the dynamics of fruit-specific Pho1a expression was similar to that in red-fruited tomatoes. During fruit development, in red-fruited Solanum lycopersicum, sucrose level was low, the monosaccharide content increased; in green-fruited S. peruvianum, the sucrose concentration increased and those of monosaccharides decreased. In both species, the starch content and Pho1a expression were downregulated. The evolutionary topology based on Pho1a sequences was consistent with the current division of tomatoes into red-fruited and green-fruited species, except for S. habrochaites.

Evaluation of germplasm in Solanum section Lycopersicon for tomato taste improvement

The genetic potential of accessions from Solanum section Lycopersicon (S. lycopersicum L., S. lycopersicum var. cerasiforme, S. pimpinellifolium L., and S. habrochaites Knaap & Spooner) for breeding tomato taste has been studied in three environments with clonal replicates. The environment clearly affected the accumulation and level of variation of sugars and acids and derived variables through a direct effect. It seems that photosynthetically active radiation would exert a major effect on sugar accumulation while in the case of organic acids the effect of temperature might be more important. Even more, important genotype × environment interactions can considerably modify the real value of germplasm, being considerably higher in wild species. The environment affected not only mean contents but also the levels of variation. Thus, the need to develop multienvironmental screening programs is suggested to identify solid sources of variation. An important intraaccession variability was also found in wild germplasm, emphasizing the need to analyze a high number of plants per accession in order to identify sources of variation. Accessions with a significant genotypic contribution to the accumulation of sucrose within the Lycopersicon group were identified and may be interesting to analyze the regulation of vacuolar invertase. Accessions with different genotypic contributions to citric, malic, and glutamic acid accumulation have also been identified. These accessions will be valuable for the development of breeding programs considering the acid component of taste. Additionally, these genetic resources will be interesting to study the regulation of the tricarboxylic acid cycle and the gamma-aminobutyric acid shunt.

Identification of genetic sources with attenuated Tomato chlorosis virus-induced symptoms in Solanum (section Lycopersicon) germplasm

The whitefly-transmitted Tomato chlorosis virus (ToCV) (genus Crinivirus) is associated with yield and quality losses in field and greenhouse-grown tomatoes (Solanum lycopersicum) in South America. Therefore, the search for sources of ToCV resistance/tolerance is a major breeding priority for this region. A germplasm of 33 Solanum (Lycopersicon) accessions (comprising cultivated and wild species) was evaluated for ToCV reaction in multi-year assays conducted under natural and experimental whitefly vector exposure in Uruguay and Brazil. Reaction to ToCV was assessed employing a symptom severity scale and systemic virus infection was evaluated via RT-PCR and/or molecular hybridization assays. A subgroup of accessions was also evaluated for whitefly reaction in two free-choice bioassays carried out in Uruguay (with Trialeurodes vaporariorum) and Brazil (with Bemisia tabaci Middle-East-Asia-Minor1—MEAM1 = biotype B). The most stable sources of ToCV tolerance were identified in Solanum habrochaites PI 127827 (mild symptoms and low viral titers) and S. lycopersicum ‘LT05’ (mild symptoms but with high viral titers). These two accessions were efficiently colonized by both whitefly species, thus excluding the potential involvement of vector-resistance mechanisms. Other promising breeding sources were Solanum peruvianum (sensu lato) ‘CGO 6711’ (mild symptoms and low virus titers), Solanum chilense LA1967 (mild symptoms, but with high levels of B. tabaci MEAM1 oviposition) and Solanum pennellii LA0716 (intermediate symptoms and low level of B. tabaci MEAM1 oviposition). Additional studies are necessary to elucidate the genetic basis of the tolerance/resistance identified in this set of Solanum (Lycopersicon) accessions.

Wild tomato endosperm transcriptomes reveal common roles of genomic imprinting in both nuclear and cellular endosperm.

Genomic imprinting is a conspicuous feature of the endosperm, a triploid tissue nurturing the embryo and synchronizing angiosperm seed development. An unknown subset of imprinted genes (IGs) is critical for successful seed development and should have highly conserved functions. Recent genome-wide studies have found limited conservation of IGs among distantly related species, but there is a paucity of data from closely related lineages. Moreover, most studies focused on model plants with nuclear endosperm development, and comparisons with properties of IGs in cellular-type endosperm development are lacking. Using laser-assisted microdissection, we characterized parent-specific expression in the cellular endosperm of three wild tomato lineages (Solanum section Lycopersicon). We identified 1025 candidate IGs and 167 with putative homologs previously identified as imprinted in distantly related taxa with nuclear-type endosperm. Forty-two maternally expressed genes (MEGs) and 17 paternally expressed genes (PEGs) exhibited conserved imprinting status across all three lineages, but differences in power to assess imprinted expression imply that the actual degree of conservation might be higher than that directly estimated (20.7% for PEGs and 10.4% for MEGs). Regardless, the level of shared imprinting status was higher for PEGs than for MEGs, indicating dissimilar evolutionary trajectories. Expression-level data suggest distinct epigenetic modulation of MEGs and PEGs, and gene ontology analyses revealed MEGs and PEGs to be enriched for different functions. Importantly, our data provide evidence that MEGs and PEGs interact in modulating both gene expression and the endosperm cell cycle, and uncovered conserved cellular functions of IGs uniting taxa with cellular- and nuclear-type endosperm.

Assessment of the fruit-ripening-related FUL2 gene diversity in morphophysiologically contrasted cultivated and wild tomato species

Members of the Solanum section Lycopersicon are characterized by different fruit morphophysiology. The MADS-box gene FRUITFULL encodes a transcription factor involved in the regulation of flowering time and fruit development. In the present study, we identified and characterized FUL2 gene orthologs from 10 different tomato accessions in terms of polymorphism, phylogenetic relatedness, and expression pattern. The identified genes had a size from 5827 bp in S. habrochaites to 6229 bp in S. peruvianum 3966 and contained eight exons. The genes demonstrated significantly higher variability in introns (960 SNPs and 76 indels) than in exons (46 SNPs); non-synonymous SNPs were translated into 22 neutral and 20 slightly deleterious amino acid substitutions (12.24% variability in protein sequences). All predicted FUL2 proteins contained the conserved euFUL motif MPP[W/C]MLRHLNG at the C-terminus and had secondary and tertiary structures common for MADS-domain transcription factors. The FUL2-based evolutionary relationships in cultivated and wild tomatoes were mainly consistent with the accession crossability. All FUL2 genes were transcribed in flowers and fruits and demonstrated two distinct expression patterns with the maximum in mature green or ripe fruits, respectively. The obtained data should contribute to understanding of genetic potential of this ripening-related transcription factor for tomato breeding programs.

LIN7 Cell-Wall Invertase Orthologs in Cultivated and Wild Tomatoes (Solanum Section Lycopersicon)

Plant acid invertases are considered to be the key enzymes in sucrose unloading and carbohydrate supply to sink tissues. Acid cell-wall invertases control sucrose transport via the apoplastic pathway during sink initiation and expansion. In this study, we identified 12 LIN7 gene homologs encoding cell-wall invertases in red- and green-fruited tomato accessions (Solanum section Lycopersicon) of self-compatible and self-incompatible species. All genes consisted of six exons and five introns, including highly conserved 9-bp exon II. Identification of 226 exonic single nucleotide polymorphisms as well as extremely high intron variability indicates a significant interspecific divergence among the examined tomato accessions. Computational prediction revealed protein structure typical for the glycosyl hydrolase family 32 and conserved catalytic sites described for other plant cell-wall invertases. LIN7 expression in mature buds and flowers confirms LIN7 role in the development of pollen tubes and grains. The variability in gene and protein sequences and species-specific differences in LIN7 expression patterns may be responsible for putative functional divergence of invertases. Furthermore, we performed phylogenetic analysis of the Solanum section Lycopersicon species based on the LIN7 gene, which clearly divided the analyzed tomato accessions into two main clusters corresponding to self-compatible and self-incompatible species and was in agreement with the separation into red- and green-fruited plants. Given that LIN7 plays an essential role in tomato fertility and fruit ripening, the characterization of protein variability within species of section Lycopersicon may be useful to evaluate the potential application of the encoding genes for tomato breeding programs.

Broad spectrum insect resistance and metabolites in close relatives of the cultivated tomato

Wild relatives of tomato possess effective means to deal with several pests, among which are a variety of insects. Here we studied the presence of resistance components against Trialeurodes vaporariorum, Myzus persicae, Frankliniella occidentalis, and Spodoptera exigua in the Lycopersicon group of Solanum section Lycopersicon by means of bioassays and comprehensive metabolite profiling. Broad spectrum resistance was found in Solanum galapagense and a few accessions of S. pimpinellifolium. Resistance to the sap sucking insects may be based on the same mechanism, but different from the caterpillar resistance. Large and highly significant differences in the leaf metabolomes were found between S. galapagense, containing type IV trichomes, and its closest relative S. cheesmaniae, which lacks type IV trichomes. The most evident differences were the relatively high levels of different methylated forms of the flavonoid myricetin and many acyl sucrose structures in S. galapagense. Possible candidate genes regulating the production of these compounds were identified in the Wf-1 QTL region of S. galapagense, which was previously shown to confer resistance to the whitefly B. tabaci. The broad spectrum insect resistance identified in S. galapagense will be very useful to increase resistance in cultivated tomato.

Incidence and developmental timing of endosperm failure in post-zygotic isolation between wild tomato lineages.

Defective hybrid seed development in angiosperms might mediate the rapid establishment of intrinsic post-zygotic isolation between closely related species. Extensive crosses within and among three lineages of wild tomatoes (Solanum section Lycopersicon) were performed to address the incidence, developmental timing and histological manifestations of hybrid seed failure. These lineages encompass different, yet fairly recent, divergence times and both allopatric and partially sympatric pairs. Mature seeds were scored visually 2 months after hand pollinations, and viable-looking seeds were assessed for germination success. Using histological sections from early-developing seeds from a sub-set of crosses, the growth of three major seed compartments (endosperm, embryo and seed coat) was measured at critical developmental stages up to 21 d after pollination, with a focus on the timing and histological manifestations of endosperm misdevelopment in abortive hybrid seeds. For two of three interspecific combinations including the most closely related pair that was also studied histologically, almost all mature seeds appeared 'flat' and proved inviable; histological analyses revealed impaired endosperm proliferation at early globular embryo stages, concomitant with embryo arrest and seed abortion in both cross directions. The third interspecific combination yielded a mixture of flat, inviable and plump, viable seeds; many of the latter germinated and exhibited near-normal juvenile phenotypes or, in some instances, hybrid necrosis and impaired growth. The overall results suggest that near-complete hybrid seed failure can evolve fairly rapidly and without apparent divergence in reproductive phenology/biology. While the evidence accrued here is largely circumstantial, early-acting disruptions of normal endosperm development are most probably the common cause of seed failure regardless of the type of endosperm (nuclear or cellular).

Sources of resistance to Potato virus Y and Pepper yellow mosaic virus in Solanum (section Lycopersicon) germplasm

Potato virus Y (PVY) is the type-species of the genus Potyvirus, family Potyviridae, being reported as a major tomato (Solanum lycopersicum L.) pathogen in several regions of the world. Pepper yellow mosaic virus (PepYMV) was originally described as a resistance-breaking Potato virus Y (PVY) isolate on Capsicum annuum L. cultivars, and afterwards it was also reported infecting tomatoes in Brazil. In the present work, a search for sources of resistance to both PepYMV and PVY was conducted in a collection of 119 accessions belonging to seven Solanum (section Lycopersicon) species. This germplasm was initially evaluated to PepYMV reaction by mechanical inoculation followed by symptom observations and ELISA. Potential PepYMV resistance sources were identified for the first time in S. habrochaites, S. peruvianum, S. corneliomuelleri, S. chilense, S. pimpinellifolium, and one accession derived from an interspecific cross (S. lycopersicum x S. peruvianum). A sub-group of 24 accessions with negative serology for PepYMV was also challenged with a PVY isolate, followed by serological and molecular detection with universal primers. Solanum habrochaites ‘L.03683’ and ‘L.03684’ were the only accessions found with stable resistance to both viruses. These results confirm S. habrochaites as the most important source of multiple resistance factor(s) to distinct Potyvirus species.