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John R. Stommel

Cultured leaf explants obtained from 36 accessions of the wild tomato Lycopersicon hirsutum were evaluated for morphogenic capacity in response to 3 cytokinins [zeatin, benzylamino purine (BA) and kinetin] in combination with indoleacetic acid (IAA). Morphogenic responses within this wild species were accession-dependent, Cotyledon tissue, in comparison to true leaf explants, were superior for callus and shoot formation. Optimal callus induction medium varied with accession, but most often contained 13.3 μM BA plus 1.7 μM IAA. Media containing 4.6 or 9.2 μM zeatin plus 0.1 μM iaa were optimal shoot induction media. Explants of L. hirsutum f. typicum accessions 126445, 127826, 128644, and 390663 and L. hirsutum f. glabratum accessions 365904, 365905, and 365906 exhibited the highest levels of shoot formation.

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John R. Stommel

Genetic characterization of anthracnose resistance in tomato (Lycopersicon esculentum Mill.) caused by Colletotrichum coccodes (Wallr.) Hughes was accomplished using populations developed from crosses between the anthracnose susceptible cultivar US28 and three resistant breeding lines (115-4, 625-3, and 88B147) that varied in their degree of anthracnose resistance and relative stage of adaptation for commercial use. These lines were of common parental lineage with resistance derived from the small-fruited L. esculentum USDA PI 272636. Anthracnose lesion diameters and fruit weight were measured in puncture inoculated fruit of parental, F1, F2, and backcross generations within each cross. Correlation coefficients between fruit size and lesion diameter were low and generally nonsignificant. Estimates of broad and narrow sense heritabilities for resistance were moderate and declined as relative anthracnose susceptibility of the resistant parent increased coincident with increasing horticultural adaptation. A simple additive dominance model, m[d][h], was adequate to explain the genetic variance for anthracnose resistance in all crosses. Genetic variance for anthracnose resistance was primarily additive. The minimum number of effective factors or loci conditioning anthracnose resistance declined during attempts to transfer high levels of resistance from PI 272636 into adapted breeding lines.

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John R. Stommel

Solanum ochranthum Dunal is a nontuber bearing wild relative of the cultivated tomato (Lycopersicon esculentum Mill.), and a potential source of new genes for disease and pest resistance. Because S. ochranthum is sexually isolated from tomato, somatic hybrids between tomato (PI 367942; L. esculentum Mill. var. cerasiforme (Dunal) A. Gray VFNT cherry × L. peruvianum (L.) Mill. backcrossed to VFNT cherry) and S. ochranthum (LA2117) were developed previously to overcome these crossing barriers. Attempts to backcross these hybrids to tomato have been unsuccessful. Pollen fertility and mitotic and meiotic studies in tomato + S. ochranthum somatic hybrids determined the cause of the sterility of the somatic hybrids and identified hybrids with moderate fertility. Chromosome counts of dividing root tip cells delineated tetraploid (2n = 4x = 48) and hexaploid (2n = 6x = 72) genotypes and aneuploidy in these hybrids. Meiotic analysis of developing microspores confirmed the presence of precocious division and laggard chromosomes at anaphase in both hexaploid and tetraploid hybrids. Bridges were observed in hexaploids at anaphase I and II and multivalent configurations were observed at diakinesis. Multivalents and univalents were evident in nearly all cells examined, proving that the two genomes are homoeologous. Aberrant microsporocytes with five to six developing microspores were noted in hexaploid hybrids. The occurrence of homoeologous pairing between chromosomes of both fusion parents is advantageous to effect recombination between these isolated species. However, the negative effects of multivalent formation and univalents likely contributed to observed sterility in these first generation fusion hybrids. Low to moderate levels of pollen fertility (0% to 52%) were found in tetraploid hybrids, while little or no viable pollen (0% to 4%) was observed in hexaploid somatic hybrids.

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John R. Stommel

Cultured leaf explants obtained from 36 accessions of the wild tomato Lycopersicon hirsutum were evaluated for morphogenic capacity in response to 3 cytokinins [zeatin, benzylamino purine (BA) and kinetin] in combination with indoleacetic acid (IAA). Morphogenic responses within this wild species were accession-dependent, Cotyledon tissue, in comparison to true leaf explants, were superior for callus and shoot formation. Optimal callus induction medium varied with accession, but most often contained 13.3 μM BA plus 1.7 μM IAA. Media containing 4.6 or 9.2 μM zeatin plus 0.1 μM iaa were optimal shoot induction media. Explants of L. hirsutum f. typicum accessions 126445, 127826, 128644, and 390663 and L. hirsutum f. glabratum accessions 365904, 365905, and 365906 exhibited the highest levels of shoot formation.

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John R. Stommel

Sugar accumulation throughout fruit development in the cultivated tomato (Lycopersicon esculentum) and a wild green-fruited species (L. peruvianum) are being examined. Results obtained using HPLC demonstrate that the fruit of L. peruvianum accessions accumulate the disaccharide, sucrose, in addition to the monosaccharides, glucose and fructose, common to L. esculentum. When detectable, sucrose in the L. esculentum cultivar FM6203 was present at very low levels throughout development. Analysis of mature fruit of L. esculentum var. cerasiforme, L. pimpinellifolium, and L. cheesmanii accessions indicate glucose and fructose as the primary storage sugars. Similar to L. peruvianum, mature fruit of the green-fruited species, L. hirsutum f. typicum and L. hirsutum f. glabratum, accumulate sucrose in addition to glucose and fructose.

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John R. Stommel

Small/miniature sweet and hot peppers (Capsicum annuum L.), such as snack peppers, are a rapidly growing class of specialty peppers. Low seed count is an important attribute for consumer acceptance of small-fruited specialty peppers. Four inbred U.S. Department of Agriculture (USDA) C. annuum breeding lines exhibiting uniformity for pod type and size and normal or reduced seed count were selected for producing F1 and segregating F2 and backcross generations. Seed content of F1 hybrids and progeny produced from the backcross of F1 hybrids to normal seed count parents exhibited unimodal frequency distributions and skewed toward the parent with normal seed count. Progeny produced from backcrosses to the reduced seed count parent exhibited bimodal population distributions representative of the respective parental phenotypes. F2 populations approximated 3:1 frequency distributions skewed toward normal-seeded parental phenotypes. Chi-square tests supported a single recessive gene model with potential modifiers controlling inheritance of reduced seed count. Genetic variants with reduced seed count facilitate seed production and propagation of specialty market class peppers.

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Yiping Zhang and John R. Stommel

The carotenoids have an important influence on tomato fruit quality and enhance the fruit contribution to human nutrition. Expression of the high pigment (hp) locus in tomato results in increased total carotenoids and increased efficiency of utilization of the polyenes. A similar mutant, dark green (dg), contains higher level of chlorophyll in immature fruit and results in darker red pigmentation, both externally and internally in ripe fruit. Random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) analyses were performed using two pairs of near isogenic lines (NILs) designed to be isogenic at the hp and dg loci. Sixty-four AFLP primer pairs and more than 1000 RAPD 10-mer primers were screened for polymorphism between each pair of the NILs. One RAPD marker was identified to be linked to the hp gene, and two AFLP primer pairs showed polymorphic fragments which distinguished the dg NILs. The markers identified in this study will be converted to allele specific SCAR (sequence characterized amplified region) markers, which are more useful in marker-assisted selection breeding programs.

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John R. Stommel and Robert J. Griesbach

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John R. Stommel and Kathleen G. Haynes

Fruit of the cultivated tomato (Lycopersicon esculentum Mill.) store predominantly glucose and fructose whereas fruit of the wild species L. hirsutum Humb. & Bonpl. characteristically accumulate sucrose. Reducing sugar and sucrose concentrations were measured in mature fruit of parental, F1, F2, and backcross (BC1) populations derived from an initial cross of L. esculentum `Floradade' × L. hirsutum PI 390514. Generational means analysis demonstrated that additive effects were equal to dominance effects for percentage of reducing sugar. It was determined that a single major gene, dominant for a high percentage of reducing sugar, regulates the percentage of reducing sugar in tomatoes. We propose that this gene be designated sucr. Only additive effects were demonstrated to be important for glucose: fructose ratios. Using L. hirsutum as a donor parent for increasing total soluble solids concentration in the cultivated tomato is discussed.