Mite responses in relation to trichomes of Lycopersicon esculentum x L. hirsutum F2 hybrids

Summary Leaflets from 11 L. esculentum x L. hirsutum F₂ individuals were bioassayed with spider mites (Tetranychus urticae). Densities of Type I, Type III, Type IV, Type V, and Type VI trichomes and the intensity of red fluorescence of the Type VI trichome tip varied widely among the 11 F₂ hybrids. Mite responses were analyzed in relation to these trichome characters and leaflet surface. Variation in Type IV density accounted for most of the variation in mite responses. Densities of Type I, V and VI trichomes also contributed to mite responses but their effects were obscured by high Type IV densities. Type VI tip fluorescence was not associated with responses of mites in bioassays of these hybrids.The investigation reported in the paper (84-10-123) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director. This material is based upon work supported, in part, by the U.S. Department of Agriculture, Agreement No. 59-2213-1-1-717-0 (Competitive Research Grants Program)     

Inheritance of foliar zingiberene contents and their relationship to trichome densities and whitefly resistance in tomatoes

The sesquiterpene zingiberene, present in leaf glandular trichomes, is reportedly responsible for the high level of arthropod resistance found in Lycopersicon hirsutum var. hirsutum. This paper reports on the inheritance of zingiberene contents and of the various types of glandular trichomes in the interspecific cross L. esculentum × Lycopersicon hirsutum var. hirsutum. Plants of L. esculentum ‘TOM-556’ (= P₁), L. hirsutum var. hirsutum ‘PI-127826’(= P₂), F₁ (P₁ × P₂) and F₂ (P₁ ×P₂) were evaluated for zingiberene contents and densitities of and glandular (types I, IV, VI and VII) trichomes. Broad sense heritabilities were high for all traits studied (0.678, 0.831, 0.996, 0.799 and 0.717 respectively for zingiberene and trichome types I, IV, VI, VII). There were significant positive genetic correlations between zingiberene contents and densities of trichomes types IV, VI and VII. Inheritance of zingiberenecontents can be explained mostly by the action of a single major locus, inwhich the allele from L. hirsutum that conditions high content is incompletely recessive over the allele from L. esculentum. Action of an incompleteley recessive allele in one major locus appears to be evident for densities of trichome types IV, VI and VII, but there is also evidence of the action of other epistatic loci for types IV and VI. F₂ genotypes selected for high zingiberene levels showed higher levels of resistance to the silverleaf whitefly Bemisia argentifolii than L. esculentum ‘TOM-556’, levels that were comparable those found in L. hirsutum var. hirsutum ‘PI-127826’ and other whitefly resistant accessions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Relationships between trichome types and spider mite (Tetranychus evansi) repellence in tomatoes with respect to foliar zingiberene contents

Zingiberene, a sesquiterpene present in glandular trichomes of Lycopersicon hirsutum Dunal var. hirsutum ‘PI-127826’, is responsible for the high level of arthropod resistance in this taxon. The current paper has the following objectives: (a) to quantify zingiberene content in tomato plants obtained from the interspecific cross L. esculentum Mill. × L. hirsutum var hirsutum; (b) to identify, classify and quantify glandular and non-glandular trichome types present in those plants; (c) to assess the level of resistance of those genotypes to spider mites (Tetranychus evansi); (d) to estimate correlations between glandular trichomes, zingiberene contents and mite repellence. Zingiberene content were quantified by a colorimetric method (Freitas, 1999); trichomes were counted from foliar paradermic slide preparations; mite resistance was assessed by a repellence test (Weston & Snyder, 1990). The results indicate that indirect selection for zingiberene content led to correlated increases both in the number of glandular trichomes (particulary type IV) and in the levels of mite repellence. These results were found both in BPX-368 [=F2 (L. esculentum ‘TOM-556’ × L hirsutum var. hirsutum ‘PI-127826’)] and in the subsequent generation BPX-368B (which represents one additional backcross to L.esculentum). Zingiberene appears therefore to be the main factor involved in mite repellence. Density of glandular trichomes in tomato leaflets markedly influences total zingiberene content. Type IV trichome density was the highest, and it was highly correlated to zingiberene content. Graduate Student; This revised version was published online in August 2006 with corrections to the Cover Date.     

Trichome characteristics of F1Lycopersicon esculentum × L. cheesmanii f. minor and L. esculentum × L. pennellii hybrids and effects on Myzus persicae

Summary The green peach aphid (Myzus persicae) is a serious pest of the tomato (Lycopersicon esculentum). Trichome-based host plant resistance of wild Lycopersicon species may offer an alternative to pesticides for management of this pest. Two wild species, Lycopersicon cheesmanii f. minor and L. pennellii, were crossed with L. esculentum and the types and densities of trichomes possessed by the F₁ hybrids measured. The effects of these trichomes on M. persicae were assessed by placing nymphs on leaves and, 96 hr later, designating each nymph as either dead, alive, emigrating off the leaf or unaccounted for. A generalised linear model was used to detect relationships between the numbers of nymphs in each designation and the densities of trichomes and leaf area. Mortality of nymphs on L. pennellii hybrids was associated with high densities of type IV and low densities of type I trichomes. Mortality of nymphs on L. cheesmanii f. minor hybrids was associated with high densities of type III trichomes. An increase in the densities of type III trichomes on L. pennellii hybrids was associated with greater numbers of nymphs emigrating off the leaf whilst unaccounted nymphs on L. pennellii hybrids were associated with increased densities of type VII and type I trichomes. Further experimentation using excised leaflets found mortality on L. pennellii hybrids may be due to nymphs being hindered by type I trichomes and increased densities of type V trichomes were associated with greater numbers of unhindered nymphs. Implications of results are discussed in the context of introducing trichome-based host plant resistance into L. esculentum.     

Zingiberene-mediated resistance to the South American tomato pinworm derived from Lycopersicon hirsutum var. hirsutum

The Lycopersicon hirsutum var. hirsutum accession PI 127826 is recognized as a good source of resistance to arthropod pests due to the action of the allelochemical zimgiberene, a sesquiterpene present in its glandular trichomes. Five genotypes were selected from the F₂ generation of the interspecific cross Lycopersicon esculentum ‘TOM-556’ × Lycopersicon hirsutum var. hirsutum ‘PI 127826’, based on their low levels (BPX-368-clone#56) or high levels(BPX-368-clone#92, BPX-368-clone#105,BPX-368-clone#179, BPX-368-clone#250) of zingiberene. The five F₂ genotypes were tested for resistance to the South American tomato pinworm Tuta absolutaalong with accession L. esculentum ‘TOM-556’ (pinworm susceptible), and the accessions L. hirsutum var. hirsutum ‘PI 127826’ and L. pennellii ‘LA716’ (resistant). The F₂ clones selected for high foliar zingiberene levels showed lower scores for leaflet lesion type(LLT), percent leaflets attacked (PLA) and overall plant damage (OPD) than the low zingiberene genotypes. The results indicated that zingiberene mediates resistance to the South American pinworm, based on feeding and on ovipositing deterrence, in populations derived from the interspecific cross between Lycopersicon. esculentum and Lycopersicon hirsutum var. hirsutum. Indirect selection for high foliar zingiberene content is suggested as an efficient technique for breeding tomatoes for resistance to the South American tomato pinworm. This revised version was published online in July 2006 with corrections to the Cover Date.     

Overcoming unilateral breeding barriers between Lycopersicon peruvianum and cultivated tomato,Lycopersicon esculentum

Summary Treatment of Lycopersicon peruvianum stigmas with an artificial medium analogous to stigmatic exudate allowed pollen germination and growth on immature pistils. Growth of Lycopersicon esculentum pollen tubes to L. peruvianum ovules, an otherwise incompatible cross, was achieved following such treated bud-pollinations. No plantlets were recovered, although a few embryos from this cross at the globularity heart stage of development were excised at 22 days after pollination, indicating the presence of crossing failures as severe as in the reciprocal cross. Hybrid plants were obtained from the reciprocal cross, using as pollen parent an L. peruvianum line selected for congruity with L. esculentum. Bud pollinations to L. peruvianum, using these interspecific F₁ hybrids as the pollen parent, allowed viable embryo development and plantlet recovery. Resulting backcross plants, which possess approximately 1/4 L. esculentum genome in a L. peruvianum cytoplasm, may facilitate further introgression of the L. esculentum nuclear genes into in foreign cytoplasm.     

Occurrence of 2,3-dihydrofarnesoic acid, a spidermite repellent, in trichome secretions of Lycopersicon esculentum × L. hirsutum hybrids

The sesquiterpenoid spidermite repellent, 2,3-dihydrofarnesoic acid, occurs in trichome secretions of LA1363, an accession of the wild tomato Lycopersicon hirsutum. To better understand the inheritance of this sesquiterpene acid and its relationship with the quantity trichome secretions, interspecific crosses were made with five inbred lines of L. esculentum. Limited seed set and frequent seedling mortality led to small populations for each generation. Concentrations of residues in leaflet washes, comprised of trichome secretions and concentrations of 2,3-dihydrofarnsoic acids were determined in F1, F2 and backcross generations. Low concentrations of this acid occurred in F1 hybrids, suggesting there was dominance for low levels of the acid. However, concentrations of 2,3-dihydrofarnesoic acid were below minimum levels of detection in two backcross generations and in one of two F2 generations. Concentrations of leaflet wash residues were high on the L. hirsutum parent and low on the L. esculentum parents. Concentrations on hybrids were generally intermediate. However, high residue concentrations were present on a few F1 and F2 hybrids, Residue and 2,3-dihydrofarnesoic acid concentrations were not correlated in F1 or in backcross generations and were uncorrelated in one of two F2 generations. In the F2 obtained by sib-mating, residue concentrations were correlated with 2,3-dihydrofarnesoic acid concentrations. Although small population sizes and perhaps other factors did not allow delineation of genetic control of the presence or abundance of 2,3-dihydrofarnesoic acid, interspecific hybrids that produce 2,3-dihydrofarnesoic acid provide the experimental plants essential for research designed to provide a better understanding of the role of 2,3-dihydrofarnesoic acid in host-plant resistance of tomato to mites and insects.     

Frequency distributions and linkage relationships of 2-tridecadone in interspecific segregating generations of tomato

Summary The levels of the naturally occurring insecticide 2-tridecadone (2TD) were measured in leaves of Lycopersicon hirsutum f. glabratum, L. esculentum, the interspecific F₁ hybrid, the F₂ and backeross generations. The wild species contains 50 times more 2TD than the cultivated tomato and the frequency distribution of the substance indicates that there is dominance for low levels of the compound. The genotype of F₂ plants was determined with respect to 6 codominant isozyme markers and 4 dominant/recessive markers. Significant associations were detected between 5 of the marker genes and the level of 2TD. We interpret these results as implying linkage between marker genes and genes that control the level of 2TD. The behavior of the gene for the determinant growth habit of the plants suggests that it has a pleiotropic effect on the level of 2TD.     

Inheritance of black leaf mold resistance in tomato

Summary Inheritance of black leaf mold (BLM) (caused by Pseudocercospora fuligena) resistance was studied in four crosses involving two resistant Lycopersicon accessions (PI134417, L. hirsutum and PI254655, L. esculentum) and four susceptible Asian Vegetable Research and Development Center tomato lines (CLN657BC₁F₂-267-0-3-12-7, CL143-0-10-3-0-1-10, CLN698BC₁F₂-358-4-13 and CL5915-93D₄-1-0-3). For each cross, six generations, i.e. P₁, P₂, F₁, F₂, BC₁F₁ and BC₁F₂ were evaluated following inoculations with isolate Pf-2 of P. fuligena. Chi-square analyses of the data based on the ratio of resistant to susceptible plants in the F₂ in three of four crosses gave a good fit to a segregation ratio of 1 R : 15 S, and BC₁F₂ data in three of four crosses gave an acceptable fit to the segregation ratio of 1 R : 63 S. The results indicate that resistance to BLM may be conditioned by two recessive genes acting epistatically in both PI134417 and PI254655.     

A novel source of resistance to the two-spotted spider mite in Lycopersicon pimpinellifolium (Jusl.) Mill.: its genetics as affected by interplot interference

We have previously found an accession of Lycopersicon pimpinellifolium (Jusl.) Mill. (`TO-937') that appeared to resist attack by the two-spotted spider mite (Tetranychus urticae Koch). L. pimpinellifolium is a very close relative of the cultivated tomato (Lycopersicon esculentum Mill.) and thereby a potential source of desirable traits that could be introgressed to the crop species. The objective of this study was to investigate the genetics of the resistance present in `TO-937'. Resistance to infestation by the spider mite was quantified in 24-plant plots of L. pimpinellifolium accessions `TO-937' and `PE-10', L. pennellii accession `PE-45', L. esculentum cultivars `Moneymaker', `Roma' and `Kalohi' (reported to be partially resistant: Stoner & Stringfellow, 1967), and the interspecific F₁ cross, L. esculentum `Moneymaker' × L. pimpinellifolium `TO-937'. Only `TO-937', the F<sub>1</sub>, and`PE-45' were found to be resistant. Resistance of `TO-937' was complete when evaluated in two small greenhouses completely planted with `TO-937' so as to simulate the genotypic homogeneity usual in commercial crops. Generations (P₁, P₂, F₁, F₂, BC₁P₁, and BC₁P₂) of a P₁ (susceptible) × P₂ (resistant) cross (`Moneymaker' × `TO-937') were studied for resistance in a single-plant per plot design. Resistance of `TO-937' was inherited with complete dominance and appeared to be controlled by either two or four genes according to whether segregation in the F₂ or the BC₁P₁, respectively, were considered. However, calculation of the number of genes involved in the resistance was complicated by negative interplot interference due to the high frequency of resistant genotypes within most of the generations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of trichomes and resistance to pod borer (Helicoverpa armigera) and their association in interspecific crosses between cultivated pigeonpea (Cajanus cajan) and its wild relative C. scarabaeoides

The legume pod borer, Helicoverpa armigera, is one of the most devastating pests of pigeonpea. High levels of resistance to pod borer have been reported in the wild relative of pigeonpea, Cajanus scarabaeoides. Trichomes (their type, orientation, density and length) and their exudates on pod wall surface play an important role in the ovipositional behavior and host selection process of insect herbivores. They have been widely exploited as an insect defense mechanism in number of crops. In the present investigation, inheritance of resistance to pod borer and different types of trichomes (A, B, C and D) on the pod wall surface in the parents (C. cajan and C. scarabaeoides) and their F₁, F₂, BC₁ (C. cajan × F₁), and F₃ generations has been studied. Trichomes of the wild parents (high density of the non-glandular trichomes C and D, and glandular trichome B and low density of glandular trichome A) were dominant over the trichome features of C. cajan. A single dominant gene as indicated by the segregation patterns individually will govern each trait in the F₂ and backcross generation. Segregation ratio of 3 (resistant): 1 (susceptible) for resistance to pod borer in the F₂ generation under field conditions was corroborated with a ratio of 1:1 in the backcross generation, and the ratio of 1 non-segregating (resistant): 2 segregating (3 resistant: 1 susceptible): 1 non-segregating (susceptible) in F₃ generation. Similar results were obtained for pod borer resistance under no-choice conditions. Resistance to pod borer and trichomes associated with it (low density of type A trichome and high density of type C) are governed individually by a dominant allele of a single gene in C. scarabaeoides. Following backcrossing, these traits can be transferred from C. scarabaeoides into the cultivated background.     

Resistance in Lycopersicon hirsutum f. glabratum to the greenhouse whitefly (Trialeurodes vaporariorum) increases with plant age

Summary In this study genetic variation for resistance to the greenhouse whitefly (Trialeurodes vaporariorum) of four genotypes of tomato (L. esculentum) and two subspecies of L. hirsutum was investigated. Resistance was quantified by the whitefly life history components adult survival, oviposition rate, pre-adult survival and developmental period, measured on plants inoculated with whiteflies in clip-on cages.The largest differences between species were found when life history components were measured on adult plants of about four months old. On L. hirsutum f. glabratum whiteflies had the lowest adult survival, oviposition rate and pre-adult survival. On L. hirsutum these components were intermediate whereas on all L. esculentum genotypes they were highest. The variation between plants was low compared to the variation within plants. These results indicate that single plant tests can be used to determine accurately genetic variation between individual plants in a segregating population.     

Development of Novel Brassica napus lines with canola quality and higher levels of oleic and linoleic acids derived from intergeneric hybrids between B. napus and Orychophragmus violaceus

Parents and their F₁ and F₂ progenies were evaluated for density of type IV glandular trichomes and resistance to Tetranychus urticae Koch during three stages of the plant growth. Results indicated significant differences between density of type IV glandular trichomes on F₁ and F₂ populations at different sampling times. As age of plant increased, density of type IV glandular trichomes and resistance to two-spotted spider mite increased. Density of type IV glandular trichomes on F₂ individuals showed a broad range of variability (0–42 trichomes/mm²). The estimated heritability was very high (>85%) for damage score and density of type IV glandular trichomes at different sampling times. Acylsugars content in Lycopersicon pennellii ‘‘LA2963’’ was more than two-folds high than those found in L. esculentum ‘‘Nandi’’, F₁ and F₂ populations, indicating that recessive gene(s) are responsible for the high acylsugar contents in L. pennellii ‘‘LA2963’’.     

Genetic analysis of sodium,potassium and chloride ion content inLycopersicon

Summary Plants of the salt-sensitiveL. esculentum, the halophytic wild speciesL. pennellii, their F₁ hybrids and the interspecific F₂ generation were grown in Hoagland's liquid culture containing 100 mM NaCl and 6 mM K⁺. Analysis of the Na⁺, Cl^- and K⁺ ions contents of the leaves showed, as observed also in previous studies, that the cultivated parent accumlated more K⁺ and less Na⁺ than the wild parent. A total of 117 F₂ plants were assayed for 15 electrophoretically detectable isozyme markers which map to nine of the twelve tomato chromosomes. Four loci, all with a similar quantitative effect on Na⁺ and Cl^- uptake, were identified by virtue of their linkage to isozyme markers. Two other loci were found to affect K⁺ uptake. This study demonstrates the potential value of using genetic markers in order to gain a better understanding of the genetic basis of quantitative traits associated with the response of plants to salinity stress.     

Monogenic resistance in tomato to Fusarium oxysporum f. sp. lycopersici race 3

Summary Resistance to fusarium wilt, incited by Fusarium oxysporum (Schlecht.) f. sp. lycopersici (Sacc.) Snyder & Hansen race 3 in tomato (Lycopersicon esculentum Mill.) was discovered in LA 716, a L. pennellii accession. A resistant BC₁F₃ breeding line, E427, was developed from LA 716. E427 was crossed with the susceptible cv. Suncoast and F₁, BCP₁, BCP₂ (to Fla 7155, a susceptible parent) F₂, F₃, and BCP₂S₁ seeds were obtained. Segregation for resistance following root dip inoculation over three experiments indicated a single dominant gene controlled resistance. Five of the 12 BCP₁S₁'s segregated more susceptible plants, whereas one of the 12 segregated more resistant plants than expected (P<0.05). Three of 23 F₃ lines segregated more susceptible plants than expected while 1 of the 23 had more resistant plants than expected (P<0.05). Segregation in all other lines fit expected ratios. Five of the 23 F₃'s were homozygous resistant which was an acceptable fit to expectations (P=0.1–0.5). The gene symbol I ₃ is proposed for resistance to race 3 of the wilt pathogen. Deviations from expected ratios in data reported here and for other breeding lines indicate an effect of modifier genes and/or incomplete penetrance. Plant age at inoculation and seed dormancy did not affect results.Florida Agricultural Experiment Station Journal Series No. 8101.     

Inheritance of resistance to Didymella lycopersici Klebb. in tomato (Lycopersicon Mill.)

Summary The inheritance of the resistance to Didymella lycopersici was studied on F₃- and Bc₁-lines from interspecific crosses of L. esculentum with L. hirsutum and with L. hirsutum glabratum. The resistance is not monogenic and is inherited in dominant fashion. The high h² values based on line means offer possibilities to seleet efficiently for enhanced levels of resistance.     

Tomato Spotted Wilt Virus (TSWV) resistance in tomato derived from Lycopersicon chilense Dun. LA 1938

Tomato spotted wilt virus (TSWV) resistance wasidentified in Y118 (Fla 925-2), an F₁BC₁S₆ tomato line(Lycopersicon esculentum Mill.), derived from a crosswith L. chilense Dun. (LA 1938). This line waspreviously selected for tomato mottle virus (ToMoV)resistance in Florida. Progeny from crosses betweenFla 925-2 and three different TSWV susceptible L.esculentum parents were used in TSWV resistancestudies. A total of 75 F₁ and 596 F₂ plants from allthree crosses were screened for TSWV resistance. ForF₂ plants free of TSWV symptoms, evaluations were madeusing enzyme-linked immunosorbent assay (ELISA). TenF₃ populations used for further greenhouse and fieldscreenings were selected from F₂ plants found to befree of the virus using visual and ELISA criteria ateach evaluation. One F₁ and four F₃ lines werestudied under field conditions (Stellenbosch, SouthAfrica) in which 100% of the `Flora-Dade' susceptiblecontrols were severely infected with TSWV. Theresults of the field study clearly establish that TSWVfield resistance is present in the Fla 925-2 (Y118)derived lines. Studies conducted on these linesrevealed that this resistance has the distinctcharacteristic of often `recovering' from initiallyhigh levels of virus titer in the tissue to levelsbelow detection with ELISA.     

Interactions of BA, GA3, NAA, and surfactant on interspecific hybridization of Lycopersicon esculentum × L. chilense

Time consuming and expensive tissue culture type techniques are currently used for achieving Lycopersicon esculentum × L. chilense interspecific hybrids. The objective of this study was to determine if the number of viable hybrid seed produced directly from this wide cross could be improved by optimizing female/male parent selection when treated with various hormones in the presence or absence of a surfactant (Triton X-100). Individual or combination treatments of BA(6-benzylaminopurine), GA₃ (gibberellic acid), and/or NAA(α-naphthaleneacetic acid) were applied (both with and without surfactant)to the ovaries of two L. esculentum female parents (Fla 7613 and89S) one day following pollination by nineL. chilense accessions. Hormone applications over 1-, 3-, and 5-dayperiods were made and all treatments were compared to a distilled water control. Across all experiments 21 viable hybrids from 1920 fruits (1.1%) were obtained from2128 pollinations. There were five consistent trends observed across three experiments. First, treatments utilizing NAA produced more hybrids than any other treatment hormone. Second, eight of the total 21 hybrids resulted from five consecutive days of hormone treatments (P-value ≤0.01), and six of those eight were realized when NAA was involved. Third, the use of Triton X-100 in combination with the hormone treatments significantly reduced the number of fruits/pollination (P-value ≤ 0.01), but did not significantly reduce the number of overall viable hybrids (P-value > 0.4). Fourth, the L. chilense male parent LA2759 significantly produced the most interspecific hybrids (P-value ≤ 0.04),seven hybrids from 295 fruits, compared to the male parent LA130 which did not produce any hybrids (from 293 fruits) -other accessions were intermediate. Fifth, nearly ten times more hybrids were produced from crosses with Fla 7613 than with 89S. These experiments provide a foundation for future studies to discern specific concentrations, timing and frequency of application of NAA and possibly, GA₃, to efficiently produce interspecific hybrids of L. esculentum × L. chilense. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of tomato leaf curl virus resistance in Lycopersicon hirsutum f. glabratum

Summary Inheritance of resistance to tomato leaf curl virus (TLCV) was studied in the progenies derived from interspecific crosses between TLCV resistant Lycopersicon hirsutum f. glabratum line B 6013 and five susceptible cultivars (HS 101, HS 102, HS 110, Pusa Ruby and Punjab Chhuhara) of L. esculentum. P₁, P₂, F₁, F₂, B₁ and B₂ progenies of the five crosses were artificially inoculated with local strains of TLCV by means of the vector whitefly, Bemisia tabaci (Genn.). and the disease reaction was studied in all the crosses. Reaction of parents, F₁, F₂ and backcrosses suggests that resistance derived from L. hirsutum f. glabratum B 6013 is based on two epistatic genes, one from the wild parent and one from the cultivated one, resulting in a 13:3 segragation in the F₂.     

Genetics of the viability of pollen grain produced at low temperatures inLycopersicon Mill.

Summary The different viabilities of pollen produced at low temperatures in intra- and inter-specific crosses of tomato were studied. Cultivars Red Top, Moneymaker, and Marroqui were crossed with cultivar E-15 and these four cultivars were hybridized with lines of the wild speciesL. pimpinellifolium PE-13,L. parviflorum PE-52,L. pennellii PE-47, andL. hirsutum PE-37 and PE-41. A six-generation family of the Moneymaker x PE-47 cross was obtained to carry out a more detailed genetical study of pollen grain viability at low temperatures. Pollen grain viability was evaluated during the winter via acetocarmine staining. When the parents were compared with their F₁, the intra-specific tomato crosses showed dominance to better-quality pollen, theL. esculentum x L. pimpinellifolium inter-specific crosses showed positive heterosis, while the crosses ofL. esculentum with the tolerant speciesL. pennellii andL. hirsutum showed intermediate inheritance. However, in theL. esculentum x L. pennellii family, the dominance and the non-allelic interactions (homozygosis x homozygosis) were the most important factors, so that dominance to better viability at low temperatures appeared to be the general mode of inheritance. Genetical control of pollen grain viability at low temperatures seemed to be polygenetic.