Breeding tomato for pollen tolerance to low temperatures by gametophytic selection

Haploid selection for traits related to pollen cold tolerance in tomato was performed in segregating populations derived from a Lycopersicon esculentum × L. pennellii hybrid. BC₁ populations were obtained by combining normal and low temperature treatments on two stages of pollen development: pollen formation, and germination and pollen tube growth. F₁ hybrids were cultivated under low and normal temperatures and their pollen was used to pollinate L. esculentum plants at low and normal temperatures. The four BC₁ populations obtained were tested for the quality and quantity of pollen produced at low temperatures. The population obtained by cold treatment at both stages had a significantly improved pollen germination ability at low temperatures. The two other coldselected BC₁ populations showed no differences compared with the unselected BC population. A second cycle of pollen selection, corresponding to BC₂, was applied in order to test its persistence in the subsequent generations and the possibility to further improve the character. This second cycle showed no improvement although some plants retained the high pollen germination ability at low temperatures that was observed in the first cycle. Hence, gametophytic selection of some characters related with tomato pollen performance may be feasible, at least for the first cycle of selection.     

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.     

Introgression of in vitro regeneration capability of Lycopersicon pimpinellifolium Mill. into recalcitrant tomato cultivars

The goal of this research was to study the introgression of the high regeneration capacity of Lycopersicon pimpinellifolium Mill line WV-700, in recalcitrant tomato cultivars (L. esculentum Mill cvs. Petomech, Santa Rita and VFN-8) using backcrossing. Hypocotyl explants of in vitro germinated seeds were cultivated in half strength MS medium supplemented with 5mg/l 6-BA to assess their shoot regeneration capacity. The apical shoot of the in vitro germinated seeds were cultured on a separate medium. Apical shoots from genotypes showing high regeneration rates were acclimated in a glasshouse and used as pollen donors for the next backcrossing. After four backcrossings, the material showed a similar mean fruit weight for the cultivated tomato and a high regeneration capacity similar to the wild species. It is shown that L. pimpinellifolium can be used with success as donor parent to introgress in vitro regeneration capacity to recalcitrant tomato cultivars. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Overcoming hybridization barriers between pea and some of its wild relatives

Pea would benefit from the plasticity and adaptability of its cross-incompatible relatives Pisum fulvum and Lathyrus sativus L., and we have tested reciprocal sexual crossings by manually cross-pollinating plants of genotypes of these three species. Studies of in situ germination of pollen grains on stigmata showed that pollen tubes were generally unable to germinate or could not reach the ovary. A few putative hybrid pods were nevertheless harvested, with one grain per pod germinated in vitro, then micropropagated for flow cytometry, isoenzyme, molecular (ribosomal ITS PCR-RFLP) and genomic in situ hybridization (GISH) studies. One such grain was recovered from an inter-generic cross of P. sativum x L. sativus and four from an inter-specific P. sativum x P. fulvumcross. A strong cross-incompatibility was shown between pea and grass pea, where the putative hybrid turned out to be pea. Conversely, with the interspecific, P. sativum x P. fulvum cross, flow cytometry and isoenzymes with leaf tissues strongly suggested hybridity, while molecular approaches and GISH confirmed the production of inter-specific hybrids, and without the need for a wild type P. sativum accession as a bridging cross.     

Genetics of resistance to early blight (Alternaria solani Sorauer) in tomato (Lycopersicon esculentum L.)

The genetic nature of early blight resistance in tomato was studied in three crosses at seedling and adult plant stages. A six generation mean analysis of the cross Arka Saurabh (susceptible) × IHR1939 (resistance) and its reciprocal cross revealed that the resistance to early blight was conferred by recessive polygenes at both seedling and adult plant stages. This polygenic early blight resistance revealed the importance of additive and additive × additive gene effects at seedling stage and higher magnitude of dominance and dominance× dominance gene effects at adult plant stage. Evaluation of parents, F₁, F₂ and backcross generations of IHR1816 (resistance) × IHR1939 (resistance) revealed that the early blight resistance genes in IHR1816 (Lycopersicon esculentum NCEBR-1) and IHR1939 (Lycopersicon pimpinellifolium L4394) are independent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sources of resistance to tomato-yellow-leaf-curl-virus (TYLCV) in Lycopersicon species

Summary Sources of resistance to tomato-yellow-leaf-curl-virus (TYLCV) were investigated in 16 accessions of three Lycopersicon species and 55 commercial tomato hybrids and cultivars. All commercial hybrids and cultivars were highly susceptible. Accessions of L. hirsutum, L. hirsutum f. glabaratum and L. pimpinellifolium showed a wide range of reactions. Those of L. peruvianum, LA 385 of L. peruvianum f. humifusum exhibited very high levels of resistance indicating their potential use in local breeding programs. In contrast to earlier findings, back indexing showed that all symptomless genotypes in this investigation were carriers of the TYLCV.     

Studies of pollen grain germination,pollen tube growth,micropylar penetration and seed set in intraspecific and intergeneric crosses within three Cruciferae species

Summary Pollen grain germination, pollen tube growth and micropylar penetration were investigated in intraspecific and intergeneric crosses involving Brassica napus L. ssp. oleifera (Metzg.) Sinsk. (oil-seed rape or fodder rape), xBrassicoraphanus Sageret (Raparadish) and diploid (2x) and tetraploid (4x) accessions of Sinapis alba L.(white mustard). For the reciprocal intergeneric crosses between B. napus and xBrassicoraphanus no effective barriers to pollen tube growth on stigmata or in styles were observed. The resulting low frequency of hybrid plants was mainly associated with a low rate of ovules with micropylar penetration per siliqua or with embryo abortion. Hybrid plants could be obtained without use of embryo rescue. In reciprocal crosses between B. napus and S. alba 2x or 4x incongruity barriers were observed on the stigma, in the style, and in the ovary resulting in a low frequency of ovules with micropylar penetration per siliqua. Open flower-pollination compared to bud-pollination generally was the more favourable procedure for pollen grain germination and pollen tube growth in crosses involving S. alba, but for micropylar penetration and seed set no differences were observed. Crosses between S. alba 2x () and B.napus () were found to result in a higher frequency of ovules with micropylar penetration as compared to reciprocal crosses or crosses with S. alba 4x. All reciprocal crosses between B. napus and S. alba 2x or 4x were unsuccessful when no embryo rescue was applied. Embryo rescue shortly after pollination, i.e. 2 to 5 days, however, resulted in hybrid seeds and plants, but only when applied to crosses between S. alba 2x () and B. napus (). The possible effects of the genome constitution, taxonomic distance and the parthenogenetic and parthenogenesis inducing ability of the parental genotypes on the observed malfunctions at the pre-and/or post-zygotic stage of the pollen grain-pistil interactions are discussed.Abbreviations DAP Days After Pollination - IAA Indole-3-Acetic Acid - kin Kinetin     

Unique meiotic behaviourin F1 plants from a cross between a non-tuberous and a tuberous Solanum species in section Petota

Uniformly abnormal meiotic behaviour was observed in 12 F₁-plants from a cross between Solanum etuberosum (non-tuberous) and S. pinnatisectum (tuberous). Per pollen mother cell at MI an average was found of 3.64 bivalents (all rodshaped, 1–2 per cell, heteromorphic), 16.64 univalents (scattered haphazardly on a continuous bipolar spindle) and 0.03 trivalents (all Y-shaped). Lagging chromosomes and precocious division of univalents very frequently occurs, leading to unequal distribution of chromosomes, aneuploid gametes and male sterility. Heteromorphic bivalents at MI, loops in bivalents at pachytene and non-disjunction in one hybrid plant, point to a highly abnormal meiotic behaviour. The occurrence of few trivalents is discussed.Considering that according to the literature nearly normal pairing was observed in the intergeneric F₁-hybrids Lycopersicon esculentum × Solanum lycopersicoides and L. esculentum x S. pennellii, the lack of chromosome pairing in an interspecific F₁-hybrid, of which both Solanum parents belong to the same section, is paradoxical to a plant breeder and might even be conspicuous to a taxonomist.     

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.     

Cytological mechanism of 2n pollen formation and unilateral sexual polyploidization in Lolium

Summary Plants of Lolium perenne, L. multiflorum and L. hybridum (331, 51 and 18, respectively) were screened for 2n pollen production. The screening was based on the size differences that are expected to be found between n and 2n pollen of a plant. It was found that 28 plants of perennial ryegrass-belonging to 13 cultivars-produced produced pollen grains of heterogenous size (big and small). The estimated frequency of big pollen grain production was higher than 10% in ten out of 28 plants and in three of them it reached a value of 100%. Eight plants obtained from two 4x × 2x crosses, in which the male parent had been previously identified as producing pollen grains of heterogenous size, was tetraploid. The cytological mechanism of big pollen grain formation observed in three plants consists in the lack of spindle formation and, consequently, of chromosome migration in anaphase II. From these breeding and cytological results, it was concluded that the big pollen grains observed were viable 2n pollen grains. The nuclei of the 2n pollen grains produced by this mechanism are genetically equivalent to those formed by the restitution of the second meiotic division. The value of these gametophytes in a breeding program of Lolium is discussed.     

Inheritance of resistance to fruit rot (Phytophthora parasiticaDast.) in tomato (Lycopersicon esculentum Mill.)

Summary Fruit rot disease caused by Phytophthora parasiticaDast. is a limiting factor in tomato production in Himachal Pradesh. 30 to 60 per cent fruits are damaged by this disease. Crosses were made between EC 54725 (Lycopersicon pimpinellifolium), a small tyuited type, resistant to fruit rot and four highly susceptible tomato commercial cultivars (Gola, Sioux, S12, and Lalmani). Studies of F₁'s, F₂'s and back crosses indicated that EC 54725 carries a dominant gene imparting resistance to fruit rot.     

Low-temperature-related growth and photosynthetic performance of alloplasmic tomato (Lycopersicon esculentum Mill.) with chloroplasts from L. hirsutum Humb. & Bonpl.

Growth and photosynthetic performance were analyzed in alloplasmic tomato at a high- (25/17 ^°C; HTR) and low-temperature regime (12/6 ^°C; LTR) in order to establish the role of cytoplasmic variation on low-temperature tolerance of tomato (Lycopersicon esculentum Mill.). Four alloplasmic tomato lines, containing the nuclear genome of tomato and the plastome of L. hirsutum LA 1777 Humb. & Bonpl., an accession collected at high-altitude in Peru, were reciprocally crossed with 11 tomato entries with a high inbreeding level and a wide genetic variation, resulting in a set of 44 reciprocal crosses. Irrespective of growth temperature, alloplasmic families with alien chloroplasts of L. hirsutum (h) were on average characterized by a high shoot biomass, a large leaf area, and a low specific leaf area in comparison with their euplasmic counterparts. These results do not directly point to an advantageous effect of h-chloroplasts on biomass accumulation at low temperature but rather towards a small general beneficial effect on growth and/or distribution of assimilates. Significant chloroplast-related differences in photosynthetic performance, however, were not detected at both temperature regimes, indicating that h-chloroplasts can properly function in a variable nuclear background of L. esculentum. It is concluded that chloroplast substitution is not an effective method for breeding tomato plants with improved low-temperature tolerance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Screening wild Lycopersicon species for resistance to powdery mildew (Oidium lycoperiscum)

Since the late 1980s powdery mildew, designated Oidium lycopersicum, frequently invaded the tomato crop in Western Europe. All commercial cultivars are susceptible. To screen for resistance in wild species a reliable and efficient disease test was developed. Young plants with two to three true leaves are inoculated at high relative humidity by spraying with a freshly prepared suspension of 2×10⁴ conidia, ml^–1. Symptoms are periodically evaluated according to a scale based on the percentage of leaf area with mycelium.One hundred and twenty seven accessions, representing eight wild Lycopersicon species, were screened for resistance to O. lycopersicum. A large variation in resistance was found between species. L. hirsutum was the most resistant species; L. pennellii was moderately resistant; species of the subgeneric group of L. esculentum and of the peruvianum-complex were all susceptible. L. parviflorum was classified separately due to a large variation between accessions. Except for this species, a low variation was found between accessions within species. High levels of resistance were observed in four accessions of L. hirsutum, in one of L. parviflorum and in one of L. peruvianum. This resistance is characterized by a very low disease incidence and a strongly restricted mycelium growth and lack of sporulation.     

Inheritance of groat protein percentage in Avena sativa L. × A. Fatua L. Crosses

Summary Gene action and heritability of groat protein percentage were determined in F₁, F₂, and F₃ generations of nine crosses between three Avena sativa L. cultivars and three A. fatua L. selections. Relationships among groat protein percentage, grain yield, and 100-seed weight also were evaluated. The three A. sativa parents were Dal (high grain yield and intermediate groat protein percentage), Goodland (low grain yield and high groat protein percentage), and Stout (high grain yield and low groat protein percentage). The three A. fatua parents were chosen for the study on the basis of vigorous plant growth and high groat protein percentage. The study was conducted at Madison, Wisconsin in 1979 and 1980.There was partial dominance towards low groat protein percentage. Narrow sense heritability estimates for groat protein percentage were low in Dal and Goodland crosses and intermediate in Stout crosses. In the F₂ generation, groat protein percentage was significantly higher in shattering than in nonshattering plants in 1979, but not in 1980. There were significant, positive correlations between groat protein percentage, 100-seed weight, and grain yield in F₁ and F₂ generations, but they were not large numerically. Relationships among these traits were either negative or nonsignificant in the F₃ generation. Although our results indicated that selection for higher groat protein percentage is possible when a low protein A. sativa cultivar is used, most of our simple cross progenies from A. sativa x A. fatua crosses had weak straw and were susceptible to crown rust (Puccinia coronata Cda. var. avenae Fraser and Led.).     

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.     

Development of tomato SSR markers from anchored BAC clones of chromosome 12 and their application for genetic diversity analysis and linkage mapping

In this study, we developed a total of 37 simple sequence repeat (SSR) markers from 11 bacterial artificial chromosome (BAC) clone sequences anchored on chromosome 12 of tomato available at Solanaceae Genomics Network. These SSR markers could group a set of 16 tomato genotypes comprising of Solanum lycopersicum, S. pimpinellifolium, S. habrochaites, and S. pennellii unambiguously according to their known species status. Clear subgroups of genotypes within S. lycopersicum were also observed. A subset of 16 SSR markers representing the 11 BAC clones was used for developing genetic linkage maps of three interspecific F₂ populations produced from the crosses involving a common S. lycopersicum parent (CLN2498E) with S. pennellii (LA1940), S. habrochaites (LA407) and S. pimpinellifolium (LA1579). The length of the genetic linkage maps were 112.5 cM, 109.3 cM and 114.1 cM, respectively. Finally, an integrated genetic linkage map spanning a total length of 118.7 cM was developed. The reported SSR markers are uniformly distributed on chromosome 12 and would be useful for genetic diversity and mapping studies in tomato.     

Effect of selection for pollen grain size on various traits in common bean

Summary Selection among microgametophytes usually exploits variation in pollen grain germination. Studies of variation in pollen grain size in common bean (Phaseolus vulgaris L.) suggested that selection for size might lead to changes in sporophytic traits. To determine whether microgametophytic selection based on size would affect pollen grain size in subsequent generations or sporophytic traits that were correlated with pollen grain size, pollen grains from three crosses were separated into two size categories by sieving and then used to pollinate cv. Diacol Calima. Selection resulted in changes in pollen grain diameter for pollen from F₁, F₂ and F₃ plants for all crosses. In vitro germination indicated no differences between vigor of large and small grains, but extraction and sieving reduced germinability. F₁ seed from two of the crosses with size-selected pollen varied in weight according to pollen grain size, but in subsequent generations, the effect disappeared. Both size categories of selected pollen resulted in F₂ progeny with reduced numbers of seeds per pod as compared to controls, suggesting that the size selection process may have resulted in indirect selection for traits reducing seed set. The overall results suggested that genes determining pollen grain size in bean have little or no effect on sporophytic traits such as seed size and seed yield.     

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.     

Reaction of Lycopersicon cultivars and wild accessions to tomato leaf curl virus

Summary 1201 tomato cultivars, breeding lines and accessions of Lycopersicon species were screened for tomato leaf curl virus (TLCV) under field and laboratory conditions during summer seasons of 1986 to 1989. Two lines of L. hirsutum (PI 390658 and PI 390659) and 2 lines of L. peruvianum (PI 127830 and PI 127831) were resistant to TLCV infection. These accessions did not produce any leaf curl symptoms either in field or after inoculation by whitefly Bemisia tabaci with TLCV. Adult whiteflies died within 3 days after releasing on resistant accessions (PI 390658, PI 390659, PI 127830), whereas the whiteflies survived upto 25 days on susceptible tomato cultivars. Under field conditions 0–4 and 5–25 adult whiteflies were observed on resistant and susceptible cultivars respectively. Hybridization was effected using the popular tomato cultivars Arka Sourabh, and Arka Vikas, as the female parents and the resistant Lycopersicon wild species as the pollen parents, to incorporate the resistant gene(s) into the edible tomatoes.