Multigenerational outbreeding effects in Chinook salmon (Oncorhynchus tshawytscha)

Outbreeding, mating between genetically divergent individuals, may result in negative fitness consequences for offspring via outbreeding depression. Outbreeding effects are of notable concern in salmonid research as outbreeding can have major implications for salmon aquaculture and conservation management. We therefore quantified outbreeding effects in two generations (F₁ hybrids and F₂ backcrossed hybrids) of Chinook salmon (Oncorhynchus tshawytscha) derived from captively-reared purebred lines that had been selectively bred for differential performance based on disease resistance and growth rate. Parental lines were crossed in 2009 to create purebred and reciprocal hybrid crosses (n = 53 families), and in 2010 parental and hybrid crosses were crossed to create purebred and backcrossed hybrid crosses (n = 66 families). Although we found significant genetic divergence between the parental lines (F_ST = 0.130), reciprocal F₁ hybrids showed no evidence of outbreeding depression (hybrid breakdown) or favorable heterosis for weight, length, condition or survival. The F₂ backcrossed hybrids showed no outbreeding depression for a suite of fitness related traits measured from egg to sexually mature adult life stages. Our study contributes to the current knowledge of outbreeding effects in salmonids and supports the need for more research to better comprehend the mechanisms driving outbreeding depression.     

Combining ability analysis over F1-F5 generations in diallel crosses of bread wheat

Summary Combining ability studies for grain yield and its primary component traits in diallel crosses involving seven diverse wheat cultivars of bread wheat (Triticum aestivum L.) over generations F₁-F₅ are reported. The general and specific combining ability variances were significant in all generations for all the traits except specific combining ability variance for number of spikes per plant in the F₅. The ratio of general to specific combining ability variances was significant for all the traits except grain yield in all the generations. This indicated an equal role of additive and non-additive gene effects in the inheritance of grain yield, and the predominance of the former for its component traits. The presence of significant specific combining ability variances in even the advanced generations may be the result of an additive x additive type of epistasis or evolutionary divergence among progenies in the same parental array. The relative breeding values of the parental varieties, as indicated by their general combining ability effects, did not vary much over the generations. The cheap and reliable procedure observed for making the choice of parents, selecting hybrids and predicting advanced generation (F₅) bulk hybrid performance was the determination of breeding values of the parents on the relative performance of their F₂ progeny bulks.     

Genetic mapping of a fertile tiller inhibition gene, ftin, in wheat

Increased fertile tiller number is a very important trait in high-yielding wheat (Triticum aestivum L.) lines. Uncovering the fundamental biological progress of fertile tiller formation would improve our understanding of the genetic nature of this trait and increase wheat yield. However, there is no suitable genetic material for studying the genetic mechanism of wheat fertile tiller formation. We report here the development of a fertile tiller inhibition line, Pubing3558, which was derived from a cross between common wheat and wild grass Agropyron cristatum. Pubing3558 possesses normal tillering ability in the seedling stage but reduced fertile tiller formation in the reproductive growth phase. A cross between Pubing3558 and the wheat cultivar Jing4841 with multiple fertile tillers was performed in order to map the fertile tiller inhibition gene (ftin). The F₁ population of the cross between Pubing3558 and Jing4841 showed a normal phenotype similar to that of the parent Jing4841. The F₂ population segregated with a 3:1 Mendelian ratio in 2-year replicates, which was further proven by the segregation ratio of the F₃ population. Genetic analysis uncovered that the fertile tiller inhibition trait in Pubing3558 is controlled by a single recessive gene. Using bulked segregant analysis methods, we located the ftin gene on wheat chromosome 1AS, which is linked closely to markers Xcfa2153 at a genetic distance of 1.0 cM. A combination of phenotype tests and genetic mapping demonstrated that the ftin gene is a newly identified gene in wheat. Overall, our results suggest that Pubing3558 will be valuable genetic source for studying the biological process of fertile tiller formation.     

Haplotype preference in lymphocyte differentiation. II. F1 hybrid helper T cells generated with antigen-bearing parental macrophages can cooperate with B lymphocytes of either parent.

Carrier-specific helper T cells were generated in F₁ hybrid mice by either conventional immunization procedures or by repeated immunizations with antigen-bearing macrophages derived from either F₁ or parental donors. The F₁ helper T cells generated in these various ways were then analyzed for their capacities to help hapten-primed B lymphocytes derived from each of the two parental strains as well as from F₁ donors in the development of secondary anti-hapten antibody responses. These analyses were conducted using two different types of in vivo assay systems as well as a totally in vitro system. Under all circumstances, helper T cells from F₁ mice, primed either in conventional fashion or with antigen bearing parental or F₁ macrophages, were capable of interacting effectively with B lymphocytes of each parent and of F₁ origin. Moreover, in the case of F₁ helper cells primed with antigen-bearing parental macrophages, there was no evidence of preferential helper activity for parental B lymphocytes corresponding to the type of macrophage used for sensitization; this was true irrespective of whether in vivo or in vitro assay systems were employed. The relevance of these findings and others which are either similar to, or discordant with, them to the general question of genetic restrictions in macrophage-T lymphocyte interactions is discussed.     

Cytotoxicity of autosensitized lymphocytes restricted to the H-2K end of targets

Lymphocytes from rodents cultured on syngeneic fibroblasts become cytotoxic against syngeneic but not against allogeneic target cells. We investigated whether known antigens are involved in the phenomenon and the data indicate that H-2 antigens must be shared between sensitizing fibroblasts and responder lymphocytes to generate autocytotoxic cells. Furthermore, the cytotoxicity of autosensitized lymphocytes is restricted to target cells identical with respect to theK and/orI regions. F₁ hybrid lymphocytes cultured on parental fibroblasts develop cytotoxicity towards sensitizing cells. In contrast, parental lymphocytes cultured on F₁ hybrid fibroblasts will not damage the F₁ cells, although they are cytotoxic against both syngeneic and allogeneic parental cells. In addition, parental or F₁ hybrid lymphocytes cultured on parental fibroblasts are not cytotoxic against F₁ hybrid target cells. Fibroblasts heterozygous for theK end only, are also resistant to the cytotoxic action of such lymphocytes. Thus it seems that H-2 antigens, specifically theK end, antigens have a significant role in the phenomenon of autosensitization.     

Molecular mapping of Yr53, a new gene for stripe rust resistance in durum wheat accession PI 480148 and its transfer to common wheat

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most damaging diseases of wheat worldwide. It is essential to identify new genes for effective resistance against the disease. Durum wheat PI 480148, originally from Ethiopia, was resistant in all seedling tests with several predominant Pst races in the US under controlled greenhouse conditions and at multiple locations subject to natural infection for several years. To map the resistance gene(s) and to transfer it to common wheat, a cross was made between PI 480148 and susceptible common wheat genotype Avocet S (AvS). Resistant F₃ plants with 42 chromosomes were selected cytologically and by testing with Pst race PST-100. A total of 157 F₄ plants from a single F₃ plant with 2n = 42 tested with PST-100 segregated in a 3 resistant: 1 susceptible ratio, indicating that a single dominant gene from PI 480148 conferred resistance. Using the F_3:4 population and the resistance gene-analog polymorphism (RGAP) and simple sequence repeat (SSR) markers, the gene was mapped to the long arm of chromosome 2B. SSR marker Xwmc441 and RGAP marker XLRRrev/NLRRrev ₃₅₀ flanked the resistance gene by 5.6 and 2.7 cM, respectively. The effective resistance of the gene to an Australian Pst isolate virulent to Yr5, which is also located on 2BL and confers resistance to all US Pst races, together with an allelism test of the two genes, indicated that the gene from PI 480148 is different from Yr5 and should be a new and useful gene for resistance to stripe rust. Resistant common wheat lines with plant types similar to AvS were selected for use in breeding programs.     

Genomic relationships between hexaploid Helianthus resinosus and diploid Helianthus annuus (Asteraceae)

Genus Helianthus comprises diploid and polyploid species. An autoallopolyploid origin has been proposed for hexaploid species but the genomic relationships remain unclear. Mitotic and meiotic studies in annual Helianthus annuus (2n = 2x = 34) and perennial Helianthus resinosus (2n = 6x = 102) as well as the F₁ hybrids between both species were carried out. Chromosome counting confirmed the hybrid origin of the latter plants and their tetraploid condition. Bivalents in hybrids ranged from 12 to 28 ( $ \bar{x} $  = 20.8). Univalents, trivalents and quadrivalents were also observed. Meiotic products comprised dyads, triads and normal tetrads and pollen grains were heterogeneous in size. These observations suggest the occurrence of 2n pollen in addition to the expected n. Genomic in situ hybridization (GISH) of total H. annuus DNA on H. resinosus chromosomes rendered weak but uniform signals; similar hybridization pattern was observed using three other annual species. Hybridization with H. annuus probe performed on root tip cells of F₁ H. annuus × H. resinosus hybrids revealed 17 chromosomes with a strong hybridization signal. GISH in hybrid meiocytes distinguished chromosomes from parental species and revealed autosyndetic pairing of H. resinosus chromosomes, allosyndetic pairing in bivalents, trivalents and quadrivalents, and the presence of univalents derived from parents, H. annuus and H. resinosus. Results obtained from classical and molecular cytogenetics do not support H. annuus as a direct ancestor of H. resinosus. The occurrence of allosyndetic pairing and the relatively high fertility of the F₁ hybrids point to the possibility that useful genes could be transferred from H. resinosus to cultivate sunflower, although the effective rate of recombination has not been evaluated. GISH method proved effective to recognize parental chromosomes in H. annuus × H. resinosus progeny.     

Inheritance of triterpene methyl ethers in Cortaderia (Gramineae)

The distribution of triterpene methyl ethers in several generations of interspecific hybrids of Cortaderia indicates dominant gene control of their synthesis. The hybrid C. richardii × C. toetoe is an exception because synthesis of α- and β-amyrin methyl ethers is suppressed in F₁ and F₂, but is restored in the backcross F₁ × C. toetoe; this backcross generation was heterozygous for genes for the amyrin methyl ethers, and on selfing segregated in a simple Mendelian ratio.     

Genotype-specific environmental impact on the variance of blood values in inbred and F1 hybrid mice

Mice are important models for biomedical research because of the possibility of standardizing genetic background and environmental conditions, which both affect phenotypic variability. Inbred mouse strains as well as F₁ hybrid mice are routinely used as genetically defined animal models; however, only a few studies investigated the variance of phenotypic parameters in inbred versus F₁ hybrid mice and the potential interference of the genetic background with different housing conditions. Thus, we analyzed the ranges of clinical chemical and hematologic parameters in C3H and C57BL/6 inbred mice and their reciprocal F₁ hybrids (B6C3F1, C3B6F1) in two different mouse facilities. Two thirds of the blood parameters examined in the same strain differed between the facilities for both the inbred strains and the F₁ hybrid lines. The relation of the values between inbred and F₁ hybrid mice was also affected by the facility. The variance of blood parameters in F₁ hybrid mice compared with their parental inbred strains was inconsistent in one facility but generally smaller in the other facility. A subsequent study of F₁ hybrid animals derived from the parental strains C3H and BALB/c, which was done in the latter housing unit, detected no general difference in the variance of blood parameters between F₁ hybrid and inbred mice. Our study clearly demonstrates the possibility of major interactions between genotype and environment regarding the variance of clinical chemical and hematologic parameters.     

Molecular mapping of leaf rust resistance gene Lr15 in hexaploid wheat

Leaf rust is a widespread and commonly occurring rust disease of wheat. Genetic resistance is the most economical method of reducing losses due to leaf rust. Lr15 has been shown to be present on wheat chromosome 2D and is reported to be a seedling resistance gene. However, tightly linked markers associated with Lr15 have not been reported to date. To identify molecular markers linked to Lr15, an F₂ mapping population of Thatcher × Thatcher-Lr15 was generated. Available wheat simple sequence repeat markers were utilized in parental screening and polymorphic markers were used to analyze the entire population of 221 plants. Phenotypic evaluations of the F₂-derived F₃ progenies with Puccinia triticina Eriks. pathotype 162A (93R15) confirmed the monogenic inheritance of Lr15. The linkage group representing chromosome 2DS was constructed at LOD 4.0 which revealed the closest flanking markers Xgwm4562 and Xgwm102 at a distance of 3.1 and 9.3 cM, respectively. Furthermore, utilization of these flanking markers in combination has successfully identified wheat lines with or without Lr15. These markers could potentially be useful in gene pyramiding with other genes to enhance rust resistance in wheat.     

Regional differences in the abundance of native, introduced, and hybrid Typha spp. in northeastern North America influence wetland invasions

In northeastern North America, an important wetland invader is the cattail Typha × glauca, a hybrid of native Typha latifolia and introduced Typha angustifolia. Although intensively studied in localized wetlands around the Great Lakes, the distributions of the hybrid and its parental species across broad spatial scales are poorly known. We obtained genotypes from plants collected from 61 sites spanning two geographical regions. The first region, near the Great Lakes and St. Lawrence Seaway (GLSL), has experienced substantial Typha increases over the last century, whereas more modest increases have occurred in the second region across Nova Scotia, New Brunswick, and Maine (NSNB). We found that hybrids predominate in the GLSL region, thriving in both disturbed and undisturbed habitats, and are expanding at the expense of both parental species. In contrast, the native T. latifolia is by far the most common of the three taxa across all habitat types in the NSNB region. We found no evidence that the formation of backcrossed and advanced-generation hybrids is limited by the reproductive barriers that are evident in F₁ hybrids. However, although backcrossed individuals arise in both regions, they are much less common than F₁ hybrids, which may explain why the parental species boundary remains. We conclude that F₁ hybrids are playing a key role in the invasion of wetlands in the GLSL region, whereas their low frequency in the NSNB region may explain why Typha appears to be much less invasive further east. An improved understanding of these contrasting patterns of distribution is necessary before we can accurately predict future wetland invasions.     

Consequences of hybridization between Ohomopterus insulicola and O. arrowianus (Coleoptera, Carabidae) in a segmented river basin: parallel formation of hybrid swarms

A putative hybrid zone between flightless earabid beetles, (iambus (Ohomopterus) insulicola and C. (0.) arrowianus nakamurai in the Ina Valley, central Honshu. Japan, was studied using experimental hybridization and morphological analysis. The principal morphological character analysed was a functional part of the male genitalia (the eopulatory piece), which is also the key diagnostic character in taxonomy. The habitat of the earabid beetles is fragmented by the Tenryu River, which runs through the Ina Valley, and its tributaries. Ohomopterus insulicola and 0. a. nakamurai occur in the upstream and downstream areas of the river basin, respectively, and a putative hybrid swarm (0. insulicola ssp. pseiidinsulkola) is found in the boundary area on the cast side of the Tenryu River. Experimental hybridization between 0. insulicola and 0. a. nakamurai resulted in F₁ offspring that were morphologically similar to 0. i. pseudinsulicola. Pre‐zygotic reproductive isolation was incomplete, although the F₁ males had a dysfunction in sperm production and were almost sterile. Only a single F₁ pair produced F₂, Backcrosses of F₁ females of males of the parental species resulted in offspring that were similar to the parental species in genital morphology. Based on the discriminant function for the shape of the eopulatory piece resulting from the experimental hybridization, we demonstrate that similar hybrid swarms exist on both sides of the Tenryu River, but in locations 25 km apart. This demonstrates parallelism in hybridization events with similar consequences. The hybrid swarms consisted of beetles with intermediate morphological characters and did not contain obvious parental forms. The establishment of such intermediate populations may have been facilitated by selection for fertile hybrids in segregated local sites in the absence of frequent immigration of parental species. This study suggests that a segmented river basin provides an opportunity for establishing novel evolutionary lineages resulting from hybridization.     

Spawning and fertility of F<Subscript>1</Subscript> hybrids of the coral genus <Emphasis Type="Italic">Acropora</Emphasis> in the Indo-Pacific

The role of hybridization through multi-specific synchronous spawning in the evolution of reef-building corals has been discussed since the 1990s, particularly for the genus Acropora. However, F₁ hybrids have been reported as common in only one case in the Caribbean, with no evidence of mechanisms that would allow continuous reproduction of the hybrids. In this study, we report for the first time the fecundity of two F₁ hybrid colonies produced experimentally from two Indo-Pacific species, A. intermedia and A. florida. These F₁ hybrids spawned at the same time as the parental corals. Backcrossing and F₁ hybrid crossing were successful in both directions. Furthermore, more than 90% self-fertilization was achieved in an F₁ hybrid, although it was negligible in the parental corals. While it is possible that the F₁ hybrid was a chimera, these results suggest that some products of interspecific hybridization may persist as the offspring of self-fertilizing F₁ hybrids.     

Characterization of a contemporaneous hybrid zone between two darter species (Etheostoma bison and E. caeruleum) in the Buffalo River System

Hybrid zones have long intrigued evolutionary biologists and provide a natural laboratory to explore the evolution of reproductive isolation (speciation). Molecular characterization of hybrid zone dynamics can provide insight into the strength of reproductive isolation as well as the underlying evolutionary processes shaping gene flow. Approximately one-third of darter species naturally hybridize making this species-rich North American freshwater teleost fish clade an ideal system to investigate the extent and direction of hybridization. The objective of this study was to use diagnostic microsatellite markers to calculate genetic hybrid index scores of two syntopic, but distantly related darter species, Etheostoma bison and Etheostoma caeruleum. A combination of hybrid index scores, assignment tests, and mitochondrial haplotype profiles uncovered mixed ancestry in approximately 6 % of sampled adult individuals, supporting contemporaneous hybridization that was previously undocumented in E. bison. Moreover, hybrids were not limited to the F₁ generation, but encompassed the entire suite of hybrid categories (F₁, F₂ and backcross hybrids). The low number of hybrids assigned to each hybrid category represents a bimodal hybrid zone, suggesting reproductive isolation is strong (but incomplete) and also advocates for the ability of hybrids to produce second-generation hybrids and backcross into both parental species, mediating introgression across species boundaries. To this end, cytonuclear profiles of the sampled parental species and hybrids were consistent with bidirectional gene flow, although there was an overall trend of asymmetric hybridization between E. caeruleum females and E. bison males. The spatiotemporal variation in hybridization rates and resulting cytonuclear patterns expanded on in this study provide a comparative genetic framework on which future studies can begin to elucidate the underlying processes that not only generate a mosaic hybrid zone, but maintain the distinctness of species in the face of gene flow.     

Genetic interactions between wheat and rye genomes in triticale

Summary Six primary triticale lines were produced from two advanced breeding lines of Triticum durum and three inbred genotypes of Secale cereale. The wheat and rye parents and the triticale derivatives were crossed in all possible combinations within each species group. Chiasma and univalent frequency of parents and hybrids were determined. The primary triticale lines had more univalents and less chiasmata per pollen mother cell than the corresponding wheat and rye parents together. The parental wheat F₁ exhibited negative heterosis for chiasma frequency whereas all rye hybrids had much higher chiasma frequencies than their inbred parents. Triticale F₁s generally showed lower chiasma frequencies and more univalents than their parents, but the degree of pairing failure was dependent upon which of the parental species within the triticale, wheat or rye, was in the heterozygous state. F₁s with heterozygous wheat genome only showed the least reduction in chiasma number (presumably caused by gene actions within the wheat genome), while F₁s with heterozygous rye genome showed high reduction in chiasma frequency and an increase in pairing failure (induced by negative interactions between the heterozygous rye and the wheat genome in triticale). A high correlation was found between the frequency of undisturbed pollen mother cells and the frequency of aneuploids in the subsequent generation. A higher number of aneuploids occurred in those populations which were heterozygous for the rye genome.     

Gene expression analyses in maize inbreds and hybrids with varying levels of heterosis

Background  

Heterosis is the superior performance of F₁ hybrid progeny relative to the parental phenotypes. Maize exhibits heterosis for a wide range of traits, however the magnitude of heterosis is highly variable depending on the choice of parents and the trait(s) measured. We have used expression profiling to determine whether the level, or types, of non-additive gene expression vary in maize hybrids with different levels of genetic diversity or heterosis.     

Relationship between gene expression and hybrid vigor in primary root tips of young maize (Zea mays L.) plantlets

Summary To provide an insight into the molecular basis of heterosis, we investigated gene expression in primary root tips of a heterotic maize hybrid (B73 × Mo17) and its parental lines (B73 and Mo17). This analysis was carried out (i) by differential plaque hybridization of a recombinant cDNA library made to poly(A) RNA isolated from B73 × Mo17 primary root tips, and (ii) by comparing with two-dimensional gel electrophoresis proteins synthesized in vitro in the rabbit reticulocyte system by poly(A) RNA isolated, at different stages of development, from the three genotypes. The results showed that there are sets of proteins and mRNAs that are differentially synthesized and expressed in the F₁ primary root tips in comparison to the parental lines. Moreover, results from the survey of 21 major in-vitrosynthesized polypeptide variants, from mRNAs of primary root tips of the parental lines and their F₁ hybrid, indicated that in seven instances hybrid proteins translated in vitro were more abundant or possibly new. In most of the remaining cases, hybrid spots were similar in intensity to the same protein produced by one of the two parental lines.     

Induced Homoeologous Pairing for Transfer of Useful Variability for High Grain Fe and Zn from <Emphasis Type="Italic">Aegilops kotschyi</Emphasis> into Wheat

Biofortification of bread wheat by the transfer of useful variability of high grain Fe and Zn from Aegilops kotschyi through induced homoeologous pairing is the most feasible approach to alleviate micronutrient malnutrition worldwide. Deficiency of chromosome 5B in interspecific hybrids allows homoeologous pairing and recombination of chromosomes of wheat with those of the related species. The interspecific hybrid plants without 5B chromosome showed much higher chromosome pairing than did the plants with 5B. The F₁ plants without 5B chromosome were selected and repeatedly backcrossed with wheat cultivar PBW343. The chromosome number of BC₂F₁ plants ranged from 43 to 60 with several univalents and multivalents. Molecular markers and GISH analysis confirmed the introgression of U/S chromosomes of Ae. kotschyi and their fragments in wheat. The BC₂F₂ plants showed up to 125 % increase in Fe and 158 % increase in Zn compared to PBW343 with Lr24 and Yr36. Induced homoeologous pairing in the absence of 5B was found to be an effective approach for transfer of useful variability for enhanced grain Fe and Zn content for biofortification of wheat for high grain micronutrient content.     

Development of a multiple bulked segregant analysis (MBSA) method used to locate a new stem rust resistance gene (Sr54) in the winter wheat cultivar Norin 40

An important aspect of studying putative new genes in wheat is determining their position on the wheat genetic map. The primary difficulty in mapping genes is determining which chromosome carries the gene of interest. Several approaches have been developed to address this problem, each with advantages and disadvantages. Here we describe a new approach called multiple bulked segregant analysis (MBSA). A set of 423 simple sequence repeat (SSR) markers were selected based on profile simplicity, frequency of polymorphism, and distribution across the wheat genome. SSR primers were preloaded in 384-well PCR plates with each primer occupying 16 wells. In practice, 14 wells are reserved for “mini-bulks” that are equivalent to four gametes (e.g. two F₂ individuals) comprised of individuals from a segregated population that have a known homozygous genotype for the gene of interest. The remaining two wells are reserved for the parents of the population. Each well containing a mini-bulk can have one of three allele compositions for each SSR: only the allele from one parent, only the allele from the other parent, or both alleles. Simulation experiments were performed to determine the pattern of mini-bulk allele composition that would indicate putative linkage between the SSR in question and the gene of interest. As a test case, MBSA was employed to locate an unidentified stem rust resistance (Sr) gene in the winter wheat cultivar Norin 40. A doubled haploid (DH) population (n = 267) was produced from hybrids of the cross LMPG-6S/Norin 40. The DH population segregated for a single gene (χ _1:1 ²  = 0.093, p = 0.76) for resistance to Puccinia graminis f.sp. tritici race LCBN. Four resistant DH lines were included in each of the 14 mini-bulks for screening. The Sr gene was successfully located to the long arm of chromosome 2D using MBSA. Further mapping confirmed the chromosome location and revealed that the Sr gene was located in a linkage block that may represent an alien translocation. The new Sr gene was designated as Sr54.     

Course of meiosis in F1 interspecific hybrids of Lycopersicon esculentum Mill. × Licopersicon chilense Dun.

A study was conducted into the course of meiosis in F₁ interspecific hybrids of Lycopersicum esculentum Mill (mutant line Mo 638) × Lycopersicum chinense Dul. and its parental forms. An F₁ interspecific hybrid was obtained through the embryo culture technique. A decrease in the chiasma frequency and an increase in the frequency of univalents and meiotic abnormalities compared to their parental forms were detected in hybrid plants. The number of univalents and the percentage of main impairments decreased, as the height of bud tier locations increased. A conclusion was made regarding the connection between the regularity of meiosis in the examined F₁ interspecific hybrids of Lycopersicon esculentum × Lycopersicon chilense, on the one hand, and the hybrid nature of genotypes and the influence of environmental factors, on the other hand.