High alpha-tomatine content in ripe fruit of Andean Lycopersicon esculentum var. cerasiforme: developmental and genetic aspects.

A variant of Lycopersion esculentum var. cerasiforme is described that deviates from the typical form of the entire species, including cultivated tomatoes, in possessing high levels (500-5000 micrograms/g of dry weight) of the steroidal alkaloid alpha-tomatine in its ripe fruits. This biotype is restricted to a tiny enclave in the valley of Río Mayo, Department San Martín, Peru. Among 88 accessions of var. cerasiforme from its present distribution in the Andes, a 90% association was found between high tomatine and bitter flavor; within the Mayo watershed, all samples from the upper drainage had bitterness and high tomatine; the frequency of both traits decreased to low levels toward the lower end. Tomatine therefore probably is the source of bitterness. Throughout L. esculentum tomatine is present at very high concentrations in earliest stages of fruit development, thereafter decreasing rapidly to midperiod, and finally diminishing gradually to near zero at maturity as a result of catabolism to biologically inert compounds, except in the variant described here. High tomatine content does not appear to affect adversely either the natives, among whom the bitter types are popular, or individuals who sampled them in this survey. Genetic determination of high tomatine in ripe fruits is totally recessive and appears to be monogenic with interaction with genes of minor effect. The prevailing pattern of glycoalkaloid synthesis and degradation in development of solanaceous fruits suggests a mechanism to protect against predation prior to ripening but to permit it afterward as a device to promote dispersal. In consideration of the nondegradative nature of the variant, its genetic determination, and very restricted geographic distribution, mutation to this form appears to be a random event of doubtful evolutionary significance.

Sexual hybridization of Lycopersicon esculentum and Solanum rickii by means of a sesquidiploid bridging hybrid.

A sesquidiploid hybrid having two genomes of Lycopersicon esculentum and one of Solanum lycopersicoides served as a pistillate bridging parent in crosses with Solanum rickii to produce L. esculentum x S. rickii hybrid progeny. Of the four progeny obtained, one (GH2754) was diploid and three were aneuploid with extra S. lycopersicoides chromosomes. The hybrids had morphological features of both parents, but attributes of the wild parent dominated. The hybrid nature of the four progeny was confirmed by isozyme, restriction fragment length polymorphism, and cytological analyses. A mean of 9.15 bivalents was observed in pollen mother cells of GH2754. A high level of pollen abortion was seen in all hybrids. Crosses of the hybrids with staminate S. rickii yielded one backcross individual, revealing a very low, but certain level of female fertility. Colchicine treatment of GH2754 generated one promising amphidiploid hybrid, which exhibited strong preferential chromosome pairing (94% of the examined cells had 24 bivalents) and appreciable pollen fertility (43% stainable). Chromosome pairing, isozyme, and restriction fragment length polymorphism data support a very close relationship between the two Solanum spp. and a much greater distance between them and L. esculentum, but the data do not discriminate between them in respect to their distances from the latter. The cytological and molecular observations, previous reports of successful transfer of traits from S. lycopersicoides to L. esculentum, and our hybridization of L. esculentum x S. rickii suggest good prospects for gene transfer from S. rickii to L. esculentum.

Recombination in sesquidiploid hybrids of Lycopersicon esculentum × Solanum lycopersicoides and derivatives.

Sesquidiploid hybrids of L. esculentum (L) x S. lycopersicoides (S) were backcrossed to L via L. pennellii (P) as a bridging species in order to detect and measure recombination. Although use of P injected its traits into the populations, the investigated traits were proven to originate from S. The appearance of S traits in diploids in the immediate progeny of sesquidiploids but mainly of derived alien addition types proved the occurrence of recombination at rates varying from 1.6% to 16%. In subsequent BC's, these traits were inherited in dominant Mendelian fashion, except for deviations favoring recurrent parent alleles, sometimes with highly significant deviations from 1∶1. Inheritance was investigated in BC and F2 ex BC for 13 traits with strong phenotypic modifications of morphological, physiological, and isozymic nature. Monogenic determination was confirmed in most instances by tight linkages. For most of the traits, small progenies allowed only rough estimates of linkage intensities, but for Wa (gene for White anthers, universal in S), a test cross with four markers on chromosome 8 established its locus 2 cM distal to dl, proximally on 8L. Also noteworthy is the linkage of Dls, a gene determining sensitivity of flowering to long days, close to sp, situated subterminally on 6L. For the majority of traits, these manifestations of linkage proved that the appearance of S traits resulted from recombination, not alien chromosome substitution - a conclusion also reinforced by observations of chromosome pairing in alien addition types and diploid derivatives. Recombined S alleles have loci in various chromosome positions. Although they were discovered on the shorter chromosomes (nos. 6-12), hybridization barriers precluded tests with the longer chromosomes. Thus, no evidence was found for restriction of recombination to certain chromosomes or chromosomal regions. The prospects therefore appear favorable for deriving valuable traits from the S parent.

Meiosis in sesquidiploid hybrids of Lycopersicon esculentum and Solanum lycopersicoides.

We have synthesized diploid hybrids between Lycopersicon esculentum and Solanum lycopersicoides and have converted them to allotetraploids. Two sesquidiploids, having two complements of the former parent and one of the latter, have been obtained by backcrossing the former parent with the alloploid. In meiosis of the sesquidiploid the L. esculentum chromosomes exhibit strong preferential pairing, consistently forming 12 bivalents, whereas the S. lycopersicoides chromosomes remain unpaired. This chromosomal comportment conforms with expectations based on meiosis of the 2x and 4x hybrids. Condensation of the S. lycopersicoides univalents is retarded in early diakinesis but their development appears normal at later stages. Presumably as a consequence of the orderly behavior of the L. esculentum bivalents and consequent contribution to each gamete, fertility of the sesquidiploids is higher than in L. esculentum autotriploids. The normally strict self-incompatibility is somewhat relaxed in the sesquidiploids. Extra S. lycopersicoides chromosomes can be transmitted from the sesquidiploid as pistillate parent, and the aneuploid progeny are viable. Establishment of alien addition races and their utilization to transmit desired genes from S. lycopersicoides to L. esculentum are anticipated.

Tight linkage between a nuclear male-sterile locus and an enzyme marker in tomato.

The linkage relationship of a nuclear male sterile locus, ms-10, was tested with two enzyme marker loci known to be on the same chromosome (long arm of chromosome 2). The results indicate the gene order is Est-1 - 18 cM - Prx-2 - 1.5 cM - ms-10. The linkage intensity of ms-10 and Prx-2 (1.5 cM) suggests that Prx-2 might provide a selectable marker for male-sterility. In accordance with this idea, the ms-10 allele was placed in cis with a rare-allele of Prx-2 (Prx-2 (1)). Selection on the basis of the codominant Prx-2 (1) allele should allow for more rapid and efficient transfer of the recessive male sterile allele into an array of genetic backgrounds, thus promoting its use in hybrid seed production.

Isozyme monitoring of genetic variation in Lycopersicon.

Linkages with isozymic loci facilitate the investigation of certain classes of genetic variation. Due to the mapping of 20 isozymic loci on nine of the 12 chromosomes of the cultivated tomato (Lycopersicon esculentum), much progress has been made in these applications, particularly in the analysis of interspecific hybrids. Isozymes can expedite the selective elimination of inferior wild parent germ plasm in backcross transfer of desired genes to the cultivated parent. Allelic isozyme constitution also aids in identification of lines, particularly in evaluating the purity of F1 hybrid cultivars. Advantages that isozymes impart to such investigations are: (1) unequivocal classification of phenotypes, (2) detection of heterozygotes, (3) lack of epistasis between isozyme loci, (4) lack of effect of allelic isozymes per se on morphology or physiology, (5) prolific source of monogenic markers, and (6) phenotyping at early developmental stages. Each of these attributes can be exploited to great advantage, but collectively they constitute a formidable argument for monitoring genetic variation by means of isozymes. Linkages between isozyme loci and qualitative loci can be exploited as in the monitoring of Mi (gene for root-knot nematode resistance derived from L peruvianum) by the very tightly linked Aps-1(1); in similar fashion, Prx-2(1) serves as a useful marker for ms-10 (male sterility). Asp-1 monitoring in the former is more reliable than testing for nematode resistance per se; codominance of Prx-2 alleles of the latter solves problems incurred by the recessiveness of ms-10; in both instances phenotypes can be ascertained at earlier growth stages for isozymes than for economic traits. In the first backcross of the interspecific hybrid L esculentum x Solanum pennellii to the former, the segregation of four quantitative traits was monitored by allelic isozymes at 12 loci, situated on at least eight chromosomes, covering approximately 60% of the known tomato genome. At least five quantitative trait loci (QTL) were found to determine each of the four traits. Each parent contributes alleles with positive as well as negative effects, the greatest balance for stigma exsertion, the trait also exhibiting the greatest extent of transgressive segregation. Three pairs of linked isozymic loci permitted a crude form of three-point mapping of the associated QTL. interactions between QTL linked with pairs of isozymic genes were tested in all possible combinations; 18 of the 274 comparisons showed significant interactions, indicating epistasis.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for Extensive Overlap of Sporophytic and Gametophytic Gene Expression in Lycopersicon esculentum.

Male gametophyte (pollen) isozyme profiles were compared with those of the sporophyte for nine enzyme systems. Sixty percent of the structural genes coding for these enzymes in the sporophyte were also found to be expressed by the gametophyte. All the genes tested were found to be expressed after meiosis, apparently transcribed and translated in the haploid gametophytes.

The effect of isozyme selection on metric characters in an interspecific backcross of tomato - basis of an early screening procedure.

The extent of correlation was estimated between isozyme genotypes and the four widely segregating characters - leaf segment W/L ratio, stigma exsertion, fruit weight, and seed weight - in the first backcross of F1 Lycopersicon esculentum x Solanum pennellii to the former parent. The inbred parents differ in their alleles at the 12 tested isozymic loci, which are known to mark a minimum of eight of the twelve tomato chromosomes. Based on the isozyme data, a mean heterozygosity value, ¯H, was calculated which estimates the proportion of pennillii alleles in each individual. Correlations between mean heterozygosity and observed levels of each quantitative trait were highly significant and positive or negative as expected from the relative parental values. Plants with the lowest mean heterozygosity - i.e., closest to the esculentum zymotype also had mean values closest to those of this parent amongst the whole backcross population for each of the quantitative traits.Bivariate and multiple regression analysis was used to evaluate the ability of isozymes vs diagnostic morphological characters to estimate the portion of recurrent parent genes carried in each backcross individual. The results suggest that isozyme data gives better estimates than single diagnostic morphological characters and approach the level obtained by combinations of three morphological traits. Since electrophoretic determinations are made on small seedlings, selection at that stage can effect great savings of space and effort by greatly deminishing the size of the population needed at maturity. As such, isozyme selection would precede morphological selection but not replace it, thus the predictive value of these biochemical markers as well as diagnostic morphological characters could be obtained.

Isozymic gene linkage map of the tomato: Applications in genetics and breeding.

New linkage data are presented for the situation of five previously unlocated isozymic loci of the tomato and closely related species with homosequential chromosomes.Prx-1 lies on chromosome 1, where it is also linked withSkdh-1; Aps-2 is linked withGot-4 on chromosome 8;Tpi-2 has been allocated to chromosome 4; and a linkage has been detected betweenPgi-1 andEst-4, whose respective chromosome has not yet been determined. These and previously published data have been summarized in the form of an isozyme linkage map. Twenty-two loci have thus been mapped on nine of the twelve tomato chromosomes. We discuss some new applications of mapped isozymic genes. In certain types of segregations, isozymic genes are far more efficient than morphological markers in providing linkage information. They greatly expedite the cytogenetic investigation of species hybrids and can be utilized to facilitate backcross transfers of genes from wild to cultivated taxa.

Genetics of esterases in species of Lycopersicon.

Improvements in plant culture and electrophoretic technique permit detection and genetic analysis of seven esterase loci in Lycopersicon esculentum and related species with homosequential chromosomes. At all of these loci except one, each allele codes for a single anodal band, and the electrophoretic variants are inherited in monogenic fashion. For the exceptional Est-4, allozymes are 1-3 banded in various combinations at four positions, and rare recombinants in one cross appeared at a frequency of 0.0005, suggesting the existence of several very tightly linked genes. Est-2 segregated solely for intensity differences in dominant/recessive fashion; Est-3 and Est-4 behave as monomers; the remaining Est-l, 5, 6, and 7 - coding for contiguous bands in the region closest to the origin - are dimeric. The latter group are tightly linked inter se in the proximal portion of 2L (long arm of chromosome 2), the total map distance of the complex being approximately 1.5 cM; Est-2 is situated on 9L between ah and marm; Est-3 on 1L between inv and dgt; Est-4 has not yet been located. Even in the interspecific hybrids, map distances are similar to the standard values for L. esculentum. Tandem duplication is hypothesized for the origin of the Est-l, 5-7 complex, which adds another example to the growing list of linked mimic genes in the tomato genome. In respect to the position of their bands and tight inter se linkage, this series exactly parallels the EA, EB, EC esterase series in Hordeum vulgare - a fact which suggests great antiquity for this block of genes.

A pseudoduplication in Lycopersicon pimpinellifolium.

A double-banded variant of Prx-2 encountered in a single wild plant in a previous survey of the peroxidases of Lycopersicon pimpinellifolium was subjected to genetic analysis. Segregations obtained from crosses of this true-breeding type with various alleles of Prx-2 yielded anomalous segregations that are not compatible with a single locus model and suggest activity of alleles of a gene at another, independent locus. A critical test was afforded by studying simultaneous segregation for the apparent duplication and for two alleles at the neighboring Prx-3, a gene which proves to be very tightly linked with Prx-2. The results of the new test revealed independence between the double-banded type and Prx-3, hence also between the former and Prx-2. The most tenable hypothesis stipulates that an independent gene mPx2(1) modifies post-translationally the product coded by Prx-2(+) to yield two electrophoretically separable isozymes. These results reveal the risk of assuming, without appropriate genetic tests, that gene duplication is necessarily the cause of multiple banding.

Peroxidase complex with concomitant anodal and cathodal variation in red-fruited tomato species.

Four groups of bands (a-d) are controlled by 19 alleles of the Peroxidase-4 (Prx-4) complex in the red-fruited tomato species, Lycopersicon esculentum and L. pimpinellifolium. Heterozygotes can be detected by virtue of codominance in all combinations except a few in which bands of single groups are absent ("semi-null" alleles). No recombinations were detected in 7419 F(2) segregants of 53 different combinations of alleles. A maximum fiducial limit (P = 0.01) of 0.08% crossing-over between any Prx-4 band groups is estimated. Variation of the anodal b bands is absolutely associated with that of the cathodal d band in respect to presence versus absence and direction of migration. In respect to the origin of these variants, the probability of 18 instances of simultaneous mutation of genes at two loci, always in such complete agreement, is so remote that no more than one locus could conceivably govern b and d. The disposition of a is not similarly associated with that of the other bands, while that of the faint-staining c could not always be reliably resolved. The negation of all save extremely low recombination rates and the observed concomitant variation of b and d strongly support the concept of single locus control of all Prx-4 banding, this hypothesis being espoused until rejection should be required be required by future research. Models of single locus control of several isozymes are discussed.

Genetic and biosystematic studies on two new sibling species of Lycopersicon from interandean Perú.

All available accessions of the entity previously named "L. minutum" and described by Chmielewski were investigated biosystematically. All such lines can be unequivocally classified into two elements that are morphologically distinguished, chiefly by differences in flower size and exsertion of the stigma. The stigmas of the large-flowered type are strongly exserted; those of the smaller, slightly or not at all. Both forms are sporadically dispersed in the central and northern Peruvian Andes, east of the continental divide. They are sympatric and often cohabit in the Apurimac-Ayacucho-Cuzco region. Despite this overlap of ranges and intermingling, intermediate types have not been found in nature, although they can be produced experimentally by reciprocal crosses between the two forms. The F1 hybrids are highly fertile, but seeds produced by self-pollination germinate poorly.Variation at the individual, populational, and higher levels was assessed in progeny tests of wild plants by analysis of 14 enzyme loci, of which eight proved to be polymorphic. The two taxa could be entirely distinguished by alternative alleles of Got-3; perfect agreement was also found in the sympatric region for alleles of Prx-3; a variable degree of differentiation exists at the other polymorphic loci. For the small-flowered taxon, all tested individuals were homozygous, and all members of a single population had identical genotype; furthermore, only limited differences at two loci were found in the eight tested accessions. In contrast, the larger-flowered type exhibits considerable variation in terms of intra- and interpopulational polymorphy, heterozygosity, and other evidence of outcrossing. With the aforementioned exception, all allozymes detected in the smaller-flowered form are known in the larger-flowered type. All evidence from flower morphology and measures of variability consistently alludes to appreciable outcrossing in the latter and strict autogamy in the former. Considerations of all lines of evidence lead to the conclusion that the self-pollinated entity is sufficiently differentiated from the outcrosser to deserve specific status. The former is named L. parviflorum, the latter, L. chmielewskii, and Latin diagnoses are presented. It is most likely that L. parviflorum evolved sympatrically from L. chmielewskii by virtue of its acquiring autogamous reproduction - an isolating mechanism that is apparently reinforced by poor reproductivity of the interspecific hybrids.

Four peroxidase Loci in red-fruited tomato species: genetics and geographic distribution.

The banding patterns of certain anodal peroxidase variants of red-fruited tomato species are governed by alleles at four loci-two alleles per locus. Alleles at three loci code for modified enzyme migration patterns and are codominant in heterozygotes; those at the fourth locus code for presence or absence of a band. No evidence of linkage was detected in preliminary tests between four of the six possible combinations of loci. All variant alleles-i.e., those not represented in the standard genotype of Lycopersicon esculentum-exist in the wild L. pimpinellifolium from coastal Peru; all but Prx-3(n) are also known in L. esculentum from the sympatric region but are rare or absent elsewhere. Between the distributions of alleles of Prx-1 and those of Ge, the gamete-eliminator locus, a significant association exists, which probably does not owe to genetic linkage. The tendency of alleles of Prx loci, as well as those of cm, Ge, h, and Od, to be shared between wild and cultivated taxa in the sympatric region but seldom elsewhere, in addition to published correlated evidence, suggests that the wild alleles tend to substitute in cultivated forms as a result of introgression. In respect to the number of common alleles, cultivated tomatoes more closely resemble the wild L. esculentum var. cerasiforme than L. pimpinellifolium.