Barbara E. Liedl

QTL analysis of pest resistance in the wild tomato Lycopersicon pennellii: QTLs controlling acylsugar level and composition.

Some accessions of Lycopersicon pennellii, a wild relative of the tomato Lycopersicon esculentum, are resistant to a number of important pests of cultivated tomato due to the accumulation of acylsugars, which constitute 90% of the exudate of type-IV trichomes in L. pennellii LA716. An interspecific F2 population, created by the cross L. esculentum x L. pennellii LA 716, was surveyed for acylsugar accumulation and subjected to RFLP/QTL analysis to determine the genomic regions associated with the accumulation of acylglucoses, acylsucroses, and total acylsugars, as well as with acylglucoses as a percentage of total acylsugars (mole percent acylglucoses). Data were analyzed using MAPMAKER/QTL with and without a log10 transformation. A threshold value of 2.4 (default value for MAPMAKER/QTL) was used, as well as 95% empirically derived threshold values. Five genomic regions, two on chromosome 2 and one each on chromosomes 3, 4 and 11, were detected as being associated with one or more aspects of acylsugar production. The L. esculentum allele is partially dominant to the L. pennellii allele in the regions on chromosomes 2 and 11, but the L. pennellii allele is dominant in the region on chromosome 3. Throughout this study, we report the comparative effects of analytical methodology on the identification of acylsugar QTLs. Similarities between our results and published results for the genus Solanum are also discussed.

Unilateral incongruity in crosses involvingLycopersicon pennellii andL. esculentum is distinct from self-incompatibility in expression, timing and location

Both interspecific and intraspecific mechanisms restrict the exchange of genes between plants. Much research has focused on self incompatibility (SI), an intraspecific barrier, but research on interspecific barriers lags behind. We are using crosses betweenLycopersicon esculentum andL. pennellii as a model with which to study interspecific crossing barriers. The crossL. esculentum×L. pennellii is successful, but the reciprocal cross fails. Since the cross can be successfully made in one direction but not the other, gross genomic imbalance or chromosomal abnormality are precluded as causes. We showed that the lack of seed set observed in the crossL. pennellii×L. esculentum is due to the inability of pollen tubes to grow more than 2–3 mm into the style, whereas S1 crosses show continued slow pollen tube growth but, also, fail to set seed. These results indicate that the unilateral response is a barrier distinct from SI, differing from SI in the timing and location of expression in the style. We therefore suggest that this unilateral response in theL. pennellii×L. esculentum cross is more accurately referred to as “unilateral incongruity” (UI) rather than interspecific incompatibility. Periclinal chimeras were used to determine the tissues involved in UI. The results of crosses with the available chimeras indicate that the female parent must beL. pennellii at either LI (layer 1) or both LI and LII (layer 2) and the male parent must beL. esculentum at either LII or both LI and LII to observe UI similar to that seen in theL. pennellii×L. esculentum cross. Pollinations with a mixture of pollen fromL. pennellii and from transgenicL. esculentum plants harboring a pollen-specific GUS reporter gene marker were used to ascertain whether the growth of the pollen tubes of either species was modified as a possible means of overcoming UI. We found no evidence of communication between the two types of pollen tubes to either enhance or restrict all pollen tube growth.

Interspecific crossing barriers in Lycopersicon and their relationship to self-incompatibility

Fertilization in plants is the culmination of a complex set of processes beginning with the initiation of pollen germination, involving the development of the pollen tube and its interactions with the female sporophyte or gametophyte, and resulting in the production of viable seed. Several mechanisms limit successful fertilization within and between species. The intraspecific mechanisms encourage outcrossing within species, and the interspecific mechanisms affect speciation and limit the exchange of genes between species.

A novel anther-expressed adh-homologous gene in Lycopersicon esculentum

Two novel tandemly-oriented open reading frames (ORFs) with homology to alcohol dehydrogenase (ADH) were isolated from tomato. The predicted amino acid composition for each of the two tandem adh genes indicates the presence of 22 and 21, respectively, of 22 amino acids conserved in ADH proteins from plants and animals. However, comparison to known plant adh genes reveals a significantly lower similarity indicating that they belong to a novel class of ADHs. According to mapping data, the adh-homologous ORFs do not represent either of the previously studied adh1 or adh2 genes of tomato. The tandem genes, termed adh3a and adh3b, mapped to a distal region of the long arm of chromosome 4, unlike adh1, which maps closer to the centromere. Adh3a and adh3b have over 90% similarity to each other at the nucleotide and putative peptide levels. The adh3a gene has ten exons and nine introns with the transcription initiation site 57 bp upstream of the translation start. A putative TATA box and polyadenylation site have been identified. Adh3a is transcribed and, according to cDNA sequence analysis, fully processed in the late stages of anther development. According to transformation analysis, tissue-specific regulatory elements reside within the -448 to +724 region. The termination codon of adh3a is separated from the putative adh3b translation start site by 789 bp of intervening sequence. The 5′ untranscribed sequences of each gene contain a stretch of 68 bp with 78% similarity. Within this stretch are sequences which are homologous to sequences found in anaerobically-induced or pollenexpressed genes from various plant species.

Evaluating self-incompatibility in Chrysanthemum: The influence of ovule number

SummaryThe impact of ovule number on seed set calculations for self-incompatible (SI) species was investigated. Diploid Chrysanthemum was chosen for this study because accurate counts of the potential number of ovules could be made. Individuals in populations of C. carinatum, C. coronarium, C. c. subsp. spatiosum, and C. segetum were crossed in complete diallels. All species exhibited similar results. Therefore, only the diallel data from C. coronarium subsp. spatiosum were presented. The seed set data with and without ovule counts were processed by SIGMAS, a computer program designed to analyze SI data. Incorporation of the actual number of ovules into seed set diallels provided the most realistic representation of values for self-incompatibility studies. Data derived from equations excluding ovule counts might lead to inaccurate genetic interpretations. Ovule counts were significant between and within genotypes for self (disc and ray florets), but not cross (ray florets only) pollinations. The disc florets in self-pollinations were found to be responsible for increasing the variability in ovule number. The statistics indicate that the disc and ray florets composed two distinct populations. At the diploid level with a single daisy flower type, the disc floret numbers were variable, whereas ray florets were relatively static. This was not the case with polyploid chrysanthemums, where both ovule populations were dynamic and interactive. The conservative nature of percent pseudo-self-compatibility (%PSC) deems it necessary to obtain an accurate measure of female fertility. Values for this could be obtained using a bulk pollination or a tester with unmatched S alleles.

Distinguishing between self-incompatibility and other reproductive barriers in plants using male (MCC) and female (FCC) coefficient of crossability

SummaryIt is difficult to assess self-incompatibility (SI) as a phenomenon or to distinguish it from other reproductive barriers. Our objective was to demonstrate a method of separating reproductive barriers and other factors affecting SI expression. The analysis consists of four parts: (1) calculating male (MCC) and female (FCC) coefficients of crossability using seed set from a diallel crossing design; (2) determining significant male and female differences using a 1∶1 paired Chi-square test; (3) constructing scatter diagrams of FCC versus MCC; (4) performing linear regression analysis between FCC and MCC. Diallel data from eight genera with various reproductive barriers were used to illustrate and validate the usefulness of this method. SI can be distinguished from other reproductive barriers (incongruity, inbreeding depression, post-fertilization phenomena) using 1∶1 Chisquare and regression analysis. In addition, the incorporation of ovule counts is crucial for demarcating the SI phenomenon in species where ovule numbers vary between self- and outcross pollinations. Male and female steriles can be identified with the MCC/FCC equations and subsequently removed from further analysis.

Alterations of the manifestations of hybrid breakdown in Lycopersicon esculentum L. pennellii F2 populations containing L. esculentum versus L. pennellii cytoplasm

Reproductive abnormalities reduced the percent stainable pollen, and fruit and seed set in interspecific F2 populations derived from crosses of Lycopersicon esculentum and L. pennellii but were not observed in parental lines and interspecific F1 populations. The degree to which these reproductive abnormalities were expressed in the interspecific F2 populations was affected by cytoplasm. Reproduction was impeded in interspecific F2 populations containing L. esculentum cytoplasm (F 2 Le ) by reduction in pollen production, the lack of fruit set and a high proportion of parthenocarpic fruit among plants capable of fruit set. The F2 populations containing L. pennellii cytoplasm (F 2 Lp4 ) showed a reduced frequency of reproductive abnormalities at all stages of reproductive development, resulting in higher values for percent stainable pollen, fruit and seed set and higher proportions of the F 2 Lp4 populations being capable of setting fruit or seed than F 2 Le populations. The major barrier remaining in F 2 Lp4 populations was reduced fruit set compared to parental lines. The barrier to fruit and seed set observed in the F 2 Le populations, and to a lesser extent in the F 2 Lp4 populations, occurs around the time of fertilization or early embryonic development. The effect of L. pennellii cytoplasm on barriers in the F 2 Lp4 populations is proposed to be due to an interaction between cytoplasmic and nuclear genes during fertilization of the F1 plants to produce F2 populations and may also affect subsequent generations.

Statistical differentiation between self incompatibility and pseudo-self compatibility in Petunia hybrida Hort, using female and male coefficient of crossability

Discriminating styles (DS), pollen-mediated pseudo-self compatibility (PMPSC), and general pseudo-self compatibility (PSC) phenomena were investigated by re-analyzing data from Petunia hybrida where known S genotypes were used. This demonstrated how female coefficient of crossability (FCC)/male coefficient of crossability (MCC) scatter diagrams and regression analyses aid in identifying and quantifying PSC within an self incompatible (SI) population. One of the female testers was identified by statistics to be SI, not DS, in contrast to what was reported in the original report, where all the plants were assumed to have operating DS. In addition, none of the females expressed PMPSC. Based on regression analysis and chi-square tests, a threshold between 27% and 31% PSC was estimated to be necessary for expression of DS. The presence of DS was also required to test for the existence of PMPSC as reported previously. The upper left-hand quadrant of the FCC/MCC scatter diagram which contains all the deviants from the theoretical SI model, is the location expression of DS has been identified. Placement for PMPSC deviants is not possible, due to the interrelationship with DS. Percent PSC did not directly equate with the different types of PSC phenomena but was useful for identifying and ranking DS in female parents. The compatible tester used in this experiment did not always produce the highest outcross seed set with the females as expected. Therefore, due to the confounding effects of the different types of PSC, it is important to choose the compatible testers with care. Regression analyses of FCC/MCC values indicated that S2.2 and S1.2 male testers did not behave in a similar fashion to S1.1 testers. It is hypothesized that this disparity could be the result of the expression of a general PSC gene, different from the DS or PMPSC genes, which is linked to the S2 allele. Since these general PSC effects associated with the S2 allele are minor in comparison to DS and PMPSC, it was necessary to distinguish the difference using statistical analysis.