L. S. Boiteux

Genetic basis of resistance against two Tospovirus species in tomato (Lycopersicon esculentum)

SummaryInheritance studies were conducted to determine the genetic basis of resistance in tomato (Lycopersicon esculentum) against two distinct Tospovirus species (tomato spotted wilt virus and tomato chlorotic spot virus) belonging to the ‘tomato spotted wilt virus complex’ identified in Brazil. F1, backcrosses and F2 populations were developed using the resistant parent ‘CNPH Tx 405’ and susceptible parent ‘IPA-5’. Segregation ratios indicated that the inheritance of resistance to both viruses fit a single dominant gene-model. Preliminary tests using data from 50 pairs of vegetatively propagated F2 plants (inoculated separately with each virus) indicated that the same gene (probably the Sw-5 gene) is controlling the resistance to both viruses. This gene displays a peculiar mode of action, closely resembling a vertical resistance response, but acting against a large number of phenotypic variants and species within the genus Tospovirus.

Inheritance of a resistance specific to tomato spotted wilt tospovirus in Capsicum chinense ‘PI 159236’

SummaryInheritance studies were conducted to determine the genetic basis of resistance in pepper against one Tospovirus isolate classified as tomato spotted wilt virus (TSWV). F1, backcrosses and F2 populations were developed using the resistant parent Capsicum chinense ‘PI 159236’ (CNPH 679) and the susceptible parent C. annuum ‘Magda’ (CNPH 192). Segregation ratios strongly indicated that the resistant response (a localization, hypersensitive-like reaction) to TSWV fits a single-dominant gene model. Under our experimental conditions, the penetrance of this gene was very high. This gene (tentatively named Tsw) is highly effective only against TSWV isolates. The resistance governed by the Tsw gene was not effective against isolates belonging to tomato chlorotic spot virus (TCSV) and groundnut ring spot virus (GRSV), two other previously described Tospovirus species.

Sources of resistance to tomato spotted wilt virus (TSWV) in cultivated and wild species of Capsicum

SummaryA germplasm collection of 70 cultivars and wild species of Capsicum was evaluated for resistance to tomato spotted wilt virus (TSWV) under field (natural inoculum) conditions. Different levels of resistance to the disease caused by this virus were observed among the tested lines. High degree of field resistance was detected in two Capsicum baccatum var. pendulum, two C. chinense, and three C. annuum lines. Controlled greenhouse tests were also carried out to confirm the resistant reaction of these seven field selected lines. These lines were mechanically inoculated with two serologically distinct isolates of TSWV obtained from different geographic regions of Brazil. The two C. chinense lines (‘CNPH 275’ and ‘PI 159236’) were virtually immune against one specific (TSWV-BsB) isolate but were susceptible to another isolate (TSWV-SP) used in this assay. Sources of resistance for both isolates were not found. Our experimental results strongly indicate there exists a number of genetic mechanisms (probably including action of vertical and horizontal genes) to TSWV resistance in Capsicum spp.

Inheritance of adult-plant resistance to Phytophthora capsici in pepper

SummaryInheritance studies were conducted to determine the genetic basis of adult-plant resistance in pepper (Capsicum annuum L.) to Phytophthora capsici. F1, backcrosses and F2 populations were developed using the resistant parent Criollo de Morellos 334 and susceptible parents Agronômico 10-G and Yolo Wonder. Pepper plants, at 36 days post-emergence, were inoculated near the base of the stem with an inoculum suspension of 5×104 zoospores/ml. Segregation ratios in the F2 generation of 13 resistant to 3 susceptible plants fit a 2-gene model for resistance with dominant and recessive epistasis.

Pepper yellow mosaic virus, a new potyvirus in sweetpepper, Capsicum annuum

Summary. A potyvirus was found causing yellow mosaic and veinal banding in sweetpepper in Central and Southeast Brazil. The sequence analysis of the 3′ terminal region of the viral RNA revealed a coat protein of 278 amino acids, followed by 275 nucleotides in the 3′-untranslated region preceding a polyadenylated tail. The virus shared 77.4% coat protein amino acid identity with Pepper severe mosaic virus, the closest Potyvirus species. The 3′-untranslated region was highly divergent from other potyviruses. Based on these results, the virus found in sweetpepper plants could be considered as a new potyvirus. The name Pepper yellow mosaic virus (PepYMV) is suggested.

Estimativa indireta de teores de licopeno em frutos de genótipos de tomateiro via análise colorimétrica

Indirect estimation of lycopene concentration in fruits of tomato genotypes via chromaticity values Lycopene (C 40 H 56 ) is a red carotenoid pigment, which naturally accumulates in flowers, fruits and roots of some vegetable and fruit crops. The unique properties of the lycopene molecule make this pigment one of the most powerful antioxidants available. Tomato (Lycopersicon esculentum Mill.) fruits and processed tomato products are the main sources of lycopene in the human diet. There is a growing amount of experimental data indicating that lycopene intake is associated with decreased risk of many types of degenerative and cardiovascular diseases as well as cancers in humans. In addition, lycopene concentration is associated with improved visual perception of tomato products. Therefore, there is a strong demand to increase the total amount of lycopene in tomato cultivars for either fresh market or processing. The development of cultivars for improved lycopene content is dependent upon the availability of simple yet accurate methodologies to quantify this carotene in tomato fruits and processed tomato products. The main objective of this work was to estimate the degree of correlation between the total lycopene content in tomato fruits measured spectrophotometrically and the chromaticity values (L*, a* and b*) obtained using a tristimulus colorimeter. Colorimetric readings were taken using three sampling strategies: (1) external fruit wall; (2) internal fruit wall and (3) the whole homogenized fruit pulp. For the achromatic component L* (relative darkness or lightness), the obtained R 2 range from 0.86 to 0.91), with these correlation values being consistently higher when the chromaticity readings were taken in whole homogenized fruit pulp. These results confirmed the feasibility of obtaining precise indirect estimation of lycopene content from chromaticity readings. The methodology described here could be useful for large scale selection of tomato lines with improved levels of lycopene without of high costs and of residue disposal problems associated with the employment of organic solvents in the standard spectrophotometric methods.

Resistance to potato virus Y (pathotype 1–2) in Capsicum annuum and Capsicum chinense is controlled by two independent major genes

SummaryThe genetic basis of the resistance to a pepper strain of potato virus Y (PVY pathotype 1–2) was investigated in two lines: Capsicum annuum ‘Criollo de Morellos 334’ and Capsicum chinense ‘PI 159236’. F1, backcrosses, and F2 populations were produced using C. annuum ‘Magda’ as the susceptible parent. Segregation ratios indicated that the resistance in C. annuum ‘Criollo de Morellos 334’ is governed by a nuclear, single dominant gene with a very high but incomplete penetrance. On the other hand, the resistant response of C. chinense ‘PI 159236’ is associated with the presence of a major recessive gene with an apparently complete penetrance. The failure in recovering or detecting PVY from/in the inoculated leaves of both resistant genotypes is strongly suggestive that both genes are acting by preventing the multiplication of the virus in the leaf tissue. Both mechanisms of resistance closely resemble either an operational immunity or a single-cell hypersensitive reaction to PVY pathotype 1–2. The name Ry 1–2 is suggested for the resistance gene from ‘Criollo de Morellos 334’. The gene for resistance to PVY pathotype 1–2 identified in C. chinense ‘PI 159236’ may be the same designated etc1, which is mentioned to be allelic to the etav locus. The phenotypic expression of the etav gene was characterized in C. annuum ‘Avelar’ as a tolerant but not immune or hypersensitive reaction against some potyviruses in Florida. These data suggest that a critical reexamination of the allelic relationships at the locus et should be performed.

Reaction of tomato hybrids carrying the Ty-1 locus to Brazilian bipartite Begomovirus species

The number of tomato-infecting begomoviruses has increased in Brazil after the introduction of the polyphagous vector Bemisia tabaci biotype B. The Ty-1 locus, introgressed from Lycopersicon chilense, controls tolerance to species of the monopartite Tomato yellow leaf curl virus (TYLCV) complex in Europe and the Middle East. However, little information is available about the Ty-1 effectiveness against species of the bipartite Begomovirus complex occurring in Brazil. Heterozygous (Ty-1/ty-1) and homozygous (ty1/ty-1) hybrids were evaluated for reaction to Begomovirus isolates under open field conditions in two growing areas in Central Brazil. Test plants were evaluated under natural inoculation with high vector pressure. Evaluation was done using a disease assessment scale (DAS) varying from 1= no symptoms to 4= severe symptoms. Systemic infection was evaluated via polymerase chain reaction (PCR) using ‘universal’ Begomovirus primers. In the trial #1, the hybrids (Ty-1/ty-1) and (ty-1/ty-1) had 35% and 95% of plants with symptoms and 75% and 100% of plants with positive PCR, respectively. In the trial #2, only 20% of the Ty-1/ty-1 hybrid plants were symptom-free with both hybrids displaying 100% of plants with positive PCR. This reaction of the Ty-1 hybrid to bipartite Begomovirus species was similar to that reported in Europe and the Middle East to the TYLCV complex with a large number of plants being neither virus-free nor symptom-free. On the other hand, symptom expression of the Ty-1 hybrid was significantly milder than ty-1/ty-1 hybrids in both trials (DAS = 1.35 vs. 2.70 for the trial #1 and DAS = 2.05 vs. 3.95 for the trial #2). Nucleotide sequencing indicated the presence of isolates genetically related to Tomato rugose mosaic virus (ToRMV) in the trial #1 and a mixed infection of ToRMV and Tomato yellow vein streak virus in the trial #2. Therefore, the Ty-1 locus seems to control a “tolerance” response to distinct Begomovirus species. Resistance gene clusters is a common feature in the tomato genome, particularly at the chromosome 6 where Ty-1 is located. Therefore, additional studies are necessary to confirm if this tolerance to a range of begomoviruses is controlled by Ty-1 alone or a by the action of distinct, closely linked genes.

A Chromosome-Specific Estimate of Transmission of Heterozygosity by 2n Gametes in Potato

Polyploid plants are formed when numerically unreduced (2n) gametes participate in fertilization. Based on cytological and genetic analyses, modes of 2n gamete formation have been determined for a number of plant species. Gametes formed by a first-division restitution (FDR) mechanism contain nonsister chromatids near the centromere, whereas those formed by second-division restitution (SDR) contain sister chromatids. These mechanisms differ in the proportion of heterozygous loci they transmit intact to offspring. This paper estimates the transmission of heterozygosity on an individual chromosome basis through pachytene analysis of chromosomes of haploids (2n = 2x = 24) of Solanum tuberosum Andigena Group (2n = 4x = 48), a South American cultivated potato. Transmission of heterozygosity by FDR and SDR 2n gametes was calculated for 6 different cytogenetic assumptions. FDR was more than twice as effective as SDR in transmission of heterozygosity under all 6 scenarios. Rates of transmission of heterozygosity were similar in each situation. Transmission of heterozygosity by FDR was also compared with transmission of heterozygosity by tetrasomic inheritance and found to be approximately 50% more effective.

RAPD linkage map of the genomic region encompassing the root-knot nematode (Meloidogyne javanica) resistance locus in carrot

Abstract Inheritance studies have indicated that resistance to the root-knot nematode (Meloidogyne javanica) in carrot inbred line ’Brasilia-1252’ is controlled by the action of one or two (duplicated) dominant gene(s) located at a single genomic region (designated the Mj-1 locus). A systematic search for randomly amplified polymorphic DNA (RAPD) markers linked to Mj-1 was carried out using bulked segregant analysis (BSA). Altogether 1000 ten-mer primers were screened with 69.1% displaying scorable amplicons. A total of approximately 2400 RAPD bands were examined. Four reproducible markers (OP-C21700, OP-Q6500, OP-U12700, and OP-AL15500) were identified, in coupling-phase linkage, flanking the Mj-1 region. The genetic distances between RAPD markers and the Mj-1 locus, estimated using an F2 progeny of 412 individuals from ’Brasilia 1252’×’B6274’, ranged from 0.8 to 5.7 cM . The two closest flanking markers (OP-Q6500 and OP-AL15500) encompassed a region of 2.7 cM . The frequency of these RAPD loci was evaluated in 121 accessions of a broad-based carrot germplasm collection. Only five entries (all resistant to M. javanica and genetically related to ’Brasilia 1252’) exhibited the simultaneous presence of all four markers. An advanced line derived from the same cross, susceptible to M. javanica but relatively resistant to another root-knot nematode species (M. incognita), did not share three of the closest markers. These results suggest that at least some genes controlling resistance to M. incognita and M. javanica in ’Brasilia 1252’ reside at distinct loci. The low number of markers suggests a reduced amount of genetic divergence between the parental lines at the region surrounding the target locus. Nevertheless, the low rate of recombination indicated these markers could be useful landmarks for positional cloning of the resistance gene(s). These RAPD markers could also be used to increase the Mj-1 frequency during recurrent selection cycles and in backcrossing programs to minimize ’linkage drag’ in elite lines employed for the development of resistant F1 hybrids.