Elsa L. Camadro

Substitutes for genome differentiation in tuber-bearing Solanum: interspecific pollen-pistil incompatibility, nuclear-cytoplasmic male sterility, and endosperm

The cultivated potato, Solanum tuberosum L. (2n=4x=48), has a very large number of related wild and cultivated tuber-bearing species widely distributed in the Americas. These species, grouped in 16 taxonomic series, range from the diploid to the hexaploid level. Polyploid species are either disomic or polysomic, and sexual polyploidization via genetically controlled 2n gametes has played a major role in their evolution. Species are separated in nature by geographical and ecological barriers. However, there are several examples of sympatric species that share the same niches but do not readily cross (i.e., the diploids S. commersonii and S. chacoense in certain areas of Argentina). External barriers alone are, therefore, not sufficient to explain species integrity. In addition, there is no strong evidence indicating that genome differentiation is important in the group. In this review we present evidence supporting the assertion that interspecific pollen-pistil incompatibility, nuclear-cytoplasmic male sterility, and the endosperm are major forces that strengthen the external hybridization barriers allowing, at the same time and under specific circumstances, a certain amount of gene exchange without jeopardizing the integrity of the species.

Phenotypic instability and epigenetic variability in a diploid potato of hybrid origin, Solanum ruiz-lealii

BackgroundThe wild potato Solanum ruiz-lealii Brüch. (2n = 2x = 24), a species of hybrid origin, is endemic to Mendoza province, Argentina. Recurrent flower malformations, which varied among inflorescences of the same plant, were observed in a natural population. These abnormalities could be the result of genomic instabilities, nucleus-cytoplasmic incompatibility or epigenetic changes. To shed some light on their origin, nuclear and mitochondrial DNA of plants with normal and plants with both normal and malformed flowers (from here on designated as plants with normal and plants with abnormal flower phenotypes, respectively) were analyzed by AFLP and restriction analyses, respectively. Also, the wide genome methylation status and the level of methylation of a repetitive sequence were studied by MSAP and Southern blots analyses, respectively.ResultsAFLP markers and restriction patterns of mitochondrial DNA did not allow the differentiation of normal from abnormal flower phenotypes. However, methylation patterns of nuclear DNA discriminated normal and abnormal flower phenotypes into two different groups, indicating that abnormal phenotypes have a similar methylation status which, in turn, was different from the methylation patterns of normal phenotypes. The abnormal flower phenotype was obtained by treating a normal plant with 5-Azacytidine, a demethylating agent, giving support to the idea of the role of DNA methylation in the origin of flower abnormalities. In addition, the variability detected for DNA methylation was greater than the detected for nucleotide sequence.ConclusionThe epigenetic nature of the observed flower abnormalities is consistent with the results and indicates that in the diploid hybrid studied, natural variation in methylation profiles of anonymous DNA sequences could be of biological significance.

Assessment of somaclonal variation in asparagus by RAPD fingerprinting and cytogenetic analyses

Abstract RAPD and cytogenetic analyses were carried out to detect somaclonal variation in somatic embryo-derived plants from two elite genotypes (‘265’ and ‘357’) of Asparagus officinalis L. cv. Argenteuil. These two approaches were chosen for their capacity to detect, respectively, changes at the level of DNA sequence or chromosomal structure. Seventy-seven plants regenerated from three embryogenic lines (‘265-1’, ‘265-2’ and ‘357-1’) exhibited no intraclonal variation for 157 RAPD markers, even though two plants from line ‘357-1’ presented dwarf phenotypes. Cytogenetic analyses revealed that the dwarf plants were mixoploid; two other, phenotypically normal plants from line ‘357-1’ were aneuploids and presented many meiotic irregularities. The remaining regenerants from this and the other lines were genetically stable with regard to the donor plants. Molecular markers and cytogenetic analysis are complementary approaches that should allow a correct assessment of somaclonal variation in asparagus.

Agronomic characterization of diploid hybrid families derived from crosses between haploids of the common potato and three wild Argentinian tuber-bearing species

Five diploid hybrid families derived from crosses between haploids of three Argentinian cultivars and three diploid wild species,Solanum gourlayi, S. chacoense andS. spegazzinii, were evaluated in a field experiment in Balcarce. All plants were androsterile and, thus, could not be screened for 2n pollen production. Data on agronomic tuber traits were statistically analyzed. Differences among families were non-significant for tuber number/plant, but were significant for tuber weight/plant and dry matter content (with values above 21.0% in two of them). The relative frequencies of tubers in the categories established for size, skin texture, and eye depth were family dependent, but they were familiy independent in the categories established for tuber shape and skin color. Great morphological variability was detected for all the traits studied, and plants with high yield and high dry matter content were identified. These families can be used as sources of germplasm in breeding and to explore heterotic responses in 2x × 4x crosses, given the capacity of some plants to produce 2n eggs, as found in a related investigation.ResumenCinco familias híbridas diploides derivadas de cruzamientos entre tres haploides de cultivares argentinos y las especies silvestresS. gourlayi, S. chacoense yS. spegazzinii, fueron evaluadas en un ensayo de campo en Balcarce. Todas las plantas fueron androestériles, y no pudieron ser tamizadas por producción de polen 2n. Los datos sobre características agronómicas de los tubérculos se analizaron estadísticamente. Las diferencias entre familias no fueron significativas para número de tubérculos/planta, pero fueron significativas para peso de tubérculos/planta y para contenido de materia seca (con valores superiores a 21.0% en dos familias). Las frecuencias relativas de tubérculos en las clases que se establecieron para tamaño, textura de piel y profundidad de ojos fueron dependientes de las familias, pero fueron independientes de las familias para forma de tubérculo y color de piel. Se detectó gran variabilidad morfológica para todos los caracteres estudiados y se identificaron plantas de altos rendimientos y contenido de materia seca. Estas familias pueden ser usadas como fuentes de germoplasma en el mejoramiento genético y para explorar respuestas heteróticas en cruzamientos 2x × 4x, dada la capacidad de algunas plantas para producir oósferas 2n, como se detectó en un experimento relacionado.

Pollen-style compatibility relations in natural populations of the wild diploid potato species Solanum spegazzinii Bitt.

Solanum spegazzinii Bitt., a wild diploid potato species endemic to Argentina, possesses resistance to adverse biotic and abiotic agents. Northwestern Argentinean populations grouped according to their morphological variability (G1 = localities East of the Calchaquí Valley, G2 = Calchaquí Valley and G3 = locality Belén) presented problems in seed set following controlled crosses. Thus, to evaluate if the morphological variability previously reported was sustained on a genetic basis, three introductions per group and five plants per introduction were used in controlled crosses (genotypic combinations). These were classified according to the number of seeds produced per fruit. Pollen-style compatibility relations were studied in those crosses that did not set seeds or that produced less than 30 seeds each. Within groups, G1 produced seeds normally, whereas hybridization barriers were found in 71.4% and 50.0% of the combinations within G2 and G3, respectively; on the other hand, from 46.7% to 73.3% of the combinations among groups exhibited similar barriers. Variability in the expression of the sites of pollen tube inhibition in the style was observed both within and among groups. Speculations are made on the possible hybrid origin of the populations.