Nuclear DNA content, chromatin organization and chromosome banding in brown and yellow seeds of Dasypyrum villosum (L.) P. Candargy

Comparative seed longevity under genebank storage and artificial ageing: a case study in heteromorphic wheat wild relatives

Seed longevity is a complex trait that depends on numerous factors. It varies among species and populations, and within different seed morphs produced by the same plant. Little is known about variation in longevity in different seed morphs or the physiological and molecular basis of these differences. We evaluated the longevity and oxidative stress status in heteromorphic seeds aged in two different storage conditions. We compared controlled ageing tests (seed storage at 45°C and 60% relative humidity; a method of accelerated ageing used to estimate longevity in genebank conditions) with storage in a genebank for up to 40 years (-18°C and 8% seed moisture content). We employed as study species two wild wheats characterized by seed heteromorphism: Aegilops tauschii and Triticum monococcum subsp. aegilopoides. We estimated the ROS content and the expression of genes coding for enzymes related to the H₂ O₂ scavenging pathway. Results confirmed that seed longevity varies between different seed morphs. Different storage environments resulted in different longevity and survival curves. ROS levels, even if with variable patterns, were higher in several aged seed lots. We observed consistency in the expression of two genes (GSR and CAT) related to ROS scavenging in the late phase of pre-germinative metabolism. Differences in seed longevity between morphs were observed for the first time under genebank conditions. Our results suggest also that controlled ageing tests should be used with caution to infer ranks of longevity under cold storage.

Genomic relationships between Dasypyrum villosum (L.) Candargy and D. hordeaceum (Cosson et Durieu) Candargy

The origin and genomic constitution of the tetraploid perennial species Dasypyrum hordeaceum (2n = 4x = 28) and its phylogenetic relationships with the annual diploid Dasypyrum villosum (2n = 2x = 14) have been investigated by comparing the two genomes using different methods. There is no apparent homology between the conventional or Giemsa C-banded karyotypes of the two Dasypyrum species, nor can the karyotype of D. hordeaceum be split up into two similar sets. Polymorphism within several chromosome pairs was observed in both karyotypes. Cytophotometric determinations of the Feulgen-DNA absorptions showed that the genome size of D. hordeaceum was twice as large as that of D. villosum. Both the cross D. villosum x D. hordeaceum (crossability rate 12.1%) and the reciprocal cross (crossability rate 50.7%) produced plump seeds. Only those from the former cross germinated, producing sterile plants with a phenotype that was intermediate between those of the parents. In these hybrids (2n = 21), an average of 13.77 chromosomes per cell paired at meiotic metaphase I. Trivalents were only rarely observed. Through dot-blot hybridizations, a highly repeated DNA sequence of D. villosum was found not to be represented in the genome of D. hordeaceum. By contrast, very similar restriction patterns were observed when a low-repeated DNA sequence or different single-copy sequences of D. villosum or two sequences in the plastidial DNA of rice were hybridized to Southern blots of the genomic DNAs of the two Dasypyrum species digested with different restriction endonucleases. By analyzing glutamic-oxaloacetic-transaminase, superoxide dismutase, alcohol dehydrogenase, and esterase isozyme systems, it was shown that both Dasypyrum species shared the same phenotypes, which differed from those found in hexaploid wheat. In situ hybridizations using DNA sequences encoding gliadins showed that these genes were located close to the centromere of three pairs of D. villosum chromosomes and that they had the same locations in six pairs of D. hordeaceum chromosomes. We conclude that the autoploid origin of D. hordeaceum from D. villosum, which cannot be defended on the basis of chromosomal traits, is suggested by the other findings obtained by comparing the two genomes. Key words : Dasypyrum hordeaceum, Dasypyrum villosum, phylogenetic relationships.

Intraspecific genotypic diversity in plants

Variations in the nuclear DNA, mainly as a result of quantitative modulations of DNA repeats belonging to different sequence families of satellite DNA and to the activity of transposable elements, have been assessed within several angiosperm species. These variations alter the amount and organization of the DNA and therefore the genotype, rather than the genome proper. They take place on an evolutionary time scale as the result of selection processes after the occurrence of uncontrolled events in the genome or may be due to direct responses of plant genomes to environmental stimuli that occur under plant-level control within a short developmental period of a single generation. These DNA changes are correlated to changes in the developmental dynamics and phenotypic characteristics of the plants, and the capability to carry out genotypic variation is an evolutionary trait that allows plant species to adapt to different environmental conditions, as well as to the variability of conditions in a given environment. The link between developmental and environmental stimuli and repetitive DNA that elicits the intraspecific diversity of plant genotypes may provide models of evolutionary change that extend beyond the conventional view of evolution by allelic substitution and take into account epigenetic effects of the genome structure.

Identification of the chromosome complement and the spontaneous 1R/1V translocations in allotetraploid Secale cereale × Dasypyrum villosum hybrids through cytogenetic approaches

Genome modifications that occur at the initial interspecific hybridization event are dynamic and can be consolidated during the process of stabilization in successive generations of allopolyploids. This study identifies the number and chromosomal location of ribosomal DNA (rDNA) sites between Secale cereale, Dasypyrum villosum, and their allotetraploid S. cereale × D. villosum hybrids. For the first time, we show the advantages of FISH to reveal chromosome rearrangements in the tetraploid Secale × Dasypyrum hybrids. Based on the specific hybridization patterns of ribosomal 5S, 35S DNA and rye species-specific pSc200 DNA probes, a set of genotypes with numerous Secale/Dasypyrum translocations of 1R/1V chromosomes were identified in successive generations of allotetraploid S. cereale × D. villosum hybrids. In addition we analyse rye chromosome pairs using FISH with chromosome-specific DNA sequences on S. cereale × D. villosum hybrids.

Does genome size in Dasypyrum villosum vary with fruit colour?

Dasypyrum villosum (2n=14), a Mediterranean grass species of the Triticeae, exhibits intraindividual fruit colour polymorphism from pale yellow to almost black. Several studies have reported differences between the plants emerging from pale and dark fruits. They include histone content in root meristem nuclei, cell cycle duration, heterochromatin banding pattern, frequency of a tandemly repeated sequence, and nuclear genome size. In the present study, we examine whether the reports of genome size being up to 1.24-fold larger in seedlings from the lighter caryopses are reproducible. In all, 29 accessions from various countries, totaling 186 plants, were investigated for genome size using flow cytometry with propidium iodide as the DNA stain. Individuals differed 1.12-fold at most and accessions 1.07-fold. The mean genome size (1C-value) was 5.07 pg or 4954 Mbp. Within-accession comparisons of seedlings derived from light and dark caryopses were insignificant (P>0.100). Thus, we found no evidence for a modificatory genome size plasticity in D. villosum. In the light of our data, the previously reported genome size variation, up to 1.66-fold within populations and 1.67-fold between populations, appears unrealistically high. Suboptimal technical procedures for quantitative Feulgen staining are probably responsible for these earlier observations.

Characterization of Dasypyrum villosum (L.) candargy chromosomal chromatin by means of in situ restriction endonucleases, fluorochromes, silver staining and C-banding

The distribution of C-banded heterochromatin was determined in an inbred line of Dasypyrum villosum. Practically no difference in chromosome morphology or band distribution could be observed within the chromosomes of the same pair. Heterochromatin bands, revealed by Giemsa banding, were characterized by means of their differential reaction to fluorochromes, silver staining and in situ digestion with different restriction endonucleases. The results clearly indicate that in D. villosum two different classes of heterochromatin with different chromosomal local-ization exist: one is evidenced by both C-banding and DAPI staining and has mainly telomeric distribution, the other is evidenced only by C-banding and has mainly centromeric distribution.