A sample-sequencing strategy combined with slot-blot hybridization and FISH was used to study the composition of the repetitive component of the sunflower genome. One thousand six hundred thirty-eight sequences for a total of 954,517 bp were analyzed. The fraction of sequences that can be classified as repetitive using computational and hybridization approaches amounts to 62% in total. Almost two thirds remain as yet uncharacterized in nature. Of those characterized, most belong to the gypsy superfamily of LTR-retrotransposons. Unlike in other species, where single families can account for large fractions of the genome, it appears that no transposon family has been amplified to very high levels in sunflower. All other known classes of transposable elements were also found. One family of unknown nature (contig 61) was the most repeated in the sunflower genome. The evolution of the repetitive component in the Helianthus genus and in other Asteraceae was studied by comparative analysis of the hybridization of total genomic DNAs from these species to the sunflower small-insert library and compared to gene-based phylogeny. Very little similarity is observed between Helianthus species and two related Asteraceae species outside of the genus. Most repetitive elements are similar in annual and perennial Helianthus species indicating that sequence amplification largely predates such divergence. Gypsy-like elements are more represented in the annuals than in the perennials, while copia-like elements are similarly represented, attesting a different amplification history of the two superfamilies of LTR-retrotransposons in the Helianthus genus.
Two repeated DNA sequences isolated from a partial genomic DNA library of Helianthus annuus, p HaS13 and p HaS211, were shown to represent portions of the int gene of a Ty3 /gypsy retroelement and of the RNase-Hgene of a Ty1 /copia retroelement, respectively. Southern blotting patterns obtained by hybridizing the two probes to BglII- or DraI-digested genomic DNA from different Helianthus species showed p HaS13 and p HaS211 were parts of dispersed repeats at least 8 and 7 kb in length, respectively, that were conserved in all species studied. Comparable hybridization patterns were obtained in all species with p HaS13. By contrast, the patterns obtained by hybridizing p HaS211 clearly differentiated annual species from perennials. The frequencies of p HaS13- and p HaS211-related sequences in different species were 4.3x10(4)-1.3x10(5) copies and 9.9x10(2)-8.1x10(3) copies per picogram of DNA, respectively. The frequency of p HaS13-related sequences varied widely within annual species, while no significant difference was observed among perennial species. Conversely, the frequency variation of p HaS211-related sequences was as large within annual species as within perennials. Sequences of both families were found to be dispersed along the length of all chromosomes in all species studied. However, Ty3 /gypsy-like sequences were localized preferentially at the centromeric regions, whereas Ty1/ copia-like sequences were less represented or absent around the centromeres and plentiful at the chromosome ends. These findings suggest that the two sequence families played a role in Helianthusgenome evolution and species divergence, evolved independently in the same genomic backgrounds and in annual or perennial species, and acquired different possible functions in the host genomes.
Nuclear DNA amounts were measured by Feulgen cytophotometry in Sorghum bicolor cv. 610 plants early exposed to 150 mM NaCl, a treatment known to induce an increased tolerance to salinity in plants carrying this genotype. In salt-treated plants, the percentages of 8C, 16C, and 32C nuclei in roots in the primary state of growth were 21.9%, 13.3%, and 4.3%, respectively. By contrast, in nonsalinized plants, only 3.5% of the nuclei had an 8C content and no higher DNA contents were observed. The salt treatment induced chromosome endoreduplication during the differentiation of cells in the root cortex, where 41.2% of the cells displayed a DNA content higher than 4C (versus 1.3% in control plants). No enhancement of endopolyploidy was observed in cells of the root vascular cylinder or the leaves of the salt-treated plants. In another S. bicolor genotype (DK 34-Alabama), noncompetent for salt adaptation, the same NaCl treatment did not induce chromosome endoreduplication in root cortex cells. Endopolyploidy may be considered as a part of the adaptive response of S. bicolor competent genotypes to salinity.
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.
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