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.

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BackgroundGenome divergence by mobile elements activity and recombination is a continuous process that plays a key role in the evolution of species. Nevertheless, knowledge on retrotransposon-related variability among species belonging to the same genus is still limited. Considering the importance of the genus Helianthus, a model system for studying the ecological genetics of speciation and adaptation, we performed a comparative analysis of the repetitive genome fraction across ten species and one subspecies of sunflower, focusing on long terminal repeat retrotransposons at superfamily, lineage and sublineage levels.ResultsAfter determining the relative genome size of each species, genomic DNA was isolated and subjected to Illumina sequencing. Then, different assembling and clustering approaches allowed exploring the repetitive component of all genomes. On average, repetitive DNA in Helianthus species represented more than 75% of the genome, being composed mostly by long terminal repeat retrotransposons. Also, the prevalence of Gypsy over Copia superfamily was observed and, among lineages, Chromovirus was by far the most represented. Although nearly all the same sublineages are present in all species, we found considerable variability in the abundance of diverse retrotransposon lineages and sublineages, especially between annual and perennial species.ConclusionsThis large variability should indicate that different events of amplification or loss related to these elements occurred following species separation and should have been involved in species differentiation. Our data allowed us inferring on the extent of interspecific repetitive DNA variation related to LTR-RE abundance, investigating the relationship between changes of LTR-RE abundance and the evolution of the genus, and determining the degree of coevolution of different LTR-RE lineages or sublineages between and within species. Moreover, the data suggested that LTR-RE abundance in a species was affected by the annual or perennial habit of that species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-017-4050-6) contains supplementary material, which is available to authorized users.

Cytophotometric, karyological, and biochemical analyses were carried out in the meristems of seedlings obtained from seeds collected from 35 natural populations of hexaploid Festuca amndinacea in Italy. Highly significant differences between populations were observed in the amount of nuclear DNA (up to 32.3%). These changes are linked to variations in the amount of heterochromatin and in the frequency of repeated DNA sequences, and particularly of a fraction of them. Differences between populations in the arm ratios and total length of the chromosomes were also observed. The genome sizes of the populations are correlated positively with the mean temperature during the year and with that of the coldest month at the stations, and correlate negatively with their latitudes. The intraspecific genome changes observed are discussed in relation to other pertinent data to be found in the literature and in relation to their possible role in environmental adaptation.

This paper reports a procedure for isolating triploid and tetraploid olive plants, which have not been produced before either artificially or in nature. These polyploids were isolated from two mixoploid somatic mutants obtained earlier by treating 'Frantoio' and 'Leccino' plantlets with gamma radiation. The mixoploid mutants exhibit increased thickness of leaf, stem and root tissues, short internodes, a modified leaf lamina shape and a strong resistance to Spilocaea oleagina (Cast.) Hugh. In addition, they produce a mixture of normal drupes and some abnormally large ones, almost twice normal size. The variation in the nuclear DNA content of the mixoploid mutants is closely correlated with variation in their pollen size, crop capacity and the production of large fruit. Triploid genotypes with 69 chromosomes were isolated by germinating the seeds of these large fruits, collected from both the mixoploid mutants. Tetraploid plantlets, with 92 chromosomes, were obtained from cv. 'Frantoio' by selecting in vitro, during several prohferation phases, the shoots with obvate leaf shape which occurred among the shoots with normal lanceolate or intermediate leaf shape.Genetic improvement of the olive tree by breeding has had only limited success (Lavee et al. 1986), principally because knowledge of the genetic basis and heritability of economic characters is limited. It is difficult to determine the genetic control of these characters because of the high genetic variability, the long-term juvenile phase and the outcrossing habit of the species to the widespread occurrence of self-incompatibility. The heterozygous nature of olives results in highly variable offspring which differ from the parents in many characters, making it difficult and tedious to select plants with good quahty. However, high heterozygosity could be an advantage when attempts are made to improve characters by induced mutations without substantially affecting the rest of the genotype of the new cultivar.The induction of genetic variability in high-value agronomic olive cultivars by mutagenesis has been the main goal of many researchers in an attempt to obtain trees suitable for planting at high density in ohve groves. Roselh and Donini (1982) obtained a compact 'Ascolana tenera' mutant by this approach but it was not of agronomic value. Petruccioh et al. (1974) isolated numerous somatic mutants from the cultivars 'Leccino' and 'Erantoio'. Only three were particularly interesting because of their compact or dwarf vegetative habit. Later, Pannelli et al. (1990Pannelli et al. ( , 1992, characterized those plants and three have been named EC (from 'Erantoio') and LC (from 'Leccino') for their compact habit, and LD (from 'Leccino') because it was dwarf. The cropping capacity of EC was low, while LC cropped regularly. Both mutants produced some fruits of large size, almost double that of normal ones (with higher frequency in LC than EC). LD flowered regularly, but fruit set was reduced by the lack of pollen in the environment during the late blossoming (1-2 w...