Proceedings of the XLVI Italian Society of Agricultural Genetics - SIGA Annual Congress

Giardini Naxos, Italy - 18/21 September, 2002

ISBN 88-900622-3-1

Poster Abstract - 3.34

ANALYSIS OF COPIA- AND GYPSY-LIKE RETROTRANSPOSON SEQUENCES IN THE GENUS HELIANTHUS AND IN OTHER ASTERACEAE

MAESTRINI P.*, NATALI L.*, SANTINI S.**, GIORDANI T.*, MINELLI S.***, CIONINI P.G.***, CAVALLINI A.*

*) Dipartimento di Biologia delle Piante Agrarie, Sezione di Genetica, Pisa

**) Department of Biology, University of Konstanz, Konstanz, Germany

***) Dipartimento di Biologia Cellulare e Molecolare della Università, Sezione di Citologia e Genetica, Perugia

Asteraceae, genome evolution, Helianthus, retrotransposons

Retrotransposons are mobile genetic elements, of putative retroviral origin, medium-highly repeated (generally from 1,000 to more than 100,000 copies per haploid genome, and in the largest genomes even millions of copies), interspersed within the genome and ubiquitous in eukaryotes, both in animals and in plants. During species evolution, retrotransposons have occupied many different sites in the genome, through the production of an intermediate RNA which is retrotranslated to extrachromosomal DNA by some enzymes (autonomously produced by the retrotransposon) and then inserted in a new site. Such a replicative transposition mechanism has lead to a rapid increase of retrotransposon copy number.

Among retrotransposons, elements with long-terminal-repeats (LTRs) and without LTRs can be distinguished. LTRs are direct repeats situated at the extremities of the retrotransposon. LTR-retrotransposons are distinguished into two major groups, Ty1/copia and Ty3/gypsy, according to sequence similarity and to gene order. The differentiation among subfamilies within gypsy- or copia-retrotransposon families is still poorly understood.

Two repeated sequences, pHaS13, 469 bp in length, and pHaS211, 752 bp, were isolated from a partial genomic library of H. annuus. Sequence comparisons revealed similarity of pHaS13 to the int gene of Ty3/gypsy retrotransposons and of pHaS211 to the RNAse-H gene of Ty1/copia retroelements. The frequency of pHaS13-related sequences varies in the genome of annual species. By contrast, no significant differences in the frequency of these sequences occur among perennial species where pHaS13-related sequences are less redundant. About an 8-fold variation in the frequency of Ty1/copia-like sequences can be observed among Helianthus genotypes. The frequencies in each genome of pHaS13 and pHaS211 are not correlated. FISH analysis of pHaS13 or pHaS211 probes to the metaphase chromosomes of Helianthus species showed scattered labelling over all chromosomes, indicating large dispersal of both Ty3/gypsy- and Ty1/copia-like sequences. However, Ty1/copia-like sequences are less represented at centromeric chromosome regions, contrary to Ty3/gypsy-like sequences, and have preferential telomeric localisation.

Retrotransposons distribution in the genome of different Helianthus species was studied by means of RFLP analyses. The Southern blot hybridisation patterns of pHaS13 or pHaS211 sequences to DNA digests of Helianthus species were utilised to build cladograms. Both cladograms agree in splitting the genomes studied into annuals and perennials, but differences occur between gypsy- and copia-trees. Such differences indicate that the retroelements were presumably active after Helianthus speciation. The different position of Helianthus annual species using retrotransposon sequences is probably related to the nature of retrotransposable elements, which may change their position independently of species evolution time.

Finally, we have analysed the occurrence of pHaS13 and pHaS211 in the genome of other Asteraceae belonging to different tribes: our sequences appear to belong to specific copia- or gypsy-subfamilies, since they hybridise only to the DNA of certain species and are highly redundant only in Helianthus.