Plant & Animal Genome VI Conference

W34

EVOLUTION, ORGANIZATION AND CYTOGENETIC LOCALIZATION OF AN ALPHOID-LIKE SATELLITE DNA IN NORWAY SPRUCE: MICRO- VS. MACRO-SATELLITES

Dipartimento di Produzione Vegetale e Tecnologie Agrarie, Università degli Studi di Udine, I-33100 Udine, Italy

A highly repeated DNA sequence family, PATR140, with tandemly arranged repetitive units (monomers) of approximately 140 bp has been identified in Norway spruce (Picea abies K.) in the frame of a project aimed at the isolation of highly repeated sequences. We sequenced 19 different clones corresponding to a total of 56 monomers. Little sequence divergence is visible among the monomers but there is considerable monomer length variation due to insertion/deletions in a poly(A/T) stretch. The length of this microsatellite region varies between 15 and 40 bp. A few internal tandem duplications of portions of the monomer are also observed, representing an additional source for length variation. The use of the PATR140 clones as probes onto genomic Southern blots shows that the repeat is tandemly arranged and that the monomers are sometimes organised into a higher order structure with repeated arrays ranging in size from 1.3 to 4 kb. This may be due either to the presence of higher order monomers or to the interspersion of PATR140 repeats with non-PATR140 sequences. A high degree of methylation of the arrays is also observed. Fluorescent in situ hybridisation to metaphase chromosomes and interphase nuclei reveals that the PATR140 sequences are located around the centromere on three chromosome pairs in Norway spruce. The presence of a hypervariable microsatellite within such a satellite sequence offers a unique opportunity to analyse the balance between the contrasting effects of the mutational mechanisms generating the hypervariability and of unequal crossing over that tends to create homogeneous repeat arrays.