The contribution of short repeats of low sequence complexity to large conifer genomes

The abundance and genomic organization of six simple sequence repeats, cons isting of di-, tri-, and tetranucleotide sequence motifs, and a minisatelli te repeat have been analyzed in different gymnosperms by Southern hybridiza tion. Within the gymnosperm genomes investigated, the abundance and genomic organization of micro- and minisatellite repeats largely follows taxonomic groupings. We found that only particular simple sequence repeat motifs are amplified in gymnosperm genomes, while others such as (CAC)(5) and (GACA)( 4) are present in only low copy numbers. The variation in abundance of simp le sequence motifs reflects a similar situation to that found in angiosperm s. Species of the two- and three-needle pine section Pinus are relatively c onserved and can be distinguished from Pinus strobus which belongs to the f ive-needle pine section Strobus. The hybridization pattern of Picea species , bald cypress and gingko were different from the patterns detected in the Pinus species. Furthermore, sequences with homology to the plant telomeric repeat (TTTAGGG)(n) have been analyzed in the same set of gymnosperms. Telo mere-like repeats are highly amplified within two- and three-needle pine ge nomes, such as slash pine (Pinus elliotfii Engelm. var. elliottii), compare d to P. strobus, Picea species, bald cypress and gingko. P. elliotfii var. elliottii was used as a representative species to investigate the chromosom al organization of telomere-like sequences by fluorescence in situ hybridiz ation (FISH). The telomere-like sequences are not restricted to the ends of chromosomes; they form large intercalary and pericentric blocks showing th at they are a repeated component of the slash pine genome. Conifers have ge nomes larger than 20 000 Mbp, and our results clearly demonstrate that repe ats of low sequence complexity, such to (CA)(8), (CA)(8,) (GGAT)(4) and (GA TA)(4), and minisatellite- and telomere-like sequences represent a large fr action of the repetitive DNA of these species. The striking differences in abundance and genome organization of the various repeat motifs suggest that these repetitive sequences evolved differently in the gymnosperm genomes i nvestigated.