A CYTOMOLECULAR LOBLOLLY PINE KARYOTYPE

¹ Forest Tree Molecular Cytogenetics Laboratory, Southern Institute of Forest Genetics, USDA Forest Service, 23332 MS Highway 67, Saucier, MS 39574

² Department of Soil & Crop Sciences, Texas A&M University, College Station, TX 77843

A cytomolecular loblolly pine karyotype has been developed based on fluorescent in situ hybridization (FISH) using 18S-28S rDNA, 5S rDNA, Arabidopsis-like telomere repeat probes and DAPI positive bands. Somatic chromosomes were prepared from colchicine treated root meristems using an enzymatic digestion technique. Eight pairs of long metacentric, one pair of short metacentric, two pairs of near sub-metacentric and one pair of smaller sub-metacentric chromosomes have been identified. Combinations of telomere repeat, 18S-28S rDNA, 5S rDNA and DAPI positive bands uniquely discriminate all the chromosomes. Seven intercalary and ten proximal 18S-28S rDNA sites were identified. All intercalary 18S-28S rDNA sites appeared to be major sites. Four pairs of metacentric chromosomes showed major pericentromeric 18S-28S rDNA sites and one of these lacked the intercalary site. Chromosome 1 contained an intercalary major 5S rDNA site. Two other independent metacentric pairs of chromosomes showed distal minor 5S rDNA sites. All but one chromosome showed DAPI positive bands. DAPI positive bands were denser or heavier at pericentromeric regions than at intercalary regions, however light intercalary bands were observed in most chromosomes. Telomere repeat signals were observed, as expected, towards the end of each chromosomal arm. With the exception of one or possibly two pairs, minor to major telomere signals were observed near or around the centromeres. Statistical analysis on chromosome arm lengths (short and long), and intercalary telomeric and rDNA positions has been carried out. A similar karyotype was developed for slash pine for direct comparison with loblolly pine as well as a comparison with a previously published slash karyotype (Doudrick et al. 1995, Journal of Heredity 86:289-296). A homoeologous group has been developed between slash and loblolly pines. Significant variation was noticed with respect to telomeric repeat sites between these two species but not for rDNA sites.