Mechanisms of cytoplasmic inheritance in western white pine

Abstract

The chloroplast genome is predominantly paternally-inherited in conifers whereas mitochondria may be maternally-inherited. Molecular approaches do not reveal the mechanisms by which this is achieved. A recent study of mechanisms of organelle inheritance in Pseudotsuga menziesii has complimented earlier work on fertilization in conifers by demonstrating that, in the archegonium prior to syngamy, maternal plastids are transformed into large inclusions and mitochondria concentrate in a perinuclear zone. Paternal plastids and mitochondria are transferred with the male gametes and contribute to proembryo formation. In the present study, male and female gametophyte development, fertilization and proembryo development were examined inPinus monticola to determine mechanisms of plastid and mitochondrial inheritance.Second year female cones were collected from mid-April to mid-June in 1987 and 1988 from trees growing in Victoria, B.C., Canada. Ovules were removed and median portions dissected out and fixed under vacuum for 2h-overnight in cold 2.5% glutaraldehyde and 2% formaldehyde in 50 mM P-buffer, pH 7.2. Tissues were buffer-washed for lh, post-fixed in 1% buffered OsO4 for lh and buffer-washed overnight. Dehydration in a graded acetone series preceded infiltration in low viscosity resin and then 60-70 nm sections were cut on a Sorvall MT5000 ultramicrotome using a diamond knife. Sections were collected on uncoated mesh or formvar-coated notchdot grids, stained with uranyl acetate and lead citrate and examined on a Phillips EM300 at 60 kV or JOEL JEM-1200EX at 60 kV.