Chloroplast DNA Sequence Homologies among Vascular Plants

Area of Science:

• Plant molecular evolution
• Genomics
• Molecular biology

Background:

• The chloroplast genome's evolutionary conservation is crucial for understanding plant lineage divergence.
• Investigating sequence conservation provides insights into the functional and structural stability of organellar DNA.

Purpose of the Study:

• To investigate the extent of sequence conservation within the chloroplast genome of higher plants.
• To determine the evolutionary relationships and divergence patterns of chloroplast DNA across various plant groups.

Main Methods:

• In vitro labeling of supercoiled chloroplast DNA from pea seedlings to create a probe.
• DNA reassociation experiments using the labeled probe with total DNAs from angiosperms, gymnosperms, and lower vascular plants.
• Hydroxylapatite assay and S1 nuclease analysis to quantify sequence homology and heteroduplexes.

Main Results:

• Chloroplast DNA sequences are highly conserved throughout vascular plant evolution.
• Flowering plants showed distinct cross-reactivity levels with the pea chloroplast probe, with broad bean and barley as extremes.
• Conserved sequences are interspersed with divergent sequences on a fine scale, with approximately 30% sequence relatedness observed across diverse species.
• The proportion of chloroplast DNA in total DNA remained constant despite variations in plant genome size.

Conclusions:

• Fundamental evolutionary changes in angiosperm chloroplast genomes involve rearrangements of conserved sequences relative to divergent ones.
• These rearrangements may differ in extent between monocotyledons and dicotyledons.
• The study provides estimates of interspersed conserved and divergent DNA within the chloroplast genome.