The salvage/turnover/repair (STOR) model for uniparental inheritance in Chlamydomonas: DNA as a source of sustenance

Abstract

The non-Mendelian inheritance of chloroplast genes in Chlamydomonas has engaged researchers for decades and has prompted numerous debates regarding molecular mechanisms and evolutionary significance. The hallmarks of chloroplast inheritance in Chlamydomonas are reviewed here, including observations on vegetative haploid cells, somatic hybrids, meiotic zygospores, and vegetative zygotes resulting from sexual reproduction. Models invoked to explain the typical uniparental maternal inheritance of chloroplast genes, and which center upon the presumed existence of sex-specific protectors and destroyers of chloroplast genomes, are briefly discussed. In an effort to bring together the diverse observations on chloroplast gene inheritance in somatic as well as sexual cells, a model is proposed that focuses on organelle DNA turnover as a source of sustenance for the cell during periods of starvation. The salvage/turnover/repair (STOR) model for chloroplast inheritance in Chlamydomonas proposes that as a consequence of the high ploidy of the chloroplast genome, many copies are dispensable; their degradation would provide nucleotides for recombination, repair, RNA synthesis and cell metabolism. The STOR model offers an alternative view of uniparental inheritance as a phenomenon of direct selective benefit to the organism rather than simply being of selfish benefit to the chloroplast genome. These concepts may also have application to other lower eukaryotes that have sexual reproduction coupled with an extended dormancy.