On the Edge of Dispensability, the Chloroplast Genes.

The polypeptides encoded by the chloroplast genes and some nuclear genes form the thylakoid NADH dehydrogenase (Ndh) complex, homologous to the mitochondrial complex I. Except for Charophyceae (algae related to higher plants) and a few Prasinophyceae, all eukaryotic algae lack genes. Among vascular plants, the genes are absent in epiphytic and in some species scattered among different genera, families, and orders. The recent identification of many plants lacking plastid genes allows comparison on phylogenetic trees and functional investigations of the genes. The genes protect Angiosperms under various terrestrial stresses, maintaining efficient photosynthesis. On the edge of dispensability, genes provide a test for the natural selection of photosynthesis-related genes in evolution. Variable evolutionary environments place Angiosperms without genes at risk of extinction and, probably, most extant ones may have lost genes recently. Therefore, they are evolutionary endpoints in phylogenetic trees. The low number of sequenced plastid DNA and the long lifespan of some Gymnosperms lacking genes challenge models about the role of genes protecting against stress and promoting leaf senescence. Additional DNA sequencing in Gymnosperms and investigations into the molecular mechanisms of their response to stress will provide a unified model of the evolutionary and functional consequences of the lack of genes.