Continuity and Discontinuity of Chloroplasts and Cyanobacteria

Abstract

Genome sequencing opened a new way of search for chloroplast origin. The term “genome” originally meant the whole set of genes of an organism, but now it means the entire DNA of an organism. Most eukaryotes are diploid, and have two sets of genome. In organisms such as human having sex chromosomes, a genome consists of a set of autosomes (22 in human) plus X and Y sex chromosomes. In the 2000s, genome sequences of various model organisms such as human, Arabidopsis thaliana (thale cress), Escherichia coli, and budding yeast were determined. Then, new generation sequencers drastically cut the cost of sequencing and allowed sequencing of genomes in non-model organisms, namely, actual organisms studied in biology laboratories, at a reasonable cost. We are now in the post-genomic era. Additionally, various large-scale data, such as transcriptomes (data of all transcripts), proteomes (data of all proteins), and metabolomes (data of all metabolites), became available at least in model organisms. These advances enabled studies on biologically interesting phenomena by wisely combining all these “omics” data, not only in model organisms but also in non-model organisms. Comparative genomics is a new technique that the progress in computer technology realized. This technique enables to establish relationship between genomic data of different organisms, and to apply knowledge obtained in an organism to other organisms. It reveals many homologous proteins in all sequenced organisms, for use in phylogenetic analysis. In this way, post-genomic studies provided enormous new data for the study on the origin of organelles, which could, perhaps, change fundamentally the endosymbiotic theory on the origin of chloroplasts and mitochondria. This is a benefit of the post-genomic re-analysis over the studies performed in the latter half of the twentieth century.