  The origin of organelles (mitochondrion, chloroplast and nucleus) remains
enigmatic. The endosymbiotic hypothesis that chloroplasts, mitochondria and
nuclei descend from the endosymbiotic cyanobacterium, bacterium and
archaebacterium respectively is dominant yet uncompelling, while our discovery
of de novo organelle biogenesis in the cyanobacterium TDX16 that had acquired
the genome of its green algal host Haematococcus pluvialis overturns this
hypothesis. In light of organelle biogenesis in the cyanobacterium TDX16 in
combination with the relevant cellular and molecular evidence, we propose
genome hybridization hypothesis (GHH) that the origin of organelles and origin
of eukaryotes as well as the diversification of organelles and speciation of
eukaryotes are unified and achieved by genome hybridization: the endosymbiotic
cyanobacteria or bacteria obtain genomes of their archaebacterial or eukaryotic
hosts and hybridize with their own ones resulting in expanded genomes
containing a mixture of hybrid prokaryotic genes and eukaryotic genes, and thus
the cyanobacteria or bacteria have to compartmentalize to accommodate different
genes for specialized function of photosynthesis (chloroplast), respiration
(mitochondrion) and DNA preservation (nucleus), and consequently turn into
photosynthetic or heterotrophic eukaryotes. Accordingly, eukaryotes and their
organelles are of multiple origin, while the formation of cancer cells is the
speciation of eukaryotes as cancer cells are new species of unicellular
eukaryotes arising from bacteria. Therefore, GHH provides a theoretical
framework unifying evolutionary biology, cancer biology and cell biology and
directing the integrated multidisciplinary research.
