Gene transfers from organelles to the nucleus: How much, what happens, and why none in Elysia?

Gene transfers from organelles to the nucleus are important in evolution and they are quantitatively great. Studies from higher plants and algae have shown that a substantial proportion of plant genomes, more than 15% in some species, consists of genes that were ultimately acquired from the cyanobacterial ancestor of plastids. Looking back into early eukaryote evolution, among heterotrophic eukaryotes, well over half of the sequences that have prokaryotic homologues are derived from eubacterial rather than archaebacterial genes. It can be argued that these ultimately stem from the proteobacterial ancestor of mitochondria. Reflecting more recent genome evolutionary processes, all sequenced eukaryotic genomes from species that have DNA in their mitochondria contain fragments of mitochondrial or plastid DNA in their nuclear chromosomes. The haploid human genome contains about 260,000 bases of sequences recently derived from insertions of mitochondrial DNA (mtDNA), 12 human loci are polymporphic for mtDNA insertions and five human mtDNA insertions cause disease (Hazkani-Covo et al. 2010). The process of gene transfer into the chromosomes is well characterized and entails non-homologous end joining mechanism, not cDNA intermediates as was once thought. Gene transfers from organelles to the nucleus have become a very commonplace phenomenon in biology. Because endosymbiotic gene transfer is so well documented for plastid origins, there has long been an expectation that gene transfer underpins the biology of the stolen chloroplasts (kleptoplasts) in the photosynthetic sacoglossan slugs from the genus Elysia. However, genome wide expression data show that no expressed genes in these photosynthetic slugs have been transferred from algae.