Genomic perspectives on the birth and spread of plastids

Abstract

The endosymbiotic origin of plastids from cyanobacteria was a landmark event in the history of eukaryotic life. Subsequent to the evolution of primary plastids, photosynthesis spread from red and green algae to unrelated eukaryotes by secondary and tertiary endosymbiosis. Although the movement of cyanobacterial genes from endosymbiont to host is well studied, less is known about the migration of eukaryotic genes from one nucleus to the other in the context of serial endosymbiosis. Here I explore the magnitude and potential impact of nucleus-to-nucleus endosymbiotic gene transfer in the evolution of complex algae, and the extent to which such transfers compromise our ability to infer the deep structure of the eukaryotic tree of life. In addition to endosymbiotic gene transfer, horizontal gene transfer events occurring before, during, and after endosymbioses further confound our efforts to reconstruct the ancient mergers that forged multiple lines of photosynthetic microbial eukaryotes.

Diatom centromeres suggest a mechanism for nuclear DNA acquisition

“…DNA transfer from plastid and mitochondrial genomes to the nucleus, which include noncoding nuclear plastid DNA's (NUPTs) and nuclear mitochondrial DNAs (NUMTs), occurred at the time of endosymbiont acquisition and later in evolutionary time and likely occurs quite frequently in present time (65,(67)(68)(69)(70)(71). This is particularly true for photosynthetic organisms, where the majority of genes required for plastid function are actually housed in the nuclear genome, with a greatly reduced plastid genome from the original acquired state (72,73). For example, in Arabidopsis nearly 18% of nuclear genes were found to be of cyanobacterial origin (74), whereas similar findings of 6-12% cyanobacterialderived genes have been found to comprise unicellular-algae nuclear genomes (75)(76)(77).…”

Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista

“…The groups investigated here and their relationships to the SAR and Archaeplastida supergroups represent a complex mixture of photosynthetic and heterotrophic eukaryotes, as well as lineages for which we have little evidence as to whether they harbour a plastid or not [13,54]. Many of these lineages possess plastids bounded by three or four membranes, which are the result of eukaryote-toeukaryote endosymbioses where heterotrophic organisms acquired plastids from red algae [55].…”

“…Owing to their possible link to haptophytes [11], centrohelids may help to shed light on one of the most puzzling aspects of plastid evolution: the origin and evolution of complex red plastids [13,14]. Centrohelids are heterotrophs, and no permanent plastid has ever been observed [15], although kleptoplasty has been reported [16].…”

Parasites and their (endo)symbiotic microbes

“…They arose by the acquisition of a cyanobacterium by a cell already containing a mitochondrion and subsequently spread throughout eukaryotic lineages by several rounds of secondary and tertiary symbioses (Archibald, 2015b). …”