The Transfer of the Ferredoxin Gene From the Chloroplast to the Nuclear Genome Is Ancient Within the Paraphyletic GenusThalassiosira

Ferredoxins are iron-sulfur proteins essential for a wide range of organisms because they are an electron transfer mediator involved in multiple metabolic pathways. In phytoplankton, these proteins are active in the mature chloroplasts, but thepetFgene, encoding for ferredoxin, has been found either to be in the chloroplast genome or transferred to the nuclear genome as observed in the green algae and higher plant lineage. We experimentally determined the location of thepetFgene in 12 strains ofThalassiosiracovering three species using DNA sequencing and qPCR assays. The results showed thatpetFgene is located in the nuclear genome of all confirmedThalassiosira oceanicastrains (CCMP0999, 1001, 1005, and 1006) tested. In contrast, allThalassiosira pseudonana(CCMP1012, 1013, 1014, and 1335) andThalassiosira weissflogii(CCMP1010, 1049, and 1052) strains studied retained the gene in the chloroplast genome, as generally observed for Bacillariophyceae. Our evolutionary analyses further extend the dataset on the localization of thepetFgene in the Thalassiosirales. The realization that thepetFgene is nuclear-encoded in theSkeletonemagenus allowed us to trace thepetFgene transfer back to a single event that occurred within the paraphyletic genusThalassiosira. Phylogenetic analyses revealed the need to reassess the taxonomic assignment of theThalassiosirastrain CCMP1616, since the genes used in our study did not cluster within theT.oceanicalineage. Our results suggest that this strains' diversification occurred prior to the ferredoxin gene transfer event. The functional transfer ofpetFgenes provides insight into the evolutionary processes leading to chloroplast genome reduction and suggests ecological adaptation as a driving force for such chloroplast to nuclear gene transfer.