Recurrent Loss, Horizontal Transfer, and the Obscure Origins of Mitochondrial Introns in Diatoms (Bacillariophyta)

We sequenced mitochondrial genomes from five diverse diatoms (Toxarium undulatum, Psammoneis japonica, Eunotia naegelii, Cylindrotheca closterium, and Nitzschia sp.), chosen to fill important phylogenetic gaps and help us characterize broadscale patterns of mitochondrial genome evolution in diatoms. Although gene content was strongly conserved, intron content varied widely across species. The vast majority of introns were of group II type and were located in the cox1 or rnl genes. Although recurrent intron loss appears to be the principal underlying cause of the sporadic distributions of mitochondrial introns across diatoms, phylogenetic analyses showed that intron distributions superficially consistent with a recurrent-loss model were sometimes more complicated, implicating horizontal transfer as a likely mechanism of intron acquisition as well. It was not clear, however, whether diatoms were the donors or recipients of horizontally transferred introns, highlighting a general challenge in resolving the evolutionary histories of many diatom mitochondrial introns. Although some of these histories may become clearer as more genomes are sampled, high rates of intron loss suggest that the origins of many diatom mitochondrial introns are likely to remain unclear.