Abstract
Diatoms are a monophyletic group of eukaryotic, single-celled heterokont algae. Despite years of phylogenetic research, relationships among major groups of diatoms remain uncertain. Here we assess diatom phylogenetic relationships using the plastid genome (plastome). The 22 previously published diatom plastomes showed variable genome size, gene content and extensive rearrangement. We report another 18 diatom plastome sequences ranging in size from 119,120 to 201,816 bp. Plagiogramma staurophorum had the largest plastome sequenced so far due to large inverted repeats and a 2971 bp group II intron insertion in petD. The previously reported loss of psaE, psaI and psaM genes in Rhizosolenia imbricata also occurred in the closely related species Rhizosolenia fallax. In the largest genome-scale phylogeny yet published for diatoms based on 103 shared plastid-coding genes from 40 diatoms and Triparma laevis as the outgroup, Leptocylindrus was recovered as sister to the remaining diatoms and the clade of Attheya plus Biddulphia was recovered as sister to pennate diatoms, strongly rejecting monophyly of two of the three proposed classes of diatoms. Our study also revealed extensive gene loss and a strong positive correlation between sequence divergence and gene order change in diatom plastomes.