Sulcozoa revealed as a paraphyletic group in mitochondrial phylogenomics

Abstract

Recently, phylogenomic analyses have been used to assign the vast majority of eukaryotes into only a handful of supergroups. However, a few enigmatic lineages still do not fit into this simple picture. Such lineages may have originated early in the history of eukaryotes and are therefore of key importance in deduction of cellular evolution. In this study, we focus on two deeply diverging lineages, Diphyllatea and Thecamonadea. They are classified in the same phylum, Sulcozoa, but previous multigene phylogenetic analyses have included only one of these two lineages. It is therefore unclear whether they constitute one group or two distinct lineages. The study of rare genomic changes reveals that both have the fused dihydrofolate reductase (DHFR) and thymidylate synthase (TS) genes (i.e. DHFR–TS), which are separated in all other unikonts that have been investigated, indicating a possible close relationship. Their phylogenetic positions have implications for the classification of Sulcozoa and the early eukaryote evolution. Here we present a phylogenomic analysis of these species that include Illumina and 454 transcriptome data from two Collodictyon strains. A total of 42 mitochondrial proteins, which correspond to orthologs published from Thecamonas trahens (Thecamonadea), were used to reconstruct their phylogenies. In the resulting trees, Collodictyon appears as sister to Amoebozoa, whereas Thecamonas branches as the closest relative of Opisthokonta (i.e. the animal, fungi and unicellular Choanozoa). In contrast, the position of another early diverging eukaryote, Malawimonas, is unresolved. The separation of Collodictyon and Thecamonas in our studies suggests that the recently proposed Sulcozoa group is most likely paraphyletic. Furthermore, the data support the hypothesis that the two supergroups Opisthokonta and Amoebozoa, which comprise a great diversity of eukaryotes, have originated from a sulcozoan ancestor.