Microheriella maris unites Archaeplastida and Cryptista in phylogenomics
There are still many "orphan" protists of which phylogenetic affiliation has not yet been determined by previous studies in the eukaryotic tree. Determining their phylogenetic positions has the potential to fill the gaps in eukaryotic diversity and it contributes to reveal true phylogenetic relationships among major lineages that still remain to be resolved well. Microheliella maris was originally described as a member of the phylum Heliozoa, but previous large-scale phylogenetic analyses have not been able to confidently place this organism within any previously described species or lineages. In this study, we analyzed the alignment of 319 genes and demonstrated that M. maris represents the basal lineage of Cryptista. Here, we propose a new clade name "Pancryptista", which includes M. maris and Cryptista. In addition, the results of the analysis of 319 genes showed that M. maris is an important taxon to recover the monophyly of Archaeplastida and the sister relationship between Archaeplastida and Pancryptista, so we call these clades "CAM clade" collectively here. Cryptista tends to be attracted to Rhodophyta depending on the taxon sampling in the phylogenetic analysis, and this particular phylogenetic "signal" may have prevented the stable recovery of Archaeplastida monophyly in previous studies. Based on our detailed molecular phylogenetic analyses using 319 genes, we hypothesize that many genes in Cryptophyceae, the internal lineage of Cryptista, have been partly recombined with homologous genes transferred from red algal endosymbionts during secondary endosymbiosis and have a faint phylogenetic affinity with the genes of Rhodophyta.