Protists are a diverse and loose grouping of disparate eukaryotic microorganisms. Thy do not have much in common besides a relatively simple organization - either they are unicellular, or they are multicellular without specialized tissues. This simple cellular organization distinguishes the protists from other eukaryotes. They diverged after Archaea and Bacteria evolved but before plants, animals, or fungi appeared on Earth.
Given their mostly single-celled nature, discovering and describing new species has been difficult, and many lineages contain a relatively small number of formally described species, despite their critical importance as pathogens, environmental quality indicators, and markers of past environmental changes.
It would seem natural to use DNA barcoding in order to shed more light on the taxonomy of protists especially because of the lack of diagnostic morphological features, but this has been hampered by the extreme diversity within the group. So far nobody was able to find a single set of molecular markers that works for all lineages as the genetic divergence observed between and within major protistan groups greatly exceeds that found in plants, fungi, and animals.
|Proposed workflow by ProWG|
But there is hope. The Protist Working Group (ProWG), initiated by the Consortium for the Barcode of Life (CBOL ) has assessed the efforts to identify the barcode regions across all protist lineages and now introduced a two-step barcoding approach to assess protistan biodiversity. A suggested workflow (see figure on the right) follows already well established best practice and adds a standardized barcoding step. They propose the V4 region of 18S rDNA as the universal eukaryotic pre-barcode. In a second step group-specific barcodes will then have to be defined separately for each major group of protists. Much of this work is still to be done, however, having a standard to start with is a very big step forward (politically probably the biggest).