Abstract
Euglenozoa is a species-rich group of protists, which have extremely diverse lifestyles and a range of features that distinguish them from other eukaryotes. They are composed of free-living and parasitic kinetoplastids, mostly free-living diplonemids, heterotrophic and photosynthetic euglenids, as well as deep-sea symbiontids. Although they form a well-supported monophyletic group, these morphologically rather distinct groups are almost never treated together in a comparative manner, as attempted here. We present an updated taxonomy, complemented by photos of representative species, with notes on diversity, distribution and biology of euglenozoans. For kinetoplastids, we propose a significantly modified taxonomy that reflects the latest findings. Finally, we summarize what is known about viruses infecting euglenozoans, as well as their relationships with ecto- and endosymbiotic bacteria.
Highlights
It is generally accepted that Euglenozoa belong to the most unusual eukaryotes [1,2,3]
The euglenids are sometimes further subdivided into Euglenida and Symbiontida [3], both groups are usually treated together due to their morphological similarity, and we still cannot compare their genomic features in the absence of such data from the latter taxon [7]
Some kinetoplastids are known to form cysts, which help them to survive adverse conditions, for example, pass through the digestive system of an animal and settle in its faeces after their discharge [37]. Some of these flagellates become very tolerant to harsh environments even at the active stage. This resulted in a series of records of free-living kinetoplastids (Parabodo caudatus, Dimastigella trypaniformis and Procryptobia tremulans) from stool and urine samples or urine-impregnated animal cage bedding sometimes misinterpreted as evidence of their parasitic nature [51,52,53,54]
Summary
It is generally accepted that Euglenozoa belong to the most unusual eukaryotes [1,2,3]. Studied so far only in kinetoplastids, the paraflagellar rod increases propulsion of the cell [18], and participates in morphogenetic and metabolic roles, as well as in environmental sensing [19] While all these synapomorphies were probably present in the euglenozoan common ancestor, euglenids, diplonemids 2 and kinetoplastids have acquired significant differences over the course of evolution. The dearth of high-quality data for nuclear genomes of euglenids and their absence in the case of diplonemids are due to the large size and repetitive character of the latter The transcriptomes from both groups contain an extremely high number of protein-coding genes, probably reflecting their metabolic versatility [6,13]. The nomenclature of kinetoplastids and diplonemids follows the ICZN, while for some euglenid groups, the ICN is used by default with the valid names according to the ICZN indicated
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