Katablepharids

Genus Katablepharis is currently polyphyletic, and species appear both in the freshwater clade and the marine clade that includes three other genera. Katablepharis here is to show the type species K. phoenikoston on the assumption that it's included in the hitherto detected freshwater clade.

Introduction

Katablepharids are heterotrophic flagellates, which have an oval or cylindrically ovate cell with one anterior and one posterior flagellum emerging from a shallow subapical groove.  

Katablepharids have long been classified as a class within cryptophytes since the first description by Skuja (1939) until recently, when Vørs et al (1992) emended them as a class incertae sedis.  Recently Okamoto and Inouye (2005a, 2005c) demonstrated that they are an independent protistan group distantly related to cryptomonads (algae) and goniomonads (bacterivorous flagellates) based on molecular phylogeny of SSU rDNA and alpha tubulin.

According to the chromalveolata hypothesis, cryptophytes belong to a supergroup Chromalveolata together with heterokontophytes, alveolates, and haptophytes.  Horizontal gene transfer of plastid genes favors the single origin of the plastid among the chromoalveolates algae.  Since katablepharids are distant sister of cryptophytes, it is interesting to know whether katablepharids still retain some remnant of the secondary plastid.

Characteristics

Katablepharids share some characters with crypromonads and goniomonads, such as the cell shape, two flagella emerging from a subapical groove and the ultrastructurally similar ejectisomes near/inside the goove; however, their morphology, such as a bilayer sheath around cell and flagellar surface, tubular mitochondrial cristae, and the presence of a complex feeding apparatus is quite distinctive.

Discussion of Phylogenetic Relationships

The original description of katablepharids dates back to 1939 by Skuja, when he described Kathablepharis phoenikoston (the type species) from freshwater and K. hyalurus from marine environments.  As the early descriptions were based on light microscopical observation, they used to include several protist genera of uncertain affinity, such as Phyllomitus spp. (kinetoplastids) and Cryptaulax spp. (incertae sedis).  More than half a century after the first description,  Vørs (1999) emended the class to have three genera, Katablepharis, Leucocryptos, and Platychilomonas. Later the new marine genera Hatena (Okamoto & Inouye 2005b, 2005c) and Roombia (Okamoto et al 2009) were established. Hatena is included in a marine katablepharids clade, while Roombia represents the earliest branch of all katablepharids.

Katablepharis officially has 6 freshwater species including the type species K. phoenikoston, and 3 marine species. There are ultrastructural studies on a marine strain “K. clone-G2” that has never been officially identified nor described. Leucocryptos, Platychilomonas, Hatena and Roombia are monospecific genera, all of which are marine environment.

Availability of the culture strains has been limited, and this is why this taxon has long been neglected.  Part of the reason are the requirements of the prey cell, which makes the establishment of the culture more complicated.  However, because of their phylogenetic and ecological importance, there are continuous efforts to culture these eukarivorous protists.

Environmental Sequences

Since the first molecular data were provided (Okamoto & Inouye 2005a, c), a number of environmental sequences have been assigned to katablepharids (Šlapeta et al 2006). Interestingly, Šlapeta et al (2006) observed a distinct grouping of marine and freshwater species, although there are no clues from the taxon sampling as to the nature of the environmental sequence data.  Nevertheless, this still implies the diversity of katablepharids and their ecological importance, as well as a possible need for re-examination of their taxonomy.

Symbiosis in Progress

Hatena arenicola is most likely to be in the midst of plastid acquisition via secondary endosymbiosis (Okamoto and Inouye 2005b, 2005c), alternating “plant” phase and “predator” phase.  Most of  the cells from the environment are in the “plant” phase, possessing a Nephroselmis sp. as an endosymbiont. In this phase, Nephroselmis sp. looks like a plastid under the light microscope, with the eyespot always located at the apex of the host cell. However, the symbiont still retains its nucleus. Moreover, their cell division cycles have not been harmonized, as the symbiont cell is always inherited by only one of the daughter host cells.  After the cell division, the daughter cell which has lost the symbiont (“predator” phase) needs to uptake a new symbiont from the cell apex, where the feeding apparatus resides.  After the uptake, the symbiont Nephroselmis sp. needs to re-establish the morphological association of the eyespot with the host cell apex, and needs to be enlarged more than ten-fold in size.

Nomenclatural Problem

There had been an issue about the genus name Katablepharis/Kathablepharis.  In the first description, Skuja used the spelling Kathablepharis for the genus name.  However, according to the International Code of Botanical Nomencrature (ICBN), this must be corrected as Katablepharis based on the name's etymology (“trailing flagellum” in Greek; kata = downwards, blepharis = eyelash).  On the other hand, according to the International Code of Zoological Nomencrature (ICZN), the spelling must be Kathablepharis regardless of the etymology. Recently, Okamoto et al (2009) published a correction of the name to Katablepharis under ICZN.

Other Names for Katablepharids

• Katablepharidae

References

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