DESCRIPTION OF THE PHYLUM CRYPTOMONADA (MARGULIS AND SCHWARTZ 1998)

EUKARYA> CHROMALVEOLATA> HACROBIAE> CRYPTOMONADA

Cryptomonada (krip-to-mo-NA-da) is made from two Greek roots that mean hidden (kryptos -κρύφιος) and unit (monada -μονάδα). In this case, the reference is to a cell (a unit) with a hidden gullet, also called a crypt.

INTRODUCTION TO THE CRYPTOMONADA The cryptomonads are among the most ubiquitous of aquatic organisms. Their general form is that of a unicell with a subapical gullet (text with tooltip) A flagellar pocket is an invagination of the cell within which the flagellar insertions occur. Extensions of the flagellar pocket form the undulating membrane in trypanosomatids. This is sometimes called the reservoir, the crypt, the cytostome (inappropriately), or gullet. from which emerge two unequal flagella (Figure 1). The cell covering is a pellicle (text with tooltip) A pellicle is a complex outer cellular covering that occurs within the bounds of the plasmalemma. Often synonymous with the term theca, a pellicle defines such groups as the euglenoid-kinetoplastid clade amd the Kingdom Alveolatae. made of proteinaceous hexagonal plates. Members of this phylum are about evenly split between autotrophic and heterotrophic taxa. Those that are photosynthetic have at least one chloroplast that has an outer membrane which embraces the large nucleus. All of them have specialized organelles called ejectosomes which line the gullet and occur between the pellicular plates. When deployed, the ejectosomes produce ribbon-like structures that might serve in defense or procuring food. Cyanomonas (see Figure 2) is a cryptomonad that has endosymbiotic cyanobacteria (cyanelles) which function as chloroplasts. They are tiny unicells, but, in large populations, they can have a great impact on local ponds. Pfiester and Holt (1978) witnessed the death of 2,000 catfish in a pond in which a bloom of Cyanomonas occurred. Likely, the cyanelles generated the toxin. Cryptomonads, though very common, are nearly unknown with regard to life history and other aspects of their biology. Part of the problem is an inability to distinguish species easily because they do not seem to undergo sexual reproduction and the species are both variable and maddeningly similar. Lee (1980), Grell (1973), Kudo (1966), Sze (1986), Lee et al. (1985), Sleigh et al. (1984), Gillott (1990), and Margulis and Schwartz (1988) all agree that the cryptomonads make up a small natural group of taxa. Kudo (1966), Grell (1976) and Lee et al. (1986) lump the cryptomonads into a phylum with most of the flagellated unicells. This system is a modification of Margulis and Schwartz (1988 and 1998) in which the cryptomonad phylum is designated Pr-7 and Pr-11, respectively. Baldauf (2003a) presents a consensus view of recent molecular-ultrastructural data in which the cryptomonads occur in a clade with the haptophytes and are associated with the heterokont clade. Burki et al. (2007) and Hackett et al. (2007) also show the monophyly of cryptomonads and haptophytes with a sister relationship to the Rhizaria, the Chromalveolates, or both. Analyses of Burki et al. (2009) and Okamoto et al. (2009) confirm that the centrohelids are sisters to the cryptophytes and haptophytes in a group that Okamoto et al. (2009) call Hacrobia (this is equivalent to our kingdom-level taxon, Eukaryomonadae). Figure 3 shows a molecular phylogeny based on Hsp90 (Okamoto et al. 2009), but the three taxa can emerge in any of the three possible topologies with other molecular analyses (e.g. LSU, SSU, genomic comparisons; Okamoto et al. 2009 and Burki et al. 2009). Zhao et al. (2012) in a broad analysis of eukaryotes by comparing 124 genes by bayesian methodology did find a sister relationship between Hacrobia and Heterokontae + Alveolatae. Hacrobia/Eukaryomonadae is not universally accepted as a monophyletic group. Reed et al. (2009), in an examination of the chromalveolates, found that while the cryptomonads and haptophytes did group together, they did not branch with the Heterokontae or Alveolatae, but instead emerged within the Archaeplastida. Burki et al. (2012), using 258 genes, found that the haptomonads and cryptomonads were mot monophyletic, and the cryptomonads were associated with the archaeplastids. Conversely, the analysis of Hampl et al. (2009), in their comparison of 143 proteins for 48 taxa that encompassed the supergroups of the eukaryotes, challenged the Hacrobia hypothesis and suggested that the haptomonads were sisters to or emerged from within the Archaeplastida while the cryptomonads remained associated with the chromalveolates. The chaos may be related to the incorporation of other enslaved eukaryotic genomes in the Chromalveolates (sensu lato) and Rhizaria (Archibald 2008). Indeed, many cryptomonads still have the remnants of the enslaved nucleus, the nucleomorph (Archibald 2009a and b). Also, because Baurain et al. (2010) found evidence that the cryptophytes, haptophytes, and heterokonts (stramenopiles) acquired their plastids from different eukayotic sources, the phylogenetic signal of the host lines might be further confounded.

FIGURE 1. Cryptomonas, a photosynthetic cryptomonad with a pair of laminate chloroplasts. Image from Systematics Biodiversity Image Archive	FIGURE 2. An illustration of Cyanomonas, an organism with cyanelles rather than chloroplasts. Image from Pfiester and Holt (1978)

FIGURE 3. A cladogram showing the relationship of the cryptomonads (in shaded box) with the rest of the Eukaryomonadae (taxa in bold). The relationship is a simplified view of the molecular phylogeny generated by Okamoto et al. (2009) based on Hsp90.

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By Jack R. Holt. Last revised: 03/04/2013