DESCRIPTION OF THE PHYLUM XANTHOPHYTA (ALLORGE EX FRITSCH 1935)

EUKARYA> CHROMALVEOLATA> HETEROKONTAE> XANTHOPHYTA

Xanthophyta (zan-THA-fa-ta) is derived from two Greek roots that mean yellow or blonde (xanthia -ξανθιά); and plant (phyto -φυτό). The reference is to the distinctive yellowish appearance due to the characteristic secondary photosynthetic pigments.

INTRODUCTION TO THE XANTHOPHYTA Though relatively small in number of taxa (diversity), the xanthophytes exhibit a large range in form (disparity) from amoeboid and coccoid unicells (Figure 1) to filaments (Figures 2 and 3). Life cycles are as varied with isogamy (text with tooltip) Isogametes are gametes that are equal in size. , anisogamy (text with tooltip) Anisogamous (adj.) describes sexual reproduction in which the gametes are structurally siimilar, but not identical. , and oogamy employed within the phylum. The common name for the group is the yellow-green algae, but they range in color from grass-green to brown-green depending on the relative abundance of accessory pigments, particularly the xanthophylls (text with tooltip) Xanthophyll is an oxygenated carotenoid secondary photosynthetic pigment that occurs in many of the photosynthetic eukaryotes. . Van den Hoek et al. (1995) claim that the taxa of this phylum rarely grow anywhere in abundance. However, we frequently see the siphonaceous filament, Vaucheria, (also called water felt) in great abundance near seeps and springs. The appearance of Vaucheria, which grows as a mat of dark green filaments on wet soil and on submerged sediments, strongly resembled to certain green algae as well as some of the water molds like Saprolegnia (Bold et al. 1988). Unlike green algae, Vaucheria does produce the typical heterokont ( heterodynamic flagella (text with tooltip) Heterodynamic flagella occur on the same cell but beat with different patterns (e.g. anterior-posterior). ) motile cell, eyespot (text with tooltip) An eyespot is a light-sensitive structure that does not form an image. This can be part of an organelle as in the chloroplast of certain microbial eukaryotes. It can be an elaborate structure that involves a light-sensitive swelling at the base of a flagellum (as in the euglenoids) or it can be a multicellular structure as in planarians. , chlorophyll c (text with tooltip) Chlorophyll C is a variant of Chlorophyll A. and a secondary photosynthetic pigment in the many of the photosynthetic heterokonts and dinoflagellates. , and storage products. The vegetative filaments are siphonaceous (text with tooltip) Siphonous (adj) describes a filament that has no cross walls. and branched. Zoospores (text with tooltip) A zoospore is an asexual spore that is motile. Zoo- (pronoumced zo-o) is a prefix that means moving. are produced at the ends of certain filaments, and they have many heterokont pairs of flagella on each zoospore. The gametangia are distinctive, a large oogonium (text with tooltip) Oogonium is a specialized structure (gametangium) that contains the ovum or egg of an oogamous taxon. (or oogonial cluster) surrounded by two or more short antheridial branches (text with tooltip) An antheridial branch (n.) filament that bears gametangia that produce sperm or spermatia. (Figure 2). The antheridia produce many small biflagellate heterokont sperm that can fertilize an egg, which produces a zygote. Usually, the zygospore (text with tooltip) Sexual spores produced by fusion of gametangia. is a resting stage. Tribonema (Figure 3) is nonmotile in the vegetative state and forms short filaments that can be abundant in the cool months as floating light green mats and loosely-attached clumps on submerged vegetation. The filaments are unbranched and the cell walls are made of overlapping H-pieces (see Figure 3), which is reminiscent of the overlapping wall structure of the diatoms. Their life history has vegetative fragmentation, asexual heterokont zoospores, and isogamous (text with tooltip) Isogamous (adj) describes sexual reproduction in which the gametes are structurally identical. sexual reproduction. The systematics of the xanthophytes has been quite chaotic over the past 30 years. They have been placed into thier own phylum (e.g. Hibberd 1990a; Margulis and Schwartz 1988, Pr-9 and 1998, Pr-14; Sleigh et al. 1984). Scagel et al. (1982), Bold and Wynne (1985), Lee (1980, 1995, 1999) and Sze (1986) consider the xanthophytes to be a class of the chrysophytes. Protozoologists like Grell (1976), Kudo (1966) and Lee et al. (1985) consider only the motile forms like Olisthodiscus (which is a Raphidiophyte). The phylogeny of Taylor (1976) and analysis of Dodge (1973) place the xanthophytes as a distinct group which is associated with the chrysophyte complex (chromophytes). In addition, the xanthophytes seem clearly related to the Oomycota although Beakes (1989) gives a non-committal review of the evidence which links them to the Oomycota (as well as the Eustigmatophyta). Sogin and Patterson (Tree of Life Project) indicate that they form a natural group and are sisters to the Phaeophyta. More recent treatments of the heterokonts (e.g. Baldauf 2003) show the heterokonts as a natural group associated with the cryptomonad-haptomonad group and a sister to the alveolates. More recent analyses (e.g. Andersen 2004; Brown and Sorhannus 2010; Yang et al. 2012) have placed the xanthophytes at the crown of the photosynthetic heterokonts as a sister group to the Phaeophytes (Figure 4). Maistro et al. (2009) examined the coccoid, filamentous, and siphonaceous taxa of the xanthophytes by multi gene analysis and identified four clades that they call Botrydiopsalean (B), Chlorellidialean (C), Tribonematalean (T), and Vaucherialean (V), as shown in Figure 4. They did not include flagellates or amoeboid taxa in their study. Surprisingly, the clades did not conform strictly to morphology and, therefore, are equivalent to the classical taxonomic groupings. Taxa of the Botrydiopsalean clade are unicellular (coccoid) and the Vaucheralean clade (only Vaucheria) are siphonaceous, but the taxa included in the Tribonematalean and Chlorellidalean clades range from unicells to branched filaments with high levels of support. They do make a point to say that they are not proposing a formal taxonomy.

FIGURE 1. A vegetative cell of Ophiocytium, which means “adder cell”. Image shows the characteristic point at one end and rounded tip at the other. Image from Systematics Biodiversity Image Archive	FIGURE 2. Two oogonia and one antheridium on a filament of Vaucheria. Note the siphonaceous nature of the filament. Image by Jan Kastovsky, http://www.sinicearasy.cz	FIGURE 3. Filaments of Tribonema whose cell walls are constructed of overlapping H-pieces. Image from Systematics Biodiversity Image Archive

FIGURE 4. A cladogram showing the relationship of the xanthophyte clades (in shaded box) within the heterokonts (taxa in bold). Oc is the Ochrophyta clade, the photosynthetic taxa. The topology of this cladogram was informed by Andersen (2004), Brown and Sorhannus (2010), and Yang et al. (2012). The definition and discovery of the xanthophyte clades was by Maistro et al. (2009).

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By Jack R. Holt. Last revised: 02/24/2014