DESCRIPTION OF THE PHYLUM ANTHOCEROTOPHYTA

DESCRIPTION OF THE PHYLUM ANTHOCEROTOPHYTA (STOTTLER AND CRANDALL-STOTTLER 1977)

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ANTHOCEROTOPHYTA LINKS

Anthocerotophyta (an-tha-se-ro-TA-fa-ta) is made of three Greek roots that mean flower (anthos -άνθος); horn (keras -κέρας); and plant (phyto -φυτό). The reference is to the horn-like sporangium or fruiting body. The phylum name is formalized from a common genus, Anthoceros.

INTRODUCTION TO THE ANTHOCEROTOPHYTA

The hornworts, in the gametophyte stage, resemble the thallose hepatics; however, when grown in uncrowded conditions, the gametophytes tend to be almost circular. The thallus structure of hornworts is simple, much simpler than Marchantia. Typically, they have an epidermis and air chambers. Some taxa enter into a symbiotic relationship with Nostoc, which enters the air chambers through cracks in the ventral surface. In general, the gametophyte has a rather greasy appearance. Another unique feature of this phylum is the occurrence of massive plastids opposed to the small lenticular ones of other embryophytes.

They never have gametophores ; rather, archegonia and antheridia occur on the upper surface of the thallus (see Figure 1). The archegonia and antheridia, however, are not well differentiated from the thallus (Figure 1-G&H). For example the archegonium is reduced to neck canal cells embedded in undifferentiated thallus tissue. The situation is similar for the antheridia. Despite the apparent simplicity of the gametangia, the sporophyte is quite complex (Figure 1-A-C). The sporophyte emerges by the growth of cells from an intercalary meristem at the top of the foot . The developing capsule appears horn-like with relatively little fertile (or sporogenous) tissue. The sporophyte capsule also has photosynthetic tissue and stomata (Figure 1-B&C). The capsule is somewhat indeterminate with spores at different developmental stages and a continual dehiscence along its longitudinal slits (Figure 1-A,D&E). Spores are shed with the aid of elaters (Figure 1-F).

LEIOSPOROCERTALES (Clade EL)

Gametophytes are thalloid and solid (no internal openings) with no mucilage clefts. Sporophytes are very large with characteristic grooves on either side of the developing capsule. The sporogenous region of the sporophyte is massive with isobilateral spores (not tetrads) scattered among the elaters (pseudoelaters), which are relatively large and may be made of more than one cell. The columella is not distinct. The capsule has large stomata.

Leiosporoceros is the only genus in the class, which is the sister to the rest of the living hornworts. They occur from Mexico, through Central America to Ecuador.

ANTHOCEROTOPSIDA (Clade ES)

This clade includes all of the other hornworts. The gametophyte thalli are variable, but typically thalloid and with internal openings (termed cavernous by Frey and Stech 2005). The sporophytes have a distinctive and obvious columella; spore tetrads tetrahedral. Elaters (pseudoelaters) are present but short (Frey and Stech 2005). Sporophytes have a distinctive and obvious columella.

ANTHOCEROTALES (Clade AS)

The gametophytes are typically thalloid and spongy (cavernous) with mucilage canals and lamellae on the upper part. Stomates occur on the gametophyte thallus and on the sporophyte capsule. The antheridia, which open apically, occur in large groups and have thick walls with four tiers (Renzaglia 1978). Spores are dark (brown to black). The most common genus in North America is Anthoceros (Figure 4). Some Anthoceros species are notable in that they have a symbiotic relationship with Nostoc sp., which is a source of fixed nitrogen.

PHAEOCEROTALES (Clade AI: >1A)

The gametophytes are thalloid, but, unlike Anthocerotales, they are solid. The multiple antheridia occur in each antheridial chamber in most species. However, the antheridial jacket is made of irregular, rather than tiered, layers. Stomata can be present or absent. Spores are opaque (yellow to black).

Phaeoceros, the dominant genus of this order, has a global distribution, but is polyphyletic (Crandall-Stotler et al. 2008). Notothylas (Figure 5), another genus in this order, has sporophyte capsules that lie prostrate on the gametophyte thallus.

DENDROCEROTALES (Clade AI: 1A)

The gametophytes are variable (cavernous to solid). Antheridia occur singly in their own antheridial chambers. Their antheridial walls are made of irregular layers of cells. There are no stomata, either on the gametophyte or sporophyte. The capsules contain elaters (pseudoelators) with helical thickenings (Frey and Stech 2005). The spores are transparent.

This order has three genera: Dendroceros (Figure 6), Megaceros, and Nothoceros, all of which have a southern continental distribution (except one species of Megaceros that occurs in the Himalayas).

SYSTEMATICS OF THE ANTHOCEROTOPHYTA

The taxonomy of the hornworts has been fairly standard. It has almost always held a rank equal to that of the mosses and liverworts (e.g. Scagel et al. 1982; and Bold et al. 1987). Although this group seems to reflect an early divergence of the archegoniate embryophytes, the analysis of Marin and Melkonian (1999) suggests that the hornworts are sisters to the leafy liverworts. If so, the synapomorphy that unites them may be longitudinal lines of dehiscence. On the other hand, Pena et al. (2008) claim that the xyloglycans of mosses and liverworts are distinctly different from those of hornworts and vascular plants, suggesting that the hornworts are sisters to the vascular plants.

Shaw and Renzaglia (2004), Stotler and Crandall-Stotler (2005), and Frey and Stech (2005) demonstrate that the taxonomy of the hornworts is complex for such a small group. Leiosporoceros is a sister group to the rest of the hornworts with one or two clades. The systems of Stotler and Crandall-Stotler (2005) and Frey and Stech (2005) differ in the ordinal structure of the Anthocerotopsida. Stotler and Crandall-Stotler (2005) have two orders while Frey and Stech (2005) reorganize the the class into three orders. We have followed the system of Frey and Stech (2005) because it most closely approximates the analyses of Duff (2006) and Crandall-Stotler et al. (2008).

LITERATURE CITED

Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York.

Crandall-Stotler, B. J., R. E. Stotler, W. T. Doyle, and L. L. Forrest. 2008. Phaeoceros proskaueri sp. nov, a new species of the Phaeoceros hallii (Austin) Prosk. –Phaeoceros pearsonii (M. Howe) Prosk. Complex and the systematic affinities of Paraphymatoceros Hassel. Fieldiana: Botany, N.S. 47: 213-238.

Dittmer, H. J. 1964. Phylogeny and Form in the Plant Kingdom. Van Norstrand Company, Inc. New York.
Duff, R. J. 2006. Divergent RNA editing frequencies in hornwort mitochondrial nad5 sequences. Gene. 366: 285-291.

Frey, W. and M. Stech. 2005. A morpho-molecular classification of the Anthocerotophyta (hornworts). Nova Hedwigia. 80 (3-4): 541-545.

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York.

Marin, B. and M. Melkonian. 1999. Mesostigmatophyceae, a new class of streptophyte green algae revealed by SSU rRNA sequence comparisons. Protist. 150: 399-417.

Pena, M. J., A. G. Darvill, S. Eberhard, W. S. York, and M. A. O’Neill. 2008. Moss and liverowrt xyloglycans contain galacturonic acids and are structurally distinct from the xyloglycans synthesized by hornworts and vascular plants. Glycobiology. 18(11): 891-904.

Renzaglia, K. S. 1978. A comparative morphology and developmental anatomy of the Anthocerotophyta. Journal of the Hattori Botanical Laboratory. 44: 31–90.

Scagel, R. F., R. J. Bandoni, J. R. Maze, G. E. Rouse, W.B. Schofield, and J. R. Stein. 1982. Nonvascular Plants. Wadsworth Publishing Co., Belmont, California.

Schofield, W. B. 1985. Introduction to Bryology. Macmillan Publishing Co. New York.

Shaw, J. and K. Renzaglia. 2004. Phylogeny and diversification of bryophytes. American Journal of Botany. 91(10): 1557-1581.

Stotler, R. E. and B. Crandall-Stotler. 2005. A revised classification of Anthocerotophyta and a checklist of the hornworts of North America, north of Mexico. The Bryologist. 108(1): 16-26.

By Jack R. Holt. Last revised: 03/20/2013


FIGURE 3. Cross section of Leiosporoceros sporophyte. Note that the columella is indistinct and that tetrads are scattered with other cells, presumably elaters (pseudoelaters). The grooves on either side of the capsule are its characteristic sutures. Image from Shaw & Renzaglia (2004)	FIGURE 4. Anthoceros growing with the horn-like sporophytes emerging from the thalloid gametophytes. Image from http://www.csun.edu/~hcbio028/Anthoceros.jpg	FIGURE 5. Notothylas, a hornwort with very short and reduced sporophyte capsules that lie prostrate on the gametophyte thallus. Image from http://www.uni-bonn.de/Aktuelles/Pressemitteilungen/318_02/bilder/Notothylas.jpg	FIGURE 6. Dendroceros sp., grows here on the bark of a tree in Brazil. Note the small, erect sporangial capsules. Image by Ziffer and placed in the Public Domain.

	FIGURE 2. MAJOR CLADES OF THE ANTHOCEROTOPHYTA EL = Elaters long ES = Elaters short AS = Antheridia wall stratose AI = Antheridial wall irregular >1A = >1 Antheridium per chamber 1A = 1 Antheridium per chamber
FIGURE 2. MAJOR CLADES OF THE ANTHOCEROTOPHYTA. The structure of this cladogram comes from Duff (2006) and Crandall-Stotler et al. (2008). The description of the clades is an interpretation of Frey and Stech (2005) and Stotler and Crandall-Stotler (2005).

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