Phylum Anthocerophyta PDF

**Topic: Phylum Anthocerophyta** I. **Course Objectives** The learning objectives for this week are the following: a. b. c. d. e. II. **Introduction** - The hornworts, or **Anthocerotae**, are a monophyletic group comprising a second major lineage of land p...

**Topic: Phylum Anthocerophyta** I. **Course Objectives** The learning objectives for this week are the following: a. b. c. d. e. II. **Introduction** - The hornworts, or **Anthocerotae**, are a monophyletic group comprising a second major lineage of land plants. - The phylum's common and scientific names (from the Greek ***keras***, horn) refer to the long, tapered shape of the sporophyte. At typical sporophyte can grow to about 5cm high. - Hornworts are frequently among the first species tto colonize open areas with moist soils; a symbiotic relationship with nitrogen-fixing cyanbacteria contributes to their ability to do this (nitrogegn is often in short supply in such areas). - An emerging, albeit surprising, consensus based on recent molecular phylogenies is that hornworts are the closest extant relatives of tracehophytes. - Three new genera have been named, increasing the number of hornwort genera to 14, namely *Leiosporoceros, Anthoceros, Sphaerosporoceros, Folioceros, Hattorioceros, Mesoceros, Paraphymatoceros, Notothylas, Phaeoceros, Phymatoceros, Phaemegaceros, Megaceros, Dendroceros,* and *Nothoceros.* III. **Phylogeny** - Duff *et al.* (2007) reported a more comprehensive molecular phylogeny utilizing three genes, one each from the nuclear, mitochondrial, and plastid genomes, and up to 62 hornwort samples, representing 12 of the 14 genera and one third of the recognized species. - Several major features of hornwort relationships are: 1. There is significant genetic distance between three lineages of honrowrts: *Leiosporoceros, Anthoceros* s. lat., and the remaining hornworts; 2. Taxa formerly recognized as belonging to *Phaeoceros* are polyphyletic and consequently, three new genera were segregated from this genus: *Phymatoceros, Paraphymatoceros,* and *Phaeomegaceros;* 3. American species of *Megaceros* and *Dendroceros*, suggesting a new genetic status to this *Nothoceros*/American *Megaceros* alliance; and 4. A sister relationship exists between *Phaeoceros* s. str. and *Notothylas*. IV. **Classification** *(See uploaded PDF e-book entitled Taxonomical Classification of Phylum Anthocerophyta)* V. **Anatomy and Development** - **Peculiarities** in the structure and development of the **sprophyte, chloroplasts, gametangia**, and ***Nostoc* colonies**, among other traits, distinguish this small assemblage of bryophytes from all other land plants. - Based on the **leafless habit** of the gametophyte, hornworts were traditionally included within liverworts and were viewed as having an affinity with simple thalloids. With phylogenetic reconstructions pointing to a sister relationship between hornworts and tracheophytes, a thorough reappraisal of morphological transformations across cryptogams in both generations is warranted. - - The **vegetative gametophyte** of hornworts is a flattened thallus, with or without a thickened midrib (Fig. 3.2.A,B). - Growing regions that contain **solitary apical cells** and immediate derivatives are located in **thallus notches** and are covered by **mucilage** that is secreted by epidermal cells (Fig. 3.2.C, D). - The apical cell and immediate derivatives contain well-developed **chloroplasts that are intimately associated with the nucleus** (Fig.3.2C). - The **wedge-shaped apical cell** of most taxa segments along four cutting faces: two lateral, one dorsal, and one ventral (Fig. 3.2E). The resulting growth forms tend to be **orbicular** and the thallus in cross-section gradually narrows from the central to lateral margins. - In comparison, the **hemi-discoid apical cell** (Fig. 3.2D, F) of *Dendroceros* cuts along two lateral and one basal face and is responsible for producing a **ribbon-shaped thallus** with an enlarged midrib and monostromatic wings (Fig. 3.2B). - At the cellular level, hornworts are known to contain **solitary chloroplasts with central pyrenoids** (or starch-free areas) and **channel thylakoids**, features shared with algae but found in no other land plants (Fig. 3.3A-D).*Leiosporoceros* has **plastids that lack a pyrenoid** but often contain a **central aggregation of large grana surrounded by starch** (Fig. 3.3F). - The classical hornwort pyrenoid transversed by thylakoids, which separate lens-shaped to elongated subunits giving the appearance of "multiple pyrenoids" (Fig. 3.3A, B). - Chloroplast structure in *Dendroceros* deviates from that of a typical hornworts in that the **pyrenoid is spherical** and **contains irregularly shaped subunits** with regularly spaced electron-opaque inclusions (Fig. 3.3G). - Chloroplasts of *Megaceros* **lack pyrenoids** and as many as 14 per internal thallus cell (Fig. 3.3E). - **Cell division** in all hornworts is **monoplastidic** and involves plastic division and morphogenetic migration that is tightly linked with nuclear division. - **Spindle microtubules** originate from an aggregation of electron-dense material at the poles, suggesting the vestige of algal-like centriolar centrosomes. - The **thickened thallus** of the hornwort gametophyte **lacks differentiation** (Fig. 3.4A), except for the occurrence of rather extensive schizogenous **mucilage canals** in species of Anthocerotaceae (Fig. 3.4B), and *Dendroceros* (sub-genus *Apoceros*). - In *Megaceros*, **epidermal cells are smaller** than in internal parenchyma cells (Fig. 3.4A). - Unlike the sporophyte epidermis, all epidermal cells of the gametophyte contain chloroplasts (Fig. 3.4A,B,D). - **Mucilage-filled cells** are abundant and scattered among photosynthetic parenchyma in most taxa (Fig. 3.4A). - **Band-like wall thickenings** (Fig. 3.4C) and **primary pit fields** may occur in cells of the thallus that subtend archegonia and later the sporophyte foot. - **Vesicular-arbuscular endomycorrhizas** are common in internal thallus cells of most taxa, and swollen, terminal tubers characterize some genera and species (Fig. 3.4E). - **Rhizoids** are unicellular, smooth and may have branched tips. They are typically ventral in position and may develop from the outer cell derived from a periclinal division of an epidermal cell. - A distinctive feature of anthocerotes is the occurrence of apically derived **mucilage clefts,** primarily on the ventral thallus (Figs. 3.2B, 3.4D). Two cells that resemble stomatal guard cells surround a pore, which lacks the ability to close and open. The function of these mucilage clegts as the **site of entry for *Nostoc*,** the colonial endosymbiont that is found in all hornworts. In most species, clefts are regularly produced from apical derivatives and each may attract the phycobiont. - Once in the mucilage-filled internal chamber, the *Nostoc* increases in size and forms a discrete spherical colony (Fig. 3.5A). Thallus outgrowths penetrate the algal colony (Fig. 3.5B). In *Leiosporoceros,* clefts are produced only in the sporeling; *Nostoc* enters and forms an intimate association directly behind the apical cells (Fig. 3.5C-E). As the thallus elongates through apical segmentation, so too does the *Nostoc* colony within an advancing narrow schizogenous canal (Fig.3.5D,E). Unknown elsewhere in plants, these branching *Nostoc* canals run through the central thallus (Fig. 3.5C) and are visible to the naked eye as dark green strands. - All honrworts have a symbiotic relationship with **Cyanobacteria** (blue-greens), which live inside cavities of the thallus. This relationship is found in a few thalloid liverworts as well, but not in mosses. Interestingly, honrworts and liverworts may also have a symbiotic association between the gametophytes and a fungus, similar to the mycorrhizal association with the roots of vascular plants. - **Fragmentation, regenerant formation,** and **gemmae production** have been reported in various taxa. Under adverse environmental conditions or simply as a means of **asexual reproduction**, some genera or species of hornwort produce **nutrient-filled tubers** as perennating bodies (Fig. 3.4E). - Gametangia are produced along the dorsal thallus midline. **Archegonia are exogenous**, i.e. they develop from surface cells, and ultimately are sunken in thallus tissue (Fig. 3.6A-C). In addition to the **central cells** of the archegonium, the archegonial initial gives rise to a one- to two-layered **venter** (Fig. 3.6B), and six rows of **neck cells** that slightly protrude from the thallus surface and are overarched by a layer of mucilage (Fig. 3.6C,D). Two to four **cover cells** cap the canal until the egg reaches maturity, at which time they are dislodged from the neck (Fig. 3.6A). **Venter cells are smaller** than the surrounding parenchyma; they are less vacuolated and contain a prominent nucleus with nucleolus and associated flattened plastid (Fig. 3.6B). The ventral canal cell and egg originate from the venter canal cell and contain dense cytoplasm including abundant lipid reserve and a single elongated undifferentiated plastid that encircles the nucleus (Fig. 3.6D). - **Antheridia** are referred to as **endogenous** because they develop from subepidermal cells and ultimately are positioned within internal thallus chambers (Fig. 3.7A). One to 80 antheridia are enclosed in each sunken chamber (Fig. 3.7A, B). The antheridial initial elongates without apical cell involvement and four primary spermatogones with eight peripheral jacket initials are produced in the formative stages of organogenesis. Thousands of minute spermatozoids are produced in each antheridium (Fig. 3.7A, C). When antheridia are mature, the plastids of the jacket layer typically have converted to orange-colored chromoplasts. - The **mature spermatozoid** is extremely small (approx.. 3.0 µm in diameter), coiled, biflagellated, and symmetrical. Both flagella insert at the anterior extreme of the cell over a spline of 12 microtubules are directed posteriorly. Spermatozoids contain an anterior mitochondrion, a cylindrical nucleus with mid-constrictions, and a posterior mitochondrion associated with a plastid containining one starch grain. Unlike sperm cells in all other archegoniates which are sinistrally coiled, the hornwort cell is **dextrally coiled** (Figr. 3.7D). - Following fertilization, the first division of the zygote is longitudinal and the **endothecium** of the embryo gives rise to a **central columella**. The **amphithecium** forms the sporogenous tissue, assimilative layer, and epidermis (Fig. 3.8A). The **foot** matures before the remaining histogenic regions (Fig. 3.8A, B). the basal meristem is established early in development and is unifacial, producing cells above the foot that differentiate upwardly. Growth of overarching gametophytic tissue occurs as the embryo develops, thus forming a protective **involucre** that in most taxa is ruptured with continued maturation of the sporophyte (Fig. 3.2A). numerous archegonia are produced in an acropetal fashion and thus young plants will bear young sporophytes at different stages of development. - Although **globose in general structure**, the anatomical organization of the foot is highly variable among species. Palisade-like epidermal cells surround the **relatively small foot of *Anthoceros*** whereas the **massive foot of *Megaceros*** contains thousands of small undifferentiated cells. Collectively, the cells at the interface between generations compose the **placenta** through which the sporophyte obtain nourishment. **Transfer cells** with elaborate wall labyrinths that facilitate intercellular transport are restricted to gametophyte cells (Fig. 3.8D, E). A distinctive feature of the hornwort placenta is the occurrence of abundant **protein crystals** between gametophyte and sporophyte cells in *Folioceros*, and some species of *Phaeoceros, Notothylas, Dendroceros,* and *Megaceros*. These crystals likely derive from gametophytic cells and may be a source of amino acids and for developing sporophytes. - At maturity, the **aerial sporophyte** is an **elongated cylindrical spore-bearing region** which includes an epidermis, assimilative layer, sporogenous tissue, columella, and basal meristem (Fig. 3.9). because of the programmed divisions from the basal meristem, **spore production is continuous throughout the growing season,** with spore maturation progressing from the base to the apex of the sporophyte. In *Notothylas*, the basal meristem functions for a limited period; the sporophyte remains small and is frequently retained within the protective tissue of the gametophyte. - **Stomata** that resemble those of mosses and tracheophytes occur in the sporophyte of many hornworts. **Guard cells** are characterized by inner (ventral) wall thickenings and apparently they are the only epidermal cells that contain prominent plastids, especially amyloplasts (Fig. 3.9D, E). epidermal cells typically are elongated and less commonly isodiametric in some species that lack stomata (Fig. 3.9G). At the tip of the sporophyte, where spores are mature, walls of epidermal cells are thickened along the outer tangential and radial walls (Fig. 3.9A, C). - An **assimilative (photosynthetic) layer** of variable thickness (4-13 layers) underlies the epidermis (Fig. 3.9A-C). **Substomatal cavities** and **prominent intercellular spaces** characterize the outer assimilative layer in taxa with stomata (Fig. 3.9A). The inner region in species with thick assimilative layers is compacted, with smaller cells and chloroplasts than those in the spongy outer layer. In taxa **without stomata** such as ***Megaceros*** and ***Dendroceros***, there is **no spongy layer**, i.e. the assimilative layer lacks intercellular spaces (Fig. 3.9C). - The **sporogenous tissue** is situated between the assimilative region and the columella. The **columella** usually comprises **16 cells in four rows of four** in cross-section but may contain as many as 40 irregularly arranged cells (Fig. 3.9B, C, F). Sporogenous tissue is bathed in mucilage and consists of **sporogenous cells or spores with pseudoelaters interspersed** (Fig. 3.9A, C). - **Spore shape, wall ornamentation and pseudoelater architecture** are variable across taxa and are widely used in taxonomy (Fig. 3.10A-F). **Pseudoelaters** are multi-cellular and range from thin-walled and isodiametric to elongated with tapering ends and evenly-thick or spirally thickened walls (Fig. 3.10B, C, E, F). These sterile cells do not undergo meiosis and are interspersed among sporogenous cells, thus separating sporocytes during differentiation. Sporophyte further facilitates **tetrad development** that involves enlargement of nascent spores and the development of a sculptoderm. - **Spore ornamentation and color** offer the main characters to deliminate honrowrt taxa. ***Leiosporoceros*** is the only known hornwort with nearly **smooth, bean-shaped spores** that are arranged in **bilateral alterno-opposite tetrads** (Fig. 3.10A). because of the arrangement, the proximal surface of these spores **exhibits a modified Y-shaped mark**. The remaining hornworts have **tetrahedral**, sometimes **cruciate, tetrads** (except *Hattorioceros*). Variability in distal wall ornamentation is seen in *Anthoceros* where it ranges from **spinose** and **punctuate** (*A. punctatus* group) to **lamellose** (*A. angustus*). The sculptoderm on proximal faces is generally less ornate, but shows considerable variability,e ven within species of *Anthoceros* (e.g. **hollows** in *A. punctatus,* **lamellae** in *A. cavernosus,* and **warts** in *A. tuberculatus*). *Spaerosporoceros* and *Folioceros* have **rounded spores with inconspicuous trilete ridges** (Fig. 3.10B). in *Phaeoceros*, species of the *laevis-carolinianus* group have **spinose spores with conspicuous cingulum** (Fig. 3.10 C). Spores of *Hattorioceros* are strikingly different from other hornworts: they are **small** (\