Haplomitriopsida

Taxobox
fossil_range = Early Permian to recent
name = Haplomitriopsida
regnum = Plantae
divisio = Marchantiophyta
classis = Haplomitriopsida
classis_authority = Stotler & Stotl.-Crand.
subdivision_ranks = Orders
subdivision =
*Haplomitriales
*Treubiales
synonyms = Treubiopsida

Haplomitriopsida is a newly recognized class of liverworts comprising fifteen species in three genera. Recent cladistic analyses of nuclear, mitochondrial, and plastid gene sequences place this monophyletic group as the basal sister group to all other liverworts.cite journal
last = Heinrichs | first = Jochen
coauthors = S. Robbert Gradstein, Rosemary Wilson, & Harald Schneider
year = 2005
title = Towards a natural classification of liverworts (Marchantiophyta) based on the chloroplast gene rbcL
journal = Cryptogamie Bryologie | volume = 26 | issue = 2 | pages = 131–150 ] cite journal
last = He-Nygrén | first = Xiaolan
coauthors = Aino Juslén, Inkeri Ahonen, David Glenny, & Sinikka Piippo
year = 2006
title = Illuminating the evolutionary history of liverworts (Marchantiophyta)—towards a natural classification
journal = Cladistics | volume = 22 | issue = 1 | pages = 1–31
doi = 10.1111/j.1096-0031.2006.00089.x ] cite journal
last = Forrest | first = Laura L.
coauthors = Christine E. Davis, David G. Long, Barbara J. Crandall-Stotler, Alexandra Clark & Michelle L. Hollingsworth
year = 2006
title = Unraveling the evolutionary history of the liverworts (Marchantiophyta): multiple taxa, genomes and analyses
journal = The Bryologist | volume = 109 | issue = 3 | pages = 303–334
doi = 10.1639/0007-2745(2006)109 [303:UTEHOT] 2.0.CO;2
doilabel = 10.1639/0007-2745(2006)109[303:UTEHOT]2.0.CO;2 ] cite journal
last = Renzaglia | first = Karen S.
coauthors = Scott Schuette, R. Joel Duff, Roberto Ligrone, A. Jonathan Shaw, Brent D. Mishler, & Jeffrey G. Duckett
year = 2007
title = Bryophyte phylogeny: Advancing the molecular and morphological frontiers
journal = The Bryologist | volume = 110 | issue = 2 | pages = 179–213
doi = 10.1639/0007-2745(2007)110 [179:BPATMA] 2.0.CO;2
doilabel = 10.1639/0007-2745(2007)110[179:BPATMA]2.0.CO;2 ] The group thus provides a unique insight into the early evolution of liverworts in particular and of land plants in general.

Description

Plants of "Treubia" grow as a prostrate leafy thallus. The leaves extend like wings on either side of the midrib, or may be folded upwards and pressed close together, giving the plants a ruffled appearance. By contrast, "Haplomitrium" grows as a rhizome with erect leafy stems. The thin, rounded leaves are arranged around the upright stems, giving the appearance of a soft moss. The species "Haplomitrium ovalifolium" of Australia often has bifid leaves that are asymmetrical, somewhat like those in "Treubia".cite book
last = Schuster | first = Rudolf M.
date = 1992
title = The Hepaticae and Anthocerotae of North America
edition = Volume 5 | pages = 294-297, 333 | isbn = 0-914-86820-9 ]

"Haplomitrium" has a number of unique characters that distinguish it from other liverworts, such as lacking rhizoids. The vegetative stems possess a central water-conducting strand with large perforations derived from plasmodesmata. [cite journal
last = Bartholomew-Began | first = Sharon E.
year = 1991
title = A morphogenetic re-evaluation of "Haplomitrium" Nees (Hepatophyta), Jumgermanniopsida)
journal = Bryophytorum Bibliotheca | volume = 41 ] This central strand is surrounded by a cylinder of cells that conduct food throughout the plant. Such an arrangement is evocative of the xylem and phloem found in vascular plants. Although some thalloid liverwort species in the Pallaviciniaceae also possess a central conducting strand, [cite journal
last = Hébant | first = C.
year = 1977
title = The conducting tissues of bryophytes
journal = Bryophytorum Bibliotheca | volume = 10 ] "Haplomitrium" differs in having a food-conducting layer and in producing no callose.

"Treubia" also has features that differ from those found in other bryophytes,cite journal
last = Duckett | first = Jeffrey G.
coauthors = Anna Carafa, & Roberto Ligrone
year = 2006
title = A highly differentiated glomeromycotean association with the mucilage-secreting, primitive antipodean liverwort "Treubia": clues to the origins of mycorrhizas
url = http://www.amjbot.org/cgi/content/full/93/6/797?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=Duckett&andorexacttitle=and&andorexacttitleabs=and&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&volume=93&resourcetype=HWCIT
accessdate = 2007-10-20
journal = American Journal of Botany | volume = 93 | issue = 2 | pages = 797–813
doi = 10.3732/ajb.93.6.797 ] such as the differentiation of five identifiable zones in the stem midrib. Unlike other leafy species, the oil bodies in its cells are restricted to certain clusters of cells, as they are in the Marchantiopsida. These oil body clusters appear as dark spots in the leaves when the plant is held up to the light. [cite book
last = Allison | first = K. W.
coauthors = John Child
date = 1975
title = The Liverworts of New Zealand
location = Dunedin, NZ | publisher = University of Otago Press | pages = 232-233]

Diversity

Living representatives of the group exhibit an essentially Gondwanan distribution with its center of diversity in Australasia. Such a distribution implies that the modern genera radiated prior to the beginning of the Cretaceous when Gondwana broke apart. Schuster proposes that species distributed in the northern hemisphere "rafted" on the Indian subcontinent to Asia, then spread across the Bering Strait into North America.Schuster, Rudolf M. (1983). "Phytogeography of the Bryophyta". Pages 463-626 "in" R. M. Schuster (ed.), "New Manual of Bryology" (Japan: Hattori Botanical Laboratory). ISBN 4-938163-3045.]

Most species in the Haplomitriopsida are found in south of the equator, though there are northern ones. The genus "Treubia" is restricted to the southern hemisphere, while "Apotreubia" has one species in New Guinea and another disjunct between eastern Asia and British Columbia. The genus "Haplomitrium" exhibits a wider distribution, with species in both North and South America, northern and central Europe, the Himalayas, Japan, and Australasia.

Classification

Class Haplomitriopsida includes two orders, each with one family. The group as a whole comprises fifteen species in three genera. A fourth genus, "Gessella", is known only from Permian fossils. The orders, families, and genera are as follows:

* Order Haplomitriales
** Family Haplomitriaceae
**:"Gessella" †
**:"Haplomitrium" (8 species)
* Order Treubiales
** Family Treubiaceae
**:"Apotreubia" (2 species)
**:"Treubia" (6 species)

An additional fossil "Treubiites kidstonii" previously has been compared to the extant genus "Treubia". However, upon re-examination of the material, specimens were determined to be more like "Blasia" and not at all to resemble "Treubia" as previously thought. [cite book
last = Schuster | first = Rudolf M.
date = 1992
title = The Hepaticae and Anthocerotae of North America
edition = Volume 5 | pages = 527 | isbn = 0-914-86820-9 ] Accordingly, "Treubiites" is now assigned to the Blasiales rather than the Haplomitriopsida.

References

External links

* [http://www.biology.duke.edu/bryology/LiToL/ Liverwort Tree of Life]
* [http://bryophytes.plant.siu.edu/treeforweb.html Simplified phylogeny of the liverworts]
* [http://www.mobot.org/plantscience/bfna/V3/HaplHaplomitriaceae.htm Information on family Haplomitriaceae]