Dioon
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Dioon is a genus of plants in the Zamiaceae family within the order of the cycads (Cycadales), that is, they are seed plants (Spermatophyta). The elevenor so species are distributed from Mexico via Honduras to Nicaragua . They are also called double palm ferns . The still clearly recognizable leaf character of the sporophyll gives it a special position within thecycads. It is the genus with the most primitive features and therefore probably the most primitive evolutionarily within the department.
description
The species of the genus Dioon are perennial , evergreen cycads. The cylindrical stem axis is partly in the ground. It is pachycaul , that is, thickened and made of soft wood. Above-ground ramifications are rare. The leaf roots remain after the leaves have died or they fall off, so that a soft bark becomes visible. Often many small shoots form next to a mother plant. The largest species is Dioon spinulosum , which can grow to a height of over 16 meters and a trunk diameter of 40 centimeters. However, typical specimens of other species are only 3 to 6 meters high or remain even smaller. Dioon species can get very old, possibly even over 1000 years.
As with other cycads, coral-like roots are sometimes formed. Such roots grow upwards from the primary root and branch out strongly just below the soil surface. The individual roots are thickened rhizomatically and are inhabited by cyanobacteria of the genus Nostoc . They are mainly used to fix nitrogen. There also appears to be a symbiotic relationship with arbuscular mycorrhizal fungi . The Dioon species have a chromosome number of 2n = 18.
leaves
The leaves are pinnate in pairs and are spiraling on the stem axis. Some of these leaves are initially wrapped in protective leaves called cataphylls during their development . In contrast to other cycads such as Stangeria eriopus , the juvenile plumage is neither curled in a transverse nor a longitudinal direction, but rather straight. The lower leaflets are often reduced to their petiole. The petioles are thornless and thickened at the base.
The leaflets are simple, often with a prickly edge. The nerve consists of many bifurcated nerves without a recognizable midrib. The nerves arise directly from the edge of the leaf spindle ( rachis ) and then run towards the pinnate axis. The blades are hairy, at least on young leaves. The hairs ( trichomes ) are colorless, branched or simple.
The stomata for gas exchange ( stomata ) are either only on the underside of the leaf, or in some species on both sides. Glands are not colored differently and are difficult to identify. The cells of the leaf epidermis are elongated parallel to the pinnate axis.
Cones and seeds
All Dioon are dioecious , that is, male and female genitals are never together on one, but only on different individuals. The male cone is stalked. The male cone scales (= microsporophylls ) are flattened and curved up at the sterile tip. They are arranged in a spiral around the pin axis. Each microsporophyll has a multitude of pollen sacs on its underside (abaxial side) . These open with slits from which the pollen then escapes. The pollen grains are streamlined and have only one germinal furrow (monosulcat). The male cones fall off after a year.
The female cones are also stalked, they can stay on the plant for more than a year. The female scales (= megasporophylls ) are more leaf-like than those of other cycads, but they are most similar to those of Cycas . The scales are flattened, widened and curved at the top. Each sporophyll has two, rarely three, ovules . These stand straight to the axis (orthotropic), are inclined inwards to this (inverse) and hang on the side of the thickened scales facing the axis. The opening at the top of the ovules ( micropyle ) is very small and only leaves a narrow opening on the nucellus . The megaspore shell thickens from 3 to 4.5 μm in the young ovule to 9-10 μm in the mature seed. The number of archegonies varies from one to ten. The nucleus of the egg cell is unusually large. The largest female cones of the genus shows Dioon spinulosum with a length of up to 80 centimeters and a diameter of up to 30 centimeters.
The seeds are spherical, egg-shaped or ellipsoid and reach a diameter between about one and four centimeters. They are surrounded by a fleshy, white or cream-colored seed coat. The seed coat consists of three layers, a fleshy outer and inner layer, with a layer of stone cells between them . The embryo is straight with two cotyledons , these are fused at the top. The embryo carrier ( suspensor ) is very long and twisted in a spiral. The seeds are furrowed radiosperm . The germination occurs kryptokotylar, that is, the cotyledons remain in germination in seeds.
Occurrence
Dioon are found in Central America between the 15th and 29th parallel north and are therefore part of the neotropical flora kingdom . The circulation area includes Honduras , Nicaragua and parts of Mexico ; it is limited by orographic conditions (altitude structures) and restricted to the north-south mountain ranges in the region. Despite the widespread distribution and the ecological plasticity of the genus, it is not a component that determines vegetation.
The locations of the Dioon species are found primarily in the tropical rainforest and in tropical moist forests, but also in tropical deciduous rainforests, coniferous rainforests, on dry, stony slopes, in canyons and on dunes near the coast.
Spread in prehistoric times
At the beginning of the Cenozoic Era , about 65.5 million years ago, the range of Dioon probably included most of North America. This is also indicated by fossil finds in northern Alaska from the Eocene (33.9 - 55.8 mya ). While the climate at the beginning of this geological age was still very warm, the most recent ice age began around 2.8 million years ago with the glaciation of the North Pole. The distribution of Dioon then shrank very much.
In the Paleocene (55.8 to 65.5 mya) Dioon was next to Zamia and Anemia the only Neotropical genus today that was still present in the flora of Alaska. During the Tertiary , until about 2.6 million years ago, Dioon died out in almost all of North America. Only a few relic stocks remained in a small area in Central America, where the diversity center of the genus still lies today. It was not possible to spread to South America because the two continents were not connected until about 3 million years ago. After that, the size of the seeds was the main obstacle to Dioon's further spread to the Caribbean islands or further south.
ecology
Whether fertilization in Dioon takes place through wind pollination or through zoophilia , through animals, has not yet been conclusively clarified. The nature of the pollen (light, dry, very large number) tends to favor wind pollination, and the number of fertilized ovules in the immediate vicinity of a pollen source also increases.
On the other hand, the inverse position of the ovules speaks against wind pollination, since it is difficult for the pollen to blow there and mainly falls on the sterile sections in the cone. Pollination by the weevil Rhopalotria mollis has already been demonstrated for cycads of the genus Zamia . Species from the genus Rhopalotria could also be found in the cones of Dioon , which also contained a large number of beetles of the genus Pharaxonotha that feed on pollen. These are further indications of zoophilia .
Danger
The genus is particularly endangered by habitat destruction, the International Union for Conservation of Nature and Natural Resources (IUCN) lists ten of the eleven species in a hazard category on its red list . One way Dioon caputoi , is considered "threatened with extinction" ( critically endangered ), three as "high risk" ( endangered ) ( Dioon holmgrenii , sonorense Dioon and tomasellii Dioon ) and five as "endangered" ( vulnerable ) ( Dioon califanoi , Dioon merolae , Dioon purpusii , Dioon rzedowskii and Dioon spinulosum ). Dioon edule is listed as a kind of "warning list" ( near threatened ). For Dioon mejiae not have enough data for an estimate of the risk.
Locations are mainly destroyed by clearing and reclamation of the land. Dioon species are popular with international collectors. Specimens of plants are dug up and sold. According to a 1992 report by the Washington Convention on Endangered Species , the trade in Dioon edule individuals reached a level of around 5,800 specimens and 1,600 in Dioon spinulosum - this does not include illegal trade.
Another possible threat to the genus is the lid scale louse (Diaspididae) Aulacaspis yasumatsui . The insect originally comes from Southeast Asia and attacks cycads there. The pest has now been introduced to Florida, California, India and Great Britain, where it causes great damage to cycads. There he was also found at Dioon edule . If the species were to penetrate Central America, it could drastically reduce or even destroy the populations of Dioon in a short time .
Systematics
The first description was in 1843 by John Lindley as Dion . The generic name was given because of the two ovules per megasporophyll: Dioon comes from the ancient Greek prefix δι- di- (= two) and ώόν ōión, ōón (= egg).
The leaf-like character of the sporophylls gives Dioon a special position within the cycads. Dehgan & Dehgan called the genus the most primitive of the cycads and suggested placing them in a family of their own . A molecular genetic analysis of chloroplast - DNA revealed that there at least by the most primitive type of neotropical is cycads.
External system
Classically, the Zamiaceae family is divided into two subfamilies with four tribes. Dioon is the only genus in the tribe of the Diooeae, which together with the Encephalarteae forms the subfamily of the Encephalartoideae. This classification was made according to morphological criteria. A morphological investigation of the relationships between fossil and recent taxa resulted in the following cladogram:
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A molecular biological analysis on a section of the 26S ribosomal DNA revealed a somewhat similar cladogram that would change the structure of the Zamiaceae significantly. Dioon would therefore be the basic genus of the family.
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Internal system
Norstog & Nichols 1997 divided the genus Dioon into two groups according to morphological aspects ( Dioon sonorense was not included in this study because it was only described in 1997). The first clade consists of Dioon spinulosum , Dioon mejiae and Dioon rzedowskii . It is characterized by massive trunks, large fronds and large cones. The second clade contains Dioon edule , Dioon merolae , Dioon holmgrenii , Dioon purpusii , Dioon califanoi , Dioon caputoi and Dioon tomasellii . These are characterized by less robustness, shorter stem axes and smaller tenons.
A phylogenetic study from 1993 confirmed these clades with one difference - Dioon caputoi was assigned to the other clade.
As of 2018, the genus Dioon contains 11 to 14 species :
- Dioon argenteum T.J.Greg. & al. : It was first described in 2003 and is so far only known from the Mexican state of Oaxaca .
- Dioon califanoi De Luca et Sabato : It occurs in the Mexican states of Puebla and Oaxaca.
- Dioon caputoi De Luca, Sabato et Vázq.Torres : It occurs in the Mexican states of Puebla and Oaxaca.
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Mexican double palm fern ( Dioon edule Lindl. ): There are one or two varieties, depending on the author:
- Dioon edule var. Angustifolium (Miq.) Miq. : Some authors also call it a separate species Dioon angustifolium Miq. viewed. It occurs in the Mexican states of Tamaulipas and Nuevo León .
- Dioon edule var. Edule : It occurs in northeastern Mexico.
- Dioon holmgrenii De Luca, Sabato et Vázq.Torres : It only occurs in the Mexican state of Oaxaca.
- Dioon mejiae Standl. et LOWilliams : The home is northern Honduras .
- Dioon merolae De Luca, Sabato et Vázq.Torres : It occurs in the Mexican states of Oaxaca and Chiapas .
- Dioon planifolium Salas-Mor., Chemnick & TJGreg. : It was first described in 2016 from the Mexican state of Oaxaca.
- Dioon purpusii Rose : It occurs only in the Mexican state of Oaxaca.
- Dioon rzedowskii De Luca, A.Moretti, Sabato et Vázq.Torres : It only occurs in the Mexican state of Oaxaca.
- Dioon sonorense (De Luca, Sabato et Vázq.Torres) Chemnick, TJGreg. et Salas-Mor. : It occurs only in the Mexican states of Sonora and northern Sinaloa .
- Dioon spinulosum Dyer ex Eichler : It occurs only in the Mexican states of Veracruz and Oaxaca.
- Dioon stevensonii Nic.-Mor. & Vovides : It was first described in 2009 and only occurs in the Mexican states of Michoacán and Guerrero .
- Dioon tomasellii De Luca, Sabato et Vázq.Torres : It occurs only in the Mexican states of Durango , Jalisco and Nayarit .
- Dioon vovidesii Gut. Ortega & Pérez-Farr. : It was first described in 2018 from the Mexican state of Sonora.
Evolution of the genus
The oldest fossil finds date from the Eocene and were found on the Kupreanof Island in Alaska . However, it is considered likely that Dioon goes back as far as the Jura (145.5 - 199.6 mya) or even further, as fossils of other closely related but more developed cycads have already been identified from this era. In the Pleistocene (0.011784–1.8 mya) and Holocene (today –0.011784 mya) the species probably carried out several north-south migrations and each followed the warm climatic conditions.
One theory suggests that the recent Dioon species evolved from a number of vicariism events in the early Cenozoic, with different species representing each other under different ecological conditions. These events must have followed one another very closely, which precluded an accumulation of synapomorphies , that is, not many traits could be newly acquired.
The recent types of Dioon are traced back to three archetypes. Dioon mejiae is considered to be the most progressive species of the genus in evolutionary terms . Dioon spinulosum and Dioon rzedowskii are traced back to a second archetype, which was probably better adapted to mesian conditions, that is, to areas of medium humidity. It was able to establish itself on the Atlantic coast of Oaxaca . The remaining species presumably stem from a third archetype adapted to xerotic, i.e. dry, conditions.
use
In Honduras, the female cones of Dioon mejiae are picked to harvest the seeds. These are processed into tamales or tortillas by around 33,000 indigenous people and replace the maize flour. The fronds are traditionally used on Palm Sunday .
swell
literature
- Knut J. Norstog, Trevor J. Nichols: The biology of the cycads . Comstock Publishing , Ithaca (New York) 1997, ISBN 0-8014-3033-X , pp. 275-285 .
- Loran M. Whitelock: The Cycads . Timber Press , Portland 2002, ISBN 0-88192-522-5 .
Individual evidence
- ^ A b Charles J. Chamberlain: The Ovule and Female Gametophyte of Dioon . In: Botanical Gazette . tape 42 , no. 5 , November 1906, p. 321-358 , JSTOR : 2465496 .
- ↑ Jack B. Fisher, Andrew P. Vovides: Mycorrhizae Are Present in Cycad Roots . In: The Botanical Review . tape 70 , no. 1 , 2004, p. 16-23 , doi : 10.1663 / 0006-8101 (2004) 070 [0016: MAPICR] 2.0.CO; 2 .
- ↑ Aldo Meretti: Karyotypic Data on North and Central American Zamiaceae (Cycadales) and their Phylogenetic Implications . In: American Journal of Botany . tape 77 , no. 8 , August 1990, p. 1016-1029 , doi : 10.2307 / 2444573 .
- ↑ a b c A. Moretti, P. Caputo, S. Cozzolino, P. De Luca, L. Gaudio, G. Gigliano Siniscalco, DW Stevenson: A phylogenetic analysis of Dioon (Zamiaceae) . In: American Journal of Botany . tape 80 , no. 2 , February 1993, p. 204-214 , doi : 10.2307 / 2445041 .
- ↑ a b Sergio Sabato: Evolutionary Trends in Dion (Zamiaceae) . In: American Journal of Botany . tape 72 , no. 9 , September 1987, pp. 1353-1363 , doi : 10.2307 / 2443507 .
- ↑ Karl J. Niklas , Knut Norstog: Aerodynamics and Pollen Grain Depositional Patterns on Cycad Megastrobili: Implications on the Reproduction of Three Cycad Genera (Cycas, Dioon, and Zamia) . In: Botanical Gazette . tape 145 , no. 1 , March 1984, p. 92-104 ( abstract ).
- ↑ Andrew P. Vovides: Insect Symbionts of Some Mexican Cycads in Their Natural Habitat . In: Biotropica . tape 23 , no. 1 , March 1991, p. 102-104 , doi : 10.2307 / 2388697 .
- ^ Interpretation and Implementation of the Convention Trade in Plant Specimens . In: CITES Secretariat (Ed.): Eighth Meeting of the Conference of the Parties . Kyoto 1992, p. 29 ( full text [PDF; 282 kB ] 2nd - 3rd March). Full text ( memento of the original from June 16, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Greg Hodges, WN Dixon: The Poliaspis Cycad Scale Poliaspis cycadis Comstock (Hemiptera: Diaspididae): A new exotic scale insect for Florida . Florida Department of Agriculture and Consumer Services , June 1, 2007 ( online ( December 2, 2010 memento on the Internet Archive )). The Poliaspis Cycad Scale Poliaspis cycadis Comstock (Hemiptera: Diaspididae): A new exotic scale insect for Florida ( Memento of the original dated December 2, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Edwards's Botanical Register or Flower Garden and Shrubbery . London, Volume 29, 1843, p. 59. ( biodiversitylibrary.org )
- ↑ Helmut Genaust: Etymological dictionary of botanical plant names. 3rd, completely revised and expanded edition. Nikol, Hamburg 2005, ISBN 3-937872-16-7 , p. 209 (reprint from 1996).
- ^ Bijan Dehgan, Nancy B. Dehgan: Comparative pollen morphology and taxonomic affinities in Cycadales . In: American Journal of Botany . tape 75 , no. October 10 , 1988, pp. 1501-1516 , doi : 10.2307 / 2444700 .
- ↑ De Luca et al: Molecular systematics of cycads . In: P. Vorster (Ed.): Proceedings of the Third International Conference on Cycad Biology . Stellenbosch 1995, p. 131-137 .
- ↑ a b Guido Grimm: Cycadales cladistic analysis of fossil and recent taxa based on morphological and molecular data . Diploma thesis at the University of Tübingen, 2000 ( full text ).
- ↑ KJ Norstog, TJ Nichols: The biology of the cycads . Cornell University Press , Ithaca, New York 1997, ISBN 0-8014-3033-X .
- ↑ a b c d e f g h i j k l m n o p q r Rafaël Govaerts (Ed.): Dioon. In: World Checklist of Selected Plant Families (WCSP) - The Board of Trustees of the Royal Botanic Gardens, Kew . Retrieved March 15, 2019.
- ↑ Tom M. Harris: Cones of Extinct Cycadales from the Jurassic Rocks of Yorkshire . In: Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences . tape 231 , no. 577 , September 19, 1941, pp. 75-98 ( abstract ).
- ^ T. Walters, TJ Gregory, J. Chemnick, R. Osborne: Hypotheses on the relationship between biogeography and speciation in Dioon (Zamiaceae) . In: Cycad classification: concepts and recommendations . Montgomery Botanical Center , Miami 2004 ( abstract ).
- ↑ Mark Bonta, Oscar Flores Pinot, Daniel Graham, Jody Haynes, German Sandoval: Ethnobotany and conservation of Tiusinte (Dioon mejiae Standl. & LO Williams, Zamiaceae) in northeastern Honduras . In: Journal of Ethnobiology . tape 26 , no. 2 , 2006, p. 228-257 , doi : 10.2993 / 0278-0771 (2006) 26 [228: EACOTD] 2.0.CO; 2 .
Web links
- Entry to The Cycad Pages (English)
- Description on conifers.org . (English)
- Dioon on the IUCN Red List of Threatened Species2018. Retrieved on 2019-03-14.
