Dendrosenecio

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Dendrosenecio
Dendrosenecio kilimanjari
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Subclass: Asteridae
(unranked) Euasterids II
Order: Asterales
Family: Asteraceae
Subfamily: Asteroideae
Tribe: Senecioneae
Genus: Dendrosenecio
(Hauman ex Hedberg) B. Nord. (1978)
Type species
Dendrosenecio johnstonii
Range of Dendrosenecio
Range of Dendrosenecio
Species

See text.
Synonyms

Senecio L.
Sources: ING,[1] NCU-3e[2]

Dendrosenecio originally listed as the genus Senecio, always a member of the family Asteraceae, are the Giant groundsel or "giant cabbage trees" endemic to the ten of the tall, equatorial mountains of eastern Africa.[3][4] "The most conspicuous plants of the high East African mountains,"[5] easily distinguished by sparse woody superposed branches crowned by huge leaf rosettes. These giant old mountain plants deviated so much from the other Senecio,[5] they were given their own genus in 1978.[1] The communities of giant Dendrosenecio and giant lobelias found on these African mountains are an exceptional example of parallel or convergent evolution and repeated convergent evolution between these two groups; providing evidence that the unusual features of these plants are an evolutionary response to a challenging habitat and an environment which can be easily described for biogeographic analysis.[6]

Contents

[edit] Terminal taxa

Many of the specimens of Dendrosenecio housed at the herbarium do not contain the location;[7] including at least one which was acquired at Kinshasa where there is no mountain but there is a trading post.[8] Given the confusion of the samples and the added confusion of the names with the name changes and the varieties and subspecies, the following list will be used for articles about this genus. The species found on Mount Kenya are by far the best example for the simplification. Dendrosenecio keniodendron is the species which grows at the highest of altitudes, Dendrosenecio keniensis is found at the lower altitudes of the range where the species grows and Dendrosenecio battiscombei grows at the same altitudes as D. keniensis but in the wetter environments. The other mountains which are not tall enough to have a "big one at the top" have the two, one species for the drier land and one for the damper environments or just one because the environment is not so extreme. This simplification works extremely well as an introduction to the giant groundsel of East Africa with one exception, Kilimanjaro who has the one species that lives at the top and only one species that lives below; subspecies and varieties living in the moister environments.

Kilimanjaro
Mount Kenya
Ruwenzori Range and Virunga Mountains
Aberdare Range
Cherangani Hills
Mount Meru
Mount Elgon

[edit] Distribution

Groundsels of several species are found throughout the world as common roadside weeds, but nowhere except in the highlands of Africa do they exhibit such large tree forms.
Theodore Roosevelt 1914[10]

The mountains of central and eastern Africa are an almost ideal model system for studying speciation and adaptation in plants. The mountains rise far above the surrounding plains and plateaus,[6] tall enough to reach above the treeline[11] forming "islands in the sky" or isolated habitats.[6] These predominantly volcanic peaks further simplify the model by their age and arrangement around the Lake Victoria basin and proximity to the equator.[6]

Gridded Adaptive Speciation Studies
Each mountain has a vertical gradient of precipitation and temperature fluctuations.[12]Mount Kilimanjaro at 5,985 metres (19,640 ft), Mount Kenya at 5,198 metres (17,050 ft) and Ruwenzori at 5,109 metres (16,760 ft)) are the three tallest mountains in Africa; each tall enough to support altitude based layers of vegetative zones.[13] Each mountain providing its own vertically placed array of isolated habitats.[6]
Located from 50 kilometres (31 mi) to 1,000 kilometres (620 mi) around the equator, the environmental fluctuations occur as daily events[6] of warm days and cold nights and are consistent throughout the year[12] or as Hedberg described this unique situation: "summer every day, winter every night".[14] In addition to the simplified environmental variables, these mountains are easily described for biogeographic analysis as their age and arrangement around the Lake Victoria basin make it easy to disentangle the effects of time and position.[6]
Vegetation zones
Simplified grid system. Oldest mountain on the left.
Simplified grid system. Oldest mountain on the left.[15][6]


In the altitudes between 3400 meters (11,000 feet) and 4500 meters (15,000 feet) some of the most extreme examples of adaptations can be found, which include:
  • Massive leaf rosettes in which leaf development occurs in a large "apical bud"
  • Water storage in the pith of the stem
  • Insulation of the stem by retaining withered and dead foliage
  • Secretion and impoundment of ice-nucleating polysaccharide fluids (a natural anti-freeze)
  • Nyctinastic leaf movement (the leaves close when it gets cold)[6]
At altitudes below 3400 meters (12,000 feet)the daily temperature fluctuations are less extreme, the average daily temperature steadily increases, and the growth forms and ecology of the Dendroseneico reflect the increased influence of biotic factors (such as competition for light) over abiotic factors (such as nightly frost).[6]
Dendrosenecio keniodendron on Mount Kenya.
Dendrosenecio keniodendron on Mount Kenya.
3400-3800 meters (11,000-12,000 feet)
Given the name Afro-alpine region by Hauman in 1955.[16] There is a sharp boundry at 3400 meters (3000 meters on the North side) that separates the forest from the lower alpine zone[11], the environment is a moorland (low growing vegetationon acidic soils) and it is here that the Dendrosenecio start to grow among the mountain tussocks and sedges.[17]
Dendrosenecio keniensis grows in this region on Mount Kenya. A variety or subspecies of Dendrosenecio johnstonii live within this altitude range on all three of the tallest mountains.
3800-4500 meters (12,000-15,000 ft)
The upper moorlands; this is where most of the D. brassica make their homes on all three of the mountains, living with tough dwarf shrubs.
4300-5000 meters (14,000-16,000 ft)
Dendrosenecio woodlands, where each mountain has its own special variety. Dendrosenecio keniensis on Mount Kenya, Dendrosenecio kilimanjari on Mount Kilimanjari and other species each on their own mountain.
4500 meters-peak (15,000 ft)
Populations of Dendrosenecio start to dwindle. Mount Kenya has the least vegetation in its upper parts due to its freezing temperatures.


Dispersal and establishment
Disbursal and establishment -- descending altitude, descending time
Kilimanjaro
Dendronsenecio kilimanjari
D. johnstonii
Aberdare Range
D. battiscombei
D. brassiciformis		 Mount Meru (Tanzania)
D. meruensis
Mount Kenya
D. keniodendron
D. keniensis
D. battiscombei		 Cherangani Hills
D. cheranganiensis		 Mount Elgon
D. elgonensis
Aberdare Range
D. keniodendron		 Virunga Mountains
D. erici-rosenii
Mitumba Mountains
D. erici-rosenii
Ruwenzori Range
D. erici-rosenii
D. adnivalis
Biogeographic interpretation of the molecular phylogeny suggests that in the most recent one million years, the first giant senecios established themselves at higher elevations of Mount Kilimanjaro and became the species D. kilimanjari. As they moved down that mountain, adapting to live in the different environment at the lower altitudes of Mount Kilimanjaro, they became a new species, D. johnstonii. Some seeds found a way to Mount Meru and established themselves as the species D. meruensis, others found a way to get from Mount Kilamanjaro to the Aberdare Range and established themselves as D. battiscombei. D. battiscombei migrated into the wet alpine habitat on the Aberdares resulted in the formation of the species D. brassiciformis. Dispersal from the Aberdares to Mount Kenya established a second isolated population of D. battiscombei. Altitudinal speciation on Mount Kenya resulted in the formation of D. keniodendron and the "dwarf" D. keniensis. Dispersal from Mount Kenya back to the Aberdares established a second insular population of D. keniodendron. Dispersal from the Aberdares to the Cherangani Hills established two subspecies of D. cheranganiensis: D. cheranganiensis subsp. cheranganiensis and altitudinal (sub)speciation into the web alpine habitat resulted in D. cheranganiensis subsp. dalei. Dispersal from the Aberdares to Mount Elgon established D. elgonensis which is a point where several subspecies diverge and disperse: from Mount Elgon to the Virunga Mountains established D. erici-rosenii; from Mount Elgon to Mount Kahuzi (Mitumba Mountains) established a second population of D. erici-rosenii and dispersal from the Virunga Mountains to the Ruwenzori Range established a third population.[6]

[edit] Nomen confusum

A Lobelia in the foreground and a Dendrosenecio in the midground.
A Lobelia in the foreground and a Dendrosenecio in the midground.

Studies of the giant Senecio of Africa suffered from an unfortunate mix-up in some of the samples that were collected in 1922. The collected materials united the leaf of Lobelia gregoriana with the inflorescence of S. keniensis and the other way around also.[18] This confusion of materials crossed samples between not just genus but also families as Lobelia is of the family Campanulaceae and Senecio is of the family Asteraceae. At that time, Senecio keniensis was rejected as a confused name (nomen confusum) based on the muddled samples from which made it impossible to select a single specimen,[19][20] but that practice is no longer permitted and the replacement name S. brassica is superfluous and other names that were based on this basionym are similarly illogical and incorrectly deduced. Examples: Fries and Fries (1922) cited the confused material for S. brassica; Hedberg (1957) selected a single specimen from among the syntypes that associated S. brassica with Fries & Fries (1305).[9]

[edit] Cytological Uniformity

Little variation was found in molecular phylogeny among the 40 recorded giant senecio collections (40 assessions), yet as a group they differ significantly from Cineraria deltoidea,[21] the closest known relative.[22] The gametophytic chromosome number (is the number of chromosomes in each cell) for the giant Dendrosenecio is n = 50, and for the giant lobelias (specifically Lobeliaceae, Lobelia subgenus Tupa section Rhynchopetalum[23] ) it is n = 14. Only five of the 11 species of giant senecio and three of the 21 species of giant lobelia from eastern Africa remain uncounted. Although both groups are polyploid, Dendrosenecio is presumed to be decaploid (ten sets; 10x) and the Lobelia more certainly tetraploid (four sets; 4x), their adaptive radiations involved no further change in chromosome number. The cytological uniformity within each group, while providing circumstantial evidence that they descended from a single ancestor and simplifying interpretations of cladistic analyses, provides neither positive nor negative support for a possible role of polyploidy in evolving the giant-rosette growth-form.[24]


[edit] References

  1. ^ a b Index Nominum Genericorum database (HTML). International Code of Botanical Nomenclature. Smithsonian Institution (1978). Retrieved on 2008-05-04.
  2. ^ Botanic Garden and Botanical Museum Berlin-Dahlem (1978). Entry for Dendrosenecio (HTML). Names in Current Use for Extant Plant Genera. Freie Universität Berlin. Retrieved on 2008-05-04.
  3. ^ Knox, Eric B.; Jose L. Panero (1998). "Evolution of the giant senecios revisited; comparison of ITS and cpDNA phylogenetic estimates." (HTML). American Society of Plant Taxonomists Abstract Index. Department of Biological Sciences, Rutgers University. 
  4. ^ Knox and Jeffrey D. Palmer, Eric B.; Jeffrey D. Palmer (Dec., 1995). "The Origin of Dendrosenecio within the Senecioneae (Asteraceae) Based on Chloroplast DNA Evidence". American Journal of Botany 82 (12): pp. 1567–1573. Botanical Society of America. 2446185. 
  5. ^ a b Hedberg, Olov (12 April 1969). "Growth Rate of the East African Giant Senecios". Nature 222 (222): 163–164. doi:10.1038/222163a0. “The most conspicuous plants of the alpine belt on the high East African mountains are a group of peculiar giant groundsels, characterized by sparsely sympodially branched woody stems crowned by huge leaf rosettes, each of which finally produces a large terminal inflorescence. These plants deviate so much from ordinary senecios that they have been allotted a separate subgenus, Dendrosenecio.” 
  6. ^ a b c d e f g h i j k Knox, Eric B. (2004). "Adaptive radiation of African montane plants", in Ulf Dieckmann: Adaptive Speciation (HTML), Cambridge University Press, 476 pages. ISBN 0521828422. Retrieved on 2008-03-29. 
  7. ^ Aluka. Dendrosenecio [family COMPOSITAE] (HTML). African Plants. Ithaka Harbors, Inc. Retrieved on 2008-05-15. “Flora of Tropical East Africa, Vol Part Part 3, page 547 (2005) Author: H. Beentje, C. Jeffrey & D.J.N. Hind”
  8. ^ Aluka. Entry for Dendrosenecio erici-rosenii subsp. erici-rosenii [family COMPOSITAE] (HTML). African Plants. Ithaka Harbors, Inc. Retrieved on 2008-05-15. “Flora of Tropical East Africa, Vol Part Part 3, page 547 (2005) Author: H. Beentje, C. Jeffrey & D.J.N. Hind”
  9. ^ a b Aluka. Entry for Dendrosenecio keniensis (Baker f.) Mabb. (HTML). African Plants. Ithaka Harbors, Inc. Retrieved on 2008-03-28. “Flora of Tropical East Africa, Vol Part Part 3, page 547 (2005) Author: H. Beentje, C. Jeffrey & D.J.N. Hind”
  10. ^ Roosevelt, Theodore; Edmund Heller [1914] (2007-09-18). "LOGICALLY", Life-histories of African Game Animals (HTML), 1, C. Scribner's Sons. Retrieved on 2008-03-28. “Groundsels of several species are found throughout the world as common roadside weeds, but nowhere except in the highlands of Africa do they exhibit such large tree forms.” 
  11. ^ a b Bussmann, Fainer W. (June 2006). "Vegetation zonation and nomenclature of African Mountains - An overview". Lyonia. 
  12. ^ a b Weischet, Wolfgang; Endlicher, Wilfried (2000). Regionale Klimatologie Teil 2 Die Alte Welt: Europa - Afrika - Asien (Regional climatology, Part 2: The old world: Europe - Africa - Asia), 625. ISBN 9783443071196. 
  13. ^ Hedberg, Olov (1955). "Vegetation belts of the East -African mountains.". Proceedings of the Linnean Society of London (Botany) 165: 134–136. 
  14. ^ Hedberg, Karl Olov (1964). "Features of afroalpine plant ecology.". ACTA PHYTOGEOGRAPHICA SUECICA 49: 1–144. 
  15. ^ Africa Ultra-Prominences (HTML). 84 Mountains with prominence of 1,500m (4,921 ft.) or greater. PEAKLIST]. Retrieved on 2008-05-05.
  16. ^ Hauman, L. L. (1955). "La "region afroapline" en phytogeographie centro africaine". Webbia XI: 467–489. 
  17. ^ Hedberg, Olov (1951). "Vegetation belts of the East -African mountains.". Svensk Botanisk Tidskrift 45: 141–196. 
  18. ^ Aluka. Isotype of Senecio keniensis Baker (HTML). African Plants. Ithaka Harbors, Inc. Retrieved on 2008-03-29.
  19. ^ Natural Resources Conservation Service (NRCS). Frequently Asked Questions -- definition for nomen confusum (HTML). The PLANTS Database. United States Department of Agriculture,. Retrieved on 2008-03-28. “nomen confusum (Latin): confused name. Based on heterogenous elements from which it is impossible to select a lectotype.”
  20. ^ Aluka. Entry for Dendrosenecio keniensis (Baker f.) Mabb. (HTML). African Plants. Ithaka Harbors, Inc. Retrieved on 2008-03-30. “Flora of Tropical East Africa, Vol Part Part 3, page 547 (2005) Author: H. Beentje, C. Jeffrey & D.J.N. Hind”
  21. ^ Aluka. Cineraria deltoidea Sond. (HTML). African Plants. Ithaka Harbors, Inc. Retrieved on 2008-03-30. “Flora of Tropical East Africa, Vol Part Part 3, page 547 (2005) Author: H. Beentje, C. Jeffrey & D.J.N. Hind”
  22. ^ Knox, Eric B.; Jeffrey D. Palmer (October 24, 1995). "Chloroplast DNA Variation and the Recent Radiation of the Giant Senecios (Asteraceae) on the Tall Mountains of Eastern Africa". Proceedings of the National Academy of Sciences of the United States of America 92 (22): pp. 10349–10353. National Academy of Sciences. doi:10.1073/pnas.92.22.10349. 
  23. ^ Aluka. Isotype of Tupa rhynchopetalum (A. Rich.) Hochst. (HTML). African Plants. Ithaka Harbors, Inc. Retrieved on 2008-03-30.
  24. ^ Knox, Eric B.; Robert R. Kowal (July 1993). "Chromosome Numbers of the East African Giant Senecios and Giant Lobelias and their Evolutionary Significance". American Journal of Botany 80 (7): pp. 847–853. Botanical Society of America. doi:10.2307/2445604. 

[edit] External links