Dating dispersal and radiation in the gymnosperm gnetum
They find that the current clades of Gnetum began diverging in late Oligocene to mid-Miocene time, with most species in the large Asian clade appearing in Miocene-Pliocene time. Won and Renner (2006) apply molecular clock analyses to a large data set including the Asian, African and American clades and evaluating genetic material from the chloroplast, mitochondria, and nucleus. This was the first known instance of horizontal gene transfer between higher plants; it only affects an Asian clade of Gnetum; and molecular clock reasoning suggests that it happened only about 2 to 5 million years ago.
Gnetum is a genus of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales.
They are tropical evergreen trees, shrubs and lianas.
Dioecious, evergreen, mostly woody vines, rarely shrubs or trees; stems with swollen nodes.
The mechanism of such horizontal gene transfer remains unknown, but Won and Renner (2003) state that "agents that have been implied are viruses, bacteria, fungi, and plant cell-piercing insects," all of which have been implicated in other instances of horizontal gene transfer.
The female strobilus bears a droplet of sugary, sticky fluid that in time retracts into the strobilus, carrying any captured pollen grains to the nucellus.
This unusual mechanism works best in high humidity settings and has been suggested as one reason why Gnetum is restricted to rainforest habitats.
Both male and female megastrobili terminal or lateral, sometimes arranged in dense, cauliflorous clusters on old stems. Gnetum arboreum Foxworthy is restricted to a small portion of Luzon, Philippines (Markgraf 1930, in Carlquist 1996b).
Leaves opposite, petiolate, without stipules, simple, elliptic, with pinnate veins and entire margins; usually with drip tips. microcarpum Blume, Gnetum montanum Markgraf, and Gnetum ula Brongniart.
The genus has been subdivided by Markgraf (1930, in Carlquist 1996b) as follows: About 19 species of Asian and Indomalesian lianas, incl. Seeds drupelike, enclosed in a red, orange, or yellow, fleshy (rarely corky) false seed coat; female gametophyte tissue copious, succulent. The following species are found in Malesia or SE Asia (Carlquist 1996b): Gnetum costatum, G. Gnetum ridleyi Gamble is restricted to a small area in Pahang, Malaysia (Markgraf 1930, in Carlquist 1996b). gnemon, and as the remaining species of the genus are all woody vines, they are presumably not usable in dendrochronology. "Lignans are of the guaiacyl-syringyl-type; the antiasthmatic properties for which G.
Each megastrobilus consists of a straight axis above a basal pair of opposite, connate bracts; the axis bears usually three to six superposed cupules, each of which contains several to many male or female strobili. Gnetum contractum Markgraf is found in Travancore, India (Markgraf 1930, in Carlquist 1996b).
Unlike other gymnosperms, they possess vessel elements in the xylem.Tags: Adult Dating, affair dating, sex dating