Teosinte
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Z. diploperennis |
The teosintes make up a group of large grasses of the genus Zea found in Mexico, Guatemala and Nicaragua. There are five recognized species of teosinte: Zea diploperennis, Zea perennis, Zea luxurians, Zea nicaraguensis and Zea mays. The last species is further divided into four subspecies: ssp. mays, ssp. huehuetenangensis, ssp. mexicana, and ssp. parviglumis, the last three also are teosintes. The species group into two sections, sect. Luxuriantes, with the first four species, and sect. Zea with Zea mays. The former section is typified by dark-staining knobs made up of heterochromatin that are terminal on most chromosome arms, while most subspecies of sect. Zea may have 0 to 3 knobs between each chromosome end and the centromere and very few terminal knobs (except ssp. huehuetenangensis which has many large knobs). The two perennials are thought to be one species by some.
The label "teosinte" came with the first Guatemalan accession and seems not to have been used in Mexico. Curiously some members of the related genus Tripsacum may be locally referred to as "teosinte" as may a cycad.
There are both annual and perennial teosinte species. Zea diploperennis and Z. perennis are perennial, while all other taxa are annual. All species are diploid (n=10) with the exception of Z. perennis, which is tetraploid (n=20). The different species and subspecies of teosinte can be readily distinguished based on morphological, cytogenetic, protein and DNA differences and on geographic origin. The most distinctive teosinte is Z. mays ssp. huehuetenangensis which combines a morphology rather like Z. m. ssp. parviglumis with many terminal chromosome knobs and an isozyme position between the two sections. The tallest, oddest and most threatened is Z. nicaraguensis which thrives in flooded conditions along 200 meters of a coastal estuarine river in NW Nicaragua.
Zea mays ssp. mays (maize or corn) is the only domesticated taxon in the genus Zea, according to one evolutionary model, derived directly from Zea mays ssp. parviglumis, with up to 12% of its genetic material obtained from Zea mays ssp. mexicana through introgression.
As would be expected, teosinte strongly resembles maize in many ways. Some populations of Zea mays ssp. mexicana display mimicry within cultivated maize fields, having evolved a maize-like form as a result of the farmers' selective weeding pressure. All but the Nicaraguan species of teosinte may grow in or very near corn fields, providing opportunities for introgression between teosinte and maize. First- and later-generation hybrids are often found in the fields, but the rate of gene exchange is quite low.
Teosintes are distinguished from maize most obvously by their numerous branches each bearing bunches of distinctive, small female inflorescences. These spikes mature to form a two-ranked 'ear' of five to ten triangular or trapezoidal, black or brown disarticulating segments, each with one seed. Each seed is enclosed by a very hard fruitcase, consisting of a cupule or depression in the rachis and a tough lower glume. This protects them from the digestive processes of ruminants that forage on teosinte and aid in seed distribution through their droppings. Teosinte seed exhibit some resistance to germination but will germinate almost immediately if treated with a dilute solution of hydrogen peroxide.
In some areas of Mexico, teosintes are regarded by maize farmers as a noxious weed, while in a few areas farmers regard it as a beneficial companion plant, and encourage introgression. Virtually all populations of teosinte are either threatened or endangered: Zea diploperennis exists in an area of only a few square miles; Zea nicaraguensis survives as approx. 6000 plants in an area 200 x 150 meters. The Mexican and Nicaraguan governments have taken action in recent years to protect wild teosinte populations, using both in-situ and ex-situ conservation methods. There is currently a large amount of scientific interest in conferring beneficial teosinte traits, such as insect resistance, perennialism and flood tolerance, to cultivated maize lines, although this is very difficult due to linked deleterious teosinte traits.