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ORIGIN AND DOMESTICATION OF MAIZE
A. Corn
1. Several different varieties are in use
a) sweet corn, flint corn, dent corn, flour corn, popcorn
2. One of largest crops in the world
a) 5.5 billion bushels/year
1) more than 3 times our production of wheat
b) world corn crop estimated at $40 billion/year
c) some eaten as ears, Corn Flakes, Fritos, popcorn
d) most consumed as it has been transformed by being major feed
for beef, pork, and chicken
1) cows produce milk, butter, cheese, etc.
2) hens lay eggs
B. History of research
C. Manglesdorf's hypothesis (first published in 1939)
1. Cultivated maize descended from a now extinct wild
pod-popcorn
a) kernels were individually enclosed in, and protected by, a
pod or chaffy shell
2. Teosinte is a mutation of maize
a) Teosinte
1) Name comes from Nahuatl "teocintli" - grain of the gods
2) Maize's closest relative
a> maize and annual teosinte have ten pairs of chromosomes with
nearly identical structures
b> are often 100% interfertile
c> both classified as subspecies of Zea mays
3) practically indistinguishable morphologically except for
female inflorescences
a> teosinte inflorescences are clustered and lateral on the
stout primary branches
1> in well-grown, branched teosinte, position of maize ear is
always occupied by a tassel
b> maize inflorescences are terminal
4) also, teosinte is distichous (two rows of single grains),
while maize is polystichous (four to many rows of paired grains)
a> hybrids suggest a developmental series, although it has not
been noted in the wild
b) once thought to be a cross between maize and Tripsacum
1) Tripsacum
a> Maize's second-closest relative
c) question is which came first
1) if teosinte came first, that man's role as selective agent
is likely
2) Manglesdorf believes that maize came first
a> earliest cobs come from from Tehuacan
b> characteristics of these differ from teosinte
1> many-ranking cupule compression
2> soft rachis
3> paired female spikelets
c> form is considered to be more primitive than teosinte
d> teosinte is considered to be inedible
1> however, teosinte kernels can be popped (Beadle)
2> Iltis has photographed children chewing cornstalks
a: suggests that sweet stems were more important as food than
the ripe grain
1: larger ears became important only later
b: chewed quids found in Tehuacan caves
d) Manglesdorf favors gradual process of natural speciation of
teosinte from primitive maize, which excludes man from the
selective process
1) supported by pollen cores from the Valley of Mexico dating
to 80,000 BP with supposed maize pollen
3. Teosinte and Tripsacum backcrossed with maize to result in
domesticated form
4. Historical note:
a) On the strength of this hypothesis, Manglesdorf was given
most distinguished plant professorship in nation at
Harvard's Botanical Museum
b) Published nearly 100 papers to support hypothesis
D. Archaeological research
1. Herbert W. Dick (Harvard) excavated at Bat Cave in 1948
a) found corn cobs and other remains
b) charcoal "pooled" for dating
c) published paper in Scientific American on "oldest ear of
corn"
2. MacNeish and Tehuacan Valley Project
a) recovered 23,000 specimens of corn
b) worked together with Manglesdorf and Walton Galinat
c) concluded lower levels had primitive popcorn (a pod corn)
1) this earliest corn was thought to be wild
E. Alternate hypothesis
1. Questions raised by Iltis
a) if maize evolved gradually from teosinte, why have no
intermediates been found?
b) if teosinte was domesticated for its grains, why have none
of the durable fruitcases of teosinte been found
contemporaneous or predating the earliest archaeological
maize?
c) given the hardness and concavity of teosinte fruitcases, why
are the glumes of the earliest archaeological maize soft and
thin and cupules relatively shallow?
d) if teosinte ears became transformed into maize ears, why do
both modern and archaeological maize ears often exhibit
staminate "tails"?
e) compared with the gradual evolution documented for all other
cereals, how did maize arise so suddenly, from ancestors
difficult to identify?
1) appearance is sudden in the archaeological record
2. Presented by Iltis at "Origin of Corn Conference" at U. of
Illinois in 1969
a) Manglesdorf left in a huff
1) decided to look at maize, teosinte, and Tripsacum under the
SEM
2) found that grains of maize and teosinte were uniformly
distributed, while grains of Tripsacum were clumped
3) hybrids of maize and Tripsacum were intermediate, as were
backcrosses with even one Tripsacum chromosome
4) Manglesdorf accepted this as proof that notion of maize +
Tripsacum = teosinte was wrong
3. Annual teosinte
a) Chalco teosinte (Zea mays mexicana)
1) edible only as green inflorescence or stems
4. Perennial teosinte
a) form of teosinte reported from Jalisco in 1910
1) Zea perennis
2) tetraploid which could not interbreed with diploid teosinte
or corn
b) searched for in vain by Iltis in 1960
c) pleaded with local botanists to launch a search
d) rediscovered in 1976 by Rafael Guzman
e) additional species, of diploid teosinte, Zea diploperennis,
also discovered
1) Zea diploperennis
2) genetic qualities may result in perennial maize
a> some examples have already survived three seasons
F. Teosinte hypothesis
1. Presented by Iltis in "Science" in 1983
a) problems with previous hypothesis
1) domestication of stepwise accumulation of traits caused by
single-gene mutations typical of crop selection required
evolution of five or six mutations, each responsible for one
major character
a> includes selection for softening of glumes and flattening of
fruitcases
2. Maize was domesticated from some older form of teosinte
3. Notion of punctuated equilibrum
a) Darwinian evolution
4. Catastrophic Sexual Transmutation Theory
a) corn ear is feminized central spike of tassel terminating
primary lateral teosinte branches
1) teosinte is ancestral to corn, however, teosinte ear is not
ancestral to corn ear
b) caused by hormonal shift within plant
c) resulted in:
1) feminization of apical male inflorescences (tassels) on
primary lateral branches
a> terminal position with pairs of soft-glumed spikelets
pre-adapted tassel spikes to turn into ears
2) supression of growth of normal female ears of teosinte
3) allocation of nutrient resources to new corn ear
a> caused apical meristem to become peristichous by
condensation twisting through slippage of rachid initials
d) selection of cultivar attributes by humans continued
1) domestication of maize began after sexual transmutation
produced free-grained "proto-ears", allowing grain
utilization
a> grains in maize were free from the beginning
2) only nonfragmentation, husking, and increases in
condensation, grain size, and rachid number are due to human
intervention
e) key is catastrophic change in plant
1) corn is not the result of a step-by-step accumulation of
single-gene mutations, but a gross re-expression of existing
mutations
2) may have been caused by cold, viruses, mycoplasmas, fungi,
or other pressures
3) Barbara McClintock discovered that extracellular process
could trigger changes in chromosomal DNA in corn
f) catastrophe could have been lethal if not for man's
intervention
g) conclusion
"The catastrophic sexual transmutation theory resolves
almost all paradoxes in maize evolution and archaeology,
establishes consistent morphological criteria by which valid
maize phylogenies may be developed, allows a plausible
interpretation of maize ear morphology and anatomy, and
promises experimental verification by environmental and
genetic manipulation."
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