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IV. Multiple Domestications in the Origin of Corn

IV. Multiple Domestications in the Origin of Corn

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EVOLUTION OF CORN



215



(Galinat, 1979) in the Maize News Letter, I do not now think it is the only

teosinte ancestor of corn. The study of Doebley does not include South

American corn, of which the highland corn such as Confite Morocho

appears to me to be related to Palomero Toluqueiio of Mexico in cob

morphology (reduced cupules and short rachillae) and cytology (reduced

knobs), with the geographic divergence occurring at some early time

before the former lost its hairiness and the latter acquired its fasciation and

its extreme pubescence. The hairy sheath trait does occur in Chalco

teosinte but it is not as strong as in Palomero Toluqueiio. The isozyme

studies of Bolivian corn by Goodman and Stuber (1983) were able to

separate the lowland Coroico types from the highland Andean complex of

corn, although they did not include teosinte. Doebley (1990) places all of

the races of Mexican corn off to the right of 2. mays ssp. mexicana and

partly above but mostly to the right of ssp. parviglumis, with a few accessions of corn intermediate between the two teosintes. Although none of

the corn tested is currently within the range of ssp. mexicana, the shift of

most corn to the right of even parviglumis suggests that shift is a product

of domestication and at one early time there was considerable overlap with

mexicana. Unless more molecular data excluding a double domestication

of different teosintes become available, one must continue to take my data

on cob and plant morphology as well as that of Kato on chromosome knob

complexes seriously. My data may not exclude independent domestications

of the same teosinte, but they are consistent with domestication from these

two different subspecies of teosinte. Because almost everyone seems to

agree that corn did not originate in a single macromutation, it also seems

probable that many independent farmers were involved and they had

various abilities to manipulate the few key domestic genes within teosinte

into producing the corn phenotype. Thus, many steps and pathways probably existed simultaneously. This scenario is different from Doebley

(1990), who concluded “the conversion of teosinte into maize is so improbable that it is difficult to imagine that it happened several times.”

The evidence from both cob morphology and from plant habit of at least

two independent domestications of teosinte has been described by Galinat

(1988a, 1991a). A review of this new evidence is presented here.



A. THE

EVIDENCE

FROM COBMORPHOLOGY

Apparently two very different morphological systems evolved independently to expose the kernel from within its teosinte fruitcase during the

domestic origin of corn. Although the two exposure systems are structurally different, they both achieve the same outcome of releasing the kernel

for easy shelling (Fig. 2).



2 16



WALTON C . GALJNAT



400



New England: 6 0 0



- ---



OOO?



New M e x i c o : 1200

Peru:



1500



Colombia:

Mexico:



2000

2500



-- - x l c o : 2500



New M e x i c o : 3 0 0 0



----



7 c



Mexico: 3000



I

eticall



Tehuacen, Mex.: 7 2 0 0



Oaxeca, M e x . : 8 0 0 0



--- - - --



!x.: 8000



f

Figure 2. Cross-sections from the ears of corn representing about 8OOO years of evolution

after domestication. Exposure of the kernel from the fruitcase of teosinte appears to have

been accomplished independently during two domestications, probably from different

teosintes, as indicated by the pattern of heterosis. The age is given in years before present

(B.P.). (a) Teosinte (Z. mays ssp. parviglumis, race Balsas) is ancestral to most corn; kernel

exposure is by rachilla elongation, with the cupule remaining unreduced. (b) The oldest

known corn from Tehuacan, Mexico (7200 years B. P.) usually had paired female spikelets,

sometimes in four ranks (eight rowed). (c) An increase in cob diameter and the level of

ranking, as with this 12-rowed ear of Chapalote, increased productivity. (d) When the larger

vascular supply evolved in Chapalote, recombined with the less demanding eight-rowed ear,

at first the surplus photosynthate became deposited as cob induration. (e) Following selection

for increased kernel size, the photosynthate was redirected into larger endosperms as in the



EVOLUTION OF CORN



217



1. Kernel Exposure by Rachilla Elongation with

the Cupule Unreduced



The cobs of most modern corn are thick and round in cross-section due

to greatly elongated rachillae, sometimes erroneously called pedicels,

borne in many ranks. The rachillae extend along the floor of welldeveloped cupules, which are collapsed down upon them and fused in a

very hard and strong configuration (Fig. 3).

The long female rachillae are due to the combined action of two recessive or incompletely dominant genes (sg I and sg 2), depending on the

background (Galinat, 1969). They not only occur in most modern corn but

also in an evolutionary pathway extending back to the oldest known

corn cobs from Tehuacan, Mexico (Fig. 2). It has been proposed that this

rachilla elongation represents one of two different systems for kernel

emergence from the teosinte fruitcase that were basic to the origin of corn

by the domestication of teosinte (Galinat, 1988a).

2. Kernel Exposure by Cupule Reduction with the Rachilla

Remaining Short



Instead of exposing the kernel by thrusting it up out of the teosinte

fruitcase by rachilla elongation, the remains of another system to accomplish the same ends are apparent in another racial grouping of corn,

namely, reducing the enclosing fruitcase (Fig. 2). The best examples of this

system remain in Confite Morocho of Peru and Palomero Toluqueno from

the valley of Mexico. With a greatly reduced or sometimes a totally

suppressed cupule, productivity in grain was achieved by evolving a long

narrow kernel as in Gourd Seed or Shoepeg of the southeastern United

Northern Flints of New England (1300A.D.) and the giant Cuzco cornof Peru (1500 A.D.).

(f) Teosinte ( Z . mays ssp. mexicana, Chalco) appears to be ancestral to another pathway,

with kernel exposure by cupule reduction and the rachilla remaining short. (g) A hypothetical

race of Mexican eight-rowed corn, almost without cupules and with short rachillae, that is

similar to the Peruvian race, Confite Morocho, known from archaeological remains about

4000 B. P. at Ayacucho (see k). (h) Palomero Toluquefio is an ancient indigenous race from

the Mexico City area that is considered here to have evolved soon after domestication of

Chalco teosinte. It has reduced cupules, short rachillas, and elongate kernels. (i) Pepitilla is

clearly related to Palomero Toluqueno. It is ancestral in varying degrees to the Southern

Dents, especially to Shoe Peg, with its narrow, deep kernels, and to Gourd Seed, with its

broad, deep kernels. These races all have reduced cupules and short rachillas. (j) The Corn

Belt Dent is known to be a hybrid between the Northern Flint (e) and Southern Dent (1)

pathways. (k) Confite Morocho of Peru has some similarities to both the earliest corn at

Tehuacan (b) and the hypothetical domesticate of Chalco (f). Adapted from Galinat (1988b),

with permission.



218



WALTON C . GALINAT



Figure 3. Longitudinal sections of the female spike showing two exposure systems of the

teosinte kernel during two domestications and increasing levels of condensation. (a) Tripsacurn floridanurn showing elongate internodes with interspaces about equal to internode

length. (b) Nobagame teosinte showing reduced interspace causing a slight triangulation of

the fruitcase. (c) Confite Morocho with a loss of interspace and cupule reduction resulting

in kernel exposure by one system. (d) Inbred A158 with condensation resulting in a folding down of the cupule apex, together with rachilla elongation, thrusting the kernel outward in

another, different system of kernel exposure. (e) Inbred B2 with cupules starting to close.

(f) Inbred B2 with cupules fused floor to roof.



EVOLUTION OF CORN



219



States. Because of the near absence of the cupule and the short rachillae

attached to it, these cobs are slender, hence the name “string cob.” By

extracting Sg I and Sg 2 genes from Confite Morocho and transferring

them to eight-row sweet corn, I developed a slender cob inbred, W401,

which had been important in slender cob sweet corn hybrids such as

“Candystick.”



B. THE

EVIDENCE

FROM PLANT

HABIT

When the plant habits from the two pathways of corn races based on cob

differences in cupule and rachilla development are arranged in the same

sequences, supportive evidence for this double origin of corn becomes

evident. Starting from two different subspecies of teosinte, pawiglumis,

race Balsas (Fig. 4a), and mexicana, race Chalco (Fig. 4d), there are two

different branching patterns that evolve stepwise toward the Corn Belt

Dent (Fig. 4g).

The branching pattern starting with parvigfumis teosinte is basal. It has

links represented in Fig. 4 as the indigenous flour corn of the American

Southwest and upper Missouri areas (Fig. 4b) followed by the New England flint corn from Rhode Island (Fig. 4c). In this sequence the ear

appears as if it were derived from an elevated tiller. This branching habit

can be duplicated by the grassy tiller (gt) gene in a primitive background. It

is described by Shaver (1967) in a modern corn background as causing the

production of numerous small tillers at the base of the main culm that

terminate in small female inflorescences (sometimes called “ground ears”).

In the presence of teosinte germplasm, the gt gene produces elongate tillers

at the base of the plant that terminate in male tassels (W. C. Galinat,

unpublished data).

In a parallel pathway starting with mexicana teosinte, the branching

pattern is lateral. The links in Fig. 4 are represented by Palomero

Toluqueiio (Fig. 4e) followed by the United States’ Southern Dents, represented by Gourd Seed (Fig. 4f). In this sequence the ear is either high or

centrally located on the plant. The Corn Belt Dent (Fig. 4g), as a hybrid

between these two pathways, has productivity refinements indicated here

as reduced tassels and erect upper leaves.

The mexicana branching habit is duplicated by the teosinte branched (tb)

gene in a modern corn background. It is located on the long arm of

chromosome 1 between the f and bm 2 loci (Burnham, 1961). It is described as producing many slender lateral branches usually ending in a

one-spike tassel. Although it produces abundant pollen, it rarely develops

seed. The reason for this is that the primary effect of the tb gene is to



Figure 4. The plant habits from the two pathways of corn races based on cob morphology

lend suport for the double origin of corn.They form two series of plant habits that may be

termed the basal branching type and the lateral branching type, both of which lead to the

Corn Belt Dent (g). (a) Teosinte subspecies pawiglumis, race Balsas, under good growing

conditions. Note the proliferation of tillers at the base of the plant. (b) The indigenous flour

corn of the upper Missouri area is close to one type of the first corn, pre- or proto-Chapalote.

Its ear is near the ground, where it comes from an elevated and condensed tiller. (c) The

Northern Flint (Rhode Island Flint). The upper ear-bearing node has elevated to a central

position on the plant, but a trail of its ascent remains. (d) Teosinte subspecies rnexicana, race

Chalco. Note that the branching is lateral and each branch terminates in a tassel with ears

borne below as on the main stalk of corn. (e) Palomero Toluqueiio. This multieared popcorn

has a barren zone both above and below the ear-bearing region. (f) Southern Dent (Gourd

Seed). The ear is high on the plant and tillers are suppressed. (g) The Corn Belt Dent. This

hybrid between the Northern Flint and Southern Dent pathways shows extreme heterosis. It

has productivity refinements indicated here as reduced tassels and erect upper leaves. These

line drawings are adapted from my painting showing the same sequences in dramatic color.



EVOLUTION OF CORN



22 1



increase the level of maleness and from this flows the free elongation of the

lateral branches. The tb or the gt gene may have a practical use to increase

maleness in the pollen parent of crossing fields (W. C. Galinat, unpublished data).

The question of how a single gene mutation such as the tb gene discovered by Burnham (1961) can undercut in one step a polygenic trait of

increased femaleness, reduced lateral branches, and evolution of the husk

enclosure of the ear will be treated later.



V. INTERPATHWAY HETEROSIS

If the high productivity of the Corn Belt Dent is at least partly a product

of the ancient double domestications of two different teosintes during

independent origins of corn, as the morphological evidence from the cob

and plant type suggest, then this insight would help in breeding for optimum heterosis .

Interpathway heterosis has long been recognized and used systematically

to increase productivity in the breeding of hybrid corn. It is just that the

ancient roots of the heterosis have never been traced all the way back to a

double domestication from different teosintes. According to the traditional

system for breeding hybrid corn, various inbred lines were developed by

repeated self-pollination, at first within the various open-pollinated

varieties of Corn Belt Dent. Then the different inbreds were intercrossed

and tested for maximum productivity in the Fl hybrids. At first the inbreds

were so unproductive that the commercial production of hybrid seed had to

be based on the double-cross system involving four inbreds, as invented by

Jones (1917, 1918). But now with second or higher cycles of inbred development, the inbreds themselves have been upgraded in productivity to a

point that a switch to the even more productive and uniform single-cross

hybrids has been made. But the varieties of Corn Belt Dent were already

known to be derived from hybrids between the Northern Flints and Southern Dents (Wallace and Brown, 1956). As might be expected, it was

discovered that the most productive hybrids were coming from crosses

between the most Northern Flint-like inbreds such as Mo 17 and C 103 out

of a Lancaster background crossed by the most Southern Dent-like inbreds

such as B 73 out of Iowa Stiff Stalk Synthetic, or before this Wf 9 out of

Reid’s Yellow Dent.

The parviglurnis to Northern Flint and the rnexicana to Southern Dent

pathways have diverse derivatives now widely scattered from Canada to

southern Argentina. The corn at these two hemispheric extremes had to



222



WALTON C. GALINAT



adapt to short growing seasons by evolving early flowering. Yet the corn

from these extremes in geographic separation is different in other respects.

In Southern Argentina, the early-flowering flour corn called “Coya” has

reduced cupules and short rachillae, whereas its early-flowering counterpart from the Gasp6 Peninsula in Canada has pronounced cupules and

elongate rachillae. These are the same two traits attributed to the two

independent domestications of different teosintes. If the proposed interpretation that the heterosis of modern hybrid corn relates to these two

different domestications, then, with this understanding, we are able to

exploit these two sources of early flowering while still capturing the interpathway heterosis in breeding short-season dent hybrids. If the predicted

changes in rainfall patterns in the United States follow global warming,

then the corn belt may be moving further north where such early-flowering

dent hybrids would be adapted and important.



VI. HUSK ENCLOSURE OF THE EAR

The husk system is a condensed lateral branch that permanently encloses

the ear terminating that branch. The development of these lateral branches

is characteristic of the mexicana subspecies of teosinte and is reflected in

the phenotype of the tb gene. The entrapment of the ear within the leaves

of the branch that it terminates results from precocious female development (protogyny) that is associated with a high level of feminization, as

explained later.



A. THE

PRODUCT

OF PROTOGYNY

That the husk enclosure of the corn ear is a female secondary sex trait

resulting from precocious female development has not always been readily

understood or obviously apparent. The common but mistaken assumption

has been that corn is protandrous based on the late date of silk emergence,

which is delayed by about a week while the silks elongate in their effort

to reach above their husk enclosure for pollen exposure. The result is a

false type of protandry that is only an artifact of artificial selection for the

unnatural husk enclosure that characterizes the corn ear.

The inheritance of this false protandry cannot be treated as if it were a

real sexual difference independent from husk length and style (silk) elongation or ability for style emergence, as in the studies of Landi and Frascaroli



EVOLUTION OF CORN



223



(1986). Nor can false protandry be used as if it were a real thing resulting

from the suppression of teosinte ears during an origin of the corn ear from

the teosinte tassel (Iltis, 1983). False observations can only lead to false

conclusions.

The husk enclosure is a female secondary sex trait that is expressed in

the vegetative phase just below the ear. Its completeness as a protective

device for the entire ear depends upon the prococious onset and duration

of female development. For example, if the ear switches its sex expression

from a lower female region to an upper male region, then at that point in

time the internodes in the shank below elongate and tend to elevate the ear

out of its husk enclosure. This appears to have been the situation with

some of the oldest tassel-tipped ears from Tehuacan and in certain primitive lving races (e.g., Confite Morocho, Pollo, and Nal Tel). But when the

ear is exclusively female, as in the advanced races of modern corn, then the

shank and all of its internodes remain short as if precocious female development were producing an early and continuous feedback of an inhibitory hormone acting primarily on internode elongation during the vegetative phase. The result is that the ear becomes trapped within its own bud,

which then forms a tight permanent enclosure of husk leaves. Genetic

factors determine whether female expression will be prococious as in corn

or latent as in toesinte, and then this has feedback that regulates internode

elongation in the plant habit below.



B. THE

HORMONELIKE

CONTROL

The apparent hormonelike nature of the female-generated feedback that

produces the husk enclosure of the corn ear is suggested by the restoration

of internode elongation in the shank following removal of the immature

(baby) ear or by the genetic masculinizing of the primary inflorescencesteosinte branched (tb) gene. The external application of gibberellic aid

(GA) to four genetically recessive dwarfs of corn was found to restore

normal internode elongation (Phinney , 1956) and the same hormone spray

was found to restore the normal monoecious condition to genetically

gyneocious squash plants (Shifriss, 1985). An increase in the level of femaleness as measured by the frequency of female flowers in squash may

be produced by spraying with Ethrel(2-chloroethylphosphoric acid), which

apparently changes to ethylene in the plant. It not only increases the yield

in cucumber and butternut squash but it may be used as a chemical aid in

producing F1 hybrids in these crops (Coyne, 1970). This flexibility in

shifting from one sex to the other in squash suggests that their flowers are

potentially hermaphroditic. It is just a matter of activating or deactivating



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IV. Multiple Domestications in the Origin of Corn

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