Tải bản đầy đủ - 0 (trang)
II. Transformation by Domestication and Isolation

II. Transformation by Domestication and Isolation

Tải bản đầy đủ - 0trang

206



WALTON C.GALINAT



at all, would occur in only a few hundred years when directed by the mind,

eye, and hand of humans acting to ensure their own survival. Under

imposed isolation where they were selectively planted, and harvested

based on purposeful nurtured, human intervention, these crops would

avoid genetic “swamping” by the wild populations and, thereby, would not

lose their domestic advantages. It is not as if the transformation of teosinte

into corn involved a series of improbable mutations, as would be the case

in the slow process of natural evolution, but rather involved a situation

wherein an observant, intelligent people selected the most useful variants

out of large populations of teosinte in the wild and then concentrated them

into isolated, evolutionary pools. All that they had to observe and act upon

was that offspring tended to resemble their parents.

Man transported a primitive type of corn not much different from the

oldest Tehuacan corn to South America some 4000 or more years ago,

where it freely diverged in complete isolation from teosinte. The interspace

trait due to interspace (is) gene character of the first Tehuacan corn survived as a relic trait in “Coroico” and related races under the hand of the

Guarany Indians on the eastern slopes of the Andes. The slender (string)

cob due to the Sg I and Sg 2 genes with their short rachillae and reduced

cupules found in some of the early Tehuacan corn survived in “Confite

Morocho” from Peru. There was a delay of 2000 or more years before corn

could spread into most of the area now comprising the United States

because of its lack of adaptation to longer daylengths and shorter growing

seasons, and eventually to a lack of human technical means to plow the

dense prairie sod in the area now occupied by the United States corn belt

(Galinat, 1985a).

Isolation resulting from such geographic diversity has always played an

important role in both natural evolution and evolution under domestication. Because of the widespread distribution of corn with the spread of

farming, corn had differentiated into over 300 races by the time of

Columbus. The differences between these races are mostly quantitative ear

traits involving gradual changes in productivity and adaptation, in contrast

with the few qualitative ear traits that are simply inherited and may rapidly

transform teosinte into corn under domestication. The taxonomy of the

races of corn in Latin America has been described in a series of publications over a 25-year period starting geographically with Mexico

(Wellhausen et at., 1952); this has been reviewed by Goodman and Brown

(1988). These races gradually became adapted to numerous sites far from

corn’s original home, apparently southern Mexico, as far south as southern

Argentina, and as far north as the Gasp6 Peninsula of Canada. Isolation

into these diverse habitats together with the selective preferences imposed

by man resulted in the present incredible array of races, each of which



EVOLUTION OF CORN



207



acquired certain adaptive, chromosomal, and genetic characteristics. This

enormous diversity, now extant in germplasm banks, is the rich raw material that is available to the modern corn breeder. Now the genetic isolation

is brought about through the use of paper bags by the breeder to control

the pollinations, and the selection of improved designs is directed by the

power of human intelligence and imagination.

In considering teosinte as a food plant, variants that would double and

then quadruple its productivity per ear could hardly escape the eye, mind,

and hand of America’s first native peoples. The key traits of corn are not

only nonadaptive in the wild, but their total combination is lethal in nature.

Corn’s key traits must have been selected out of natural variation within

teosinte and transported into isolation, for example, in irrigated garden

plots, in the Tehuacan Valley. The first introduction to the Tehuacan

Valley may have been in trade of salt for grain, and the grain was then

grown by a desperate people who would invent irrigation.



A. THE

SEPARATION

BY KEYTRAITS

To the traditional three key traits separating corn from teosinte, which I

and others have described before (e.g., Galinat, 1983, 1988a) and which

will be reviewed only briefly here, I shall now add a fourth trait involving a

complex block of genes partially controlling the fruitcase enclosure versus

naked exposure of the kernel. In an effort to keep the list of keys to a

minimum, background factors such as interspace (is) and string cob (Sg I

and Sg 2) are not included. The new four-part listing of keys is as follows:



1. Solitary female spikelets in teosinte in comparison with paired ones

in corn.

2. Two-ranked central spike in teosinte in comparison with a manyranked one in corn.

3. Shattering rachis (cob) in teosinte in comparison with a nonshattering one in corn.

4. Fruitcase-enclosed kernel in teosinte in comparison with an exposed

kernel in corn. Kernel exposure may occur by two different systems, as

described later.

1. Inheritance of Solitary Versus Paired Female Spikelets



The reactivation of the suppressed second female spikelet in teosinte

immediately doubled the productivity of grain per spike and, thereby,

made it more adaptable as a human (and bird) food plant and less adapted



208



WALTON C . GALINAT



to natural survival. The pairing of spikelets opens up the fruitcase, giving

easier access to the grain. Protection replacement evolved later from a

husk enclosure system of the ear as a whole that requires human intervention to release the seed.

In some segregations from corn-teosinte hybrids, the single female

spikelet is expressed as a single recessive gene ( p d ) , as is known from

the early studies of Collins and Kempton (1920), and its most commonly

agreed-upon location is on chromosome 3 (Langham, 1940). Other reported locations (chromosomes 4, 7, and 8) apparently result from the

interactions of other genes (Mangelsdorf, 1947; Rogers, 1950). In many

key trait derivates, stability to p d expression is associated with the interspace and string cob traits (Galinat, 1991b). It has even been suggested

that the dominant allele ( P d ) of p d had no role in the origin of the corn

ear because, according to this hypothesis, the paired female spikelets came

from transmutation of the teosinte tassel, which already has a paired

condition of its male spikelets, into the corn ear (Iltis, 1983). Because both

tassel and ear diverge developmentally from the same primordial inflorescence, not one from the other, the suggestion of Iltis (1983) is not valid.

The solitary spikelets of the teosinte ear are not removed by removing

the teosinte ears, but by changing the gene that produces them.



2. Inheritance of Two-Ranked Versus Many-Ranked SpikeIets

The proliferation into four or more ranks of spikelets was another

system for at least doubling the productivity per spike. When this system

was combined with that of pairing, the productivity per spike was at least

quadrupled. At this point, humanity obviously had a new and important

food plant. But it was not a one-time event. Domestication would be in

many stages simultaneously in the hands of many farmers of various

abilities.

Two-ranking versus four-ranking (decussate) is controlled by a single

recessive gene (tr) when expressed in the central spike of modern eightrowed corn. But two-ranking also normally occurs in the lateral branches

of a tassel with a many-ranked central spike and in the secondary ears on a

plant with a many-ranked main ear, or just in stunted corn. It is those who

favor a wild corn (i.e., Mangelsdorf, 1986; Wilkes, 1986) this time who

would also remove a portion or a whole inflorescence (the central spike of

the tassel or the main ear) in order to derive, and in my mind ridiculously

so, teosinte out of corn.

The tr gene is located on the short arm of teosinte chromosome 2, as

shown in the data of Rogers (1950), and confirmed with interchanges be-



EVOLUTION OF CORN



209



tween corn chromosome 2 and its partial homologue Tripsacurn chromosome 9, which carries the short loci of Zea chromosome 2, including

rr (Galinat, 1973). In segregations for molecular markers, the data of

Doebley et al. (1990) agree in placing a major gene controlling “RANK” on

the short arm of chromosome 2. A minor position for tr on chromosome 1

was also reported by Langham (1940) and Rogers (1950), but in my material

this factor just involves reduced condensation, a factor known to be associated with changes in kernel row number (Anderson and Brown, 1948).

3. Inheritance of Shattering Rachis (Cob) Versus

a Nonshattering One



Selective pressure against abscission layer development, or shattering, is

unconscious and automatic when harvesting is at the whole-ear level in

corn or any other cereal, such as the small grains. Even when harvesting

is performed by beating a plant against an animal skin, intact ears are

favored, as the whole ear flies into the skin. With shattered ear types, there

are losses before, during, and after the operation that tend to reduce their

frequency in harvested grain.

Nonshattering is somewhat complex as a key trait separating corn from

teosinte. It involves at least two and sometimes three components, each

under control of a different incompletely dominant gene. Rind abscission

extends through the rind at the node between the apex of the cupule and

the divergence of the glume cushion above. When an ear with just rind

abscission is snapped in cross-section, the abscission layer looks like a

donut with white pith at the center. When pith abscission is added, the

whole in the donut is lost. Pith abscission (Ph) appears to be linked to rind

abscission (Ri) far out near the end of the short arm of chromosome 4

(Galinat, 1975b). There is a distinct layer across the pith in cobs of the

Durango teosinte chromosome 1derivative of A158. This gene, designated

ab, is separate from Ph and stronger in expression. Another factor that

prevents shattering is really a condensation factor. It holds the rachis

together by a fusion of the apex of the cupule to the glume cushion above.

As heterozygotes in this back-cross to a su gl tester, the expression of Ph

and Ri was poor and more frequent toward the tips of the ear, where

condensation is slightly relaxed.

The nonshattering rachis becomes semilethal in teosinte because it inhibits seed dispersal. But the reciprocal condition, partial abscission layers

in the corn cob, may be tolerated because the comparatively high level of

condensation in corn prevents complete rind abscission through a fusion

of the apex of the cupule to the glume cushion above. Pith abscission is



2 10



WALTON C . GALINAT



ineffectual in the absence of complete rind abscission, so modern corn can

cope with some gene flow from teosinte for either one alone of these two

abscission factors because of their individual incomplete effect on the corn

cob shattering.



4. The Chromosome 4 Difference

That the often-described block inheritance on the short arm of chromosome 4 (4s) controlling certain difference between the ears of corn and

teosinte might have originally started with a monogenic change has been

suggested by Kermicle (Doebley ef al., 1990). Apparently they were considering a single mutation with pleiotropic effects rather than a compound

locus, if not a polygenic complex. The most obvious effects of the 4 s factor(s) from teosinte upon corn, as described by Mangelsdorf and Reeves

(1939), Rogers (1950), Sehgal (1963), and Galinat (1963), include an upward inclination of a short rachilla, cupule enlargement, and an induration of both outer glume and cupule, all of which are important to the

teosinte fruitcase. The effects of the counterparts to these traits from all or

a portion of corn 4s upon kernel exposure and threshability of teosinte

have not been studied, despite the fact that the teosinte background is

essential to analyze a teosinte-to-corn transformation. An assumption that

corn 4 s alone will transfer the corn glume character and rachilla elongation

and relaxation into teosinte is probably not valid. The teosinte background

would still carry other genes for glume induration on chromosomes 1and 7

spikelet inclination on chromosome 1, and other modifying genes that alter

the expression of corn 4s (W. C. Galinat, unpublished data).

There remain numerous questions about 4s to resolve. Did its importance evolve just because there happened by chance to be a cluster of

floral genes in this area? Did the block inheritance result from recent

postdomestication evolution as a means to protect teosinte against harmful corn introgression that would destroy its fruitcase? In addition to tight

linkage, the integrity of the complexes may be enhanced by inhibitors to

crossing-over, such as cryptic rearrangements and chromosome knobs, or

,by close linkage with gametophyte genes. In the latter case, the Guatemalan teosintes are known to carry a gametophyte (Ga) allele within their

chromosome 4 complex (D. Duvick, personal communication, 1984).

More recently other teosintes (2. mays ssp. rnexicana, races Central

Plateau, Chalco, Amecameca, and Los Reyes, but not Nobogame, as well

as one accession of ssp. huehuefenangensis) were all found to carry some

type of Ga system on 4 s which would serve to protect its linked teosinte

segment against corn introgression. Kermicle and Allen (1990) note that

one of these, the TIC-CP (teosinte incompatibility to corn-Central



EVOLUTION OF CORN



211



Plateau) system, is unknown in corn and, therefore, it may belong solely to

teosinte as a protector of its associated segment if not of genomic integrity

in general. It is of interest that the miniature teosinte of ssp. parviglumis,

race Balsas, which Doebley (1990) places closest to corn, does not carry

a 4s Ga system for protection against corn. Perhaps its small size is

adequate.

According to the linkage data of Rogers (1950), the key traits of the

Guatemalan (Florida) teosinte do not have as tight a control by block

inheritance as do the Mexican teosintes. In the case of this teosinte, it

would seem, therefore, that evolution for block inheritance may still be

underway.

Investigations into the nature and evolution of teosinte 4s into corn 4s

involves many aspects. Corn 4 s or various known components of it could

be transferred to teosinte in order to determine their domestic value. The

teosinte 4s could be dissected and then its parts reconstructed in a corn

background with suitable marker genes on either side in order to reveal its

components as reciprocal cross-overs.

The breaking of tight linkages requires large populations when studied with conventional marker genes segregating at the whole plant level,

which makes the analysis difficult. Such a study may be more amenable to

analysis with molecular markers. I have isolated the chromosome 4 difference several times with classical marker genes and some of these have

different effects, suggesting that crossing-over had occurred between independent factors within the segment for glume induration and spikelet

inclination. This may happen unconsciously with small populations. Intercrosses between them are being made.

Other essential traits controlled on the short arm of chromosome 4 include rind and pith abscission near the end of the arm (Galinat, 1975b) and

apparently genes involved with rachilla elongation and cupule development. The historic tunicate locus lies down on the long arm of the same

chromosome. Even the strongest allele (Tu) at this locus, which usually

has monstrous effects, was once considered as representing wild corn

(Mangelsdorf and Reeves, 1939; Mangelsdorf, 1948), a viewpoint to which

Mangelsdorf adhered for over 30 years (Mangelsdorf, 1974).



B. ISOLATING

MECHANISMS

The various isolating mechanisms that protect teosinte against harmful

introgression by corn gennplasm have already been described in detail

(Galinat, 1988a). Briefly, they are (1) isolation by differences in flowering

time, (2) isolation by geographic separation, and (3) isolation by block

inheritance, including tight linkage and gametophyte factors.



212



WALTON C. GALLNAT



C. THE

TEMPORARY

ROLEOF THETUNICATE

Locus

Apparently a weak tunicate allele (tu w) played a temporary role as an

attractant to domestication during at least one of corn’s origins, but it was

usually discarded in favor of the chromosome 4s factors for reduced

induration and rachilla elongation and downward relaxation. This tu w

allele still lingers on in at least one primitive race, Chapalote, which has

evolutionary roots tracing back to the oldest Tehuacan corn. The tu w

allele derived from Chapalote does have value in vestigial glume corn

by its modifying action in restoration of the tassel glumes to Vg plants,

thereby allowing pollen production on VgVg plants with glumeless ears

(Galinat, 1966).

The tu w allele produces soft, slightly elongate female glumes together

with slightly elongate rachillae. They are broader but not as long as the

male glumes borne on the same plant. They could not be a product of

transmutation by a male to female secondary sex trait as implied in the Iltis

hypothesis. The tu w allele is known to have similar effects upon teosinte in

producing “soft-shelled” fruitcases with partly emergent kernels that are

freely threshable, as envisioned by Emerson and Beadle (Beadle, 1972).

Genetic cross-over analysis with su gl 3 show that this allele is at the

tunicate locus (Mangelsdorf and Edwardson, 1953).



III. TIME REQUIRED FOR

TRANSFORMATION UNDER DOMESTICATION

Any disagreements that remain over the origin of corn seem to revolve

around the time frame available and that required for the transformation

of teosinte into corn.



A. THE

POWER

OF DOMESTICATION

The power of domestication in rapidly selecting new types is described

by Darwin (1859) as “that which enables the agriculturist, not only to

modify the character of his flock but to change it altogether. It is the

magician’s wand, by means of which he may summon into life whatever

form and mold he pleases.” Darwin (1859) elaborates

Domestic races often have a somewhat monstrous character; by which I mean, that,

although differing from each other, and from other species in the same genus, in several

trifling respects, they often differ in an extreme degree in some one part, both when

compared one with another, and more especially when compared with the species under

nature to which they are nearest allied.



EVOLUTION OF CORN



213



The sudden appearance of a new exotic nitch such as domestication provides an opening for extremely rapid evolution, especially for annual plants

such as teosinte, where there is (1) a short generation time, (2) abundant

variability, and (3) isolated places under the eye, hand, and mind of

farmers to direct the evolution.

But some traditional taxonomists and even some evolutionists, in unexpected contradiction with Darwin’s observations and the basic understanding of both plant breeders and geneticists, do not accept the rapidity of

change in the case of the origin of corn by domestication of teosinte. For

example, Mangelsdorf (1947) states “If maize has originated from teosinte,

it represents the widest departure that comes within man’s purview,” and

then suggests that “One must indeed allow a considerable period of time

for its accomplishment, or one must assume cataclysmic changes, of a

nature unknown, have been involved.”



B. A CONSIDERABLE

PERIOD

OF TIME,

AN

UNNECESSARY

ASSUMPTION

The assumption of a considerable period of time as one of the

Mangelsdorf options for a teosinte-corn relationship was picked up by

Goodman (1988) as “involving multiple events long before the era of plant

domestication” and earlier by Randolph (1972, 1976), who would even

retain the obsolete generic name of Euchlaena for teosinte. Doebley (1990)

would like to have a considerable period of time for “a series of improbable mutations” as if he were thinking of natural evolution instead of his

stated support for domestication. He would gain extra time by removing

the time frame set by the known archaeological record. Accordingly,

Doebley (1990) accepts a reevaluation of the age of the oldest Tehuacan

specimens by Bruce Benz, who claims they are 2000 years younger than

previously thought (Benz and Iltis, 1990).



C. CATACLYSMIC

CHANGES

OF AN UNKNOW

NATURE,

A FALSE

TENET

The assumption of a cataclysmic change, of a nature unknown, as an

option for a teosinte origin of corn by Mangelsdorf was picked up by Iltis

(1983) in his CSlT (cataclysmic sexual transmutation theory) theory by

which a macromutation would transmute the teosinte tassel into the corn

ear in one giant step. The theory was intended to show instant corn production and thereby explain the lack of connecting links, in the archaeological records, that are older than the oldest known corn, and the fact



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

II. Transformation by Domestication and Isolation

Tải bản đầy đủ ngay(0 tr)

×