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VII. Current Direction of Corn Evolution and Where It Is Going

VII. Current Direction of Corn Evolution and Where It Is Going

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225



EVOLUTION OF CORN



A. THE

ROLEOF INCREASED

FEMALENESS

The more advanced and productive races of corn have evolved a higher

level of femaleness in which the optimal productivity comes from increases

in both the number of female spikelets per ear and increases in kernel size.

This increased female productivity comes at the expense of male productivity in that the number of male spikelets and amount of pollen production

are reduced. This is a reversal from that of primitive corn and the wild

populations of most teosintes, although a single teosinte plant may have

100 or more small inflorescences that are male or female or mixed. In corn

as in other monoecious crops such as in the Cucurbitaceae, the increased

femaleness was associated with selection for increased productivity. As

higher levels of femaleness increased productivity in corn, the ear became

increasingly precocious, with the consequence that it also became more

deeply entrapped within husk leaves and, thereby, was unable to disperse

its own seed without assistance from humans. Although the key traits of

corn, including paired female spikelets and many ranks of spikelets, are

factors for increased productivity in that each one doubles the yield of

grain per spike, they are inherited independently from increased feminization, rather than being a product of it, as proposed by Iltis (1983). According to the hypothesis of 11th (1983), the genes pd-Pd for paired versus

single female spikelet had no role in the origin of the corn ear because the

toesinte tassel has paired spikelets and, according to his hypothesis, the

first ear of corn was a sexually transmuted tassel. But teosinte tassels

feminized by either tassel seed genes or just by terminating short tillers

carry only single female spikelets, not paired ones as expected by Iltis

(1983). Increased feminization has been important to the origin and productivity of corn but not in the form suggested by Iltis (1983).

But to know how to extend this part evolution into the future, we must

discover and appreciate the pathways of corn’s history, including corn’s

origins and the genetics of ear morphology that resulted in the present

situation. It may be advantageous with some of the elite material now in

commercial use to recycle some of corn’s ancestral germplasm in order to

recover some lost trait or to enhance an existing trait, for example, the

combining ability to produce productive hybrids. To extend the increased

productivity of the large ears of modern corn, which is associated with

increased levels of femaleness, we may have to devise genetic techniques to

cope with reduced tassels in the male parent of hybrid crossing fields, such

as certain ramosa genes or the tb gene previously mentioned.



B. PARTITIONING

OF PHOTOSYNTHLTE

BETWEEN



THE SEXES



Before the origin of corn, the teosinte progenitor, then as now, made a

larger reproductive investment of resources in male inflorescences (tassels)



226



WALTON C. GALINAT



and their pollen production than it did in female inflorescences and their

kernel development. The tassels of teosinte terminate not only the main

stalk but also all primary branches, which become elongate and efficient

pollen dispensers. The high male/female ratio is adaptive to teosinte

because seed set depends upon wind for cross-pollination between scattered and sometimes widely separated plants. In many wild species, evolution favors out-crossing between widely separated and more masculine

plants because such pollinations tend to produce more vigorous and successful hybrid offspring. Under domestication, the hybrid vigor of outcrossing is ensured and controlled by the hybrid seed corn companies, first

by selective inbreeding and then by selective hybridization of two good

combining parents, usually in the 4/1 female/male planting arrangements

of a crossing field, which is sometimes modified to 6/2 female/male

arrangement, this being more amenable to mechanization.

Teosinte has two general branching patterns, one lateral and the other

basal, and each results from a higher level of maleness than that of corn, as

described earlier, under biphyletic domestications. As with other wild

monoecious plants that came under domestication, human selection for

increased productivity brought an increased feminization and an increase

in female inflorescence size and a reduction in male inflorescence number

and/or size.

Factors that favor the evolution of one large ear per corn plant include

(1) the free intraplant movement of resources (photosynthate), (2) a decrease in total construction costs for both the husk systems and cobs per

plant, with fewer but larger ears, and (3) a decrease in tassel size and

pollen costs per unit seed set, with the conserved resources partitioned into

constructing larger cobs. These factors agree with one of the theoretical

models for gamete packaging strategies for plants, in general as set forth in

mathematical terms by Schoen and Dubuc (1990).

With decreased expenditures on the husk system, protection for the

longer ear may be inadequate under factor (2), indicating that a compromise of two or more shorter ears with better protection may be a more

productive strategy. Under factor (3), a sudden one-step reduction in tassel

size with corresponding increases in ear productivity may be achieved by

the introduction of the unbranched (ub) tassel gene. With the extreme

expression of ub, the tassel is completely without branches, but usually ub

tassels have one or two short branches at the base of the central spike.

Reduced tassels may be inadequate to function as males in crossing fields,

but solutions are possible.

Both factors (2) and (3) are fixed costs that are drawn from one initial

pool of resources. A secondary pool of resources that accumulates and

becomes available over the next 2-3 weeks contributes to kernel development and cob induration.



EVOLUTION OF CORN



227



VIII. SUMMARY AND CONCLUSIONS

Research to segregate and isolate the key trait differences between

teosinte and corn in isogeneic backgrounds has already enabled us to (1)

determine the minimum number of genetic changes that are essential to

convert teosinte into corn, (2) determine the inheritance and chromosomal

location of these genes, (3) determine the modifying effects of background

genes in shaping the expression of the key trait genes, and (4) determine

from the past and current direction of corn’s evolution as to where it may

be going from here, and which loci and genes will be useful in directing the

future extension of this evolution.

That only four or five inherited key trait units separate teosinte from

corn is based on the recovery rate of parental types in F2 segregations

(Collins and Kempton, 1920; Mangelsdorf and Reeves, 1939; Beadle,

1980). One viewpoint holds that these inherited units are clusters of linked

genes that eventually evolved their partial isolating mechanisms through

tight linkage, cross-over suppressors such as cryptic rearrangements, and

chromosome knobs, or by close linkage with gametophyte genes. The Ga

silks of teosinte would have selective protection against ga corn pollen. The

evolution of some mechanism for segmental isolation would occur within

teosinte in order to conserve certain essential combinations of genes from

breaking up through crossing-over and recombination with the domestic

alleles that would reduce natural survival (Galinat, 1988a). The corn

counterparts to these blocks do not require such protection because the

fitness of the corn ear is in the eye and mind of humans. The best example

of such a teosine segment is on the short arm of chromosome 4, where

there are genes controlling spikelet inclination, rachilla elongation, outer

glume and cupule induration, and, near the end of the short arm, rachis

and pith abscission. In addition to this partial segmental isolation, other

mechanisms include differences in flowering time and geographic isolation.

The location of the two-ranking (rr) gene compared with its manyranking allele in a background of eight-rowed corn is on chromosome 2,

but tr expression may be suppressed at higher kernel row numbers. The p d

(single female spikelets) gene is probably located on chromosome 3, but its

expresssion is unstable in the presence of a thick and/or condensed cob.

The expression of abscission layer development is both unstable and complex in a background of modern corn. In addition to the genes on the short

arm of chromosome 4, condensation controlled on chromosome 1 may

result in nondisarticulation by a fusion of cupule apex to glume cushion just

above.

The near failure of internodes to elongate in the cob of most modern

corn results in an unstable expression of the p d and rr genes. With the tight



228



WALTON C. GALINAT



juxtaposition of spikelets, small fluctuations in the internal-external environment result in either a proliferation or a reduction in spikelet pairing

and/or ranking. The exception to this instability is found in certain corn

that became isolated in South America at an early time. These are Coroico

and related corns from the Bolivian lowlands, which carry the primitive

interspace (is) gene, and Confite Morocho of Peru in the highland Andes,

which carry the string cob (Sg I and Sg 2) genes. These genes remove all

tight condensation from the female spikelets, and thereby allow stabilization of p d and tr expression. It is significant that these background genes

occur in teosinte and apparently in the oldest known archaeological corn.

In this respect, the oldest corn cobs are connecting links to teosinte.

In developing key trait stocks for genetic and molecular analysis, the

background should contain the is, Sg I and Sg 2 genes in order to provide

stable expression of the key trait genes. Because the hypothesis under

analysis is that corn evolved out of teosinte, ideally we would be studying

the key trait genes of corn in a teosinte background. But the traditional

genetic and molecular markers are all in corn and the plant habit of corn is

easier to manipulate in controlling pollinations, and so the modification of

corn with these primitive genes is a reasonable compromise.

Increased femaleness started with the domestication of teosinte and has

continued throughout the millennia of corn improvement in productivity.

The consequence in recent years has been reductions in tassel development

to the extent that they tend to be inadequate as pollinators in seed production crossing fields. The increased femaleness has not only resulted in

larger, more productive ears, but the earlier precocious thrust into rapid

pistil development (protogyny) has resulted in a deeper, more permanent

entrapment within the husk leaves of the branch that the ear terminates.

When this husk system for ear protection functions in cooperation with

man as the agent for seed harvesting and dispersal, it is man who usually

gets the food as his reward instead of the birds and worms.

Because the increased femaleness, productivity, and adequate husk

coverage of the corn ear all appear to have a hormone basis involving

feedback of a female-generated signal to terminate further development in

the underlying vegetative phase, it behooves us to obtain a better understanding of its genetic and physiological basis for the continued improvement of corn both by standard plant breeding techniques and by the

laboratory techniques of biotechnology.

The possibility of a double domestication of two different teosintes

cannot be dismissed yet on the basis of the molecular comparisons made so

far. In particular, the corn of the Andean highlands has to be compared

directly to 2. mays ssp. mexicana, race Chalco, as well as ssp. parviglumis.

The latter, according to interpretations of the molecular data of Doebley

(1990), is the only teosinte progenitor of all corn.



EVOLUTION OF CORN



229



ACKNOWLEDGMENTS

This paper is from the Massachusetts Agricultural Experiment Station, University of

Massachusetts at Amherst. The research was supported in part from Experiment Station

Hatch Project No. 566 (NE-124), from CRGO Grant 88-37261-3541 (Hatch-8801056), and a

grant from Pioneer Hi-Bred International. The typing was done by Mrs. Lorraine Daley.



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This Page Intentionally Left Blank



USEOF SURFACE

COMPLEXATION

MODELS

IN SOIL

CHEMICAL S y s m s

Sabine Goldberg

USDA-ARS,

U.S. Salinity Laboratory,

Riverside, California 92S01



I. Introduction



II. Description of Models

A. Common Characteristicsof Surface Complexation Models

B. Constant CapacitanceModel

C. Triple-Layer Model

D. Stem Variable Surface Charge-Variable Surface Potential Model

E. Generalized Two-Layer Model

F. One-pKModel

111. Application of Models to Protonation-Dissociation Reactions on Oxides,

Clay Minerals, and Soils

A. Constant Capacitance Model

B. Triple-Layer Model

C. Stern VSC-VSP Model

D. Generalized Two-Layer Model

E. One-pKModel

N . Application of Models to Metal Ion Adsorption Reactions on Oxides,

Clay Minerals, and Soils

A. Constant Capacitance Model

B. Triple-Layer Model

C. Stern VSC-VSP Model

D. Generalized Two-Layer Model

E. One-pKModel

V. Application of Models to Inorganic Anion Adsorption Reactions on

Oxides, Clay Minerals, and Soils

A. Constant Capacitance Model

B. Triple-Layer Model

C. Stern VSC-VSP Model

D. Generalized Two-Layer Model

E. One-pKModel

VI. Application of Models to Organic Ligand Adsorption Reactions on Oxides

k Constant Capacitance Model



233

Advances in A p m y , Volumr 47



234



SABINE GOLDBERG

B. Triple-Layer Model

C. Stern VSC-VSP Model

VII. Applicationof Models to Competitive Adsorption Reactions on Oxides

A. Metal-Metal Competition

B. Anion-Anion Competition

C. Metal-Ligand Interactions

VIII. Incorporation of Surface ComplexationModels into Computer Codes

A. Incorporation into Chemical Speciation Models

B. Incorporation into Transport Models

IX. Summary

References



I. INTRODUCTION

A model is a simplified representation of reality considering only the

characteristics of the system important to the problem at hand. An empirical model is a description of data without theoretical basis. A chemical

model provides a description of a chemical system consistent with its

chemical properties and should be simultaneously as simple and as chemically correct as possible. The ideal model is effective, comprehensive,

realistic, and predictive (Barrow and Bowden, 1987). An effective model

closely describes observations, a comprehensive model applies to a wide

range of conditions without modification, a realistic model conforms to

accepted theories of behavior, and a predictive model can be applied to

different conditions.

Unlike empirical models, surface complexation models are chemical

models that strive to satisfy the above characteristics and to give a general

molecular description of adsorption phenomena using an equilibrium

approach. The purpose of molecular theory is to derive thermodynamic

properties such as activity coefficients and equilibrium constants from the

principles of statistical mechanics (Sposito, 1981). The surface complexation models are designed to calculate values for the thermodynamic properties mathematically and constitute a family of models having similar

characteristics. This model family includes the constant capacitance model

(Stumm et al., 1980), the triple-layer model (Davis et al., 1978), the Stern

variable surface charge-variable surface potential model (Bowden et al.,

1980), the generalized two-layer model (Dzombak and Morel, 1990),

and the one-pK model (van Riemsdijk et al., 1986). The major advancement of the surface complexation models is that they consider surface

charge. Surface charge results from protonation and dissociation reactions

as well as from surface complexation reactions of reactive surface hydroxyl



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