PROCEEDINGS OF THE
NATIONAL ACADEMY OF SCIENCES, 29(6): (1943)
ELECTRICAL CORRELATES OF PURE AND HYBRID STRAINS OF SWEET CORN*
H. S. BURR
SECTION OF NEURO-ANATOMY,
YALE UNIVERSITY SCHOOL OF MEDICINE
Communicated April 30, 1943
It has been said by numerous investigators that the key problem in biology is organization. The patterns which living organisms present form the basis of description and classification in natural history. Adequate as these are in many instances, little evidence has been forthcoming concerning the forces which impose this stable and characteristic pattern on the dynamic flux which is unique in living systems. The geneticists have been extraordinarily successful in showing that many of the properties of the pattern can be related to structural elements in the chromosomes, including even molecular linkages. The mechanism by means of which the genes control form is still hidden. It is at this point that the key problem, the pattern of organization, arises.
The search for an answer to the problem has fallen rather naturally into three channels. In the first, the investigations have tended to assume that the forces operating were peculiar to biological systems. This has led to a number of fruitful qualitative descriptions such as physiological gradient, biological and embryological fields. These terms include no quantitative measures and, therefore, cannot be genuinely rigorous. The most popular approach has been through chemistry. The relationship between morphogenesis and chemical entities and processes has been widely examined. Out of such studies has come a series of descriptions of the building blocks of which protoplasm is composed, but so far it has failed to provide significant evidence of how, in the midst of constant chemical change, the design remains so astonishingly constant. The third channel is still relatively unexplored. A few sporadic and still unsuccessful attempts have been made to influence the shape of living things through physical force. However, the development of modern techniques of electrical measurement has made it possible to explore the electrical correlates of this design more effectively.
In 1932 Burr,1 and in 1935 Burr and Northrop,2 proposed a working hypothesis to the effect that the electrical signs to be found everywhere among living things indicate the existence of an underlying electrodynamic field whose characteristic forces impose pattern on protoplasm. In the past seven or eight years, a number of studies coming from Burr's laboratory have tended to support the validity of the working hypothesis. More particulariy, in 19413 he showed that in the developing frog's egg a characteristic electrical pattern could be determined in the unfertilized egg, and in the fertilized egg before the advent of the characteristic form of the embryo. The axis of this electrical pattern was found to predict the longitudinal axis of the nervous system. These findings suggest that there is a close relationship between the electrical pattern and the bilateral symmetry of the embryo. Genetics has shown the structural pattern to be closely associated with chromosomal arrangement. It follows, therefore, that in all probability, there is a very close relationship between electrical pattern and the genetic constitution of the living organism. To investigate this problem is difficult in the animal kingdom. In the plant world, however, the situation is simpler for genetic controls are more readily imposed and the consequences more readily observed.
With the cooperation of the authorities of the Connecticut Agricultural Experiment Station, it has been possible to study the electrical patterns in several pure and hybrid strains of sweet corn. The corn seeds used were from strains which have been under study for some time. These strains differ considerably in genetic constitution and in the degree of hybrid vigor shown in crosses between them.
The seeds were collected in 1942 and studied during the winter of 1942-1943. Four inbred strains were studied, and three hybrids. The inbreds used were C6, C13, P39, and a mutant of P39, C30. The hybrids were P39×C13, C30×C13, and C6×C13. C6 is the seed parent of Whipcross C6.2, with ears set low on stalk and well formed. It is resistant to bacterial wilt and rust. C13 is a Golden Early Market inbred. It is used as the seed or parent pollen of Marcross, C13.6. It is highly resistant to bacterial wilt. P39 is a mid-season yellow sweet corn, inbred, of unknown origin. C30 is a semi-dwarf mutant of P39. It is normal in appearance though much reduced in size. It is completely recessive to the P39. It has been shown by Singleton4 to differ by only a single gene from P39.
The hybrids studied have shown considerable differences in hybrid vigor in the field. When P39 is crossed with C13, the average weight of the ears is significantly less than in the hybrid C30×C13. This was true in 1940 and again in 1942. In 1941, however, there was no difference. In spite of the dwarfism of C30, the hybrids from this line seem to have slightly more hybrid vigor than P39 hybrids. The hybrid C6×C13, known as "Marcross," is still the outstanding early season hybrid (Singleton, personal communication). In this material, therefore, there are available four stable pure strains of significantly different properties with which to correlate electrical patterns. The three hybrids between them show gradation of hybrid vigor which is least in P39×C13 and greatest in C6×C13. If electrical patterns have any significance, electrical correlates of these differences should be manifest.
The technique employed was standard, consisting of the determination of a standing potential between the two opposite ends of the longitudinal axis of the kernel. The point of the attachment of the kernel to the cob was invariably positive and the opposite pole negative. Contact with the system was made by silver-silver chloride electrodes and measurements made with a microvoltmeter and galvanometer. More than 2000 measurements were made under carefully controlled conditions with the results shown in table 1.
A statistical analysis† shows that the F factor is 6.4. The requirement for 1% significance is 3.07. It is safe to conclude, therefore, that the means of all the strains differ significantly from each other. A second set of determinations was carried out using a larger number of kernels but with a smaller number of measurements for each kernel, and the means were found to check closely with those shown in table 1. Aside from the generally different means the most striking finding was the very great difference between the mean of the single gene mutant C30 and the parent stock P39. It is remarkable that the change of a single gene in P39 should produce such a profound and significant change in the over-all pattern of voltage difference.
STATISTICAL ANALYSIS OF POTENTIAL DIFFERENCES IN SWEET CORN SEEDS
Means in millivolts
|F=6.4. 1% Sig., 3.07. 5% Sig., 2.23.|
The conclusion seems to be inescapable, that there is a very close relationship between the genetic constitution and the electrical pattern. If further studies should confirm this conclusion, it seems very probable that one of the ways the chromosomes impart design to protoplasm is through the medium of an electrodynamic field.
No less interesting than the electrical correlates of the pure strains is the relationship between the potential differences and hybrid vigor. The electrical studies show a significant relationship between the potential difference and the degree of hybrid vigor, a relatively high potential difference being found in association with a high degree of hybrid vigor in the field; and a lower difference with a lower degree of vigor.
The search for a significant measure of hybrid vigor in plants has yielded confusing and inconclusive evidence. The findings here reported suggest that by means of the measured potential difference it may be possible to predict hybrid vigor.
Electrical correlates have been found between different inbred strains of sweet
corn even when the difference is due only to a single gene.
2. The magnitude of the potential difference is positively correlated with the degree of hybrid vigor.
* Aided by a grant
from the Fluid Research Funds of the Yale University School of Medicine.
† The aid of Dr. Chester I. Bliss and Professor I. L. Child is gratefully acknowledged.