Biological Bulletin 18(6): 285-337 May 1910 (Corr. Dec. 1910)
DETERMINATION OF DOMINANCE AND THE MODIFICATION OF BEHAVIOR IN
ALTERNATIVE (MENDELIAN) INHERITANCE, BY CONDITIONS SURROUNDING OR
INCIDENT UPON THE GERM CELLS AT FERTILIZATION.1
1A paper presented at the meeting of the American Naturalists in Boston, Dec., 1909.
WILLIAM LAWRENCE TOWER
|The Point of View||285|
|Experiments in Analysis||293|
|Crosses Between L. signaticollis and L. diversa||293|
|Crosses Between L. signaticollis and L. undecimlineata||296|
|Experiments in Synthesis||307|
|Crosses Between L. undecimlineata and L. signaticollis||308|
|Crosses Between L. undecimlineata, L. oblongata and L. multitaeniata||312|
|Neo-Mendelism — The Factorial Hypothesis and Germ-plasm Theories||323|
|Dominance — Dominance discussed on basis of examples in this paper||329|
|Relation of External Conditions to Dominance||333|
|Explanation of Plates||338|
The Point of View.—The mutation theory of DeVries, the rediscovery of Mendel's paper on hybrid peas, and the work of the Mendelian hybridologists have focused the attention of biologists upon the characters of organisms as basal units for investigation in the effort to understand the modus operandi of evolution. The most conspicuous outcome is the growth of the "unit character" hypothesis, which, in the main, has been adopted by the Neo-Mendelians as the fundamental assumption necessary
for their practice and theory, because of the fact that the characters of organisms often stand out sharply and behave with distinctness in the processes which follow reproduction.
That many of the characters in organisms are distinct and behave in the sharp, alternative fashion described by the Mendelians, there can be no reasonable doubt. To deny the existence of these sharply defined, predicable behaviors in inheritance is to deny the evidence of our senses, and accuse a considerable body of reputable workers of inability to make accurate observations. The fundamental question is, do these unit characters, or lesser entities, occupy in the organism the mosaic relation and have the capacity for the mosaic rearrangement which is assumed by most Mendelians and by the followers of DeVries?
We must not be diverted from the main question by any fancied injury to our biological orthodoxies by the Neo-Mendelians' array of factors, determiners, allelomorphs, gametic couplings, latencies, etc., because these represent only the symbols of processes at present unknown, although the results of these unknown processes are observable, predicable, capable of control and modification.
Because a character is sharp and distinct, or may be apparently removed, is no necessary reason for supposing that it may be taken away as an entity, and no reason whatsoever for believing it to be conditioned by a subsidiary mass; rather, we must regard an organism essentially as we would look upon any inorganic substance, as a mass of matter in which the unit is the individual with its array of attributes or qualities, or its specific characters. It is certain, however, that there is adequate evidence to prove the truth of the situation as described by the Mendelians as far as the behavior of the attributes is concerned; but the basis and cause of this behavior is entirely unknown, and must remain unknown until we have replaced by fuller knowledge the present crude ideas of the constitution of living substance.
The situation which has developed in the last few years at the hands of the Neo-Mendelians themselves is interesting.
From the simple conditions discovered by Mendel there has arisen through the work of the last decade an array of observations tending to show that the Mendelian phenomenon is not in many instances as distinct and simple as one might wish, and at present divers kinds of variability in the behavior of characters are described and attributed, in some instances, to several different kinds of latency, to gametic coupling, to variable potency, to variable dominance, and so on. The situation essentially is this, that as investigation has progressed it has been discovered that not one, but a host of determining factors (I use the word factor as meaning something which makes possible a given result, with no idea expressed or implied as to the nature of this factor) are operative in the production of alternative inheritance; and in the attempt to preserve the letter of the law of Mendelian theory of unit characters with segregation in gametogenesis, a host of hypotheses have been developed in order to save the original theory.
Among the Neo-Mendelians the assumption is universal that all the differences that come out of crosses are entirely due to internal factors brought into the fertilized egg by the gametes as factors and determiners, to various combinations of allelomorphs, and so on. That some of these variable conditions may be due to external causes does not seem to have occurred to any of the Mendelians, and no effort has been made to eliminate in any one case what would first be eliminated in any accurate physical or chemical work, namely, the effect of surrounding conditions, and forces incident upon the reactions unquestionably going on in the developing individual.
Probably most of us will admit that the fertilization process represents the bringing together of two more or less like physico-chemical masses, and the combining of these into a new body with potentialities and capacities which then enable it to go on in a constantly increasing series of epigenetic reactions, and ontogenetic processes, and finally to evolve into an adult organism. The essence of the activity and reaction involved are beyond any question physico-chemical in their nature. This being true, the first step in the elucidation of this complex array of variability in behavior is to determine to what extent surrounding or incident forces may modify in a particular case the type of alternative inheritance which is found; when effects of these forces are known, then attack can be profitably made upon problems of internal factors. This first step I have in part accomplished in a series of experiments which form the basis of this preliminary paper.
Material.—The material upon which this series of experiments herein described was carried out consisted of three species of chrysomelid beetles of the genus Leptinotarsa, which would hybridize freely and perfectly in all directions: Leptinotarsa signaticollis Stål, a species occurring in southern Mexico at the foot of the escarpment on the western side of the Mexican plateau; Leptinotarsa undecimlineata Stål, a species confined entirely to the savannahs and lower foot hills from Tampico in Mexico, southward to Costa Rica and Panama; and Leptinotarsa diversa n. sp., which is very similar to the former but is limited entirely to the higher foot hills on the border of the Mexican plateau. The reason for choosing these three species was that any two of them, when crossed, give only certain very definite and invariable products, and I was thus enabled to eliminate many complications and possible sources of error. The contrasting characters are as follows:
Between L. signaticollis Stål (Fig. 1) and L. undecimlineata Stål (Fig. 2), in the adult, the elytra of L. undecimlineata have deep greenish black longitudinal stripes, edged with a double row of punctations, while L. signaticollis has the punctations, but not the stripes. The ground color of the elytra in L. signaticollis is grayish, while in L. undecimlineata it is white. No other characters in the adult had sufficient contrast to give sharp alternative inheritance. The larvae of the two are sharply contrasted; those of L. undecimlineata and L. signaticollis are exactly alike in the first stage, but in the second stage L. undecimlineata is white and L. signaticollis yellow, both with the same system of spots, and in the third stage L. signaticollis is yellow with well-developed tergal stripes and L. undecimlineata is pure white without stripes. These differences are well shown in text Figs. 1 and 2.
|FIG. 1. L. signaticollis Stål. (A) Adult. Showing the absence of pigment, and the presence of the impressed punctations on the elytra which border the stripes in other species. The absence of pigment may well represent the negative half of a Mendeian allelomorph. (B) Full-grown larva. Showing the arrangement of black color marks upon the sides and back of the larvae. The ground color in this state is bright chrome yellow. (C) Larva of second stage. Showing the characteristic color pattern. The ground color is a light chrome yellow.|
In L. diversa (Fig. 3) the elytra are marked by longitudinal stripes of greenish black edged with an irregular double row of punctations, and the larvae are in all stages exactly like those of L. signaticollis. There are no other simple differences between the three species that could be readily utilized in experiments of this kind, but the characters used are sharp and striking, and satisfy in every respect the conditions demanded by Mendelian hypotheses.
The elytral stripes of L. diversa and L. undecimlineata represent the present or positive character, and the lack of stripes in L. signaticollis the absence or negative factor of an allelomorphic pair, the yellow hypodermal color one member and the white the other of an allelomorphic pair: yl = (+) wh = (-); the dorsal spots the presence (+) and their absence the absence (-) of another pair of allelomorphs—when we view the characters from the present Neo-Mendelian standpoint.
|FIG. 2. L. undecimlineata Stål. (A) Adult. Showing on the elytra the presence of longitudinal pigmented bands which are bordered by rows of punctations as in L. signaticollis. The pigmented bands may be considered the positive half of a Mendelian allelomorph, so that when the two species are crossed we are following all of the Mendelian qualifications in crossing the presence and absence of the same character. (B) Full-grown larva, showing the characteristic color pattern in this stage. The ground color is pearly white. (C) Second stage larva, showing the characteristic color pattern. The ground color is also pearly white.|
A further reason for choosing this material for this investigation is the fact that when crossed it often gives most perfect Mendelian results, and in some cases perfect ratios of 1 : 2 : 1 are obtained in the second hybrid generation, while other crosses, brothers and sisters of the same material, did not give the same, but on the contrary, quite different results. For some time I was at a loss to understand the reason for this anomalous condition. No amount of crossing or investigation succeeded in disclosing the presence of any internal factor, which by its operations would produce this result. Careful examination, however, of the records which have been kept in the vivarium where these experiments were carried on, gave evidence that the differences were possibly due to the conditions surrounding the hybrid series during development. Accordingly, an array of experiments has been carried out to test this point—do the conditions surrounding or incident upon the gametes before and at fertilization and in early ontogeny influence in any way the behavior of characters? As far as this phase of the modifiability of alternative inheritance is concerned, I have essayed to investigate it in two ways: first, by the usual process of hybrid analysis, as it is practiced in all Mendelian work, in which the extracted dominants, recessives and heterozygous forms are isolated and carried on as pure cultures, and again inbred and tested in the usual ways—essentially experiments in analysis to determine germinal constitution; second, I have carried on a series of complicated experiments in synthesis, comparable to those which one would expect to go on in nature when two of these species are brought into contact and hybridized, in order to learn what the result would be if two species should come in contact under different conditions, and hybridize, and these results I shall describe under the head of Experiments in Synthesis.
|FIG. 3. L. diversa. (A) Adult. Showing the presence on the elytra of longitudinal dark stripes in exactly the same position as the dark stripes in L. undecimlineata, and in the position of the absence of stripes in L. signaticollis. (B) Full-grown larva, showing characteristic color pattern. The ground color is bright chrome yellow as in L. signaticollis. (C) Second stage larva, showing characteristic color pattern. The ground color is yellow.|
L. signaticollis ♀ X ♂ L. diversa.
Exp. No. H 409/411.
A female of L. signaticollis and a male L. diversa were allowed to reproduce under the conditions of the first experiment (Exp. No. H 409), while a certain number of their eggs were being laid (designated as Lot A). These eggs developed normally under the following conditions:
FOOD : NORMAL — UNIFORM.
|Day Av.||80° F.||± 5.4°||Day Av.||74 per cent||± 6 per cent.|
|Night Av.||75° F.||± 7.8°||Night Av.||75 per cent.||± 5.7 per cent.|
and gave typical larvae, which gave two classes of adults in the F1 generation in the proportion of approximately 1:1, or
|sig. type||mid type|
The individuals like the female parent (sig. type), when inbred, came true to the type for four consecutive generations, which was as far as they were carried; while the mid types, when inbred, gave in the F2 generation marked Mendelian ratios in the proportion of 1 : 2 : 1, the numbers for Pair B being:
|signaticollis type||mid type||diversa type|
|Observed 98 :||201 :||101|
|Expected 100 :||200 :||100|
After a few days the same pair of individuals was placed under the conditions of Exp. No. H 410:
FOOD : NORMAL — UNIFORM.
|Day Av.||75° F.||± 4.3°||Day Av.||51.2 per cent.||± 11 per cent.|
|Night Av.||51° F.||± 5.8°||Night Av.||81.5 per cent.||± 14.2 pet cent.|
and there allowed to develop another set of eggs (Lot B).
These eggs developed normally and gave in the F1 generation only a single type like the signaticollis parent, which, when inbred, gave in the F2 generation signaticollis, and continued to breed true for four consecutive generations. This experiment was repeated seven times with uniform results, confirming the conclusions drawn from the first two experiments and showing beyond doubt that the variability of behavior in the alternative inheritance of the eltyral stripes in these two species of beetles was due to conditions surrounding and incident upon the germinal materials in their most sensitive stages, before, during and immediately following fertilization. The behavior common to this type of experiment is shown in Plate III.
With this cross of an L. signaticollis ♀ X ♂ L. diversa, the determination of dominance and the ensuing type of behavior is clearly a function of the conditions incident upon the combining germ plasms. In the repetition of Exp. No. H 409/411, the experiment was varied so that in some cases it was the first laid eggs that gave the behavior of Exp. No. H 409, and in others, it was the last laid eggs, or those of the middle of the reproductive period—showing that the results are not "age results," nor due to segregations, nor orthogenesis giving one kind of germ at the start, another at the middle, and others at the close of the reproductive period.
Crosses Between L. signaticollis and L. undecimlineata.
More interesting and complicated results were obtained in crosses between L. signaticollis and L. undecimlineata, where there are contrasting characters between both larvae and adults, differences in the specific pattern as a whole, in specific spots and in the general body color. Reference to Figs. 1 and 2 will show essentially what these differences are, as far as pattern and spots are concerned. In body color of the larvae the difference is between a white in L. undecimlineata and a bright chrome yellow in L. signaticollis.
Relation of External Conditions to Dominance.
The question of dominance, while a vital one, has been somewhat wrongly attacked. I would hardly wish at the present time to attempt to account for the highly variable results that have been found in the dominance and recessiveness of characters by such explanations as germ contamination, variable potency, alternative dominance, or different types of latency, etc. It may well be true that there is variable dominance, but to what is this dominance due? It seems that many of the experiments in which variable dominance has been found and described were uncritical, and were carried on under the uncontrolled conditions of most breeding operations. In regard to variable dominance or alternative dominance, as a function of the gametic constitution of organisms, it is necessary that the operations should be carried on in such a way that surrounding or incident conditions are eliminated to the fullest extent; and experiments must be based, not upon one series, but upon parallel series of similar cultures. As far as Shull's (1908) attempt to explain this condition by various types of latency is concerned, there again the relatively gross conditions under which we are obliged to carry out most of our experiments leaves one open to criticism as to what the results observed were actually due. In other words, the question of dominance, as Bateson (1902) has suspected, is not entirely one of gametic constitution, nor is it one of external conditions, but it is a combination of the two, and this result seems to be fully borne out by the experiments cited in this paper.
Concerning the nature of the products which result from a given cross, much depends upon how dominance works and what it is that is present in the germ cells. The products may well be modified and lead to endless confusion by this very fact which I have established, of the determining and influencing of dominance by external conditions. Thus, for example, in Exp. No. H 410, external conditions determined the whole future history of that culture. By the conditions surrounding the germ cells at the initial cross the total character of the race was determined for as many generations as I cared to continue the experiment. If this is generally true, and I see no reason why it may not be, then the determination of dominance, which determines also the resulting products, is a most vital factor in evolution.
Again, the variability in products which one finds, as, for example, the differences which MacDougal (1905) found between crosses of Oenotheras made by him in New York and those made by DeVries in Holland, when they were using, as far as could be determined, identical material, has a very direct bearing upon this point. MacDougal says: "..., the very differences between the results of the hybridizations, as carried out in Amsterdam and New York, suggest that the manner in which the various qualities in the two parents are grouped in the progeny might be capable of a wide range of variation. Many indications lead to the suggestion that the dominancy and prevalency, latency and recessivity of any character may be more or less influenced by the conditions attendant upon the hybridization; the operative factors might include individual qualities as well as external conditions."
This at once suggests differences in results, and difficulties that will arise through the carrying out of like experiments under unlike conditions. This phase of the situation is set forth to some extent in the second part of this paper in the experiments in synthesis. These experiments, while not susceptible of analysis along certain lines, indicate very clearly that the operation of the principles of alternative inheritance will be productive of diversity of results under diverse conditions when using homogeneous materials. This gives a clue which may be of paramount importance in the further investigation of the problem of the origin of species in nature.
In many organisms there exists a physiological isolation, which, although the organisms may be close together, living in the same location, etc., by definite limitations in their reproductive activities prevents them from intercrossing. This may be due to incompatibility between germ cells, but quite often it is due to difficulties incident to copulation or to the penetration of sperms; that is, the difficulties are purely mechanical. Thus it might well rarely happen that an individual would arise of a character such that a cross could result; and if such a cross were made, a race having the attributes of one organism with the capacity for physiological isolation of the other, might easily arise and keep it from any further chance of intercrossing with other species.
The experiments and observations herein given warrant the general statement that conditions external to a cross are important factors in determining the results thereof. This conclusion has been worked out in both normal and hybrid crosses, in crosses between races which have been created selectively, and between forms which arose as sports; and the second series of experiments in synthesis is sufficient warrant for attributing to this factor a considerable importance in evolution.
Underlying these, there are, of course, deeper factors than those with which we are dealing. The characters which behave Mendelianwise are in the main superficial, unimportant attributes of the organism, and only rarely are they the characters which would make for success or failure in the struggle for existence; they are most often color and specific characters, which, while fixed and vigorous in their behavior, are not important in the economy of the organism. These behaviors, beyond any question as to how and why, suggest the operation of something which gives a result best described at present in factorial terms; and that there are such things as later ontogenetic factors seems highly probable in many cases and absolutely certain as regards many colors. The knowledge that we have concerning melanogenesis leaves us no alternative in this respect.
Back of all this, however, is the fundamental question of how these germ cells, how this living substance is constituted, and what is the relation in this complex of that which makes for the elaboration of tyrosin and tyrosinase in melanogenesis. What is it in this complex that localizes in a definite area the appearance of a pigment? What is it that combines into a definite pattern a series of attributes, some of which can be shifted and rearranged in the processes of hybridization? The problem of the constitution of the gametes of that which makes for form, for localization, for pattern, etc., is the fundamental problem; and as long as we fail to see clearly what the constitution of living matter is, such phenomena as these which we have been discussing must remain more or less superficial in our knowledge of the living organism.
There is nothing in the behavior of these attributes, in our ability to shift them and make new combinations, which, of necessity, commits one to any of the doctrines of preformation in pangenes or biophores, or to oneness of constitution and orthogenetic destiny. The situation, as regards alternative behaviors, should be free from the bias of biological orthodoxies, and to regard the organisms with which we are dealing as so many complex physical substances whose composition we are investigating, and among which we are seeking to determine the limits and laws of combination, will give the most rapid progress towards the end of a better understanding of the larger problems of the evolution of living substance.
THE UNIVERSITY OF CHICAGO.
April 1. 1910.