Plant genetics

Front matter

(BERT R. MANY LIBRARY

AT ORNELL UNIVER

3 1924 074 276

Preface

There is developing an increasing demand for courses in genetics adapted to the needs of botanical students, and teachers of botany have asked repeatedly for a suitable text. A number of texts on heredity and evolution have appeared, but these have contained far too little genetics. On the other hand, there are several very thorough texts on certain phases of genetics, but these have been more in the nature of monographs than texts. They have focused on too limited a field to give the reader any general appreciation of genetics as a whole and frequently have been too difficult for the uninitiated to appreciate at all. WaLTER’s Genetics is an admirable elementary text, but is not exactly adapted to the needs of the young botanist. All texts, in fact, have emphasized animal genetics more strongly than the student of botany needs, except certain books on plant breeding, which, however, are general in nature and give little attention to theoretical genetics. In short, the subject has been changing so rapidly that no one has ventured to write a general authoritative text."

The group that the present book is intended to serve primarily comprises those who intend to make botany their profession and who, although not as yet specialists, t After the manuscript of this text had gone to the printer, there appeared a book by Bascock and CLAUSEN, entitled Genetics in relation to agriculture. This is ‘‘a general authoritative text,” for it contains a treatment of both plant and animal genetics, well suited to a thorough year’s course in genetics.

v vi Preface have had general training in the fundamentals of botany. Such students, for example, are commonly found in the last undergraduate year or first graduate year of their work with no distinct purpose to become geneticists, but wishing to be able to appreciate the important current work in plant genetics.

Such a group of young botanists became evident at the University of Chicago. In their attack upon current botanical literature they frequently encountered papers dealing with plant genetics, but through lack of preparation were unable to grasp their significance. This type of literature seemed too important to be relegated entirely to the specialist, and therefore the authors of the present text organized a course of lectures to meet the need. The purpose of the lectures was not to develop professional geneticists, but merely to initiate students of botany into the point of view of working geneticists, so that they could appreciate an. important phase of botanical literature. With such a purpose there was no attempt to give a complete presentation of modern genetics, but rather to introduce the student to genetics in the simplest way. As a consequence, for pedagogical reasons, certain perplexing facts were omitted, while others were slightly adjusted so as to convey the fundamental ideas without confusion. The result of the course was so successful as to suggest the desirability of putting the lectures into text form for the benefit of teachers who wish to profit by this experience. As a reference book it is entirely inadequate, much representative material having been omitted and only enough bibliography given to put the student upon the trail. As a textbook also it has two disadvantages: (1) it

Preface vii is avowedly not exact in some of the details; (2) it is adapted definitely to young botanists with fairly thorough elementary training. The excuse for the inexactness of certain details is the pedagogical necessity. The preparation of a text for the students referred to is explained by the fact that it represents a very important group which has not been provided for. In brief, the | book is neither a technical presentation of genetics nor a general text, but a course of general lectures adapted to a special purpose.

In connection with the laboratory phase of this course the authors wish to make grateful acknowledgment to E. M. East, ALBERT BLAKESLEE, R. A. EMERSON, and G. H. Suutt for very useful illustrative material.

J.M.C.

M. C. C.

CHAPTER

Contents

. INTRODUCTION

. THE INHERITANCE OF ACQUIRED CHARACTERS . MENDEL’s Law

VI. NEo-MENDELISM (Continued)

Chapter I Introduction

It should be realized that genetics is a natural outgrowth from the study of organic evolution. The conception of evolution began as a speculation, but became scientific in connection with the work of Lamarck and Darwin. In such work the method used was that of observation and inference. Facts were observed and an explanation was devised that would relate them. Resemblances and differences among species were noted, and it was inferred that these indicated degrees of relationship. It was assumed that closely related species must have had a comparatively recent common ancestry and that more distantly related species must have had a more remote common ancestry. Through comparisons of structure and of geographical distribution systems of phylogeny have been inferred, and an outline of the evolution of the plant and animal kingdoms has been the result. All of these conclusions are based upon comparison and inference.

This method reached its extreme application in the work of Darwin, whose observations included a range of forms and an extent of time unequaled by any preceding student of evolution. It may be said that in’ Darwin’s work the method of comparison and inference reached the limit of its possibilities. The students of evolution were chiefly concerned with explaining the changes that resulted in phylogeny. In other words,

I by comparison and inference they related certain forms and then tried to explain why these forms had become different. For example, Lamarck’s explanation of the changes was that they are the results of “use and disuse”; DARWIN’s explanation was that they are due to “natural selection.”

It was in 1900 that a new method for the study of evolution was announced with the appearance of The mutation theory by DE Vries. The new method was experimentation. Instead of comparing two species and inferring that one of them had produced the other, species were bred through successive generations, under rigid control, and were observed to produce new species. The old method inferred that a certain thing occurred; the new method saw it actually occurring.

In developing this experimental method the facts of inheritance began to accumulate. Presently the facts accumulated sufficiently to be organized into theories of inheritance, and the special field of genetics was the result. In brief, therefore, genetics is the experimental study of inheritance. Aside from the interest of genetics itself, its possible applications are even more important. For example, it will probably eventually explain organic evolution; and, most important of all considerations, it will probably enable us to control plant and animal breeding in a way that will be of the greatest practical importance in agriculture.

It should be realized that, in so complex a subject as heredity, increasing experimental work must greatly increase the range of known facts and make explanations increasingly difficult. The facts of inheritance obtained from experimental work with a few simple forms

Introduction 3 may permit satisfactory conclusions; but when greater numbers and more complex forms are included, conclusions become not only difficult but sometimes impossible. It is necessary, however, to organize facts, as they are obtained, into some consistent theory that will relate them to one another and that may be the basis of further investigation. Progress in any complex subject is marked by a series of explanations, each in turn proving inadequate as facts multiply, but each in turn helping to further progress. It should be understood, therefore, that in the subsequent chapters the theories presented are not to be regarded as final, but rather as suggested explanations of the known facts. It is certain that new facts will continue to be discovered and that explanations will have to be modified to fit them; but the present explanations are necessary for coordinating the facts we have. In other words, a sharp distinction must be drawn between established facts and proposed explanations. The former are permanent, the latter are temporary.

Chapter II Earlier Theories Of Heredity

It is probable that men have thought of heredity from the earliest times, but so far as is known there was no formulation of any definite theory of heredity before the time of ARISTOTLE. Since that time there are records of numerous theories of inheritance that may well be regarded as wild speculations. They were evolved with little or no basis of fact and of course are not to be regarded as scientific. Some of these early theories are interesting but not profitable for our purpose. They developed from superstition rather than from observation.

It is evident also that inheritance in man first attracted attention; later, animals were taken into consideration. The study of inheritance in plants is of comparatively recent development, due chiefly to the fact that sex was not observed in plants until late in the seventeenth century, and heredity was thought of only in connection with sex. Even after sex in plants was announced by CAMERARIUS in 1694 there was no general belief in the claim until much later. For this reason plant inheritance became a subject of observation and theory, and the history of plant genetics began long after the discovery of sex in plants.

The earliest distinct theory of heredity which falls within the period of modern biology was formulated by Darwin (1). This was published in 1868 in his book entitled Animals and plants under domestication and was called pangenesis. In a certain sense DARWIN apologized for pangenesis, realizing that knowledge had not advanced far enough for the construction of any adequate theory. The theory of pangenesis, therefore, is to be regarded more as a suggestion than as a formulation of belief.

The theory may be summarized as involving two theses which should be kept distinct. The first thesis, in Darwin’s words, is as follows: ‘The individual cells and organs of the whole organism are represented in every germ cell and bud by definite material particles. These multiply by division, and at cell division pass on from the mother cells to the daughter cells.”” This is the essential feature of pangenesis; that is, every germ cell (egg or sperm) contains dividing particles that in cell division pass on to every cell produced. This statement agrees in general with the facts recognized today under a somewhat different terminology, it being only necessary to call these particles chromosomes. This is remarkable when it is remembered that the statement was made fifty years ago.

The second thesis is as follows: “All the cells of the body throw off particles at various periods of their development. These reach the germ cells and hand over to them any characters of the organism which they may lack.” This is known as the transportation hypothesis and was an attempt to account for certain facts which seemed to indicate the inheritance of acquired characters. It is somewhat surprising to find DARWIN explaining heredity on the basis of the inheritance of acquired characters, for his theory of the origin of species by natural selection does not call for the inheritance of acquired characters.

The theory of pangenesis proper is quite in keeping with the present point of view; but the weakness of the transportation hypothesis was so evident that it was soon set aside by biologists. In discarding the transportation hypothesis, however, biologists in general rejected also the doctrine of pangenesis, a common result when a truth is found in combination with the obviously false.

The theory of heredity which was chiefly responsible for replacing pangenesis was proposed by WEISMANN (3), whose publications appeared during the decade—1880- 1890. WEISMANN developed two companion theories: one called germinal continuity, which has to do with heredity; and the other called germinal selection, which is a very imaginative explanation of individual variation. Our concern is chiefly with the theory of germinal continuity.

According to DARWIN’s transportation hypothesis, any change arising in the organism at any time during its life would become represented by gemmules in the reproductive cells. In this way it would be possible for acquired characters to be inherited. WrEISMANN could discover no mechanism in plants or animals which could justify such a conception.

It had been known for some time that germ cells and body cells in animals remain separate during their later development, but WEISMANN seems to have been the first to point out the significance of this fact. He makes the following statement: “The difficulty or the impossibility of rendering the transmission of acquired characters intelligible by an appeal to any known force has often been felt, but no one has hitherto cast doubt upon the very existence of such a form of heredity.”’ The general belief of the time that the inheritance of acquired characters was a fact came from two sources: (1) many examples of the inheritance of acquired characters were being reported; and (2) inheritance of acquired characters was thought necessary to explain evolution. WEISMANN therefore faced two problems: (1) to explain away the reported cases of the inheritance of acquired characters; and (2) to provide a theory which would make evolution possible without the inheritance of acquired characters.

He took up individually the many reported cases of such inheritance and discredited them one after another, showing convincingly how they could be explained in some other way. He also cited from his own experience numerous cases in which the inheritance of acquired characters was distinctly absent. As a result of his investigation of these cases, he developed his theory of germinal continuity, commonly spoken of as continuity of the germ plasm, a theory which is in good standing today. In the attempt to provide a theory which would make evolution possible without the inheritance of acquired characters WEISMANN distinctly failed. To explain variation, which is the basis of evolution, he proposed the theory of germinal selection, which is even more imaginative than Dar- WIN’s transportation hypothesis. As a consequence WEISMANN’S experience was much like that of Darwin. His theory of germinal continuity has fairly well stood the test of later investigation and is still current among biologists, while his theory of germinal selection has been practically discarded. A brief explanation of these two theses is as follows.

GERMINAL CONTINUITY.—A statement of this theory in the words of the author is as follows:

I believe that heredity depends upon the fact that a small portion of the effective substance of the germ plasm remains unchanged during the development of the egg into an organism, and that this part of the germ plasm serves as a foundation from which the germ cells of the new organism are produced. There is a continuity of germ plasm from one generation to another .. . hence it follows that the transmission of acquired characters is an impossibility, for if the germ plasm is not formed anew in each individual, but is derived from that which preceded it, its structure, and above all its molecular constitution, cannot depend upon the individual in which it happens to occur; but such an individual only forms as it were the nutritive soil, at the expense of which the germ plasm grows, while the latter possessed its characteristic structure from the beginning, namely, before the commencement of growth, but the tendencies of heredity, of which the germ plasm is the bearer, depend upon its molecular structure, and hence only those characters can be transmitted through successive generations which have previously been inherited, namely, those characters which were potentially contained in the structure of the germ plasm. It also follows that those other characters which have been acquired by the influence of special external conditions during the lifetime of the parent cannot be transmitted at all.

This is the theory of germinal continuity and it is in general agreement with the results of biological work today (see fig. 1).

GERMINAL SELECTION.—The purpose of the conception of germinal selection was to construct a theory of variation, and therefore of evolution, which does not involve the inheritance of acquired characters. When

Fic. 1.—Diagram illustrating WEISyANN’s theory of germinal continuity. Three generations are represented, with cells of germ plasm shaded, and those of body plasm unshaded; germ plasm continuous from generation to generation, carried over from parent to offspring by zygote (Z); impossible for body plasm to perpetuate itself into a second generation.

Io Plant Genetics the theory was being constructed the theory of natural selection was widely accepted. It should be understood that WEIsMANN did not criticize natural selection, but he did not believe that the individual variations upon which natural selection was based were to be explained as the inheritance of acquired characters. The theory of germinal selection, therefore, was intended to explain the origin of individual variations in some other way. The explanation of the theory begins as follows:

I believe that it is possible to suggest that the origin of hereditary individual characters takes place in a manner quite different from any which has as yet been brought forward... . . In the first place, it may be argued that external influences may not only act on the mature individual, or during its development, but that they may also act at a still earlier period upon the germ cell from which it arises. It may be imagined that such influence of different kinds might produce corresponding minute alterations in the molecular structure of the germ plasm; and as the latter is, according to our supposition, transmitted from one generation to another, it follows that such changes would be hereditary. .... Without altogether denying that such influences may directly modify the germ cells, I nevertheless believe that they have no share in the production of hereditary individual characters. .... Hereditary individual differences must therefore be derived from some other source. I believe that such a source is to be looked for in the form of reproduction by which the great majority of existing organisms are propagated, sexual reproduction. .... The object of sexual reproduction is to create those individual differences which form the material out of which natural selection produces new species.

WEISMANN forestalls a criticism which was sure to come by saying that the opinion has already been expressed that deviations from the specific type are rapidly destroyed by the operation of sexual reproduc-

Earlier Theories of Heredity II tion. Such an opinion may be true with regard to specific characters, because deviations from the specific type occur in such rare cases that they cannot hold their ground against the large number of normal individuals, but the case is different, as he claims, with those minute differences which are characteristic of individuals, because every individual possesses them, although of different kind and degree. He states that cross-breeding between all the individuals of a species is impossible, hence the obliterating of individual differences is impossible.

According to WEISMANN, therefore, individual variations, and therefore the possibility of natural selection, originate in the sex process. Sex reproduction originates no absolutely new characters, but merely makes possible innumerable combinations of characters. It is evident that the pairing individuals must differ to some extent, if the result is to be a new combination of characters.

This raises the question of the origin of differences in the first sexual individuals. Sexual reproduction was derived from asexual reproduction, and, according to the theory, must have begun with variable material. How did these individual differences arise? WEISMANN offers the following explanation. He had stated that environment has no modifying influence upon the germ plasm in general, but says that “in the lowest one-celled organisms the case is erttirely different.” In this case, as he puts it, ‘parent and offspring are in a certain sense one and the same thing,” and germ plasm and body plasm have not been differentiated. In this case, therefore, variations induced by external conditions are hereditary because the germ plasm is the body plasm.

In this way individual differences arose in one-celled organisms; and when the sexual method appeared it found varying individuals ready to work upon. The mission of sex, therefore, according to WEISMANN, was to lay hold of these variations and multiply their combinations, thus providing a wider range of choice for natural selection. In this way he explains the origin of individual variation without the inheritance of acquired characters.

WEISMANN also devised a mechanism by which sexual reproduction results in multiplying combinations of individual differences. This mechanism is the basis of his theory of germinal selection and is as follows.

In WEISMANN’S time chromosomes as such had not been recognized, but they were represented in his terminology by the term idants. He conceived of each idant as composed of one or more units called ids. An id stood for a complete individual, each id being able to control the complete development of an individual. A child received both paternal and maternal ids in equal numbers; that is, father and mother were each represented by a 50 per cent influence on the offspring; each grandparent would be represented by 25 per cent of the ids in an individual; and so on, indefinitely. Asa consequence, all existing individuals must now contain as many different kinds of ancestral ids as they are capable of containing Sooner or later, owing to the accumulation of ids, ‘“‘sex reproduction can proceed only by reduction in the number of ancestral ids, a reduction which is represented in every generation.”

This is WEISMANN’s explanation of the grosser mechanism of heredity, but it was only preliminary to the theory of germinal selection. In the first place he imagined that each id is composed of determinants, which are ultra-microscopic, each of them representing, not the whole body structure as does an id, but all the characters that belong to a particular kind of cell. Each determinant in turn was conceived of as composed of still smaller units called biophores, each of which represented a single character of a cell. Biophores have the capacity to assimilate food, to grow, and to reproduce themselves in the germ plasm. This imaginary structure of the cell is spoken of as WEISMANN’S cell architecture, in which biophores are built into determinants, determinants into ids, ids into idants. In fact, this is a remarkable theoretical analysis of the structure of a chromosome.

Selection is conceived of as occurring in the following way. After fertilization the biophores are too numerous for all of them to obtain expression, and the result is a struggle for existence, a sort of ultra-microscopical warfare. As a consequence some biophores perish and others survive and are perpetuated. This struggle of the biophores for nutrition, and the almost infinite combinations that might enter into the structure of the resulting determinants, would open up a wide field for variation. This is the theory of germinal selection, a struggle and a survival of fortunate biophores, and it may well be regarded as fantastic. It has been discarded, not only because it is without foundation in fact, but chiefly because it is beyond the reach of experimental testing.

What is called Weismannism had a very wide influence for a number of years, and the continuity of the germ plasm, with its result on the doctrine of the inheritance of acquired characters, is still current. The first result of Weismannism was to overshadow DaARWwIN’s theory of pangenesis, which had unjustly shared the deserved fate of its companion theory, the transportation hypothesis. Dr Vries (2) was the first to recognize this injustice, and in 1889 published his Intracellular pangenesis. Without attempting to defend the transportation hypothesis, DE Vries showed the real value of the doctrine of pangenesis. He claimed that his theory of intracellular pangenesis is little more than a restatement of the fundamental ideas of DARwIn’s pangenesis, but in fact it contains enough new material to justify a definition.

The material particles which DE Vries conceived of as the carriers of heredity he called pangens, the equivalent of DARWIN’s gemmules. He claims that in the nucleus every kind of pangen of the individual is represented, while the cytoplasm in each cell contains only those pangens that become active in the cell. He concludes that, with the exception of the pangens concerned in nuclear activities, such as nuclear division, all pangens have to leave the nucleus in order to become active. But most of the pangens of almost every kind are represented in the nucleus, where they multiply, partly for the purpose of nuclear division, partly to pass into the cytoplasm to engage in other activities. These pangens not only pass out into the adjacent cytoplasm, but are also carried by the protoplasmic currents into the various organs of the protoplast, where they multiply and become active. All protoplasm consists of such pangens derived at different times from the nucleus, together with their descendants. Hence the theory is called intracellular pangenesis, as contrasted with Darwin’s body pangenesis, that is, distribution of the heredity particles in the cell as contrasted with distribution through the body as a whole.

De Vries formulated this theory before he had conducted the breeding experiments that led to his theory of mutation, and afterward he applied the theory to his experimental results in breeding.

1. DARWIN, CHARLES, The variation of animals and plants under domestication. London. 1890.

2. DE Vries, Hueco, Intracellular pangenesis. Chicago. 1910.

3. PouLToN, SHONLAND, SHIPLEY, WEISMANN, on heredity. Clarendon Press. 1891.

Chapter III The Inheritance Of Acquired Characters

The inheritance of acquired characters has been mentioned in its relation to WEISMANN’s theory of germinal continuity, but it deserves a somewhat fuller consideration. The idea was first developed by LaMARCK in connection with his theory of evolution, the so-called theory of appetency, or the effect of use and disuse. FRANCIS GALTON, in 1875, was one of the first to express skepticism in regard to such inheritance, but it was WEISMANN who was most influential in combating the idea. After WEISMANN’S presentation of the situation, biologists were divided into two camps in reference to the inheritance of acquired characters: (1) neo- Lamarckians, who affirm belief in the inheritance of acquired characters, and (2) neo-Darwinians, who deny it. Geneticists and embryologists, however, whose work brings them most in contact with the problem, seem to be fairly well agreed that acquired characters are not inherited.

Much of the lack of agreement in this controversy is due to the definition of an acquired character. It should be kept in mind that actual characters are not inherited, but only the determiners, which regulate the way in which the organism reacts to its environment. For example, when it is said that a child inherits its father’s nose, the statement is not meant to be literally true; it is meant that just as there was something in the body of the father that was responsible for the development of a particular type of nose, so there was a similar something in the child’s body that produced a similar result. It is merely a matter of convenience to speak of the inheritance of characters.

WEISMANN defined an acquired character as “any somatic modification that does not have its origin in the germ plasm.” This definition is not always easy to apply. Examples of acquired characters in the Weismann sense are mutilations, effects of environment, results of function (as in the use or disuse of certain organs), and many diseases that affect the bodily mechanism. WEISMANN gave three reasons for rejecting the belief in the inheritance of such characters: (1) there is no known mechanism by which somatic characters may be transferred to the germ plasm; (2) the evidence that such a transfer does occur is inconclusive and unsatisfactory; and (3) the theory of the continuity of the germ plasm is sufficient to account for the facts of heredity. This last reason has been discussed, but the other two should be considered.

When WEISMANN says that there is no known mechanism by which somatic characters can be transferred to the germ plasm, to him it is equivalent to saying that it is hard to see how the water that has gone over the dam can return and affect the flow of the water upstream. He assumes, of course, that the germ plasm is isolated from the somatoplasm very early in the development of the fertilized egg into an individual, and that when it is isolated it takes no active part in the history of the body (see fig. 1). The somatoplasm is thus merely a carrier of the germ plasm and is unable to affect the character of it any more than a rubber hot-water bag, although capable of assuming a variety of shapes, can affect the character of the water it contains (WALTER 6).

Opponents of WEISMANN object to any such view of the complete isolation of germ plasm. Studies of cell lineage in animals have shown that germ cells are set apart very early in the development of the individual, and that they certainly are not derived from distinctly somatic cells. No such statement, however, can be made in reference to plants. Germ cells in plants are derived from epidermal or hypodermal cells, which previously were distinctly somatic; and furthermore germ cells have been induced by experiment to form from almost any living tissue which is as distinctly somatic as any plant tissue can be.

Another objection to WEISMANN’S view is that every organism is a physiological as well as a morphological unity, and that cells completely insulated in such a unity would be impossible. Cytologists also have come to believe that there are protoplasmic connections between adjacent cells in practically all plant tissues, and in general physiology tends to confirm this. All of this means a growing belief that the somatoplasm can affect the germ plasm.

The reply of the Weismannians is that even though somatoplasm can affect germ plasm in this general physiological way, this is a very different thing from the ininheritance of some definite acquired character. To be inherited such a character would have to be exactly redeveloped in the germ plasm, and the influence referred to cannot be so specific as that. This of course is a theoretical answer and the question can only be decided by experimental work. A theoretical rejoinder to this answer may be suggested. It is like the voice in a telephone transmitter, which starts vibrations that make the receiver repeat the voice.