Breeding and the Mendelian discovery

Front matter

AND THE

MEN DELIAN DISCOVERY

A. D. DARBl SHIRE.

Preface

This book deals, as its title indicates, with the facts discovered by Mendel, and with the bearing of these facts, and of the theory put forward to explain them, on the science of heredity and the practice of breeding. It is intended to serve merely as an introduction to the subject. My conception of the most serviceable form of such an introduction has not been to place before the reader a sketch which takes in the whole range of discovery and speculation in this sphere of inquiry, but, rather, to open the door to an intimate familiarity with a few instances of the Mendelian phenomenon, and especially with those studied by Mendel himself. To this end I have given a fuller account of the phenomena observed by Mendel than has yet appeared in popular form : the seven pairs of characters studied by him are all figured for the first time ; and other results of his are illustrated by photographs from specimens which I have bred myself. But, in case the reader takes up the position, with which I heartily sympathise, of refusing to be satisfied with anything short of actually seeing the things which Mendel discovered,

I have given full instructions as to how such an

VI

PREFACE experiment as Mendel’s is carried out, together with the addresses of the seedsmen from whom peas bearing one or more of the fourteen characters studied by Mendel can be procured.

But although I regard a thorough knowledge of the Mendelian discovery as the proper foundation on which the attempt to breed, in the light of it, should be based, I have not limited myself to an attempt to impart this knowledge. I have also indicated the more important lessons which the practical breeder can learn from this discovery, and dealt with some of the more interesting biological questions which it raises, or helps to answer.

My endeavour, throughout, has been to eliminate from my picture of the things described everything with which these things have been invested by the human imagination ; in other words, to make my picture not the finished work of the post-impressionist, but the untouched proof of the photographer. And I hope that, in this way, I may do something, in my own sphere of interest, to counteract the prevalent tendency of that which is written, on any topic, to bear but a remote relation to what can actually be observed. This remoteness of words from actuality is especially characteristic of much of the literature on the subject of heredity^ which has accumulated during recent years. The rational imagination, in its

PREFACE

Vll flights in this sphere, has left the ground of facts below, and attained to altitudes which put the feats of airmen in the shade. I am not arguing that it is desirable that those writers to whom the ground is distasteful should cease to fly, but merely pointing out that, inasmuch as so much energy is at present devoted to enterprise of this kind, there is room for a book that will enable anyone to look closely at some of the facts which, though they constitute the ground over which a great deal of recent discussion has raged, are seldom known at first hand by those who witness, or even in some cases by those who take part in, these battles of the air.

My thanks are due, and are readily tendered to Professor de Vries for having taken the photographs which constitute Figs. 1 to 5 inclusive expressly for this book ; and to the trustees of the British Museum for permission to photograph the case of my mice which is reproduced in Plate II.

I should claim more credit than is properly mine if I did not acknowledge the help which I have received in the actual carrying out of the breeding experiments to which such value as this book may possess is in great part due. In connection with the breeding experiments with mice, now concluded, my thanks are especially due to Mr. Charles Biddolph

Vlll

PREFACE for much help which he very generously gave me. The task of harvesting and recording the peas, on the scale on which the experiment is now carried on, is made possible, and pleasant, by the assistance of Mr. Frank Sherlock ; whilst the fullness of the harvest itself is due to the energy and skill of my gardener, Mr. George Reeves.

Contents

CHAPTER

2. The Mendelian Phenomena of Inheritance ; the

Result of Crossing Tall with Dwarf Peas .

Chapter I

The practical breeder and the student of heredity both deal with the same natural process, but for very different reasons. The breeder’s object is to maintain or increase the value of the animals and plants which minister to the needs or please the fancy of mankind. The student of heredity aims at finding out how the characters of animals and plants are handed on from generation to generation. The breeder is not concerned with the interpretation of what he achieves ; he does not care how the characters of his stock are handed on from generation to generation, so long as the changes which he effects tend in the direction of an improvement. The student of heredity, on the other hand, is not concerned with the application of what he finds out ; he does not care whether the characters of his material are tending in the direction of an improvement, so long as he finds out how they are handed on from generation to generation. But there cannot, I think, be any doubt that both the practical breeder and the student of heredity would profit much by a knowledge of the methods employed and the results obtained by the other. It is a question which of the two would gain most : the man of science by the knowledge of the facts relating both to the successes and the failures of the breeder ; or the breeder by an acquaintance with the principles elucidated, and the precision of the records kept and the methods employed by the man of science. It is desirable that the student of heredity should, for the sake of his science, become interested in breeding. It is desirable that the breeder should, for the sake of his art, become interested in heredity. And, in general, it is desirable that each should recognise that he has much to learn from the other ; for though, as we have seen, the objects of their work are different, the problem which they are both investigating is the same.

My object in writing this book is to place within reach of the practical breeder some of the things found out by the scientific student of heredity, which are likely to be of service to him.

The methods of the breeder are two. They are, as a matter of fact, nearly always used in conjunction, and not separately ; but they are nevertheless essentially distinct. They are selection and crossing. Pure selection, operating on material which is not the immediate result of a cross, modifies the form of an animal or plant, and leaves it different from what it was when the selection began ; something has been added or taken away ; something created or destroyed. The primary object of crossing is to combine within one strain two desirable qualities existing in distinct strains. But an apparent result of crossing is very often a much more rapid production of something new than is brought about by selection ( see Plate II.). The novelty in such a case, however, is an illusion, and due to the recombination of characters which have been long hidden and long separated. Selection, then, creates something new ; crossing merely recombines characters which already exist. Very little — it may almost be said that nothing — is known of the causes which determine the origin of new characters, either in the domesticated state or in a state of nature. At any rate, there is not enough known to base a scientific practice of breeding upon. But the art of breeding by the recombination of already existing characters — and it is a question whether in its ultimate analysis this may not be all that the breeder can do — has been brought to a point of extreme precision by the discovery made by Mendel in 1865. And the object of this book is to make perfectly clear what that discovery was, and how it affects the practice of breeding. The manner of origin of new forms will be dealt with because a knowledge of Prof, de Vries’ observations on this process is necessary to an appreciation of the significance of Mendel’s discovery.

Ever since 1859, when Darwin effected the general acceptance of the doctrine of evolution, it has been believed that new forms have originated by the accumulation of the almost imperceptible differences between individuals, whether this has been brought about by the stress of circumstance or the skill of the breeder. In other words, the material upon which selection, both natural and artificial, has been supposed to operate has been assumed to be these imperceptible differences, the occurrence of which is referred to as the phenomenon of continuous variation.

This belief was attacked by Mr. Bateson in his “ Materials for the Study of Variation ” in 1894, and was severely shaken by the publication of Prof, de Vries’ “ Mutationstheorie ” in 1901. The belief embodied in both these works is that evolution has not proceeded by the accumulation of the differences presented by continuous variation, but by the differences which are furnished from time to time by what is called “ discontinuous variation ” by Mr. Bateson, and “ mutation ” by Prof, de Vries.

The point at issue between the Selectionist, who holds that evolution has been brought about by the selection of continuous variations, and the Mutationist, is this. It is admitted that species, as we see them, constitute a discontinuous series ; they are, with rare exceptions, distinct from and do not merge into the species most closely allied to them. The question is whether, when one species arises from another, the new species arises at one bound, i.e. in one generation, or whether it only comes into existence after all the imperceptibly small gradations, between it and the old species from which it sprang, have been traversed over the

FIG. 1 —CENOThERA LAM ARC KI AN A course of many generations. According to the former view, species cannot be said to arise by selection. If, when they have once arisen, they have defects which hamper them in the struggle for existence, where this occurs, they will doubtless drop out. But this does not constitute origin by selection. According to the latter view, on the other hand, the character of the new species is only attained by the traversing of the gradational stages between it and the parent one ; and inasmuch as it may be supposed that these gradational forms would not have left offspring if they had any defects which prevented them doing so, it may be said that the new species arises by a process of natural selection.

The view that species have originated by mutation is based on Prof, de Vries’ observations on the Evening Primrose ( (Enothera Lamarckiana) (Fig. 1). Working with this form, he was able to witness, for the first time, the actual process of the origin of new species.

For some time he had been searching in the neighbourhood of Amsterdam for plants which were giving off new forms ; but as he failed to find any he concluded that they were all in an immutable condition. At last he found a plant which appeared to be in a mutable state. This was (Enothera Lamarckiana . It was growing in a disused potato field near Hilversum, in Holland. This plant had escaped into the field from a bed in a park close by, where some annuals were grown every year. The (Enotheras were thickest at the corner of the field where they had invaded it ; and at this spot they formed a dense thicket of vigorous plants as high as a man. From this point outwards the plants became sparser, and the extreme edge of the area which they covered was occupied by an advance guard of seedlings and young rosettes.

Two new species were found in the field — (Enothera Icevifolia, characterised by its smooth, as opposed to crumpled, leaves and by its narrow petals, which do not overlap one another as do those of 0. Lamarck - iana ; and 0. brevistylis, characterised by the shortness of its style, which is so short that, when the flower is first looked into, it looks as if there were no style or stigma.

Seed was collected from the plants in the potato field at Hilversum and sown in the Botanic Garden at Amsterdam ; and rosettes — i.e. plants in their first year, which have not yet developed a stem — were also transplanted there. In this way there arose a series of pedigree cultures in which a number of other new species originated. Of these I will only refer to those which are figured in this book.

(Enothera gigas is the rarest and the finest of the new species which has arisen in the Garden at Amsterdam. It has only appeared three times ; and it is in every respect more vigorous and more heavily built than its parent 0. Lamarckiana.

The flowering spike is shown in Fig. 2 ; and its fruits, alongside those of 0. Lamarckiana , in Fig. 3. The following characteristics of 0. gigas can be made out from these figures. Its leaves are broader ; its

FIG. 2.— OENOTHERA GIGAS

Fig. 3 .—(ENOTHERA LAM ARC Kl AN A

(E. G1GAS fruits shorter but stouter ; its stem thicker ; and its petals broader, and consequently its buds fatter, than those of the parent species. The top of the stem is densely clothed with foliage ; and its appearance forms a striking contrast to the naked look of the top of the stem of 0. LamarcJciana. This feature is due to the greater breadth of the leaves, to the fact that they are bent downwards, and to the shortness of the internodes, especially at the extreme top of the stem ( see Fig. 3) in 0. gigas.

Its first appearance was as follows. In 1895 there was a crop of about 14,000 plants, constituting the fourth generation of the Lamarckiana culture. All the mutational forms had been transplanted from the crop, and most of the plants of 0. Lamarckiana had been thrown away. In the beginning of August about 1,000 of these plants were in flower, but many were still in the rosette stage. Thirty-two of the strongest of these rosettes were selected and planted out; they flowered in the July and August of the following year. One of them attracted Prof, de Vries’ attention by its thick stem, and by its large cup-shaped flowers. On August 10 all of its flowers were picked off, both the open and the withered ones, and the whole top of the plant enclosed in a paper bag to ensure self-fertilisation. It set a quantity of good seed. This plant was the parent of the new species 0. gigas. Its ancestors, for at least three generations back, were ordinary O. Lamarckiana. The numbers of seed-bearing plants in these three generations were successively only nine, six, and ten.

and the characters of 0. gigas are so striking that if one of these twenty-five plants had been a gigas it could not have been overlooked, as the seed-bearing plants were under continual and close observation.

The self-fertilised seeds of the original gigas were sown in 1897. They produced somewhat over 450 plants. All of them were exactly like their parent, with one exception, to be mentioned later ; and they constitute a type distinct, from the very outset, from 0. Lamarckiana. They bred true on their first appearance, and have continued to do so ever since.

GEnothera rubrinervis , which, unlike 0. gigas , is one of the commonest of the new species which arose from 0. Lamarckiana , is characterised, as its name implies, by the possession of bright red veins on the leaves and broad red stripes on the calyx and fruit. The flowering spike is shown in Fig. 4. The leaves are narrower than those of Lamarckiana ; but perhaps the most remarkable feature of this new species is the brittleness of the stem. If the stem of the flowering plant is hit, it breaks in two or three places with perfectly smooth surfaces of fracture. The cause of this brittleness is the extremely slight development of the bast fibres in the vascular bundles, in the stem. In all other respects rubrinervis is a healthy species, and it is the only one of the new species which produces as much pollen and seed as the parent species.

0. rubrinervis arose from 0. Lamarckiana sixty-six times in all, and not only did it always possess

Fig. 4 .—(ENOTHERA RUBR1NERV1S

(Slightly Enlarged.)

the same set of characters — the red colouring, the brittleness, the narrow leaves — but none of these characters ever appeared separately. It also, like 0. gigas , produces offspring, all of which are indistinguishable from itself.

(Enothera nanella. — This is the dwarf GEnothera. A whole plant is shown in Fig. 5. It does not attain to half the stature of the parent form. It arose 158 times during the whole course of the experiments, during which some 50,000 plants were raised, i.e. it appeared in a ratio of about 3 per cent. It also, like the other two new species already mentioned, came perfectly true to seed directly it had arisen.

0. nanella is distinguished from the other new species which I have described, first in the fact that it differs from its parent species by only one salient feature — dwarfness ; and secondly in the fact that this characteristic is commonly found in other genera of plants, as for instance the pea ( see Fig. 6), to mention one of a large number of plants which present this peculiarity. Professor de Vries has proposed to give the name “ varietal ” to characters like dwarfness, which are widely distributed throughout the vegetable kingdom, to distinguish them from specific characters which, according to him, do not recur in this way. Varieties are forms characterised by the possession of varietal characters, and species are forms characterised by specific characters thus defined. For instance, the complex of characters possessed by 0. gigas or by O. rubrinervis do not occur elsewhere in the vegetable kingdom, and these two forms are regarded as true species ; whilst the character of 0. nanella , which is regarded as a variety, not only occurs in the pea and in other plants, but also amongst the new forms which have arisen from 0. Lamarckiana. In fact the single exception referred to amongst the 450 plants raised from the first 0. gigas was a plant which had all the characters of 0. gigas , but possessed the dwarf habit of 0. nanella , and should be designated 0. gigas-nanella.

Professor de Vries has drawn this further extremely important distinction between specific and varietal characters. Specific characters, according to him, blend in inheritance; whilst varietal ones behave in Mendelian fashion when crossed. Indeed, he makes the behaviour of a particular character when crossed the test whether it is a varietal or a specific one. Professor de Vries has made a number of crosses between 0. gigas and other species of (Enothera . For instance, 0. gigas , crossed with 0. Lamarckiana , gives rise to forms which are a blend between their two parents, and moreover breed true to this intermediate character. The conclusion to which he comes in the paper in which he describes these results is stated as follows by him : “In all these respects, and in others as well, 0. gigas behaves as a true species when crossed, and not as a variety, as 0. nanella does when crossed with the same forms.” There are certain complications in the case of 0. nanella , but these need not detain us now. Full details can be found in the paper referred fig. s.—cenothera nanella

BREEDING AND HEREDITY n to below * by those who wish to follow the matter further. Having thus indicated the mode of origin and possible nature of characters which are inherited in Mendelian fashion, we may now proceed to a consideration of the Mendelian phenomenon itself.

Chapter II The Mendelian Phenomena Of Inheritance

THE RESULT OF CROSSING TALL WITH DWARF PEAS

Mendel published his results in the journal of a local scientific society in Briinn, in Austria, in 1865; but his paper remained unnoticed* until the year 1900, when attention was directed to it almost simultaneously by Correns, Tschermak and de Vries, the former two of whom had repeated some of Mendel’s experiments, and were thus able to attest the accuracy of his observations. I shall deal in Chapter XII. with the circumstances in which Mendel’s discovery was made, and with the probable causes which led to its being forgotten for so long ; and shall pass on now to a description of the Mendelian phenomena themselves.

The reader is probably familiar with two kinds of the edible pea (Pisum sativum), the tall and the dwarf. The tall peas grow to any height from three to six, or even eight, feet, and need to be supported with sticks ; the dwarf peas do not rise more than a foot or two above the level of the ground, and need not be staked.

The difference between a tall and a dwarf pea is

* With the single exception of a reference in Focke’s Pflanzenmischlinge , 1881.

FIG. 6— seedling of tall pea seedling of dwarf pea

The seeds were sown at the same time.

THE MENDELIAN PHENOMENA 13

not the same as the difference between a tall and a dwarf man. In a human dwarf everything is on a smaller scale than in the normal man. But a dwarf pea is not simply a miniature edition, as it were, of a tall one : it differs from a tall pea in one single characteristic, the length of the internodes, i.e. the sections of the stem between two nodes, or joints, where the leaves are given off. This is shown in Fig. 6, which represents a seedling of a tall, and a seedling of a dwarf pea sown on the same day. The number of nodes between the seed and the point of insertion of the two leaves which are face to face and enclose the growing point of the stem, is the same in both cases, namely five. They can all be clearly seen in the tall seedling, but the lowest one in the dwarf is difficult to make out on account of the shadow. The shortness of the dwarf pea is thus seen to be due solely to the shortness of its internodes.

If seed is saved from a dwarf plant it will give rise, when sown, to dwarf plants only ; and seed saved from tall plants will only produce tall plants. In other words, both tall and dwarf peas, when left to themselves, breed true.

It is desirable to state here that the pea is normally self-fertilised. The male and female organs and products are both present in every flower ; and what happens when a plant is “ left to itself ” is that the female products (or egg-cells) of a single flower are fertilised by the male products (the pollen grains) of the same flower. When one kind, such as tall, is crossed with another kind, such as dwarf, the pollen grains are transferred from the flowers of a tall plant to a particular part of the female organ of a dwarf plant, or vice versa. The details of this process will be described in Chapter X. The main point which I wish to be noted here is that the pea is always self-fertilised unless it is artificially cross-fertilised by the agency of man.

We have seen that dwarf plants, when left to themselves, set seed which produces dwarf plants only ; and that tails similarly breed true.

We will now consider the result of crossing a tall with a dwarf.

The plants which result from a cross between a tall and a dwarf are always tall. These tails are no shorter than their pure tall parents ; in fact, as a rule, they are a little taller, because, like most hybrids, they are very vigorous. They constitute the first hybrid generation.

These hybrid tails are allowed to self-fertilise. If the seed saved from them is sown, about three-quarters of the plants produced are tall, and about a quarter are dwarf. That is to say, in every four plants, three are tall and one is dwarf, on the average. This average result is shown in the genealogical table which forms the frontispiece, where it will also be seen that this generation — namely, the one which consists of three tails and one dwarf in every four — is called the second hybrid generation.

Let us now consider the result of sowing the seed saved from the plants composing the second hybrid generation. These plants, like those of the first

THE MENDELIAN PHENOMENA 15

hybrid generation, are allowed to self-fertilise. The seed set by the dwarfs produces only dwarfs. This is what we should expect, if we look at the tails and dwarfs in the genealogical table from the point of view of their stability in breeding, that is to say according as to whether they produce their own kind only, or both kinds, namely tall and dwarf. If we look back through the pedigree of the dwarfs of the second hybrid generation, we do not come to a dwarf until we reach the pure dwarf parent of the cross. This pure dwarf, as we know, breeds true. We might, therefore, expect that the dwarfs will breed true when they reappear in the second hybrid generation. And we see that they do.

Before we consider the offspring of the tails in the second hybrid generation, let us look back in their pedigree and see if we can form any idea as to what their offspring will be. In the case of the dwarfs of the second hybrid generation we had to go back two generations, namely, to the parents of the cross, before we met with an ancestor bearing the dwarf character. But in the case of the tails we have only to go back one generation — to the first hybrid generation — to find a tall. But this hybrid, although it resembles the pure tall externally, differs from it in its breeding properties. The pure tall produces only tails ; from the point of view of its breeding properties, it is stable. But the hybrid tall produces both tails and dwarfs, in the proportion of three tails to one dwarf in every four ; it is unstable. That is to say, if we look back through i6 the ancestry of the second hybrid generation, we find only one kind of dwarf — a pure, true-breeding or stable kind ; but we find two kinds of tails, a pure stable kind which produces tall offspring only, and a hybrid or unstable one which produces both tall and dwarf.

What, then, is the nature of the three tails of the second hybrid generation ? There are three possibilities. They may be all pure ; or all hybrid ; or, lastly, some of them may be hybrid and some pure. This latter is actually the case. Two of the three are hybrid, and produce tails and dwarfs in the ratio of three tall to one dwarf in every four. The remaining tall is pure, and produces only tails.

In the genealogical table we have supposed that each plant of the second hybrid generation produces only four offspring, because this enables us to put the average proportions of tails and dwarfs amongst these offspring most conveniently in diagrammatic form. The pure tall which produces only tails is seen at the extreme left of the second hybrid generation. In the middle are the two hybrid tails, each producing three tails and one dwarf on the average ; and at the extreme right is the dwarf, which produces only dwarfs.

Let us now glance at the actual proportions observed by Mendel himself. He does not say how many plants of the first hybrid generation he raised ; he only states that he made thirty-seven crosses between tall and dwarf. We do not, therefore, know how many of these were successful. His second

THE MENDELIAN PHENOMENA 17

hybrid generation consisted of 1,064 plants, of which 787 were tall, and 277 were dwarf, a very close approximation to a ratio of 3 to 1.

Ten seeds from each plant of a hundred only of these 787 tails were sown. Twenty-eight of these produced tails ; seventy-two produced both tails and dwarfs. This result departs considerably from the expected one. Let us examine it more closely. Of the three tails in the second hybrid generation, two should produce tails and dwarfs in the proportion of 3 to 1 ; one should produce tails only. This is one way of expressing the average result. Another way is to say that two -thirds of the tails in the second hybrid generation produce tails and dwarfs in the ratio of 3 to 1 ; and one-third tails only. Of a hundred tails in the second hybrid generation we should, therefore, expect sixty-six or sixty-seven to produce tails and dwarfs in the ratio of 3 to 1, and thirty-four or thirty-three to produce only tails, if the general statement I have made as to these ratios is true. It will, therefore, be seen that amongst the hundred tails of the second hybrid generation which Mendel tested, the number of pure tails was too low (twenty-eight as against thirty-four or thirty-three), and the number of hybrid tails too high (seventy-two as against sixty-six or sixty-seven). This brings us up against a very important question, namely, how close a fit between the so-called Mendelian expectation and the actual result must we expect ? The general rule is that the greater the numbers of individuals dealt with, in the experiment, i8 the closer will the actual result approximate to the Mendelian expectation. This is a very important question, which will be dealt with later.