Journ. of Gen. 11:
A GRAFT-INFECTIOUS DISEASE OF DATURA RESEMBLING A VEGETATIVE MUTATION.
By A. F. BLAKESLEE,
Station for Experimental Evolution, Gold Spring Harbor, N. Y.
(Received May 15, 1920.)
(With Plates II-VI.)
AT the New York meeting of the Botanical Society of America in 1916 the writer presented a paper entitled "A Recurrent Mutation in Datura Suggesting Vegetative Segregation." As reported at the next meeting of the Society, more recent experiments have shown that the peculiarity is in the nature of a disease since it can be transmitted by grafting upon normal stock. The variation involved, among changes in other morphological characters, a suppression of spines on the capsules. As observed by earlier geneticists, it had been assumed to have resulted from imperfect dominance of spininess when the normal type was crossed with a smooth-capsuled variety. In as much as the disease is transmitted through both seed and pollen and causes profound morphological changes in the individuals affected, it at first appeared to be genetic in nature. It seems appropriate therefore to publish the present paper in a journal of Genetics rather than in one devoted to phytopathology with the thought that results obtained in Datura may be of possible value to geneticists in explaining peculiarities in their breeding experiments with other species.
The Jimson Weed, Datura Stramonium1, is historically of interest to students of heredity, since before the appearance of Mendel's work on peas, Naudin (7) and Godron (6) had used this species in studies of inheritance and with it had discovered the reappearance of the contrasting parental types in the F2 generation. De Vries (8) and later Bateson and Saunders (2) used the same species soon after the rediscovery of Mendel's Law in studies on the inheritance of spininess and colour. The present writer with Avery (4 and 5) has also accumulated additional data on the inheritance of these and other characters.
1 A number of species have been established based on forms which differ genetically from D. Stramonium by single factors. Thus D. Tatula is the purple spiny form, D. laevis of Naudin and D. Bertolonii of Godron is the white, smooth-capsuled form. The contrasting pairs of genetic characters so far discovered are spiny and smooth capsules, purple and white flowers associated with purple and green stems, and many and few nodes between the cotyledons and the first fork. We have preferred to include under the name D. Stramonium all the forms which differ by single Mendelising factors as well as various named biotypes which differ from D. Stramonium at best in minor characters chiefly of leaf and habit.
Plants showing the disease have been called Quercina (abbreviated to Q.) on account of peculiarities in leaf structure. The first Quercina plant was found in cultures at the Conn. Agric. College in the fall of 1915. It was considered a spineless mutant of the purple-stemmed form. Two open-pollinated capsules had set, and two flowers later opened, which, since their stamens were devoid of pollen, were artificially pollinated by other plants. The seed thus obtained was the origin of a series of breeding experiments with the Quercina mutant as it was then called.
II. DESCRIPTION OF QUERCINA PLANTS.
In describing the distinguishing characteristics of Quercina plants it will be convenient to start with seedlings from the open pollinated capsules obtained in 1915. As will be shown under breeding experiments, Quercina plants do not breed true, but throw a small proportion of normals along with Quercina seedlings. The latter usually can be recognized at an early stage, sometimes as soon as the cotyledons have expanded. Pl. II, fig. 1 shows above three Q. plants in 3-inch pots in contrast with three below from the same pedigree which were considered normal at the time the photograph was taken. The early leaves of Q.'s tend to be narrower than normals, more or less twisted, and with indentations at the margin. Pl. II, fig. 2 shows the same plants at a later date but similarly arranged. The increasing indentation of the leaves in comparison with the normals is evident. The plant at the lower left-hand corner which previously was classed as normal has now shown its Q. nature by the character of its later leaves. At times, Q. seedlings appear normal till a relatively late stage, but in general a seedling which has the character shows it early.
A series of Q. leaves are shown on Pl. IV, fig. 5 below in contrast with a series of normal leaves above from the same line. The marked indentation of the leaves shown in the photograph is not an extreme condition. The leaves on some Q.'s may be eroded so far as to leave little more than the mid-ribs. If the Globe mutant, which has broad leaves, becomes Q., the leaves become narrower than typical Globes, but are still broader than normal narrow-leaved types. An increased indentation, however, is characteristic of all Q. leaves, whatever may be the genetic type to which the plant belongs.
The stature of Q. plants is somewhat less than that of normals. The branches are more slender and the root system is less well developed.
Typical Q. flowers are recognizable at an early stage. A series with buds is shown on Pl. III, fig. 3 in comparison with normal flowers below. The corolla of Q. flowers is split between the lobes as far down as the insertion of the filaments as shown in the opened flower on the extreme right from which the calyx has been removed. The corolla segments are incurved and twisted around the base of the flower. Frequently in the expansion of the bud they are unable to free themselves from the calyx. The third and fourth Q. buds from the left are typical of such "blind" flowers which may fall from the plant without opening. The infolding of the lobes is characteristic of early stages, and hence, when a normal bud would have its corolla nearly reaching the tip of the bud, the incurved lobes leave an empty space inside the apex of the calyx at the stage shown by the second bud from the left. The flowers of purple Quercinas are darker coloured than normals and the stem colour in such forms is also increased in intensity.
The stamens in the bud (Fig. 4) are found to be shrivelled, and in typical Q.'s produce nothing more than a few small grains which when tested have been found to be function1ess.
The stigma in normal plants is made up of two thick lobes, facing outward (Fig. 4). In Q. flowers the stigmatic surface is chiefly on the inside of the lobes and runs part way down two sides of the style.
The most conspicuous peculiarity of Q. plants, however, is the suppression of spines on the capsule. This may be complete, giving capsules as smooth as those of the inermis variety. The form of the capsule, however, is that characteristic of the variety or mutant affected, the globe mutant plants, if also Q., having flattened globose fruits. Seeds from Q. capsules are distinctly smaller than those from normals and their percentage of germination is less. The cotyledons are frequently unable to extract themselves from the seed coats without assistance and plants may die for this reason. Older seedlings also appear to be less vigorous than normals.
III. ORIGIN OF QUERCINAS IN THE FIELD.
The description which we have given of Q. plants is based on individuals which had come from Q. seed. The condition, however, breaks out in the field, generally late in the season on plants which have been produced by normal parents. The infection may be first noticed weakly expressed in a single branch, and gradually spreads as new growth occurs. Fig. 6 is a photograph of such a branch which has a normal spiny capsule at the lowest fork shown. The later capsules show a gradual suppression of spines to the one which is entirely smooth. Occasionally an infected capsule is found with one or more of the four valves smooth and the rest spiny as shown in Fig. 7 (insert, in Fig. 6). The two flowers of the infected branch, above and at the right, have the characteristic separation of the corolla into segments.
The presence of the disease may be first observed in the change in form of leaves and flowers. Purple flowers become darker in colour and often slightly mottled with lighter patches, before the infection has brought about a splitting of the corolla. As the season advances more plants show Q. branches. When normal plants have flowered and formed capsules, Q. plants are still flowering; for the Jimson Weed is normally self-pollinated in the bud, and Q. plants, from their lack of pollen, are dependent for setting capsules upon occasional off-pollination which is especially rare in such plants on account of the failure of many flowers to open.
Table I shows the number of Q. plants found in normal and Q. pedigrees in the field for the year 1916. In 1917 in the same field there were 1.67% spontaneous occurrences of Q.'s out of 5230 individuals planted in the ordinary manner. No Q.'s from seed were planted in the plot this year.
With only a single possible exception have Q.'s ever been observed to occur spontaneously in the greenhouse, where Q. plants as well as normals were under cultivation. Under greenhouse conditions plants are not grown as large as in the field. This fact would give a lessened opportunity for infection, but in the many thousand plants infection would have been expected in a few if the means of transmission were present. It is perhaps similarly because of the lack of means of transmission that Q. infections have been rare in our plots near the greenhouses, while a mile away in our hill plots they have been common. Thus, while in 1917 there were 1.7% in the 5230 plants in the regular hill plots, there were no infections in over 2000 plants in the plots near the station, including pedigrees planted close and far apart. There were at this time also 88 Q. seedlings planted in the station plot.
In that year representatives of certain of the pedigrees were grown in a "spread" plot with the individuals spaced five feet apart. Plants in these spread plots had an extremely high percentage of incidence (26.4% out of 167 individuals), while plants from the same pedigrees elsewhere in the field, but spaced the customary one foot apart, showed only the usual proportion of Q. plants (1.6% out of 643 individuals). The larger number of Q.'s in the spread plots may have been due to the relatively enormous size of these plants which were not reduced in growth by their neighbours. Each plant therefore gave a greater area exposed to infection, and infection once taking place was carried to all the numerous branches of the plant. Infection on the branch of a plant in the more crowded plots, on the other hand, was apparently unable to infect more than the few branches organically connected with it since plants adjacent to Q.'s and with their branches touching seem no more likely to acquire the infection than if adjacent only to normals. What the carrier of the virus may be is at present unknown but evidence beside that just given leads to the belief that the Q. disease is not communicated by mere contact.
Apart from its appearance in our cultures, we have found Q. plants upon two occasions. Once in Staten Island where several such plants were discovered, and once at Woodbury, Long Island, where a single Q. plant was found among a group of 182 normals. One out of five seedlings from the latter plant was a Q., but none of the four seedlings from the Staten Island material turned out Q. The writer has inspected relatively few Daturas growing wild. It is probable that a more careful search would show the disease not uncommon in wild plants.
IV. BREEDING EXPERIMENTS.
(a) Inheritance of Q. through Female Gametes.
While it was still thought that Q. was a genetic mutant, a considerable amount of breeding work was carried on combining it with all the available types of Jimson Weeds. No varietal immunity has been discovered within the species. From the four capsules produced by the first Q. plant discovered in 1915, 120 seedlings were produced, of which 53 were normal and 67 Q.'s. Breeding work with typical plants of the latter class was carried on in the greenhouse in 1915-16. Since the typical Q.'s are devoid of pollen it has been necessary to cross them with normals. Table I (p. 32) shows the results of such crosses from field records of 1916. Q.'s are produced in 79 0/ of the offspring of Q. female parents. The remaining normal plants from Q. parents do not carry the infection. The incidence of spontaneous infection in the field was even somewhat less for them (0.95%) than for normals free from Q. ancestry (1.26%)
Q. plants from Q. parents usually show their Q. nature at an early seedling stage. In 1916 the seedlings from Q. parents were labelled "Normal," "Questionable," or "Quercina" at the time of setting into the field from 3-inch pots. When the adult plants were inspected in the fall, their condition was compared with the earlier records on the pegs that remained. The results are tabulated in Table II (p. 32).
Plants classified as Q.'s in seedling stage are Q.'s when adult. The two exceptions noted out of 671 plants, if not the result of experimental error, may be due to faulty classification in the earlier record. It will be noted that in seedlings the Q. infection is determined by leaf abnormalities alone. Apart from these two possible exceptions we have not observed that the plants once infected ever recover. The infection often manifests itself, however, only after the plants have been transplanted to the field, as is shown by the 17% of apparently normal seedlings which later became Q.'s. There is usually little difficulty in distinguishing Q. plants out of infected seeds from field infected individuals, in which the character is evident later in the season and does not involve the older leaves. About as many "Questionable" seedlings turned out normals as there were normal seedlings which turned out Q. If all the questionables were included with the Q. seedlings they would have given in consequence a close approximation to the total Q.'s found in the adult stage.
Some of the plants put into the field died before final records were taken, and some of the labels were lost. The table includes only individuals with complete records. Of 77 plants not included in the table which died in the field, 24 were described on the pot labels as Q.'s, 51 as Questionables, and two only as normals. In other words 3.4% of the seedlings classified as Q.'s died in the field, 12.8% of those classified as Questionables, and only 0.7% of those classified as normals. The questionables were probably thus classified on account of not having developed far enough to allow a sure identification of their character. They were thus weaker plants when set out, hence their greater mortality. If we class all the Questionables with the Q.'s we have a mortality of 6.8% in Q.'s against 0.7% mortality in normals. Whatever the figures used, the infected plants are seen to be less likely to survive than normals. As will be noted later, the per cent, of germination of Q's is lower than that of normals, hence the final records fail to indicate the total amount of seed infection. The breeding evidence shows, however, that the Q. infection is carried by the female gametes and affects at least 79% of the seed produced by Q. parents.
(b) Investigation of Two Field-infected Plants.
In the 1916 field cultures two plants (A and B), which had become spontaneously infected and which showed the infection in different degrees in individual branches, were chosen for detailed study. The problem was to ascertain if any relation existed between the proportion of Q. offspring and the strength of the Q. character measured by the relative number of spines on the capsules from which the seedlings were derived. A smooth capsule was graded 0, and one fully armed was graded 4; 1, 2 and 3 were intermediate grades from very slightly spined to a condition with spines somewhat reduced below normal. Final records were taken of the potted plants between four and five months after planting. A considerable number of individuals were not graded on account of late germination, or on account of doubtful determination at the time it was necessary to clean out the greenhouse for other cultures. If such doubtful plants had been retained to a recordable size they would probably have increased the percentages of Q.'s. All plants that germinated however, have been included in figuring the percentages of germination. The capsules of plant B were all open pollinated, those of plant A were part open pollinated and part pollinated by hand. Tables III and IV (p. 33) give details for the two plants, and Table V (p. 34) gives summaries. Although some individual inconsistencies occur, it will be observed that the capsules which show evidence of infection give a higher percentage of Q. seedlings and a lower percentage of germination than capsules which appear normal.
A number of the capsules of plant A showed unequal distribution of spines on the four valves of the type shown on Pl. IV, fig. 7. Seeds from individual valves were sown separately with the result shown in Table VI (p. 34). Valves A and B form one carpel, and C and D form the other, of the two-celled ovary. In some cases only a part of the seeds was sown separately from the individual valves, and therefore the numbers in Tables III and VI are not in all cases identical. A correspondence between the percentage of Q. seedlings and the amount of spininess of valves from which they were derived is scarcely evident.
We conclude that the percentage of infection of the seed is only very roughly proportionate to the external evidence of the disease on the part of the plant from which they were derived and that the correspondence largely if not entirely disappears in reference to the divisions of individual capsules. In other words, if a capsule from a plant which is becoming Q. is fully armed and thereby shows no external evidence of infection, its seedlings are relatively low in Q.'s. On the other hand, if the capsule shows evidence of reduction of spines recognized by the grades employed, the seedlings are relatively high in Q.'s. So long as the capsule appears to be infected at all, the grade of spining has no close relation to the percentage of Q.'s in its offspring.
(c) inheritance of Q. through Male Gametes.
It has been shown that the Q. infection is strongly transmitted through the female gametes. The question arises as to whether it can be carried by the pollen. A difficulty lies in the fact that typical Q. plants are devoid of pollen: Occasionally, however, as a plant becomes infected in the field, some of the flowers may not be so strongly affected as the rest and in consequence may produce a varying amount of pollen. This was the case with five flowers of plant A already discussed. Table VII (p. 34) shows the proportion of Q. and normal offspring when the infection is carried by the male gametes. The pollen from flowers A 11, A 12 and A 15 was only slightly potent in transmitting the disease, as might be expected from the grade of their capsules, while the flowers A 7 and A x were more active. Unfortunately the record on flower A x is not complete. It is evident that these flowers which throw few Q.'s when selfed do not actively transmit the disease through the pollen, while A 7 transmits the disease actively both when selfed and when the infection is carried through the pollen. The high proportion of Q.'s from the self of A 15, the pollen of which was inactive, may have been due to an infection of the pistil which had not yet involved the stamens. Why pedigree 16655 should run so high in Q.'s is not clear unless a single anther of the flower was strongly infected and happened to be used in making the cross. It cannot be determined from our present records whether pollen was taken from individual anthers or mixed in making these crosses.
The experiments show that the male gametes can carry the Q. infection. Since Q. pollen infects the seed, it doubtless would infect the tissues of the plant when carried to flowers of normal plants. Insect pollination might be one means of the spread of the disease from lightly infected individuals with functional pollen.
V. INOCULATION AND GRAFTING EXPERIMENTS.
The spontaneous appearance of Q. branches on field-grown plants and the spreading of the character to new branches of affected individuals suggested that the peculiarity was in the nature of a disease. In testing this supposition a series of inoculation and grafting experiments were undertaken.
(a) Inoculation Experiments.
One of the commonest diseases of tobacco, another member of the Solanaceae, is "mosaic" or "calico" as it is called from the patches of lighter colour on the leaves. The causal organism is not known, but the infection is readily communicated from one plant to another by mere contact of healthy plants with infected leaves. Mosaic diseases of unknown nature have been discovered in other groups. It was thought that the Q. complex might be due to such a mosaic type of disease. Accordingly attempts were made to transmit the infection to healthy plants by rubbing their leaves with those of Q. plants, and by needle inoculations with the expressed juice of Q. leaves, but without success. In one experiment two cions from normal plants were left in the expressed juice from Q. leaves for 24 hours, and two other cions for 48 hours, before grafting them on to normal stock. Neither of these four cions took the disease nor caused the normal stock to turn Q. Transmission of mosaic diseases by contact in other species is often difficult of accomplishment and it is possible that more extensive trials would have given occasional success with the Q. disease of the Jimson Weed. However, mere contact of leaves cannot be a usual means of infection in nature, since in the field normal plants, adjacent to Q.'s and with their leaves touching, seem no more likely to be infected than those adjacent only to normals. The disease is easily transmitted, however, by grafting.
(b) Grafting Experiments.
Grafting of Q. branches on normal stock or the reverse union invariably has given infection. This has often resulted even when the Q. cion has died and dropped off before a perfect union has been established. Pl. V, fig. 8 is a photograph of a green-stemmed normal plant. Its two main branches were cut off a short distance above the fork and on the left branch was grafted a purple Q. cion, recognized by its eroded leaves, and on the right branch was grafted a purple normal cion. The infection passed down the one branch and up the other into the normal cion, causing the production of a capsule with a reduced number of spines and a later capsule entirely smooth. The buds in the stock below the fork also grew out with Q. characters, the one on the left shows a single capsule with reduced spines and one entirely smooth while the one on the right shows a single capsule likewise entirely smooth. The infection is usually manifest as soon as new leaves are formed and gradually spreads leaving the older parts, produced before the infection, free from abnormality and causing the new growth to be more and more distinctly Q. in appearance.
A number of other Solanaceous species, including several different genera, were tested by grafting with Quercina Jimson Weeds. None was found to be so susceptible to the disease as the Jimson Weed, however.
Datura meteloides could be infected by grafting, but the disease was slow in showing. The disease could be brought back to the Jimson Weed by grafting on it a cion of infected D. meteloides. In Pl. V, fig. 9 a cion of this species is shown grafted on the normal Jimson Weed at the left (marked Nor.) and a cion from the same plant was grafted on the Q. plant on the right (marked Q.). The infection of the D. meteloides shows as a mottling of the leaves with lighter patches and a puckering of the leaf surface. This was the condition three or four months after the graft. A few months later the leaves of the cion were free from abnormal colour and puckering of their surfaces, but had their margins deeply eroded. On such plants the flowers are devoid of pollen. Three of such infected flowers were pollinated from a normal plant of D. meteloides and selfed seed from the pollen parent were taken as a control. In comparison with the 10 individuals in the control, the 291 seedlings from the Q. capsules of D. meteloides were obviously stunted and their leaves of a paler colour. By the time of flowering, however, all of the 291 plants had recovered and showed no trace of infection except one, which had lacerated leaves, flowers without pollen, and somewhat eroded corollas. This plant, No. 18182 (1) is shown at the left in Pl. VI fig. 10 beside a normal plant from the same pedigree. The Q. disease is therefore not so readily transmitted through the seed in Datura meteloides as in D. Stramonium, although the former species acquires the disease slowly through graft infection.
Most of the species tested seem immune to the infection and unable to transmit the virus. The Petunia is a typical example. In addition to controls, eight attempts were made to infect this species by grafting with Q. Jimson Weeds, but without success. In one case, shown in Pl. VI, fig. 11, a Petunia cion was grafted on a Q. Jimson stock (Q.). A month later a cion from a normal Jimson (N.) was grafted on to the Petunia, leaving a distance of 16 cm. between the two grafts. Five and a half months after the last graft was made the Jimson cion at the top of the plant was still normal in appearance, and had produced two spiny capsules while the original stock was still Quercina. Twenty-six seeds from the second of these capsules were planted and produced 20 seedlings, all normal. It may be concluded that the Petunia neither acquires the disease, nor is able to transmit the virus through as much as 16 cm. of its stem.
An experiment similar to that just described for the Petunia was tried with the tomato. The virus failed to pass through 35 cm. of the tomato stem and infect a normal Jimson graft above it. It is possible that with a shorter distance the experiment would have succeeded, since, in one out of three trials, a cion taken from a graft on a Q. plant gave the disease to a normal Jimson when grafted on it. In this case, however, the cion was cut off from very near the junction between the tomato and the Q. stock, and it is barely possible that some of the tissue of the latter was the cause of the infection.
Both the two cions of Jerusalem cherry grafted on Q. stock produced normal flowers with pollen and fruit, but their leaves, although normal in shape and otherwise like the controls, were more or less blotched with yellowish patches. Of two grafts from these infected cions on to normal Jimsons, one transmitted the disease and the other did not.
The egg-plant gave some evidence of being susceptible to infection. Two cions were grafted on to Q. Jimsons. One grew but slightly and hence had little opportunity to show the disease. The other cion produced a vigorous growth, with many flowers with pollen and finally fruited. The leaves were normal in shape, but were puckered in a manner shown by the earlier stage of infection in Datura meteloides.
The only instance not already mentioned where a species showed any evidence of being susceptible to Q. infection through grafting was with D. Ceratocaula. Of two cions grafted on to Q. Jimson stock, one produced a flower without pollen and also two slightly abnormal buds which fell off before opening. These latter buds had their corollas more or less slit. Later flowers from this plant, however, were normal. Except for the abnormalities of these three flowers in a single plant the cions were normal in appearance.
In Table VIII are listed the species tested by grafting with Q. Jimson stock. Controls on normal Jimsons were tried in all cases.
At least one other mosaic-like disease has been found infecting Jimson Weeds. This has been called "Z." It was first noticed in two adjacent plants in the field cultures of 1917. Pl. VI, fig. 12 is a photograph of an infected individual. Infected plants are obviously 1iseased. The leaves are light in colour, mottled, more or less eroded, and strongly puckered resembling somewhat the badly diseased leaves of beans attacked by mosaic. The leaves may be reduced to merely the midribs. The capsules are deformed, with spines reduced or entirely absent. The buds are generally elongated, the flowers "confused" with corollas often split or otherwise malformed and with numerous accessory carpels frequently developed. Infection develops rapidly in the plant and soon renders it valueless for records or for the production of seeds. The disease was first serious in the field cultures, of 1919. From its spread from centres of infection it was obviously transmitted by means of contact. Attempts to communicate the disease to normal plants by rubbing them with infected leaves were successful with some exceptions. No extensive experiments have been made with this disease to discover what other Solanaceous species are susceptible. It has not yet been found possible, however, to transmit it to the tobacco (Nicotiana Tabacum) either by rubbing the leaves together or by grafting.
From smooth capsules of a Z plant 89 seeds were sown and gave 77 seedlings which remained normal. The experiments indicate that the Z disease is infectious by contact of leaves but is not carried by seed.
VI. EARLY RECORDS OF QUERCINAS.
It has not seemed profitable to search all the early records for descriptions that would indicate plants with Q. infection. Naudin (7) and Godron (6), as well as Bateson and Saunders (2), undoubtedly had them under observation. It may be stated that the writer has grown many thousand plants of the Jimson Weed, including large numbers of individuals that were heterozygous for inermis capsules, but has never observed any except Q.'s which showed a mosaic arrangement of the spines on the fruits, with some valves smooth and others more or less spiny.
Naudin (l.c., p. 49) reports in an F1 between an inermis variety (D. laevis) of D. Stramonium and the armed type, that while most of the plants had spiny fruits, others had fruits with reduced spines. Many of the capsules on three out of the 40 plants in this generation were very spiny on part of the surface while totally smooth on the rest. Naudin believed that they united thus by distinct and separate compartments the distinctive traits of the parental types (D. laevis and D. Stramonium). He calls this "hybridité disjointe" and cites in this connection the condition in the graft chimaera Cytisus Adami. He attempted to secure offspring from the smooth part of such a mosaic capsule, but due to the poor maturity of the seed only four seedlings were obtained. Of these, one was inermis, and may have been an extracted recessive or a strong Q. In a series of F2 plants from the same original cross, Naudin found six individuals out of 38 which again showed more or less well marked his "hybridité disjointe" and which were also presumably Q.'s.
Godron (l.c., p. 14) reports finding capsules partly spiny and partly smooth. He objects to Naudin's interpretation that the separation of the fruit into smooth and spiny portions is due to their origin from smooth and spiny parents, since he says that he has found this condition when both parents had spines.
Bateson and Saunders (i.e., p. 23) after discussing the intermediate colour of the flowers of the F1 between white and purple flowered forms say:
"The occurrence of intermediate forms was also occasionally noticeable in the fruits. Among the large number of capsules examined, there were some of the mosaic type, in which part of the capsule was prickly and the remainder smooth, while others, suggesting a blend, were more or less prickly all over, but the prickles were much reduced in size, and often formed mere tubercles. These mosaics occurred as rarities both on prickly individuals and on smooth ones still more rarely."
Further evidence pointing to the Q. nature of these abnormal capsules is given in their following statement
"Such intermediate fruits were most often found towards the end of the flowering season."
In a footnote (l.c., p. 24) they call attention to a plant with a single undersized smooth fruit in the F1 from the cross, purple inermis x white armed, where only armed plants heterozygous for inermis should be expected. If their suggestion of possible experimental error for the appearance of this plant be not the correct explanation it may have been a strong Q. which, both from competition with more vigorous normal individuals and from the difficulty of securing pollination, might readily be reduced to the production of but a single capsule. A single capsule on a normal plant in the field, however, would be rare under usual cultural conditions.
To one familiar with the manifestations of the Q. disease, it seems likely that the examples just given above from the literature are instances of infected plants rather than cases of blending or of so-called "mosaic" inheritance. The term "mosaic," however, may be applied to the disease, if not to the type of inheritance, since it resembles in many ways the mosaic of the tobacco, beans, and other forms for which a causative organism has not been discovered.
Mosaic diseases are not uncommon but their nature is obscure. A number have been described for the Solanaceae (cf. Allard (1)), and the mosaic or calico of tobacco is familiar to all who have grown the plant. The disease of the Jimson described in the present article differs from the others of the family in that it is carried by both seed and pollen and appears not to be transmitted artificially by mere contact or inoculation.
That grafting communicates the infection to normal plants relates the disease to infectious chlorosis of Abutilon Thompsoni and other forms investigated by Baur (3). It differs from such instances in that no vegetative function of the plant is obviously impaired. The strong Q.'s, which come from infected seed, although smaller than the normals, are as vigorous as many of the mutants. It also differs from infectious chlorosis of the mallows in that it is carried by seed.
The profound morphological changes brought about in the leaves and especially in the flowers and fruit are such that Q. individuals would be considered worthy of specific if not of generic separation if 100% of the seedlings instead of only 79% came true to the Q. complex. As the facts stand, however, there is much in the behaviour of Q. plants which suggests genetic phenomena.
1. A form called Quercina was discovered in the Jimson Weed (Datura Stramonium) occurring spontaneously like a mutation.
2. Quercina plants are distinguished from normals by greater dentation of the leaves, slitting of the corolla, absence of pollen, partial or entire suppression of spines on the capsules, and certain other characters associated with less vigorous growth.
3. In a single year's test about 1 1/4 % of the normal plants in the field took on the Quercina character by the last of the season. The Quercina character generally shows itself weakly in a single branch and gradually spreads to all the new growth.
4. The Quercina complex is transmitted by seed to about 79% of its offspring when pollinated from normal plants. The remaining 21% normal offspring do not produce Quercina seedlings in the next generation.
5. There is a rough correspondence between the strength of Quercina character in the parent and the number of Quercina plants in its offspring.
6. In plants which were becoming Quercina, pollen was obtained which transmitted the character.
7. Quercina cions grafted on to normal plants of the Jimson Weed cause the new growth of the stock to take on the appearance of Quercinas.
8. The cause of the Quercina characters is a disease transmitted by grafting.
9. Certain other species of the Solanaceae were found to be susceptible to the disease by grafting though to a less degree.
10. It has not been possible to infect plants artificially by rubbing with diseased leaves nor by inoculation of expressed juice from Quercina plants.
11. Another disease of the Jimson Weed is briefly described which is highly infectious by contact.
12. Instances in the literature are cited where the same disease apparently has been recorded but mistakenly attributed to a vegetative segregation or "mosaic" inheritance brought about by smooth and spiny varieties in the ancestry.
Quercina Jimson Weeds (Datura Stramonium). 1916 Records.
|Origin of Cultures||Number of
|(1) Washington, D.C. Inbred 5 generations:|
|(a) Purple Armed (2 lines)||13||596||584||12||2.01|
|(b) White Armed (1 line)||4||260||258||2||0.77|
|(2) Erfurt, Germany.
Inbred 4 generations:
White Inermis (2 lines)
|(3) Various Crosses (6 lines) within (1) and (2)||83||2845||2813||30||1.05|
|(4) Staten Island (4 lines)||28||770||748||22||2.86|
|(5) Bronx, N.Y. (4 lines)||4||694||689||5||0.72|
|(6) Cold Spring Harbor, N.Y. (1 line)||1||246||246||0||0.00|
|(7) All lines with Normal ancestry (15 lines)||139||5801||5726||73||1.26|
|(8) Normal Plants from Quercina Parents (1 line)||51||1781||1764||17||0.95|
|(9) Normal Plants
|(10) Quercina (1 line) crossed with Normals (5 lines)||34||1348||283||1065||79.01|
Comparison of Seedlings and Adults from Quercina Parents. 1916 Records.
|Totals||In Adult stage, 79.0% = Quercina; 21.0% = Normal.|
Offspring from Individual Capsules of Plant No. 1645 (91).
Offspring from Individual Capsules of Plant No. 1618
Summaries of Tables III and IV.
|Average Percentage Germination||Average Percentage Quercina|
|Grade||A||B||A and B||A||B||A and B||A||B||A and B||A||B||A and B|
Offspring from Individual Valves of Capsules of Plant No. 1645 (91).
|Totals||Grade 0=59.9% Q. : Grade 1=83.9% Q. : Grade 22=85.7% Q. : Grade 3=51.6% Q. : Grade 4=56.3% Q.|
TABLE VII. Transmission of Quercina through Pollen.
|16512||A 7 x A 7||Q. x Self||2||0||32||100.00|
|16624||16122 (4) x A 7||N. x Q.||4||11||86||88.66|
|16080||16122 (17) x A 7||N. x Q.||4||9||61||87.28|
|16679||16122 (17) x Self||N. x Self||4||89||0||0.00|
|16511||A11 x A11||Q. x Self||4||64||2||3.03|
|16659||16122 (13) x A 11||N. x Q.||4||25||0||0.00|
|16670||16122 (15) x A 11||N. x Q.||4||14||0||0.00|
|16510||A 12 x A 12||Q. x Self||4||52||3||5.45|
|16612||16122 (2) x A 12||N. x Q.||4||35||0||0.00|
|16628||16122 (5) x A 12||N. x Q.||4||86||6?||6.52|
|16626||1612 (5) x Self||N. x Self||4||50||0||0.00|
|16639||16122 (7) x A 12||N. x Q.||4||90||5?||5.26|
|16636||16122 (7) x Self||N. x Self||4||43||0||0.00|
|16655||16122 (11) x A 12||N. x Q.||4||30||31||50.82|
|16658||16122(13) x A12||N. x Q.||4||46||0||0.00|
|16673||16122 (16) x A12||N. x Q.||4||91||0||0.00|
|16441||1634 (11) x A 12||Q. x Q.||-||2||71||97.26|
|16501||1645 (65) x A 12||Q. x Q.||-||6||61||91.04|
|16509||A 15 x A 15||Q. x Self||4||38||23||37.71|
|16617||16122 (3) x A 15||N. x Q.||4||92||0||0.00|
|16615||16122 (3) x Self||N. x Self||4||44||0||0.00|
|16729||16169 (63) x A 15||N. x Q.||4||24||0||0.00|
|16742||16169 (69) x A 15||N. x Q.||4||91||0||0.00|
|16665||16122 (14) x Ax||N. x Q.||4||2||91||97.85|
|16662||16122 (14) x Self||N. x Self||4||43||0||0.00|
|16710||16169 (50) x Ax||N. x Q.||4||0||10||100.00|
|16709||16169 (50) x Self||N. x Self||4||29||0||0.00|
|1. Datura Stramonium (Jimson Weed)||many||Always infections (cf. text)|
|2. Datura meteloides||3||Infection, but slow in appearing; transferable back to Jimson (cf. text)|
|3. Datura ceratocaula||2||No infection evident, except three slightly abnormal flowers (cf. text)|
|4. Browallia demissa||5||No infection; cions fruited|
|5. Capsicum annuum (Pepper)||2||No infection ; one cion fruited|
|6. Lycopersicum esculentum (Tomato)||3||No infection; cions fruited; one out of three grafts back to Jimson caused infection|
|7. Nicotiana affinis||1||No infection|
|8. Nicotiana Tabacum (Tobacco)||3||No infection; clone fruited|
|9. Petunia hybrida||8||No infection; cions fruited (cf. text)|
|10. Solanum Pseudo-capsicum (Jerusalem Cherry)||2||Slight infection; back graft to Jimson caused infection in one case out of two (cf. text)|
|11. Solanum citrullifolium||2||No infection; cions with flowers and spiny burs|
|12. Solanum Dulcamara||3||No infection; cions fruited; back graft to Jimsons caused no infections|
|13. Solanum Melongena (Egg-plant)||2||One cion showed slight infection; one cion fruited (cf. text)|
|14. Solanum tuberosum (Potato)||2||No infection|
EXPLANATION OF PLATES.
|Fig. 1. Seedlings in
3-inch pots; upper row Quercinas, lower row normal controls.
Fig. 2. Later stage of same seedlings shown in Fig. 1 similarly arranged. Lower left control now seen to be Q.
|Fig. 3. Buds and
mature flowers of Jimsons; at extreme right calyx removed; upper row Q., lower
Fig. 4. Dissections of pistils and stamens; left row Q., right row normal; figure slightly enlarged.
|Fig. 5. Upper row
successive leaves from a normal plant; lower row, the same from a comparable Q.
Fig. 6. Branch of a field-infected Q. plant showing degrees of spininess from normal armed to almost entirely smooth.
Fig. 7. (Insert, in Fig. 6.) Capsule with one side smooth and other side spiny from field infected Q. plant.
|Fig. 8. Green-stemmed stock with purple Q. cion grafted
on left fork and purple normal cion grafted on right fork. Infection has caused
production of smooth capsules on the normal cion and on the two branches of the
stock below the fork.
Fig. 9. Cions of Datura meteloides grafted on to normal Jimson at left and on to Quercina Jimson at right. D. meteloides cion on Q. stock shows infection in crumpling of leaves.
|Fig. 10. Seedlings
from Quercina Datura meteloides; Q. at left, normal at right.
Fig. 11. Stock of Quercina Jimson supporting a cion of Petunia which bears at its apex a normal Jimson cion. The Petunia has failed to transmit the infection from the Q. stock to the normal Jimson cion.
Fig. 12. Jimson Weed attacked by a contact infectious disease (Z) of the mosaic type. Youngest leaves are reduced to the midrib.