GENETIC AND CYTOLOGICAL STUDIES OF MAIZE Barbara McClintock The mode of action of controlling ele- ments in maize, a main topic of recent re- ports, has continued to be studied during the past year. Attention has been directed particularly to the system of elements of which Spm (Suppressor-mutator) is a com- ponent. This system, which affects the ac- tion of genic substances at the A, locus in chromosome 3, has been described in previ- ous reports and is referred to as the g,"1- Spm system. It has been concluded that a"~ arose by insertion of an element, be- longing to the system of which Spm is a component, at the standard A, locus. This element directly affects the action of the gene substance at 41, altering it in a recognizable way, and the modified A, locus has been designated a”, It re- sponds in readily detectable ways to the presence of the independently located ele- ment Spm, and one of the responses effects changes in gene action in both somatic and germinal cells. The Spm element may undergo change in location within the chromosome complement by a_ process termed transposition. Methods of detect- ing transpositions of controlling elements have been discussed in recent publications. One method takes advantage of the link- age relation between the element and a given genetic marker; transposition of the element to a new location alters this rela- tion, and such alterations are easily de- tected. Two tests utilizing this method were conducted with Spm during the year, to reveal more about the degree of stability of its location. They will be summarized below. The tests determined the numbers of Spm elements in different parts of indi- vidual plants, and their locations in the chromosome complement. In each test, the examined individuals represented the prog- eny of a single plant that carried one Spm element at a known location. It was possi- | ble to gain some information about the frequency of occurrence of transposition of Spm and the period during develop- ment when it takes place; but in these re- spects the results of the two tests were quite different. One gave evidence of fre- quent transposition of Spm, occurring early in development. The other showed infrequent transposition, and thus indi- cated a considerable degree of stability of location. Although transposition of Spm appears to be under genetic control, the factors and conditions associated with it have not yet been recognized. The first test involved progeny of a plant in which Spm was carried in chromosome 9. This plant was a."*/a, (chromosome 3), Wx/wx (chromosome 9); and it had one Spm element in chromosome 9, as the testcross results entered in table 7, A, indi- cate. Twelve variegated plants, derived from variegated kernels in the Wx class of the first ear of this plant (see table 7, A, row 1, column 5), were tested for Spm number and location. The silks of all fertile ears produced by each plant received pollen from plants that were homozygous for a." and wx and had no Spm. Table 6 records the number of ears ob- tained from each plant and their positions on the plant, the Spm constitutions of the cells that produced these ears, and the link- age relations of Spm to Wx. All together, twenty-six ears were obtained from these 12 plants. In 1 plant, the cells that gave rise to the ear on a tiller (side branch) had no Spm, but in the cells that gave rise to the remaining twenty-five ears one or two Spm elements were present. In sixteen of 394 the ears, one Spm was present and was linked with Wx. In five ears, two Spm elements were present and one of them was linked with Wx. In four ears, one Spm was present, but it was not linked with Wx. The kernel types appearing on these three classes of ears are entered in B of table 7. It should be noted that the cells which produced the first and second ears on the main stalk, in the 5 plants from which such ears were obtained, had the CARNEGIE INSTITUTION OF WASHINGTON in number of this element in different plants and in different parts of the same plant. The second test of stability of location of Spm was made with the progeny of a plant carrying an Spm element located close ti, Y in chromosome 6. This particular loca. tion of Spm was detected initially only in one plant of a culture. That plant was a." /a."™", Y/y in constitution; and the silks of one of its ears received pollen from TABLE 6. Spm Constitution and Location in Different Plants of a Culture and in Different Parts of Individual Plants Plant No. No. of Ears ae ituti . Position of Ear Spm Constitution and ” rad Tested P cr in Plant Linkage with Wx A-6, B-1, and B6....... 1 Ist ear, main stalk 1 Spm; linked with Wx B4 oo eee 1 Ist ear, main stalk 2 Spm; one linked with Wr AS cece 2 Ist and 2nd ears, main stalk 2 Spm; one linked with Wx (both ears) B-2 and B-5 ........... 2 Ist ear, main stalk; tiller ear 1 Spm; linked with Wr ’ (both ears) Al oo, 3 Ist and 2nd ears, main stalk 1 Spm; linked with Wx Tiller ear 1 Spm; not linked with Wr AB eee 3 Ist and 2nd ears, main stalk 2 Spm; one linked with Wr Tiller ear 1 Spm; linked with Wx A4# ee, 3 Ist and 2nd ears, main stalk 1 Spm; not linked with Wx Tiller ear (all three ears) AQ occ eee 3 Ist ear, main stalk 1 Spm; linked with Wx Ear on one tiller 1 Spm; linked with Wr Ear on another tiller No Spm AT occ cece cee aes 4 Ist and 2nd ears, main stalk; ear on each of two tillers same Spm constitution. In 3 of the 7 plants from which tiller ears were obtained, how- ever, a difference was expressed between the cells of the main stalk and those of a tiller with respect to Spm constitution and location. The results indicate that in the plants of this culture the Spm element underwent frequent change of location in the chromo- some complement. The time of change was either late in the development of the germi- nal cells in the parent plant or early in development in the progeny plants. The transposition mechanism will account not only for the changes in location of Spm but also for the observed losses or increases 1 Spm; linked with Wx (all four ears) a plant that was homozygous for a:"° and y but had no Spm. The resulting ear was sectorial, in that a small sector at its base was composed of 47 kernels (21 Y:26 y) derived from cells in which Spm was absent. The cells producing the larger part of the ear carried one Spm element. Among the 329 kernels in this part of the ear, 167 had no Spm; 10 of them were Y and 157 were y. The remain- ing 162 kernels carried Spm; 153 were Y and 9 were y. Close linkage of Spm with Y was evident, for only 5.6 per cent recom- binants appeared among the 329 kernels. All fertile ears produced by 17 plants derived from the Y, Spm class of kernels DEPARTMENT OF GENETICS 395 TABLE 7. Phenotypes of Kernels on Two Ears of 1 Plant (A), and on Twenty-Five Ears Produced by 12 Plants in Its Progeny (B) Kernels in A derived from cross of 2 a,"1/a,, Wx/we x & a""1/a", wx/wx, no Spm; in 3, from cross of 9 a,"*/a,""! or a,™"1/a,, Wxfwx X Safa, wx/wx, no Spm. Phenotype of Kernel No. and Location of “eee Color Pale Color Sener Total No. Spm in 2 Parent mutation) (no Spm) (Spm present) of Kernels Wx wx Wx us Wx wx A | Spm; linked with Wx........ 0 1 26 196 197 18 438 0 0 38 81 89 29 237 B | Spm; linked with Wxr........ 1 0 418 1539 1512 356 3826 * > Spm; one linked with Wx... 0 0 79 267 594 323 1263 | Spm; not linked with Wx.... 0 0 190 168 140 174 672 * 20.2 per cent are “recombinants.” received pollen from plants that were homozygous for a: and y but had no Spm. One ear per plant was obtained from ears, close linkage of Spm with Y was ex- pressed; only in the ear produced by a tiller of one plant was there no evidence } plants, two ears per plant were obtained trom 4 plants, three ears per plant from 7 plants, and four ears per plant from 3 plants. All together, forty-four ears were produced by these 17 plants, and in all of them the cells that gave rise to the ear carried one Spm. In forty-three of these of linkage of Spm with Y. Table 8 records the phenotypes of the kernels that ap- peared on the forty-four ears. In contrast to the first-described test, this test revealed a case in which the Spm ele- ment showed a considerable degree of stability of location. As was mentioned TABLE 8. Phenotypes of Kernels Appearing on Forty-Four Ears Produced by 17 Plants Having the Constitution 4,%-1/a,"1; Wx/wx When Pollinized by Plants Homozygous for a," and wx and Having no Spm A. Phenotypes appearing on the forty-one ears produced by 15 of the plants. B. Phenotypes appearing on a partially sterile ear of one plant. Reduction in transmission of the Y-carrying chro- mosome is evident. C. Phenotypes appearing on the two ears produced by one plant; (1) main ear, (2) tiller ear. Phenotype of Kernel A Colorless with Ear ( germinal ‘Gs Sony Spots of 4, Total mutation) Anoepm) (Spm present) Y y Y y Y y Meee eee ee ee 2 2 247 4708 4551 192 9702 * Boece eee ee eee 0 0 1 91 20 1 113 CO) cece cee eee 0 0 25 203 171 11 410 C2) lee eee 0 0 65 47 48 59 219 * 4.5 per cent are “recombinants.” 396___ CARNEGIE INSTITUTION OF WASHINGTON earlier, however, nothing is yet known about the genetic or other factors that may control the time during development of a tissue when changes in location of Spm will occur, or the frequency of such changes. Types of Spm Elements The observed effects of Spm control over gene action at a:"* have been remarkably consistent, notwithstanding the various dif- ferent locations in the chromosome com- plement the element is known to occupy. Sometimes, however, the action of Spm becomes altered, producing modified types of control of a" expression. One modi- fication, which arises rather frequently, will be considered here. Occasionally there will appear, on an ear of a plant carrying ai” and Spm, a kernel with an aberrant phenotype. Instead of many deeply pig- mented spots in a colorless background, this kernel may have only one or several tiny dots of deep pigmentation in a color- less background. Plants were grown from several kernels of this type, and they and their progeny were examined to determine the nature of the change responsible for the altered phenotype. Tests showed that in such cases an Spm element is present, but its capacity to suppress gene action at a," and to induce mutation at that locus is much weakened. It has therefore been given the symbol Spm-w. In this part of the discussion, the standard Spm element will be designated Spm-s to distinguish it from the Spm-w element. Spm-w elements have been found in sev- eral different chromosomes of the comple- ment. The one to be considered here first appeared in a single kernel on an ear pro- duced by a plant carrying Spm-s in chro- mosome 5. This kernel showed only a few dots of deep pigment in a colorless back- ground. The plant that developed from it was pigmented throughout, in contrast to plants carrying Spm-s, which have streaks of deep pigmentation in a nonpigmented background. Tests revealed the presence in this plant of an Spm-w element, located in chromosome 5, which showed the same value of recombination with Pr as the Spm-s element in the parent plant. Part of the progeny of the plant was ex. amined, in turn, and tests of the different types of individuals it contained made it possible to define the weakened action of the Spm-w element. The reduced capacity of Spm-w to suppress gene action at a," is shown by the appearance of anthocyanin pigment throughout plants that carry it, The development of this pigment, how- ever, is very much retarded in comparison with that in plants that have no Spm ele- ment. Plants with Spm-w become fully colored only at late maturity, whereas plants with no Spm develop pigment «at early stages. The weakened capacity of the Spm-w element to induce mutation at a" is also shown by the phenotypes of kernels that carry it. Either mutant spots are totally absent, or just one or a few spots appear. Table 9 has been constructed to show the linkage of this Spm-w element with )’r. When both Spm-s and Spm-w are presen in the same plant, Spm-s is epistatic to Spm-w. Both elements segregate normilly at meiosis, however, as illustrated by the kernel types on the ears of the testcross recorded in table 10. Each of the 5 plants used in this testcross carried Spm-s in chro- mosome 6, closely linked with y, and Spm-w in chromosome 5, linked with Pr. Another type of Spm element has re- ceived preliminary examination, but de- scription will be postponed until more is known about its action. Spm elements with altered expressions may represent changes in state of the standard Spm-s element. If so, their origins should be comparable to the changes in state of Ac. A Modifier Element within the Spm System An element which greatly increases the rate of mutation at a." in the presence? of Spm first appeared in a kernel on one ear of a plant that was a"? Sho/as sh: DEPARTMENT OF GENETICS _ 397 TABLE 9. Phenotypes of Kernels Appearing on Ears Produced by 20 Plants from the Cross Fa" 1/a"; Pr Spm-w/pr X S ay™4/a,""; pr/pr; no. Spm-w Phenotype of Kernel with Regard to Per Cent a," Action . i Pr Total Recombinants Pale color (no Spm-w).... 00... c cece rece eee 1016 2943 3959 25.6 Colorless with one or several 4, dots or specks (Spm-w present) 00.0.0. cece cece eee ee 1780 569 2349 24.2 Colorless; no A, specks (Spm-w present)........ .... 1462 vee Total mumber of kernels....... 002002 7770 Total with Spr-w.. 0... cc cece ttt ene es 3811 in constitution and carried one Spm ele- ment. The silks of two ears of this plant received pollen from a plant that was ho- mozygous for a: and shz and had no Spm. These two ears produced a total of 738 kernels; 354 had a7 and She, and 384 were homozygous for a and she. Of the kernels in the a7 She class, 167 were uniformly pale colored (no Spm) and 187 had spots of deep color in a colorless back- ground (Spm present). All but one of the variegated kernels exhibited the number of mutant spots that is characteristically produced by the state of a:""* known to be present in the pistillate parent. One kernel, however, had a much larger number of mutant spots; and the plant grown from it showed a marked increase in number of streaks of deep pigmentation in a non- pigmented background. To investigate the reason for this altered expression of ai”, the plant was self-pollinated, was crossed reciprocally with a plant that had another state of a:”* but no Spm, and was used as a pollen parent in crosses with plants that were homozygous for a: and shz and had no Spm. The tests revealed the pres- ence in this plant of an unmodified Spm element, and also an unmodified a@,"* locus. It had an independently located ele- ment, however, which was capable of markedly increasing the frequency of oc- currence of mutation at ai.°* when Spm was also present; and both states of ai" * responded to it in this manner. Plants were grown from the various classes of kernels appearing on each of the ears produced by the above-indicated crosses; and they, in TABLE 10. Phenotypes of Kernels Appearing on Ears of 5 Plants Produced by Cross of 9 a™2/a,%- 2; Y/y Spm-s (standard Spm); Pr Spm-w/pr x Say" fa"; y/y; pr/pr; no Spm-s; no Spm-w Phenotype of Kernel with Regard to a," Action Y Pr Y pr y Pr y pr Totals Pale color (no Spm-s, no Spm-w)....... 0.00.4. 98 200 9 12 319 Many A, spots in colorless background (Spm-s present) .......0.0..0 0... 16 19 360 342 737 One or several 4, dots or specks in colorless background (Spm-w, no Spm-s)............-. 212 69 11 9 301 Colorless; no 4, dots or specks (Spm-w, no Spm-s) 0... cece cece e eens 56Y 5y 61 Total kernels 2.20.00 cece enn eben tebe e bere need 1418 Summaries: 737 Spm-s (376 Pr : 361 pr; 35 Y :702 y) 362 Spm-w; no Spm-s (223 Pr:78 pr; 337 Y :25 +) 319 no Spm-s, no Spm-w (107 Pr : 212 pr; 298 ¥ :21 y) 29.8% recombination between Pr and Spm-w 5.7% recombination between y and Spm-s 398 CARNEGIE INSTITUTION OF WASHINGTON turn, were tested for presence and absence of the modifier element, and for its inherit- ance behavior when present. These tests not only confirmed the conclusions drawn from tests of the parent regarding the pres- ence and mode of action of the modifier element; they also showed that the ele- ment could undergo change of location in the chromosome complement in somatic cells. In this respect, therefore, its behavior is much like that of some of the other con- trolling elements. The origin of this new controlling ele- ment, in the system of which the Spm element and the element at a:”-* are com- ponents, is not understood. It is known, however, that without the new element the types of mutation, their time of occur- rence during development of a tissue, and their frequency of occurrence are expres- sions of the state of a:"7 itself. The modi- fer element affects the expression of only one of these three aspects of state, namely, the frequency of occurrence of mutation. When it is present in conjunction with one of the states, the increase in mutation fre- quency is estimated to be about threefold. The Relation between a." and a” It has recently been determined that the system of elements responsible for control of gene action at a” also operates to con- trol gene action at a2”"". In this respect, the history of origin of both a” and a2™~ is of considerable significance, and so will be outlined briefly. Some years ago, in the progeny derived from self-pollination of a plant, a number of individuals exhibited variegation in leaf and sheath with regard to intensity of chlo- rophyll pigmentation. A study was com- menced to examine the expression of this variegation and also its inheritance pat- tern. In the course of study, many plants in one culture that carried the control sys- tem regulating chlorophyll expression were self-pollinated. On the ear produced by one of these plants, some kernels exhibited variegation for anthocyanin pigmentation. Plants derived from the variegated kernels also showed variegation for anthocyanin pigmentation, and tests conducted with them made it possible to associate this phenotypic expression with an alteration that had occurred at the standard A. locus in one chromosome 5 of the parent. The modified locus was designated a.""2, for in kernels carrying it spots of anthocyanin appeared in a colorless background, as if the change in gene expression was from recessive to higher alleles of Ao. Study was continued, and in its course the silks of an ear of a plant carrying the system responsible for control of mutations at a2" received pollen from a plant that was homozygous for ai, carried in chro- mosome 3, and for the standard A: locus, carried in chromosome 5. On the resulting ear, one exceptional kernel, instead of be- _ ing totally pigmented, had spots of deep pigmentation in a colorless background. The plant derived from this kernel alsa exhibited variegation for anthocyanin pig- mentation. Tests of the plant indicated the presence of the a: locus derived from the male parent and of an altered 4; locus derived from the female parent. Altcra- tion of the Ai locus must have occurreil late in development of the ear of the pistil- late parent plant, for only this one kernel exhibited modified A: action. The modi- fied locus was designated a,""7. All studies of a1” have been made with progeny of this single plant. Although similarities in expression were noted between the chlorophyll variegation originally studied and the variegations as- sociated with the modified Az and A: loc (a2"* and a,""*), investigation of the op- eration of the systems responsible for con- trol of gene action in the first two cases was suspended. Attention was concen- trated instead on examination of the sys tem responsible for control of gene action at a:""*, When the mode of operation 0! that system, the Spm-a,"* system, wis appreciated, it became clear that further consideration should be given to a2”; 10 determine whether or not changes in ex- pression of genic materials at this locus are under the control of elements belonging to the same system. Pedigree relationships as well as kinds of behavior suggested such a possibility. Study of the system operating at ae" was therefore resumed, with this viewpoint in mind. Two strikingly different states of a2" were selected for the renewed investiga- tion. Both states respond to an independ- ently located element that exhibits a Sup- pressor-mutator (Spm) type of control of gene action. With one of these states, some gene action occurs at the a2”~* locus in the absence of Spm, resulting in the appear- ance of anthocyanin in both kernel and plant, the pigment being uniformly dis- tributed within a tissue. The rate of action appears to be lower than that of the stand- ard Ae gene, for in both plant and kernel the pigment intensity is low. In the pres- ence of the Spm element, however, gene action is suppressed except in some cells where mutations at the a2” locus, initi- ated by Spm, allow the gene substance to be fully active. These mutations result in stability of expression of the genic ma- terials at the locus in subsequent cell and plant generations. The characteristics of this state of a2” are essentially similar to those of some states of ai". The expression of the second selected state of a2”! has not yet been satisfactorily analyzed, but it clearly differs from all the many known states of a:”*. In the ab- sence of the Spm element, it gives rise to deeply pigmented kernels and plants, al- though the intensity of color is not so great as that produced by the standard Az locus. When the Spm element is present, both kernel and plant arewariegated. Pigmented areas in the kernel have about the same intensity as that produced in the absence of Spm; but colorless areas may be present within the pigmented areas. The pattern of variegation in any one kernel depends upon the number of Spm elements present. When only one Spm element is present, DEPARTMENT OF GENETICS 399 the kernels have many large pigmented areas as well as some small pigmented spots. When two Spm elements are pres- ent, there are few if any large pigmented areas, and most of the kernels have only small pigmented spots. If three or more Spm elements are present, the kernels either are totally colorless or have small pigmented spots in a restricted region of the aleurone layer. The type of pigment in the colored areas, and the observed effects of dose of Spm on the pattern of their appearance, suggest that the variega- tion in this case is not related to mutation at the a” locus, as it is with the pre- viously described state of the locus. Rather, the pigmented areas seem to reflect changes affecting the Spm element, which either result in its removal or alter its capacity to suppress gene action with this state of a, The Spm element that was present in the cultures having the two states of a: is not the same as the Spm-s element car- ried in the a:"* cultures. Changes in its mode of control of gene action at a:"* sometimes occur in somatic cells, and these are often expressed in different sectors of the same plant. They affect both the sup- pressor and the mutator action of the Spm element. The nature of these changes is not yet understood; but the marked differ- ence in stability of action between this Spm element and the Spm-s element was strik- ingly revealed when the latter was intro- duced into an a2" culture. The Spm-s element exerts a consistent and stable con- trol of gene action at a2”"", of a type simi- lar to that which it exerts at a."~. On the other hand, the mode of inherit- ance of the Spm element in the a2” cul- tures is similar to that in the a.” cultures. Somatically occurring transpositions bring about loss of Spm from some cells, change of its location in others, or changes in both number and location. The several methods adopted to detect such transpositions in the a:"* cultures have also been applied here. In addition, use of the state of a2”* 400___ CARNEGIE INSTITUTION OF WASHINGTON that reflects doses of Spm has made it pos- sible to select among the kernels on an ear those that have different numbers of Spm elements. The effectiveness of this selec- tive method was shown by tests of 12 plants (group A) derived from kernels whose variegation patterns indicated the presence of more than one Spm element, and 10 plants (group B) derived from ker- nels whose variegation patterns indicated the presence of only one Spm element. One of the plants in group A had no Spm, which suggested that transposition, occur- ting in the gametophyte, had resulted in increase in number of the Spm element in the endosperm nucleus and its absence in the zygote nucleus. In tests of the remain- ing 11 plants, one testcross ear per plant was obtained from 5 plants, two testcross ears per plant from 5 others, and three testcross ears from the eleventh. In 1 plant, three or four Spm elements were present ° in the cells that gave rise to an ear on the main stalk and one on a tiller. Seven plants had two Spm elements in the cells that produced the main and the tiller ears. In the remaining 3 plants, two Spm ele- ments were present in the cells that gave rise to the main ear, but only one Spm element in the cells that gave rise toa tiller ear. Among the 10 plants in group B, one testcross ear per plant was obtained from 2 plants, two testcross ears per plant from 5 plants, three testcross ears from 1 plant, and four testcross ears per plant from the remaining 2 plants. All the ears, except two tiller ears of 2 plants, were produced from cells having one Spm. The cells that produced each of these tiller ears had two independently located Spm elements. The finding that the same system of ele- ments controls gene action at a." and at a2” is not unexpected. Similar relation- ships with respect to control of gene action have been observed at a number of differ- ent loci in cultures carrying the Ds-Ac system. Insertions of the Ds element at different gene loci have initiated control of action of the genic substance at each locus by this system of elements. The ori- gin of a:"* by modification of a standard A, locus in a culture carrying the elements responsible for control of gene action at a2"~ suggests that at both loci an element of common ancestry is present. That ele- ment may also have been present at the locus of the gene responsible for chloro- phyll variegation, for the chlorophyll vari- egate was the first-recognized member of this sequence of gene change. The possi- bility cannot be tested, however, since the cultures carrying it were discarded some years ago. It is also not unexpected to find differ. ences in expression of the Spm element in the a." and a2"* cultures, for a serics of selections was made among the a:”"! cultures before the system responsible for control of a2” action was recognized. We know, too, that changes may occur in the action of the Spm element in a:"7 cul- tures, as described previously. Moreover, other elements may appear, such as the modifier of rate of mutation at a." dis- cussed above; and these, if their presence is not recognized initially, may be respon- sible for unwitting bias in the selection of kernels and plants for subsequent study. In this connection, it is suspected, although not yet certain, that an inhibitor of Spm may be present in some of the az" cul- tures. Obviously, since the number of variables may increase during the course of a study. analyses of systems of controlling elements can sometimes be complicated and time consuming. Recognition of the different elements belonging to a control system. and of the changes that may occur in them as regards both type of action and location in the chromosome complement, requires many types of test. In order to study any one element of a system, each of the other variables, as it is recognized, must be re- moved by crossing and selection, so 25 to work with the smallest possible number of associated and interacting elements. Aberrant Behavior of a Fragment Chromosome Much effort has been expended during the past year in analysis of a structurally modified chromosome 9, of which prelim- inary investigations were reported in Year Book 55. In this modification the substance of chromosome 9 is distributed between two components. The distal third of the short arm comprises one member, referred to as the fragment chromosome. At the proximal end of this segment is a centro- mere, from which may extend a small, deeply staining piece of chromatin. The extension is often lost, however, leaving the fragment with a terminal centromere. The other component carries the proximal two-thirds of the short arm and all of its long arm, and is referred to as the defi- cient chromosome. The fragment chro- mosome shows aberrant behavior in so- matic cells. It may be lost to some cells, and undergo changes in structural organi- zation in others. It may also become at- tached to ends or centromeres of other chromosomes, or be incorporated into an- other chromosome. Both the frequency of occurrence of events leading to such consequences and the time of their occur- rence during the development of a tissue are now known to be under genetic con- trol. Interest in this structurally modified chromosome was aroused initially by the aberrant behavior of the fragment chro- mosome in somatic cells. Later it was dis- covered that this fragment could behave unexpectedly in some of the meiotic cells also, and in plants either heterozygous or homozygous for the structural modifica- tion. Although there is no conspicuous cytological evidence of the fact, it has been shown genetically that a segment of chro- matin, adjacent to the centromere in the DEPARTMENT OF GENETICS 401 fragment chromosome, duplicates a seg- ment at the end of the deficient chromo- some. Products that could be assumed to arise from crossing over between this seg- ment in the fragment and the homologous segment in the normal or in the deficient chromosome have appeared in the progeny of both heterozygote and homozygote. It is certain, however, that not all of them result from the ordered process of meiotic events that normally leads to crossing over; and this is made particularly evident in the apparent crossover products formed at meiosis in the homozygote. Often these are structurally normal chromosomes 9, but sometimes they are defective. The frequency of appearance of crossover prod- ucts in the gametes of the homozygote varies widely among the different homozy- gotes, but is constant for any one of them. The present evidence, though limited, does suggest that the variation may be an ex- pression of the genetic system that controls the time of occurrence of aberrant events altering the organization of the fragment chromosome. If so, then when this system operates in a meiotic cell the crossover mechanism may be utilized but the frag- ment chromosome itself may not be re- quired to undergo the usual preliminaries that normally control the position of cross- ing over and its frequency of occurrence at any one position. At any rate, it is cer- tain that the rules assumed to apply to crossing over in maize are not always followed by the fragment chromosome when it participates in a crossover type of event. Until more definite conclusions can be drawn regarding the mechanism respon- sible for the complicated behavior of the fragment chromosome, a review of the evidence obtained from the many tests conducted with it will be postponed. 402 CARNEGIE INSTITUTION OF WASHINGTON BIBLIOGRAPHY Burgi, E. See Hershey, A. D. De, D. N. Ultrastructure of nuclear membrane in plant cells. Exptl. Cell Research, 12, 181- 184 (1957). Demerec, M. A comparative study of certain gene loci in Salmonella. Cold Spring Harbor Symposia Quant. Biol., 21, 113-120 (1956). Demerec, M. (Editor). Advances in Genet., 8, 402 pp., 1956. Demerec, M. The Biological Laboratory, Cold Spring Harbor. 4/.B.S. Bull, 7, 20-21 (1957). Demerec, M., and B. P. Kaufmann. Drosophila Guide. 6th ed. Carnegie Inst. Wash. Publ., 44 pp., 1957. Franklin, N. See Streisinger, G. Gay, H. Nucleocytoplasmic relations in Dro- sophila. Cold Spring Harbor Symposia Quant. Biol., 21, 257-268 (1956). Gay, H. See also Kaufmann, B. P. Hershey, A. D. Chemistry and viral growth. In Currents in Biochemical Research, edited by David E. Green, Interscience Publishers, New York, 1956, pp. 1-28. Hershey, A. D. Bacteriophage T2: parasite or organelle? The Harvey Lectures, ser. Li, 229-239 (1957). Hershey, A. D. Bacteriophages as genetic and biochemical systems. Advances in Virus Research, 4, 25~61 (1957). Hershey, A. D., and E. Burgi. Genetic signif- cance of the transfer of nucleic acid from parental to offspring phage. Cold Spring Harbor Symposia Quant. Biol., 21, 91-101 (1956), Hershey, A. D., and N. E. Melechen. Synthesis of phageprecursor nucleic acid in the pres- ence of chloramphenicol. Virelogy, 3, 207- 236 (1957). Kaufmann, B. P. A portrait of the chromosome. Bios, 28, 21-35 (1957). Kaufmann, B. P. Review of Chromosomes, Sex- Cells and Evolution in a Mammal, by Phillip V. Tobias. Am. Scientist, 45, 214A-216A (1957). Kaufmann, B. P. Review of International Review of Cytology, vol. V. Science, 125, 45] (1957). Kaufmann, B. P., H. Gay, and M. J. McElderry. Effect of ribonuclease on crossing over in Drosophila. Proc. Natl. Acad. Sci. UL S., 43, 255-261 (1957). Kaufmann, B. P., and M. R. McDonald. Organi- zation of the chromosome. Cold Spring Harbor Symposia Quant. Biol., 21, 233-246 (1956). Kaufmann, B, P., and M. R. McDonald. The nature of the changes effected in chromo. somal materials by the chelating agent EDTA. Proc. Natl. Acad. Sci. U. S., 43, 262- 270 (1957), Kaufmann, B. P. See also Demerec, M.; Mc- Donald, M. R. McClintock, B. Controlling elements and the gene. Cold Spring Harbor Symposia Quant. Biol., 21, 197-216 (1956). McDonald, M. R., and B. P. Kaufmann. Produc. tion of mitotic abnormalities by ethylenedi- aminetetraacetic acid. Exptl. Cell Research, 12, 415-417 (1957). McDonald, M. R. See also Kaufmann, B. P. McElderry, M. J. See Kaufmann, B. P. Melechen, N. E. See Hershey, A. D, Moser, H. Contributions to the theory of the continuous bacterial growth apparatus. I. Kinetics of growth of homogeneous popula. tions. Proc. Natl, Acad. Sci. U. S., 43, 222- 226 (1957). Streisinger, G. Phenotypic mixing of host range and serological specificities in bacteriophages T2 and T4. Virology, 2, 388-398 (1956). Streisinger, G., and N. Franklin. Mutation and recombination at the host range genetic re- gion of phage T2. Cold Spring Harbor Symposia Quant. Biol., 21, 103-109 (1956). Streisinger, G., and J. Weigle. Properties of bac- teriophages T2 and T4 with unusual in- heritance. Proc. Natl. Acad. Sci. U. S., 42: 504-510 (1956). Weigle, J. See Streisinger, G. PERSONNEL Year Ending September 30, 1957 Buchanan, Jennie S. (Mrs.), Research Assist- ant *Burgi, Elizabeth, Associate in Research Carley, Catherine, Switchboard Operator and Computer *Clowes, Royston C., Fellow of the Damon Runyon Memorial Fund *De, Deepesh Narayan, Research Assistant Demerec, M., Director *Djordjevié, Bozidar, Research Assistant *Fillekes, John R., Maintenance Man Fisher, Agnes C., Secretary to the Director Gay, Helen, Associate in Research Goldman, Irving, Research Assistant Gross, Julian D., Research Assistant Haggerty, Michael J., Maintenance Man Hashimoto, Kazuo, Carnegie Institution Fel- low Hershey, Alfred D., Microbiologist *Howarth, Sheila, Associate in Research Jones, Henry H., Photographer *Kafer, Etta, Carnegie Institution Fellow Karossa, Judith, Research Assistant Kaufmann, Berwind P., Cytogeneticist *Kelly, Kathleen R., Stenographer Kozinski, Andrej W., Fellow of the Polish Academy of Sciences Lahr, Ernest L., Associate in Research McClintock, Barbara, Cytogeneticist McDonald, Joseph L., Janitor McDonald, Margaret R., Chemist McDonald, William T., Janitor McIntyre, Jean W. (Mrs.), Technical Assist- ant Mandell, Joseph D., Carnegie Institution Fel- low *Martinello, Marian L., Research Assistant Meissner, Richard C., Superintendent of Buildings and Grounds Miyake, Tadashi, Research Assistant *Ozeki, Haruo, Research Assistant Peckham, Leslie E., Senior Clerk Rogers, Claude F., Chief Clerk *Sengiin, Atif, Carnegie Institution Fellow Sepe, Domenico, Greenhouse Man Smith, Guinevere C. (Mrs.), Librarian, Cu- rator of Drosophila Stocks Snyder, Emmy M. (Mrs.), Technical Assist- ant *Sgmme, Randi (Mrs.), Research Assistant Streisinger, George, Associate Geneticist Thomas, René Paul-Emile, Rockefeller Foun- dation Fellow Tomizawa, Jun-ichi, Associate in Research Van Houten, William B., Engineer Wassermann, Felix, Guest Investigator White, Harry, Chief Mechanic Wilson, Carole E., Technical Assistant Yoshida, Yoko, Research Assistant DEPARTMENT OF GENETICS 403 Summer 1957 and Temporary Anderson, Richard P., Maintenance Man Baer, Harold, Guest Investigator Beckwith, Barbara, Assistant Bert, Grace R., Assistant Burtch, Ethel P. (Mrs.), Typist Cannon, W. Dilworth, Jr., Assistant Fochtmann, Grace M., Assistant Gots, Joseph S., Guest Investigator Gregory, Jack, Assistant Holden, Floyd, Maintenance Man Nevole, Nancy A., Assistant Page, Gilbert, Maintenance Man Powell, Florence (Mrs.), Assistant Simrell, Elizabeth Jane, Assistant to Li- brarian Starfield, Phoebe D., Assistant Streisinger, Lotte, Assistant Victoria, William, Maintenance Man Collaborators at Biological Laboratory Barratt, R. W., Summer Investigator Bernheimer, Alan W., Summer Investigator Calef, E., Summer Investigator Caspari, E. W., Summer Investigator Englesberg, Ellis, Bacteriologist Errera, M., Summer Investigator Franzese, Eleanor, Business Manager Granick, S., Summer Investigator Hotchkiss, Rollin D., Summer Investigator Hyde, Olive, Administrative Assistant King, James C., Geneticist Luria, S. E., Summer Investigator Maramorosch, Karl, Summer Investigator Novick, A., Summer Investigator Skaar, Palmer D., Bacterial Geneticist Wallace, Bruce, Assistant Director, Geneti- cist Watanabe, T., Summer Investigator Witkin, Evelyn M., Summer Investigator, Instructor * Resigned during the year.