VOL. 17,193l BOTANY: B.McCLINTOCK 485 THE ORDER OF THE GENES C, Sh AND Wx IN ZEA MA YS WITH REFERENCE TO A CYTOLOGICALLY KNOWN POINT IN THE CHROMOSOME BY BARBARA MCCLINTOCE BOTANY DEPARTMENT, CORNELL UNIVERSITY Communicated July 7, 1931 The monoploid complement in Zea mays is composed of ten morphologic- ally distinguishable chromosomes.i By means of trisomic inheritance it has been possible to associate certain linkage groups with particular members of the complement.2 Thus the linkage group involving the genes C (colored aleurone), sh (shrunken endosperm) and wx (waxy starch) has been associated with the next to the smallest chromosome (chromo- some 9) of the monoploid complement. It is the purpose of this paper to present evidence indicating the serial order of these genes with reference to a known point in the chromosome. In a previous report3 it was shown that in a certain strain of maize a segmental interchange had occurred between chromosomes 8 and 9. The diagram in figure 1 is similar to the one accompanying that report. By means of the close synaptic attraction of the homologous parts of chromo- somes in early meiotic prophase it has been possible to determine the region in each chromosome at which the interchange occurred, and to recognize this region in the sporocytes in plants with the chromosome complex shown in c, figure 1. The conspicuous knob terminating the short arm of chromosome 9 (n in the diagram) is found only in certain maize cultures. However, in these cultures it is a constant feature of this chromosome. Evidence presented in this paper indicates that the loci of the genes wx, sh and c lie in the region between the interchange point and the knob, wx being nearest the interchange and c farthest from it. Plants which are heterozygous for the interchange possess the chromo- somes nNIi (see diagram) and show approximately 50% sterility in pollen and eggs." At diakinesis the interchange complex appears as a ring of four chromosomes (d, Fig. 1). The four members of this ring distribute 2-by-2 at anaphase. Since in half of the sporocytes any two adjacent chromosomes in the ring go to the same pole, forming sterile combinations, and in the other half of the sporocytes and the adjacent members go to opposite poles, forming fertile combinations, six types of spores are formed, only two of which are viable: those containing n with N and those containing I with i. Occasionally the distribution at anaphase is such that 11 chromosomes, including 3 members of the ring, go to one pole and 9 to the other. In this manner viable 11-chromosome gametes are formed. Such gametes could possess the following chromosomes: (1) iIN, (2) iln, (3) nNI, (4) nNi. 486 BOTANY: B. McCLINTOCK PROC. N. A. S. Through fusion of these 11-chromosome gametes with normal gametes four types of 21-chromosome plants can be obtained. The types of chromosome cotigurations seen at diakinesis in the resulting 21-chromo- some individuals enable one to determine which of the four types of ll- chromosome gamete has functioned in any particular case. Experience has shown that fusion of a normal gamete with one of type (1) or type (2) will produce an individual which is approximately 2530% sterile;4 where- a I FIGURE 1 U.--Diagram of the two normal chromosomes which were involved in the segmental interchange. The clear portions in the chromosomes represent the spindle fibre at- tachment regions. The smaller chromosome terminated in an enlarged, deeply staining knob. The arrows indicate the places in the chromosome at which the interchange occurred to produce the situation shown in b. b.-The two chromosomes produced as the result of the segmental interchange. C.-The type of synaptic complex in mid- prophase of meiosis obtained by combining a normal chromosome complement with an interchange complement through crossing. N, larger normal chromosome; n, smaller normal chromosome; I, larger interchange chromosome; i, smaller interchange chromosome. &--The ring formed at diakinesis by the opening out of chromosomes from the complex shown in c. (Chromosomes labeled as in C.) as, fusion oi a normal gamete with a gamete of type (3) or type (4) will produce an individual which is approximately 18% sterile. In the former cases the chromosome configuration at diakinesis is frequently a closed ring of four chromosomes with the extra chromosome attached. In the latter cases the configuration is not a ring, but consists either of a "chain" of five chromosomes, a bivalent plus a trivalent or, infrequently, 2 bivalents plus a univalent. The "chain" is composed of two bivalent chromosomes, bivalent 8 (N) and bivalent 9 (n), held together by the extra chromosome (1 or i) which is synapsed at one end with 8 (N) and at the other end with 9 VOL. 17, 1931 BOTANY: B. McCLINTOCK 487 (n) ( Fig. 2). Opening out of the members of the bivalents in late pro- phase produces a chain-like structure. Plant 270n belonged to the low sterile class. This plant was suspected of having the formula nnNNI since it was noted that in those sporocytes which showed a trivalent instead of a chain of five chromosomes, the tri- valent always consisted of one long and two small chromosomes. Since no knob was present at the end of either the n or the I chromosome it was impossible to be certain from such observations. Pollen counts from sev- eral anthers indicated that the plant was approximately 18% sterile. If this plant had the formula nnNN1, four types of functional gametes would be produced: (1) nN, (2) nNN, (3) nnN, (4) nNI. Since ll-chromo- some pollen grains seldom function in competition with grains containing only the monoploid complement, selfing such a plant would produce four types of individuals. All the 20-chromosome individuals would be ordi- nary normals. Of the 21-chromosome individuals there would be three kinds. The functioning of gametes of types (2) and (3) would produce individuals trisomic for chromosomes 8 (N) and 9 (n), respectively. The diakinetic configuration shown by such individuals would be nine bivalents plus a trivalent or ten bivalents plus a univalent. No sterility above that found in normal diploid individuals (approximately 2-5y0) would be observed in pollen or eggs. The functioning of type (4) would produce an individual of the same chromosome constitution as the parent (nnNNI>. It would show a chain of five chromosomes at diakinesis and exhibit 18% sterility in pollen and eggs. Twenty-two individuals resulting from selfing plant 27011 were grown and examined cytologically. Eight of these were 20-chromosome indi- viduals. They were ordinary diploids, showed no sterility and gave di- somic ratios for c, sh and wx. Twelve of the thirteen 21-chromosome individuals must have come from gametes of type (4) since they showed 18% sterility in pollen and eggs. Cytological observations indicated that the remaining 21-chromosome individual was trisomic for chromosome 8 (N) and therefore must have involved a gamete of type (2). It was non- sterile and gave disomic inheritance for the genes c, sh and wx. On the basis of the appearance of the chromosomes at diakinesis the assumption has been made that the formula for the parent plant and all its partially-sterile offspring was nnNN1. To test this assumption several of the partially sterile offspring were crossed to ordinary normal plants whose number 9 (n) chromosomes terminated in conspicuous knobs. Chromosome counts were obtained from nineteen individuals resulting from such a cross. There were nine 20-chromosome individuals, all normals. Among the ten 21-chromosome individuals seven were partially sterile like the female parent and three were non-sterile. Two of the three non-sterile 21-chromosome individuals were examined and were found to be 488 BOTANY: B. McCLINTOCK PROC. h-. A. S. trisomic for chromosome S (N). Among the partially sterile plants two were examined for chromosome configurations at diakinesis. In each of them a chain of five chromosomes was observed. If the extra chromosome in these 21-chromosome plants were the long interchange chromosome (I), the knob should be found only in two positions.* These positions are illustrated in figure 2. The conspicuous knob allows the configuration to be easily interpreted. In every sporocyte in which a chain of five chromosomes appeared the knob was found in one or the other of the positions illustrated. In some sporocytes a trivalent plus a bjvalent appeared instead of a chain of five. The presence of the knob on one of the n chromosomes makes it possible to state that such a trivalent is composed either of nnI or NiVI. This evi- * n I N FIGURE 2 dence makes it certain that the extra chromosome in the complement of these sterile plants is the long interchange chromosome (I). The configuration which would be seen if the short interchanged chromo- some (i) were present would be a chain of five chromosomes, also. How- ever, the long arm of chromosome 9 would be associated with the inter- changed chromosome and so the knob would be where the two n chromo- somes join. No such case was seen. Chromosome I possesses nearly two-thirds of that chromosome (number 9) which carries the genes of the c-&wx linkage group. It is highly proba- ble, therefore, that the loci of some of the known genes of this linkage group lie in this portion of the chromosome. Crossover data have shown that the locus of the gene sh lies between that of c and WX. The map distance between c and sh is 3.3, that between sh and wx, 20.3.6 Un- published data which Dr. C. R. Burnham has generously allowed me to use have shown that the percentage of crossing-over between wx and the inter- change point is approximately 13 per cent, between sh and the interchange VOL. 17,1931 BOTANY: B. McCLINTOCK 489 approximately 32 per cent, between c and the interchange, approximately 33 per cent. If it could be shown that the long interchange chromosome (Q carries the genes wx, sh and c or their allelomorphs, their order with refer- ence to the interchange point could be established. The following evidence is presented in support of this claim. TABLE 1 RATIOS OBTAINED FROMSELFING PLANT 270~ COLORED EOLORL~SS Sh sh Sh sh wx wx wx wx wx wx wx wx _--- ---- 11 8 10 25 5 9 2 Plant 270n was heterozygous for the genes C, sh and wx (table 1). Among the 21-chromosome, partially sterile individuals which came from the colored, non-shrunken, non-waxy kernels of 27011, three were utilized for genetic investigations. These plants were numbered 388Ai, 3SSDi and 3SSDz. All had the formula nnNN1. The pollen of plant 3SSAi was examined for the waxy ratio and sterility (table 2). The waxy ratio in the pollen of this plant was approximately 15 11,`~ : 1 ZCY. This is in contrast to the 1 Ii/x: 1 Rex ratio found among diploid individuals or individuals which are trisomic for some chromosome other than that which carries the genes of the c-sh-wx linkage group. In most anthers the sterility approximates lSyO. For reasons as yet un- known, certain anthers show a considerably lower or higher percentage of sterility. When this pollen was used on a plant homozygous for c, sh and wx the following ratio was obtained: 2969 0 X 3SSA1 d c c Sh sh Sh sh wx wx Tl,`x wx wx wx wx wx -- 27% -- 3 0 0 -ti 172 5** When crossed to a plant heterozygous for c but homozygous for sh and wx the following resulted: 389C1 0 >G38SAi c7 C c Sh sh Sh sh wx wx wx wx wx wx wx wx xi y(j 124 T O~iT95t* In examining these data it should be remembered (1) that the functional gametes formed by plant 3SSA1 contain nN, NNn, nnN or nNI in varying 490 BOTANY: B. McCLINTOCK PROC. N. A. S. proportions according to the mode of distribution at meiosis (page 487), (2) that the spores with nI or NI abort and are the primary cause of the observed sterility and (3) that the 11-chromosome pollen grains do not function in competition with those containing the monoploid set. The ratio for waxy in the pollen and the ratios and linkage relations of the genes c, sh and wx in the backcross are distinctly abnormal. The ratios are explainable on the assumption that the interchange chromosome (I) has that portion of chromosome 9 which involves the loci of the genes c, sh and wx. Accordingly, it can be stated that plant 3SSA1 possessed two n chromosomes each with the genes c-sh-Wx and an interchange chromosome (I) with the genes C-Sh-WZY. The genie constitution of the two n chromosomes in plant 3SSD, were c-sh-wx and C-sh-Wx, respectively; chromosome I contained the genes C-Sh-wx. This is clear from the following cross to the triple recessive: d 2961s 0 X 388D1 fl c Sh sh Sh sh wx wx wx wx wx wx wx wx ii --- 21 72 11 01879 -- That one n chromosome of plant 3SSD2 contained the genes c-sh-Wx, the other n chromosome C-Sh-Wx and the long interchange chromosome (I) the genes C-Sh-wx can be seen from the following crosses: 3SSD2 0 X 3903 (homozygous c-sh-WX)~ C c Sh sh Sh sh wx wx wx wx wx wx wx wx los33-C _--- 1 0 38 0 29622(homozygous c-sh-wx) 0 X 3SSD2 3 C c Sh sh Sh sh wx wx wx wx wx wx wx wx -- __--- 194 34 2 0 5 0 130 4** 2962l(homozygous c-sh-wx) 0 x 3SSDZ fl C c Sh sh Sh sh wx wx wx wx wx wx wx wx ---- 197 2 18 3 -j- 0 Fo x** VOL. 17, 1931 BOTANY: B. McCLINTOCK 491 The above results point to the fact that the loci for the genes c, sh and wx lie in that part of the long interchange chromosome (I) contributed by chromosome 9. That the genes lie in the order wx-sh-c beginning at the interchange point is indicated by the fact that wx shows 13'%, sh 32% and c 33% crossing over from the interchange point. It is not known on which side of the spindle-fibre insertion region these genes lie. It is possible, however, that they may like in the short arm, between the insertion region and the knob, since evidence presented by Stadler6 suggests that the gene wx lies closer to the spindle-fibre insertion region than the genes sh or c. TABLE 2 POLLEN C~~N~~FROMINDIVIDUALANTHERSOF PLANT 388Al PXR CBNT WX wz *BcmTIYB ABORTIVE PER CmiT wx 991 59 129 10.9 5.6 2029 142 937 30.1 6.5 632 48 145 .17.5 7.0 701 55 164 18.0 7.2 1035 80 240 17.7 7.1 1415 69 347 18.9 4.7 The author wishes to state that no attempt has been made to discuss in detail the data herein presented. It was desired to present briefly the evidence at this time, since it lends valuable support to the argument in the paper which follows. * The position of the knob resulting from crossing-over will be described in a later paper. ** Among the plants of the c-sh-wx cultures four or five silks were exserted before the ear shoot appeared above the leaf sheath and was bagged. It is possible that the high number of c-sh-wx kernels mostly represent self-pollinations rather than double crossovers or crossovers involving all three chromosomes. Similar crosses are being repeated. 1 McClintock, B., SC&, 69, 629 (1929). * McClintock, K. B., and Henry E. Hill, Genetics, 16, 175-190 (1931). a McClintock, B., Proc. Nat. Aced. SC;., 16, 791-796 (1930). 4 Bumham, C. R., Ibid., 16,269-277 (1930). 6 Mimeographed pamphlet on linkage in maize. Cornell University. 6 Stadler, I,. J., Sci. Agr., 11, 557-572 (1931).