THE Ds LOCUS. PART ITI. TRANSPOSITION OF THE Ds LOCUS 1. Introduction Transvosition of the Ds locus is a relatively frecuent phenomenon, In the report on the origin and behavior of the c+ml locus, it was shown that oem) arose from a normal © locus as the consequence of a transposition of a Ds locus from its standard location in chromosome 9 to a position close to or within the ¢ locus. The presence of the transposed Da locus did not produce any alteration in the appearance of the chromosome in the re-ion where it had been inserted, nor did its oresence cause any reduc# tion in crossing-over between C and Sh. In this new location, Ds responded to Ac in exactly the same way that it responds to ac when in ite standard looation, The relation of this resnonse to the production of visible mutations of ceml to ¢ was discussed in the earlier report, The study of c-ml mutational phenomena sugsested that the Ac-controlled mutable loci arise because of transpositions of the Ds locus, Since these transpositions are relatively frequent, it say readily be understood why so many new Aceoontrolled mutable loci are arising in the Ds, Ac plants and why previously stable "wild-type" loci "suddenly" become unstable and mutable, Secause transpositions of Ds are related to the origin of mutable loci, it is of prime importance to determine the mechanism responsible for this transposition, How docs it oceur and why does it ocour with such relatively hich frecuencies? An extensive analysis of one case of transposition of Ds has indicated the whe probable mechanism that brings about this transposition, The analysis also suggests the reason why these transpositions are so frequent. This case of transposition of Ds will be considered in detail in this report, &. The origin of transposed De, Case I The first recognized case of transvosition of Ds arose in the oross of 4 plant (4108C-1) having the constitution wd I Sh 2g Wx Ds in one normal chromosome 9 and Jd C sh bz wx as in a nornal homolosous chromosome 9, This plant was heterozygous for ac (Ae ac), “he tynes of kernels resulting from the cross of this plant to a female pliant carrying C sh bz wx ds aoc are given in table 1. (The male parent plant (4108C#1) arose from an I - © bz, Sh = sh, wx-wx kernel on an ear coming from the cross of a C sh bz wx ds ac female plant by an Ac ac male plant having two normal chromosomes 9 with dd C Sh Bz 4x Ds and wd I Sh Bz Wx ds, respectively. The kernel from which plant 4108Cel arose was selected becsnuse it had received an I Sh Bz dix Ds chromosome, The Ds locus was introduced into this chromosome as a consequence of crossing-over in the heterozyzous male parent.) As table 1 shows, with the exception of two aberrant kernels, the types of kernels and the ratios obtained are those expected on the basis of the above given constitution of this plant. “hen this plant was crossed to a ¢c sh 32 wx ds ac female plant, the types of kernels appearing on the ear are those viven in table 2, The ratios of the various tyoes of kernels obtained in this cross likewise agree with the given senic constitution of the male parent Plant (4108¢=1; see suvplement to table 2), In both erosses, the variegation in those kernels having both Da and Ac is of the Table 1 Types of kernels appearing on ear from cross of C sh bz wx ds ace x i Sh Ba wx De ag we g C sh ba wx ds Plant 4108-1] Kernel type Number of kernels Iet bz, Sh*, Wxewx $7 I Sh Wx 59 C sh be wx 128 I sh wx 5 C Bu=C bz, Sh,Wx=wx 5 C¢ Sh Ba wx 2 I Sh wx 52 C Ba-C bz, sh, ix=wx 0 C sh Bz Wx 1 C sh ba, #xowx 22 C sh ba Wx 31 IeC Be-C bz, Sh, Wxewx 1 Origin of Plant 4806 I-c Bz,3h = 1 Not tested Totals 364 * The Shesh variegation will not be indicated in the table. It may be understood to be present, Supplement to Table 1 Types of chromatids produced by plant 4108C#1 and appearance of kernel in table 1 J a 3 4 I Sh Bz Nx De Aa ac Cc sh »bs Mex «6s Appearance of kernel Ac i-C bz ,Sh-sh, éx-wx 5? Non Cross-over I Sh B bz Wx Da chromatids ac I Sh Wx C sh bz wx ds e produced during meiosis in this slant and the tyves of gametes that will result accompanies table 3, This plant had a normal chromosone 9 carrying © ds sh bz wx ds Wor Canad and a Duplication chromosome 9 with wd and 1, ased 3h Bz ux in each of the two duplicated segments (see diacram, table 3 supplement). Synhapsis between the two chromosomes 9 usually occurs as siven in the diagram anc the crossover percentazes between the marked loci are not reduced, as the genetic analysis of the Fe plants that were heterozygous for this duplication will show, The "ds" desienstion is meanineless with respect to the presence of any true allelic loci? It has been inserted to make the constitutions easier to read ata Glance, Eecarse of the many possible classes of crossover chromatids that could arise in this plant, only the single crossovers between the two chromosomes 9 following synapsis of the distal duplicated segment in the Duplication chromosome 9 with its homologous segment in the normal chromosome 9 are siven in the supplement to table 3, Double crossovers in the region between C and wx are relatively rare and will be neglected at this point in the discussion, ‘The single erossovers, however, are frequent and are most important in interpreting the constitutions of the observed types of kernels. althoush it was realized from the tynes of kernels on the séelf-pollinated ear that Table 3 oar Types of kernels appearing on self-pollinatéen of Plant 4306 wa I Ds Sh Bz Wx Wx Bz Sh Ds Constitutions ee 2 WaC ds sh bz wx ds AG ac I kernels Cc kernels I Sh Wx 81 C Sh Bz Wx 4 I Sh Wx-wx 6 C Ba-C bz, Shesh, Wx-wx 3 Imo Bz-C ba, Shesh, Wx-wx 73 C sh bz wx 89 IeC Bz, Sh Wx I Sh wx 4 Total : 274 I-C Bz-C bz, She-sh, wx 10 178 I: 960 I sh wx 2 168 Wx : 106 wx I bz-C bz, sh wx i Supplement to Table 3 Types of chromatids produced by plant 4306. Nonecrossover ami sincle cross-over chromatids resulting from the usual type of synapsis. The expected appearance of the kernel if combined with a C sh bz wx ds chromosome in Ac ac ac or ac ac at constitution o hoa oa ee te wi ' 4 3 Os t ner # = coenew cee eee ete aminmmne Li””~*~«Ssédgs Bh bz wx ds rr én Appearance of kernel -Chromatids: ‘ wy Ao IeC BueC bz, Sh, Wx-wx Non=crossovers I De Sh Bz ax ¥#x Bz Sh Ds ~~ Duplication ac I Sh ¥x C ds sh bz wx ds Ac and ac C sah bz wx normal Ids sh bz wx ds Ac and ac I sh wx normal Region 1 \ .- Ac C BaeC bz, Sh, Wx=wx Cc De Sh Bz 4x Wx Be Sh Ds _ Duplication ac C Sh Bz Wx Ac I bee bz,sh wx Region 2 I Ds sh be wxds —~ normal ™ ge I sh wx Ac C BaeC bz, Sh, Wxr-ws C ds Sh Bz Wx Wx Bz 3h Ds Duplication ac 2 Sh Bz #x Supplement to Table 3 continued \ _- Ao I be-C bz, Sh wx Region 3 I Ds Sh bz wx ds normal “ae I Sh wx 2 Ac C BzeC bz, Sh, Wx-wx C ds sh Bz Wx Wx Bs Sh Ds ~ Duplication ac ¢ Sh Bs Wx \ Ao I-C Ba-C bz, Sh wx I Ds Sh Be wx ds Region 4 normal] ac I Sh wx 1, ac C Bu-C ba, Sh, Wx—wx ¢ as sh bz Wx Wx Be Sh De © Duplication ac C Sh Bs Wx \ _- Ae ImC Bz-C ba, Sh, Wxewx I Ds Sh Be Wx da Region 5 normal \ ae I Sh Wx 1, Ae C Bz-C bz, Sh, Wrewx C ds sh bz wx 4x Bz Sh Ds ~ Duplication ac C Sh Bea Wx Region a ' 1 Ac I-C Bz-C bz, Sh, Wx=wx Not considered Wa I Ds Sh Bz 4x Wx Bz Sh Ds in table Duplication \ ge I Sh Wx wd C ds sh bz wx ds Ac and ae 2 sh ba wx normal ~5S— (1) a Da locus had been transposed just to the right of the I locus and that (2) some chromosomal aberration involving the region to the right of the I locus had likewise occurred, the exact nature of the aberration was not clearly understood from the analysis of this ear alone, To obtain exact information on the aberration that occurred and its possible relation to a transposition of Da, some of the kernels in the various classes represented in table 3 were planted in the summer of 1948 under culture number 4628, With respect to Wd or wd and BZ or bz, the types of plants arising from these various classes of kernels are given in table 4. 3. Cytological examination of plants in culture 4628, Cytological examination was made of a number of plants in the various sub-classes of culture 4628, The sporocytes in some of these plants gave very poor figures, Those in which the constitution of ohromosome 9 was clear and readily analyzable are summarized in table 5. In subecultures D and £, the majority of plants probably had the same genic as well as chromosomal constitution as the mother plant. All the examined plants had one normal chromosome 9 and a chromosome 9 with a duplication of a mid-segment of the short arm, This duplicated segment was inserted into the short arm, Homologous synaptic association of the short arm of the normal chromosome 9 with the short arm of the Duplication chromosome 9 usually occurred slong the distal two-thirds of the arm, «4 loop configuration in the Duplication chromosomes 9 was present. Its position varied in the different sporocytes but it was usually close to or within the deep- staining region adjacent to the centromere. None of this proximal Table 4 Appearance of plants in culture 4628 obtained from selected gemmme-nf kernels on self- pollinated ear of plant 4306 (table 3) Sub-culture Appearance of kernel Wamber of desig-~ in each sub-culture kernels iad ahr Sordi Ranta arising nations from which plants arose planted A Ie-C Bz, Sh Wx 1 O No shoot developed; onl “roots B I Sh Ax-wx 2 2 white seedlings (wd/wd) Cc I Sh Wx 20 17 «4d, Be : 1 white : 2 no sermination D I-C Ba-C bz, Shesh, Wx wx 158 15 Wd, Bz (Many C bz wx areas) E I-C Bz-C Bz, Shesh, Wx-wx 10 8 Wd, Bz: 1 white : 1 no germination (Few C be wx areas) F I Sh wx 2 2 wd, Bz G I-C Bz=C bz, Sh-sh, wx 3 3 Wa, Bs (Many C bz areas} (Few C bz areas) Z I sh wx 2 2 Wa, bz d I bz-C bz, sh wx 1 1 #d; died in seedling stage K C Sh Bz Wx 2 2 Wa, Bz L C Bz-C bz, Shesh, Wx-wx 2 2 4a, Bz M C sh bz wx 20 6 Wd, bz Table 5 Constitutions of chromosomes 9 in plants of culture 4628 based upon examination of sporod:ytes at pachytene. Plant Number Constitution of chromosomes 9 Constitution of in examined plant plant from pollen examination 4628C~ 9 2 Duplicationg chromosomes 9 Wx Wx 4628C-17 1 Duplication chromosomes qx wx 1 Normal a * 4628D-10 1 Duplication “ " 4X wx 1 Normal +. a 4628D-11 1 Duplication " “ ax wx 1 Normal ~ 2 4628D-12 1 Duplication Wx wx 1 Normal 4628E- 8 1 Duplication xX wx 1 Normal 4628F- ] 2 Normal WX wx 4626F~< B 2 Normal Wx wx 4628G~ 3 & Normal wx wx 4628G~- 2 2 Normal Wx Wx 4628H~ 1 2 Normal wx wx 4628H- 2 2 Normal WX Wx 46281~ 1 1 Duplication ax wx 1 Normal abe deep-staining re-ion, however, was included in the duplicated segment, Plant 4628C~9 was homozysous for the chromosome 9 with the duplication, y comparative measurements of the short and long arms of chromosome 9 in this plant and from the appearance of the chromomeres within this arm, it was apparent that the duplication was composed of a segment approximately equivalent in length to a third of the normal short arm, It was composed only of the smaller chromomeres characteristic of the distal two-thirds of the normal short arm, It was concluded (1) from the chromomere constitution in the plant homozygous for this duplication, (2) from the synaptic phenomena in the heterozygous plants and (3) from the genic and chromosomal constitutions of the Fo population that the duplicated segment was composed of a section of the middle of the short arm of chromosome 9 and of a length approximately one-third of this arm, This segment had been inserted into a normal short arm, ‘This may be diagrammed, without reference to order of genes, as follows: Normal chromosome 9 short arms 3 apabrewore ee -{- eee ap fees : OF So Rerttey BPN Duplication chromosome 9 short arm! - 2 > mA a _ . caaeme ced woe aa wr ee of . Plant 46281e1, arising from a kernel having a cross-over ay chromatid, was heterozygous for the Duplication chromosome 9, Synaptic behavior between the normal and the Duplication chromosome 9 was similar to that described for the heterozyzous Plants in subecultures Dand E, The plants in sub-cultures ¥, G and H, on aye the other hand, had two morphologically normal chromosomes 9, No abnormalities of any kind could be seen in the chromosomes 9 of these plants, In all of these plants, however, the I, Ds, Sh and Bz loci in the normal chromosome 9 had originally been located in the Duplication chromosome 9 of the mother plant, A genetic analysis of the plants of culture 4628 has made it possible to state (1) the genic composition of the two identical seements in the Duplication chromosome 9, (2) the order of the genes in each segment and (3) the probable event that occurred in a cell of plant 4108C <1 which produced the duplication and the transposition of a Da loous from its standard position to a position immediately to the right of the I locus, The purpose of the following description is to give the evidence that allows these conclusions to be drawn. 4. The genetic analysis of the plants in culture 4628, (a), Sub- culture C,. Pollen examinations were made of a number of plants of culture 4628 (table 6), This was particularly important in sub-culture C in order to select those plants that could be expected to be homozygous for the Duplication chromosome 9, These plants should be jx Wx. Only 3 Wx Wx plants were present in this sub-culture, Because plant C=9, a ix Wx plant, had also been examined cytologically, it was selected for tests to determine the types of variegation patterns that the De loci in the Duplication chromosomes $ would produce, This plant was crossed to the following female plants: Table 6 #x and wx constitutions in plants of culture 4628 based on pollen examination Sub-culture fx We" Wx we wx wx Appearance of kernels from which plant arose eames Cc 3 14 0 I Sh vx D 0 3 0 I«C Bz +C bz, Shesh, Ax=wx E 0 1 0 I= C Bz -C bz, Shesh, (x=wx (few C be areas) ¥ 0 0 2 I Sh wx G 0 0 3 I-C Be -C bz, Shesh, -wx H 0 0 2 I= C Bz, Sh-sh, wx (few C bz areas) Z 0 Q 2 I sh wx K 0 2 Q C Sh Bz ¥x L 0 2 0 C Ba- ¢ bz, Shesh, Jx-wx * All 3 plants had a few, emall, partially filled wx staining pollen grains, This is to be expected from De® mutations (see Anmual Report, 1948), ~G— (1) ¢ ah bz wx ds, ac ac (2) C sh bz wx ds, Ac ac (3) o sh Bz wx ds, ac ac (4) C Sh Bz wx Ds / C Sh Bs wx Ds, Ac Ac (5) C Sh Bz wx os f C Sh Bz wx Ds, ac ac When crossed to C sh bz wx ds ac female plants, two tyves of kernels appeared on the ears in eoual ratios, One was I Sh dx and non-variegated. The other tyne of kernel was variegated (table 7a), All these variegated kernels had sectors that had lost the I locus, Hany of the sectors were C Bz Wx in phenotype but within the sector varlagation for C bz was oresent. These C bz areas were all wx, Within the C Bz sectors there were often large wx areas that were definitely Bz and not bz, This 1 important, as will be indicated later. There were also some C bz wx sectors, not within or associated with the C Bz sectors, It was evident that two types of events were occurring in these kernels. Both involved loss of the I locus but one gave the C Bz Ax sectors with C bz wx and Bz wx areas within them and one gave C bz wx seotors directly. Further analysis has made it clear that these two separate types of varie- gation are associated with the presence of two Ds loci in the Duplication chromosome 9, one (De) located just to the right of the I locus and one (Ds”) losated to the right of the proximal duplicated segment. A De® mutation accounts for the appearance of the indevendent C bz wx sectors; a Dst mutation gives rise to the C Bz 4x sectors with a secondary tyne of variegation within them, as described above; C eroded. mw Iealbengs - ge Iudlap ee fotlswing 3 fowwelin thy cueowline Suowalia a nen, ). Table Yaa 1 a Fi y £ C eh bz we ds ac? x LDS" 3h Ba dx Wx Bg Sh De Ao ac ¢ I Dat Sh S82 Ax x Bg Sh De® Plant 462809 Kernel types Cross I Sh ix ImC Ba-C ba; Shesh, ix-wx* 4361-5 x 46280-9 222 212 4363-10 x" " 18 16 4362C-6 x * " 273 256 Totals 513 464 * See text for soourate description of variegation. Table 7=»b e eh Bz wx ds ac 2 x «4628649 oO Kernel types Colorless Cross Colorless Sh-sh dh Wx : Wx=wx 4347-19 x 4628C-9 180 189 Table 7c C sh bz wx ds, Ac ac 9 x 46285-9 ne a not obviously . Odds variegated Speckled Ac ac ac {Ae Ac Ac and (Ac 4c ac) type ac ac ac) 4462C-11] x 46280-9 190 121 140 /- x 4462c~e, x " " 171 115 110 1 ¢ Sh Be wx non-varie- gated Totats | 3 61 36 _ &sO * Some of the kernels in this column have a few small © areas or a few specks of C. oN La PG os yom wR Rasy tages EK Se PTET unset VAST ® ‘ ‘ : a Py . Qu bo ees A i ef beetle, Table 7d C Sh Bz wx Ds / C Gh Ba wx Ds, Ac Ac 2 x 4628C6-9 ¢ I Wx I-C, Wxewx I-C, vxewx Cross not obviously van peated Ac Ac ac type Ac ac ac * 4380R-6 x 4626C-9 25 36 4 | ' ei 4 Sen Wopat on fe Deus {or dupa ahvou qe a =Q= The evidence obtained from this cross alone is not sufficient to establish the “Sonstitut ions of the chromosomes 9 in this plant (4628C-9) as given in table ?-a, The analysis of the other plants aga in culture 4628 allows ie “constitutions of the chromosomes 9 in this vliant to be designated with a high degree of certainty. The constitution of the Duplication chromosomes 9 are, then: I Ds) Sh Be #x Wx By Sh De®, That no C locus was present in the duplication chromosome 9 was indicated from the cross of 4628C-9 to ec sh Bz wx ds ac female Plants (table 7b), All of the kernels resulting from this cross were colorless, No colored spots or areas were seen in any of then, Half of them, however, were jx «- wx variegated, as expected. Ifa C locus had been present to the right of net, then its presence should have been detected in the 4x - wx variegated kernels on this ear. It seems reasonable to conclude, therefore, that no C locus was present in the Duplication chromosome 9, That none should be present will be indicated when the projected event that save rise to the duplication is discussed, Plant 4628C+9 was crossed to a C sh bz wx ds, Ao ac female plant, table 7=c, If plant 4628C-9 were Ac ac in constitution, as the evidence in tables 7a and 7-b indicate, the endosperm consti- tutions of the kernels represented in table 7eo should be 1 Ac 4c Ac 3 I Ac Ac ac : 1 Ae ac ac : 1 ac ac ace Responses of the Ds loci to Ac dosage would determine the classes of kernels appearing on this ear, The ratios of classes, with respect to variegation pattern, in table 7c, are those that could be anticipated. The observations suggest that some of the ac ac AG constitutions allow a very Light sveckled pattern of variegation to sppear--a few very late Da muta- wo Ow tions were ocourring in some of these kernels, Table 7#& gives the tyves of kernels obtained from the cross of 4628C-9 to a C Sh Bz wx Ba C Sh Bz wx Da, Ac Ao (allelic positions) female plant, again, the classes of kernels appearing on the ear following this cross are in agreement with the projected constitution of the male parent, (bd), Sub-culture D The tested plants in sub-culture D were all heterozysous for the Duplication chromosome 9, The selection of this class of kernels from the self~nollinated ear of plant 4306, would suggest that they micht have similar chromosome and cenic constitutions as the mother plant. Two of these plants were crossed to C sh be wx ds ac female plants, The results of these crosses are given in table 8, Because of the many classes of kernels that could appear following this oross, a supplement to table & has been included to show the types of chromatids that these plants could produce, if they had the siven chromosome 9 constitutions, The types of chromatids are the same as those that have been considered for the parent plant. (Supplement to table 3), In these crosses, however, a direct test for the presence of these various types of non-crossover and crossover chromtids is available, It may be seen from the suyplement to table & that crossing over would produce morphologically normal chromosomes 9 having a transposed Ns locus. The constitutions of these chromosomes, in the single cross-over classes, would be: I De! sh bz wx, I Ds 3h bz wx and I Ds 3h Bz wx, The variegation pattern that each of these chromo- somes would produce in the cross to ¢ sh bz wx ds is apparent, They appear in table 8 under the designated headings I ba=C bz, sh, wx, I baeC ba,Shesh, wx and IsC Ba«C bZ, Shesh,wx, respectively. If plants Table 8 1 gue 2 C ds sh bz wx ds T3E5zE Grogs =~ _ | Kernel type x x x x Totals 4628D~-10 4628D-10 4628D-11 4628D~11 I Sh Wx 38 55 37 70 200 IeC BeeC bz, Sh*, Wx-wx 16 27 40 59 142 T th wx 22 29 13 30 94 I-€ BzeC ba, Shi, wx 8 16 19 33 76 I bzeO ba, Sh, wx 1 1 1 0 3 I sh wx 3 5 2 8 18 I bzeC bz, sh wx 1 3 2 6 12 C Sh Bz Wx 20 16 16 27 79 C BaeC bz, Sh, Wxewx 10 7 ? 19 43 C Sh Bs wx 0 0 i 2 3 C sh bz wx 6F 115 — 108 — 220 500 Total kernels 1170 * The Shesh variegation will not be indicated in the table but may be understood to be present. 545 I: 625 C 2¢5 Was BS ky Supplement to Table & Types of chromtids produced by ¢ plants in table 8, Only single cross-overs in designated synapsed region considered. , ® 3 4+ + 4 M 5 tpsl sh Be * ax We Bz Sh Ds“ + O- ia C ds sh bz wx ee “Om Appearance of kernel in Table 8 ~ Ae I-C Bu-C bz, Sh, Vxewx Non-crossovers I Ds? sh Bz ¥x Wx Bz Sh Ds” ~ Duplication ac I Sh Wx C ds sh bz wx ds Ac and ac C ah bz wx normal Cross-overs I ds sh bz wx ds Ace and ac I ah wx normal Region 1 > AG C Beet ba, Sh, Wx—wx c ps! sh Bz Wx Wx Bz sh Ds? < 7 Duplication ac C Sh Bz Wx 1 _ AG I ba-C bz, sh wx Crossovers I De+ sh be wx ds Normal ~ ac I sh wx Region 2 C ds sh Be wx Wx Be sh De® - ac G BaeC bz, Sh, #x-wx a £ “ Duplication ~ ae C Sh Bs Wx Supplement to Table & continued Crossovers le _ Ae I be-C bz, Sh, wx I Ds~ Sh bz wx ds Region 3 Normal “ae I Sh wx - Ao C Ba-C ba, Sh, Wx-wx C ae sh Bz ax dx Be Sh Ds® — Duplication ac C Sh Bz ax Crossovers Ac IeC Bue bz, Sh, wx I Det sh Bz wx as — , , Region 4 Normal “ae I Sh wx . 2- Ao C Be-C bz, Sh, Wxewx ¢ ds sh bz vx Wx Bz Sh Ds Duplication “ae C Sh Bz wx Crossovers — ac I-C Be-C bz, Sh, Wxowx I Dest sh Bz dx ds Region 5 Normal “ac I sh Bs dx 2 AC C Bz-C bz, Sh, /x-wx C ds sh bz wx ix Bz Sh Da” ~ Duplication Nac C Sh Ba ax