SELECTION AND IDENTIFICATION OF A SPONTANEOUS ALIEN CHROMOSOME TRANSLOCATION IN WHEAT

CHAO-CHIEN JAN, JAN DVORAK, CALVIN 0. QUALSET AND

KHAIRY M. SOLIMAN

Department of Agronomy and Range Science, University of California, Davis, California 95616

Manuscript received February I, 1980 Revised copy received April 3, 1981

ABSTRACT

A wheat (Trit icum aestivum L. emend Thell) disomic addition line (2n = 6z = 44), SH1-152-2, with a pair of Elytrigia pontica (Podp.) Holub 2n = 102 70 [syn. Agropyron elongatum (Host) P.B.] chromosomes controlling blue aleurone color was crossed with a short-statured spring wheat ‘Sonora 64’ (T. uestiuum) . Isoline pairs of blue-disomic addition lines and nonblue euploid lines were produced by selecting plants segregating for blue aleurone for 12 generations. Nineteen of 20 blue aleurone lines were 2n = 44. addition lines, and one had 2n = 42 chromosomes. Several lines of evidence showed that this line had a spontaneous translocation in which the p arm of wheat chromo- some 4A was replaced by an Elytrigia chromosome arm carrying the blue aleu- rone gene. The Elytrigia chromosome in SHI-152-2 appeared to be homologous with E . pontica chromosome 4el,, which also carries the blue aleurone gene. It was concluded that the spontaneous translocation originated from simultaneous misdivision of univalents and subsequent reunion at the centromere of chromosome arm 4Aa with the Elytrigia chromosome arm.

B L U E aleurone in the endosperm of the wheat kernel has been transferred to Triticum aestivum L. emend. Thell from Elytrigia pontica (Podp.) Holub

2n = lox = 70 [syn. Agropyron elongatum (Host.) P.B.] and E. intermedia (Host.) Nevski (SUNESON and POPE 1946; POPE 1948; JENKINS 1950). Since then, interest has been expressed in the blue aleurone character as a marker for genetic studies, as an identifiable marker for wheat cultivars used as a feed grain, and as a marker to sort hybrid from nonhybrid kernels by optical reflectance for hybrid wheat production. The blue aleurone character transferred into wheat from Elytrigia, as in “Blue Baart” developed by C. A. SUNESON in the 1940’s, or as in “Blue A” developed by B. C. JENKINS (KNOTT 1958) , is associated with an addition of a pair of chromosomes having a gene for blue color development.

The usefulness of the blue aleurone character would be enhanced if the gene controlling it existed as an integral part of a wheat chromosome. Although nu- merous induced translocations of alien chromosome segments into the wheat chromosome complement have been reported (KNOTT and D V O ~ K 1976), the possibility of utilizing spontaneously occurring translocations for gene transfer has been little explored. In this paper, we report the method of isolation of a spontaneous translocation and the subsequent identification of the Elytrigia and Genetics 98: 389-398 June, 1981

390 c-c. JAN et al.

wheat chromosomes involved. Previously, we have reported on the effects of blue aleurone and/or the associated chromosome segment on grain protein and its electrophoretically identifiable components ( SOLIMAN, BERNARDIN and QUAL- SET 1980).

MATERIALS AND METHODS

SH-152-2, used as the source of blue aleurone, was selected from wheat Composite-Cross I by EL-SHARKAWY (1965). It is homozygous for blue aleurone, with 2n = 44 chromosomes. SH1-152-2 was crossed with ‘Sonora 64,’ a short-statured spring cultivar of T . uestiuum.

In the F, generation, 125 plants showing segregation for blue aleurone color (xenia) were randomly selected. These plants were expected to be monosomic additions (2n = 43), mono- somic substitutions (2n = 42) or translocation heterozygotes (272 = 42). In subsequent genera- tions through the F,,, one plant segregating for blue grains was selected from each of the F,- derived lines. Two generations were grown in the field in most years. During the generations of selecting plants segregating for aleurone color, there was no conscious selection for agronomic characters; however, only 75 of the original 125 lines remained at the F,, generation. Loss of lines was due to the failure to identify a plant segregating for aleurone color because of a small number of plants grown, or because of sterility. The 75 F,, lines were planted in the field, and homozygous blue and nonblue plants were harvested from each line. Thus, 75 pairs of isolines were established, in which each isoline pair has a blue (B) and nonblue (NB) line. The NB lines were expected to have 42 wheat chromosomes, and the B lines could be either disomic additions (en ;= %), disomic substitutions (2n 42) or translocation homozygotes (2n = 42). SOLIMAN (1975) examined 20 of the isoline pairs and found that the NB lines had 42 wheat chromosomes, as expected. Among the B lines, 19 had 44 chromosomes and one, identified as UC 66049B, had 42 chromosomes. The 42-chromosome line will be designated BA throughout the remainder of this paper. Subsequently, C. C. JAN (unpublished) identified a second 2n = 42 B line, but it has not yet been studied in detail.

The chromosomal events that resulted in the integration of blue aleurone in the 42chromo- some line were examined in four studies. These included examination of chromosome pairing in hybrids of BA as a male parent with (1) “Chinese Spring” ditelosomics and the euploid hexaploid cultivar “Combinador,” ( 2 ) Chinese Spring double ditelosomics, (3) Chinese Spring monosomics, and (4) disomic addition of chromosome 4eZ, of E. pontica into T . uestiuum cv. “Rescue” produced by LARSON and ATKINSON (1972). The Chinese Spring chromosomal variants were developed and kindly supplied by E. R. SEARS, U.S. Department of Agriculture and University of Missouri, Columbia. The segregation for blue aleurone among seeds on the F, plants (F, gen- eration) from crosses of Chinese Spring monosomics or monotelosomics with BA was determined. In these crosses, a ratio of 3 blue to 1 nonblue seeds was expected in the crosses involving the non- critical chromosomes, and a deficiency of nonblue seeds was expected in the cross involving the critical chromosome. The intensity of blue color is dosage dependent, and the heterozygous seeds have one or two doses of the blue aleurone gene. In the environment in which these plants were grown, the blue color was weakly expressed and some seeds could not be distinguished from NB seeds; therefore, progeny tests were done to confirm the genotype of the F, embryo.

Finally, C-banding of root-tip chromosomes was employed to provide further evidence con- cerning the chromosome location of the blue aleurone character. A modified procedure of IORDAN- SKY et al. (1978) was used. Root tips, about 10 mm long, were excised from 2-day-old seedlings, placed in ice-cold water and stored at 2” for 20 hr. The root tips were fixed in 45% acetic acid for 20 hr. They were then hydrolyzed in 0.2 N HCl for 5 min, washed in distilled water and treated with a 1:l mixture of 5% solution of pectinase (Sigma, purified grade) and 5% solution of cellulase (Sigma, Type I ) . The roots were treated by the enzyme mixture until a gentle tap on a cover-glass resulted in dispersion of cells (usually 30 to 45 min). Root tips were then removed from the enzyme solution and washed in distilled water. Squashes were made immediately in 45% acetic acid. The cover-glass was removed by the dry-ice method and the slides were air dried. The slides were then treated with saturated Ba(OH), for 5 min, transferred directly into

ALIEN CHROMOSOME TRANSLOCATION 39 1

1 N HC1 for 1 min, washed in distilled water and dried on the slide warmer at 35“. The slides were placed in 2 x SSC buffer, pH 7.0, for 1 hr at 60”, washed in distilled water, dried and stained with 7% Giemsa (Fisher) solution in 0.05 M KH,PO,-Na,HPO, buffer, pH 6.8. Slides were then washed in distilled water, dried and mounted in permount.

RESULTS

BA X ditelosomics and Combinador: If BA is a disomic substitution, pollen mother cells (PMC’s) with complete chromosome pairing (i.e., all chromosomes paired in bivalents or multivalents) cannot occur in these hybrids because, under normal circumstances, the Elytrigia chromosome does not pair with wheat chromosomes. In the hybrid Combinador x BA, 50% of the PMC’s had complete chromosome pairing at meiosis. In 19 of 21 hybrids from the crosses between BA and Chinese Spring ditelosomics (Table I ) , all telosomes paired with the chro- mosomes of BA, and 63% of PMC’s had complete chromosome pairing. In the hybrid with ditelosomic bAa, only a few cells could be analyzed, and these had 19” + ti” + 2’. The hybrid with 6Da was anomalous because it was monosomic. Thus, BA cannot be a disomic substitution.

TABLE 1

Meiotic chromosome pairing in the F , generation and aleurone color segregation in F, seeds from crosses between BA and Chinese Spring ditelosomics

Ditelosome Maximum pairing

~ ~~ ~ ~

No. of kernels Blue Nonblue Blue kernels, %

IAL IBL IDL

2AS ZBL 2Da

3Aia 3BL 3 Da

4Aa 4BL 4Da

5AL 5BL 5DL

6Aa 6BS 6Da

7 A L 7BL 7DS

20”+tl” 18”+tl ” + l I V

20”+tl” 18”+tl”+ 1 IV

19”+t3Iv 20”+tl” 18”ftl”flIv 19”+t3Iv

20”+tl” 18”+tl”+lIv 20”+tl” 1 8”+tl” + 1 IV

20”ftl” 18”+tl”+lIv

20”+ tl ” 18”+tl” + 1 I V

20”+t1” 18”+tl”+1Iv 20”+tl” 18”+tl“+IIv 20”+tl” 18”+tl“+lIv

20”+tl” 18”+tl”+lIv 20”+tl” 18”+tl”+lIV 20”+tl“ 18”ftl ”+ITV

19”+tl” 4-2’ 20“+tl” 18”+tl”+IIv 19”+tl”fl‘ 1 7 ” ~ t l ” + 1 I ~ + l ’

20”+t 1 ” 1 8” +tl “+l IV

20”+ tl ” 18”+ tl ”+l IV

20”+tl” 18”+tl”+lIV

21 8 169 130

141 141 85

224 137 149

153 1 985

68

296 218 230

151 176 125

246 162 341

105 84 60

92 77 65

134 85 85

46 89 47

146 109 118

82 108 83

125 63

204

67 * 67 * 68 NS

61 * 65 * 57 * 63 * 62 * 64 *

77 NS 70 NS 59’ * 67 * 67 * 66 *

65 * 62 * 60 * 66 * 72 NS 63 *

* P < 0.01 for fit to a 3 blue : 1 Nonblue ratio; NS = not significant, P 2 0.05.

392 c-c. JAN et al.

BA x 4e1, disomic addition: In the hybrids of disomic addition 4e11 x Chinese Spring, chromosome 4eZl was not paired in any of 133 cells analyzed. Yet, 44% of the PMC’s had complete chromosome pairing in the cross of BA with disomic addition del,. This shows that a substantial part of an Elytrigia chromosome is translocated to a wheat chromosome in BA. As a corollary, the relatively high frequency of pairing between the translocation chromosome and chromosome 4e11 indicated that the Elytrigia chromosome in the blue aleurone addition lines produced in Davis is homologous with the 4eZ, chromosome incorporated into wheat by LARSON and ATKINSON (1972). The Elytrigia chromosome in the Davis materials will be designed 4e1,.

Table 1 shows that BA differs from Chinese Spring by a single translocation involving chromosomes 2A and 2 0 . When BA was crossed with disomic addi- tion 4el,, two multivalents were often observed in a cell. This indicated that, in addition to the heterozygosity for the 2A/2D translocation, the hybrid was hetero- zygous for one more translocation. Since Rescue differs from Chinese Spring by a single translocation involving chromosomes 2B and 4A (LARSON and ATKIN- SON 1970), BA and Rescue must differ from each other by two translocations, 2A/2D and 2B/4A. Unexpectedly, however, in the hybrids from the cross disomic addition 4eZ, x BA, a number of PMC’s had 19”+ I”, which indicated that Elytrigia chromosome 4eZ1 was involved in the multivalent chromosome conju- gation. Since the Elytrigia chromosome 4eZ1 does not pair with wheat chromo- somes, its pairing in the quinquevalent had to be mediated by the Elytrigia-wheat translocation chromosome in BA. Hence, the Elytrigia chromosome segment in BA must be on one of the four chromosomes (2A, 2B, 2 0 or 4 A ) that differentiate structurally the chromosome complement of BA and Rescue. In the hybrids ob- served between Chinese Spring ditelosomics and BA (Table I), telosomes 2AS, 2BL, 2Da, and 4Aa paired with frequencies that did not differ appreciably from the pairing frequencies of these telosomes that are usually observed in inter- varietal wheat hybrids. Therefore, the Elytrigia chromosome segment in BA must be present in one of the opposite arms, i.e., 2AL, 2BS, 2Dp, or 4Ap.

BA X monosomics: F, segregation data, based on progeny tests, are given in Table 2. No monosomic F, plant from the cross monosomic 5B x BA was ob- tained. Monosomic F, plants from the cross monosomic ?B x BA were sterile. Among the remaining 19 F, populations, only the one produced from monosomic 4A gave the critical segregation and therefore, the blue aleurone gene must be on chromosome 4A in BA.

BA X double ditelosomics: Double ditelosomics ID and 7 0 were not available and hybrids from the crosses involving double ditelosomics I A and ? A did not have two telosomes. In the remaining 17 crosses, double telotrisomics were found as expected. In 16 of them, both telosomes were seen to pair often in a single conjugation. Hybrids from the cross double ditelosomic 4A X BA were excep- tional in that a chromosome conjugation involving both telosomes was not ob- served. In this cross, 97 PMC’s had a heteromorphic pair, composed of a complete chromosome and a telosome, plus an unpaired telosome, which was presumably 4Ap, while both telosomes were unpaired in nine PMC’s. These results indi-

ALIEN CHROMOSOME TRANSLOCATION 393

cated that it is highly probable that the entire chromosome arm 4A/3 was re- placed by an arm of the Elytrigia chromosome in BA.

Disregarding the critical crosses monosomic 4A x BA, which resulted in all blue seeds (Table 2), and ditelosomic 4A x BA, which resulted in 3 blue : 1 non- blue segregation (Table 1), 33 of the 38 remaining crosses listed in Tables 1 and 2 were somewhat deficient in the frequency of blue seeds. This suggests that the Elytrigia chromosome arm present in the translocation chromosome does not compensate perfectly for the 4Ap arm.

C-banding: Chromosome 4A is one of the wheat chromosomes that can be easily identified by its C-banding pattern (GILL and KIMBER 1974). In Ch' inese Spring, the (Y arm (Figures 1 and 2) has a small terminal band, two large proxi- mal bands and one minor centromeric band that is only seldom apparent. The /3 arm has a terminal band, two intercalary bands ar?d two proximal bands (Figure 1).

In BA, the 4Aa arm has a banding pattern essentially identical to that of the 4Aa arm in Chinese Spring (Figure 3). However, there is only a single terminal

TABLE 2

Aleurone color segregation in F , seeds from 19 Chinese Spring monosomic or monotelosomic lines crossed with BA

Aleurone color Monosome or F, plants Blue Nonblue Blue, inonotelosome no. Kernel no. Kernel no. %

1 AL IBL I DL

2A 2B 2 0

3A 3B 3 0

4A 4B 4 0

5A 5B 5D

6Aa 6BS 6LJa

7 A 7B 7 0

7 5 4

3 5 7

5

7

3 8 5

6

3

3 3 8

3 6 4

-

-

213 79

161

104 71

171

85

130

86 55 21

42

175

134 185 185

82 301 162

-

-

151 44 94

54 53 91

52

63

0 28 17

17

84

87 98

120

60 248

97

-

-

59 t 64* 63 t 66 * 57 -I- 65 t 62 1-

67 * 100 "f 66 * 55 *

71 NS

68 *

61 t 65 t 61 t 58 J- 55 t 63 t

-

-

* 0.01 < P < 0.05; t P 5 0.01; NS, not significant P 2 0.05 for fit to 3 Blue : 1 Nonblue ratio.

394 c-c. JAN et al.

FIGURE 1 .-C-banding of chromosomes in an incomplete root-tip cell of double ditclosomic 4A showing the banding pattern of 4Aa and YAP telosomes. Note the characteristic banding pattern of chromosome 5R.

FIGURE 2.-C-banding of chromosomes in n cell of ditelosomic 4Aa. The telosomcs arc indi- cated by the arrows. Note the correspondence of the banding pattern with the 4Aa telosome in Figure 1 .

ALIEN CHROMOSOME TRANSLOCATION 395

FIGURE 3.-C-banding of homozygous plant of BA. The arrows indicate the pair of chromo- somes 4A.

band in the p arm. The /3 arm in BA is thinner and stains more lightly than the other chromosomes of the complement. The absence of the proximal bands that are normally present in 4AP clearly shows that the entire 4AP arm was replaced by an Elytrigia chromosome arm in BA. Hence, the exchange point between the 4A chromosome and 4eZ2 chromosome was at the centromere.

Chromosome 5B, which can be easily identified by its characteristic banding pattern (Figure 1 ) , was used as an internal standard to compare the lengths of the chromosome arms. If the length of 5 B is assumed to be 1 .O, the average length of the 4Aa arm is 0.46 -C 0.024 in Chinese Spring and 0.48 -C 0.012 in BA. This provides additional evidence that the a arm is unchanged in BA. Using the same procedure, the p arm is 0.48 +- 0.006 in Chinese Spring, but only 0.43 * 0.015 in BA. Thus, in BA, the Elytrigia chromosome arm that replaced 4Ap is slightly shorter than the 4AP arm in Chinese Spring.

DISCUSSION

The 42-chromosome blue aleurone line isolated in FI2 from the cross SHl- 152-2 X Sonora W possesses a spontaneous translocation in which the /3 arm of wheat chromosome 4A was replaced by the arm of Elytrigia chromosome 4eZ2 carrying the blue aleurone locus. The translocation chromosome is transmitted with high frequency through gametes; thus, the translocation is of a compensat- ing type.

KNOTT provided evidence that compensating translocation induced by irradia- tion in wheat preferentially involve homoeologous chromosomes KNOTT 1968;

396 c-c. JAN et aZ.

DVOHAK and KNOTT 1977). An irradiation-induced translocation of an Avena barbata Pott. chromosome segment into chromosome 21 of A . satiua L. (THOMAS and AUNG 1977) is a notable exception to this observation. Although the alien chromosome did not appear to be homoeologous with the A . satiua chromosome, in some genetic backgrounds the translocation chromosome was transmitted normally to the progeny.

It appears that KNOTT’S finding is not limited to the irradiation-induced com- pensating translocations, but to those occurring spontaneously, as well. BAKSHI and SCHLEHUBER (1959) produced a leaf-rust-resistant line in which E. pontica chromosome 3Ag substituted spontaneously for wheat chromosome 3 0 (SEARS 1972). A spontaneous translocation was later found that involved the 3Ag chro- mosome and homoeologous chromosome 3 0 (SMITH et al. 1968). In ‘Zorba’ and several other wheat cultivars, rye chromosome I R substituted spontaneously for wheat chromosome I B (ZELLER and FISCHBECK 1971). A spontaneous translo- cation was later identified in which a segment of the rye chromosome was trans- located into wheat chromosome I B (ZELLER 1973; METTIN, BLUTHNER and SCHLEGEL 1973). In another spontaneous translocation, a segment of rye chro- mosome 5R replaced a part of the /3 arm of chromosome 4A (DRISCOLL and SEARS 1965). Although the wheat and the rye chromosomes belong to different homoeo- logous groups, there is reason to believe that chromosome 5R is partially related to chromosome 4A, as a consequence of structural differentiation of the latter chromosome (ZELLER and BAIER 1973; DVO~AK 1980). It is very likely that the spontaneous translocation in BA also involved homoeologous chromosomes. Ely- trigia chromosome 4eZ,, which present evidence showed to be homologous with chromosome 4ele, substituted spontaneously for wheat chromosome 40 and com- pensated for it (LARSON and ATKINSON 1970, 1972). Hence, chromosome 4eZ, must be homoeologous with wheat chromosomes of group LE.

Elytrigia elongata (Host.) Holub (2n = 22 = 14) chromosome 4E was shown to compensate gametophytically for chromosomes 4A and 40, while compensa- tion for chromosome 4B was poor (DVOGK 1980). Assuming that chromosome 4E and 4eZ are similar genetically, it is likely that spontaneously occurring male- substitution gametophytes lacking either chromosome 4 A or chromosome 4 0 would be superior to other kinds of substitution gametophytes and addition gametophytes. Thus, the 4el(4A) or 4eZ(40) gametes will have an enhanced chance of being transmitted to the progeny. A heterozygous plant for blue aleurone from such a mating will have 20” + 2’ chromosomes. Simultaneous misdivision and subsequent reunion of telosomes are likely to result in an arm exchange between the univalent chromosomes (SEARS 1973). Hence, it is not surprising to find that the spontaneous compensating translocations tend to in- volve homoeologues.

C. C. JAN and K. M. SOLIMAN were supported in part during this work by D. F. Jones post- doctoral fellowships. This work was also supported by grants from the California Crop Improve- ment Association. H. E. VOGT sustained the isolines throughout their development.

A L I E N CHROMOSOME TRANSLOCATION 397

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Corresponding editor: R. L. PHILLIPS

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