CYTOGENETICS OF DISOMICS, MONOTELO- AND MONOISODISOMICS AND ml, st, MUTANTS OF

CHROMOSOME 4 O F COTTON‘

J. E. ENDRIZZI AND R. BRAY2 Depariment of Plant Sciences, University of Arizona, Tucson, Arizona 85721

Manuscript received March 21, 1979 Revised copy received December 5,1979

ABSTRACT

The linkage relationships and arm locations of the ml, and st, mutants of cotton were determined with the use of monotelodisomics, a monoisodisomic and disomics. The mZ, st, mutants are more than 50 cM apart on chromosome 4. The ml, locus is in the short arm and 16 cM from the centromere, and st, is in the long arm and 48 or more CM from the centromere. Recombination in the monoisodisomic was twice the expected value and higher, but not signifi- cantly, than in its monotelodisomic counterpart. I t was concluded that the in- crease indicates a greater frequency of proximal exchange in the mono- isodisomic than in the monotelodisomic.

HIS paper reports the results of the linkage relationships and the arm loca- tions of the ml, and st, mutants on chromosome 4 of cotton, Gossypium

hirsutum L. Chromosomes that are telocentric for the long and short arms were used in the mapping studies. Recombination data involving an isochromosome for the short a m are also given.

MATERIALS A N D METHODS

In previous reports, the two mutants, ml, and st,, segregate as single gene differences, even though G. hirsutum is an allotetraploid. This, of course, is due to the fact that most Upland cottons are unimeric for the two genes. Both mutants have been located on chromosome 4 of the A genome by monosomic analysis (WHITE and ENDRIZZI 1965; ENDRIZZI 1975).

T583 mosaic leaf (m1J: Mosaic leaf is a recessive mutant that causes a multicolored or mosaic pattern of green, light green, pale yellow and white over the entire leaf or a segment of the leaf (LEWIS 1958). In the present paper, we are assigning the number “1” subscript to the ml symbol to distinguish it from i t s duplicate, ml,. The normal ME, allele was recently isolated from the African introduction Mu8b.

In the testcrosses, the nonhybrid parent was T583 (ml, St , ) in all cases except one, where TMI ( M l , St,) was used.

AGI club stigma (stJ : The homozygous recessive has an extremely small stigmatic surface located at the tip of the style. It produces fully viable pollen, but, due to the lack of a functional stigmatic area, the flowers are completely female sterile. The normal allele exhibits incomplete dominance, which enables individuals to be dasdied as S t , St, OT St , st,. Heterozygotes have

Regional Research Project 5-77. IContribution No. 2.970 from the Arizona Agricultural Experiment Station. Part of this research was done under

a Present address: Plant Breeder, Anderson Clayton, Oilseed Processing Division, Mesa, Arizona.

Genetics 94: 979-988 April, 1980.

980 J . E. ENDRIZZI A N D R. BRAY

long pointed bolls, stigma lengths intermediate between those of St, St, and st, st, and are par- tially female sterile (MCMICHAEL 1965). Stigma length and boll shape can, therefore, be used to distinguish between St , St , and St, st, genotypes. St , St, plants have the typical round or oblong boll of Upland cottons, whereas St, s t , plants have a very distinctive long boll, terminat- ing in an unusually long tip. The two hemizygous genotypes, St,- and st,-, have boll shapes typical of St, St , and St, st,, respectively. Stigmas of st, hemizygous may be intermediate in development to that normally seen in St, st, and st, sf, genotypes, but more often they tend to be more like st,si,. Homozygous recessive (s, st, ) plants are female sterile, but hemizygous re- cessive ( s t plants are partially female fertile and can be self-pollinated. The female-sterile homozygous recessive st , st, is normally maintained by pollinating, in successive generations, St, st, plants with pollen from st, st, plants. We have taken advantage of the fertility of the hemizygote to develop a true-breeding ml, st, line. This was done by constructing plants mono- somic for chromosome 4 (mono-4) and with the ml, st, gemtype. The true-breeding ml, st, is maintained by self-pollinating mono-4 plants, whose progeny will consist of two types: mono-4 progeny that are hemizygous ml, st, and disomic progeny that are homozygous ml, st,.

The mono-4 ml, st, was used in crosses with monotelodisomics 4L and 4s and with monoisodi- somic 45’. In addition, AGI club stigma M1, st, was crossed with monotelodisomic 4L (Table 1 ) . In crosses in which the chromosome deficient types are used as female, two kinds of F, progeny are recovered: disomics and monotelodisomics or monoisodisomics. Disomic F, sibs with the ml, st,/Ml, St , genotype and an F, with the Ml, st,/ml, St, genotype from the cross of AGI x T583 were used for estimating recombination between the two loci in normal homologs. (Table 1 ).

Monotelodisomics and monoisodisomics: SEARS’ (1974) nomenclature was adopted here for the deficiencies. For example, a monotelodisomic in cotton has 25“ + tl” (tl” = telocentric + com- plete chromosome). The chromosome-arm deficiency types for chromosome 4 used were the following: one monotelodisomic line deficient for the short arm (telo-4L) ; two monotelodisomic lines, each deficient for the long arm (telo-4s); and one monoisodisomic line deficient for the long arm (iso-4s) (Figures 1-8).

The tests that identified these as chromosome 4 deficiencies and their linkage relationships with the mll stl mutants are described below.

RESULTS 4 N D DISCUSSION

Cytogenetic tests for identification of chromosome deficiencies Monotelodisomic 4L: This line contains the telocentric for the long arm of

chromosome 4 that originated as a single plant among the progeny of an uniden- tified monosomic. The telocentric was identified as chromosome 4 in a cross with the T4-5 IV, translocation. In this translocation. chromosomes 4 and 5 can be easily recognized in metaphase I configurations of heterozygotes (ENDRIZZI 1974). Chromosome pairing in the critical Fl’s was primarily 24 bivalents plus a chain-of-four chromosomes that included the telocentric; in these configura- tions. it was readily apparent that the telocentric was chromosome 4.

The telocentric chromosome was also shown to be chromosome 4 in a cross with T583 ml,. In this cross, two kinds of F,’s were recovered: one had the nor- mal phenotype and were disomics and the other had the mosaic (ml) phenotype and were monotelodisomics. These results not only establish that the telocentric is chromosome 4, but that the ml, locus is in the short arm. The monotelodisomic chromosome pair occurred as two univalents in 4.3% of more than 200 meta- phase I cells,

Phenotypically, monotelodisomic 4L plants exhibit, to some degree, most of the syndrome of morphological characters expressed by mono-4 plants, which,

CYTOGENETICS OF ml, st, MUTANTS IN COTTON 98 1

2 4 6 a FIGURES 14.-Monotelo- and monoiso-disomic bivalents of cotton at metaphase I. FIGURES 1,

3 , 5 and 7 are photomicrographs and FIGURES 2,4,6 and 8 are ink drawings. The left bivalent in FIGURES 2, 4 and 6 is a drawing of the bivalent in the upper corresponding figure, whereas the remaining two in each are drawings of bivalents from other MI cells. The arrows point to the telocentric and the isochromosomes in the bivalents.

FIGURES 1. 2.-Monotelodi~mic 4L bivalents. FIGURES 3, 4.-Monotelodisomic 4s bivalents. FIGURES 5, 6.-Monoisodisomic 4s bivalents with one arm of the isochromosome and the homol- ogous arm of the normal chromosome joined by a chiasma. FIGURES 7, 8.-Monoisodisomic 4s bivalent with both arms of the isochromosome and the homologous arm of the normal chromo- some joined by chiasmata.

in this case, is the bushy habit, long peduncle and longer bracteole teeth, but not longer bolls.

Monotelodisomic 4s: Two telocentrics for the short arm were used. Each originated as a single plant, one in a mono-7 line and the other in a mono-3 line. Both telos were identified as chromosome 4 in crosses with the T4-5 IV, trans- location and with ml, st,. In the former case, pairing in the critical F, consisted primarily of 24 bivalents plus a chain-of-four chromosomes terminating with the telocentric; here, again, it was apparent that the telocentric was chromosome 4. In the cross of monotelodisomic 4s with ml, st,, the F1 monotelodisomic plants expressed the phenotype for the MI, st, alleles, whereas their F, disomic sibs had the normal (MI, S t , ) phenotype. This, of course, not only reestablished that the telo is chromosome 4 and MI, is in the short arm, but also that the st, locus is in the long arm.

Phenotypically, monotelodisomic 4s plants rather closely exhibit the major

982 J. E. ENDRIZZI AND R. BRAY

syndrome of morphological characters expressed by mono-4 plants, including the bushy habit, long peduncle, longer bolls and bracteole teeth. In 7.7% of 232 metaphase I cells, the heteromorphic pair occurred as two univalents, which is 3.4% greater than that observed for telo-4L. We have estimated that these frequencies of nonpairing have little, if any, effect on overall recombination esti- mates. It is assumed that most pollen produced by the pollen mother cells with two univalents are likely to be deficient for chromosome 4 and nonfunctional.

Monoisodisomic 4s: The monoisodisomic was isolated as a single plant in progeny of mono-4, and it was identified as chromosome 4 by the same two kinds of tests as conducted with telo-4S above. Plants with the is0 are phenotypically similar to telo-BS plants, which might be expected since the long arm is missed in both cases.

Since an isochromosome will have a tendency to synapse with itself, the fre- quency of two univalents is higher in monoisodisomics than in monotelodisomics. In the present case, the monoisodisomic occurred as two univalents in 10.5% of 372 MI cells.

Linkage analysws Disomics: Linkage studies were conducted with the ml, and st, after they

were located on chromosome 4 by monosomic tests. The data included a coupling F2 population and one coupling and one repulsion testcross population (Table 1 ) . The F, and the testcross in coupling show that the two loci are 50 or more cross- over units apart. The testcross in repulsion indicated linkage with 47.3 * 2.3% recombination; however, we doubt the linkage is real since the coupling testcross, consisting of a much larger population, did not show evidence of linkage, and a testcross population of 1000 plants is required to detect a maximum of 47% recombination at a confidence level of P = 0.05 (STEPHENS 1955). Th’ is con- clusion is supported by the fact that the monotelodisomic data show that the two loci are approximately 64 crossover units apart.

Monotelodisomic 4L: Here. in contrast to telo-4S7 the 1111, locus is hemizygous and the St, locus is heterozygous. As shown in Table 1, two genetic types of monotelodisomic F,’s were used in the testcrosses. In the first one, the normal chromosome carried the repulsion genotype MI, st, and the telocentric chromo- some carried only the St, genotype. The testcross results with this monotelo- disomic show that the recombinants were 121 MI, ml , St, St, disomics and 14 ml, ml , St, st, monotelodisomics, giving a st,-to-centromere distance of 47 map units. The frequency of pollen transmission of the telocentric could be detected easily by the appearance of mosaic ( m l ) plants in the testcross progeny at the young seedling stage, since the female parent in the testcross had the ml, ml , genotype. The percentage of mosaic seedlings was 14.4, which is the rate of pollen transmission of the telocentric 4L chromosome. This estimate may be slightly less than the true transmission rate since a small percentage of seedlings with the ml genotype do not express the mosaic phenotype in the cotyledons and early leaf stages of the seedlings. Not all mosaic seedlings were transplanted

CYTOGENETICS OF ml, st, MUTANTS IN COTTON

TABLE 1

Segregation of ml, st, in F2 and testcrosses of disomics and in zestcrosses of monotelodisomics and a monoisodisomic

983

ml ml - MI -- St St St st st 5t St St Si st st si Total ~2 P % Recomb.

Disomics F, ml st/M1 S t selfed 60 116 63 26 39 15 319 3.4 0.7-0.5 56.2 2 ( m l S t ) X ml s t /Ml S t 182 197 174 188 735 1.51 0.7-0.5 50.5 2(ml S t ) X M1 s t lml St 101 154 91 119 465 19.81 >O.Ol 47.3 +. 2.3

Monotelodisomics telo 4L 2(mZ S t ) x M1 st

St -- 121 142 10 14 287 st-centromere = 47.0

2(Ml S t ) x m l st - 411 28 __- 69 st-centromere = 52.2

St

telo 4s 2(ml S t ) x ml st

combined: 356 st-centromere = 48 -t- 2.6

-- - 6 76 2 397 481 ml-centromere = 16.2 1.7 M1

Monoisodisomic 4s 2(ml S t ) x m l st 4 80 0 284 368 ml-centromere = 21.7 & 2.1

> M1 M1

1 = 5 recombinant monotelodisomics plus 36 recombinant disomics.

to the field; therefore, the 24 ml plants given in Table 1 for this testcross should not be constructed as the actual frequency of pollen transmission for telo4L.

For the second testcross involving telo-4L, the genotype of the monotelodisomic consisted of a normal chromosome carrying ml, st, in coupling and a telocentric chromo,some carrying St, only. Here, the 41 m2,-Str St , plants in Table 1 included five nonrecombinant monotelodisomics and 36 recombinant disomics; the 28 Ml, ml, St , st, plants are nonrecombinant disomics. These data show 52.2% recombination between St, and the centromere. Combining the data of two mono- telodisomics for 4L gives a recombination estimate of 48 * 2.6 map units between St, and the centromere.

Monotelodisomic 4s: Tests described above established that the ml, locus is located in the short arm, and the st, locus is located in the long arm of chromo- some 4 . The monotelodisomic 4 s is heterozygous MI, ml, and hemizygous for st,, i.e., the normal chromosome has the ml, st, genotype, and the telocentric chromosome has the MZ, genotype only. Data in Table 1 shows that 78 recombi- nants (MI , ml, St, st, and ml , ml , St, St,) were recovered in the 481 testcross progeny, showing that the ml, locus in the short arm is 16.2 crossover units from the centromere. The pollen transmission rate of the short-arm telocentric chro- mosome was 1.7%. The plants receiving the telocentric via the male gamete were

984 J. E. ENDRIZZI A N D R. BRAY

detected by their bushy habit, long peduncle and normal boll shape. They are the six noncrossover (Mlr ml, St , St,) and the two crossover (ml, ml, St , St,) plants (Table 1 ) .

In conclusion, the data for telo-4S and telo-4L show that the ml, locus is in the short arm and 16.2 map units from the centromere, and that the st , locus is in the long arm and 48 or more map units from the centromere. The true ml- centromere map distance may be greater than 16.2, since a reduction in recom- bination in telocentrics for both the long and short arms of chromosome 6 has been observed (ENDRIZZI and KoHm 1966).

The disomic data discussed earlier showed that ml, and st, segregate inde- pendently and are, therefore, 50 or more map units apart. Recombination data for telos-41 and 4 s gave a map distance of at least 64 units between the two loci. Since 50% recombination is the maximum that can be observed in two-point linkage crosses, the 14% increase in map distance cannot be interpreted as the result of compensatory recombination. The 64 map units reflect only an approach to the true map distance between ml, and St, , which is likely to be greater than our estimate shows.

Monoisodisomic 4s: Monoisodisomic 4s F, is basically identical to monotelo- disomic 4 s F,, except that the isochromosome has two doses of the Ml, allele. Recombination in the monoisodisomic gave an mZl-to-centromere distance of 21.7, which is 5.5% greater than the 16.2 distance observed in telo-$S.

Percentage pollen transmission of the isochromosome ( 1.4) was essentially the same as for telo-4S (1.7). Nonpairing of the normal homologue and the iso- chromosome was slightly higher than that of the two chromosomes of the mono- telodisomic 4 s (10.5% us. 7.7%). A contingency test, however, showed that the two nonpairing frequencies are not significantly different. Also, the contingency test conducted with the two testcross populations revealed no significant differ- ence in their crossover values, and the two values overlap at the 95% confidence level. Statistically, therefore, the cytological and recombinational behavior of the tWG cytotypes are basically identical, even though monoisodisomic 4 s had 5.5 % more recombination than monotelodisomic 4s.

A reduction in crossing over in the centromeric region was shown to occur in monotelodisomics of cliromosomc 6 of cotton (ENDRIZZI and KOHEL 1966). Whether mono telodisomics of other cotton chromosomes behave in a similar manner is unknown. However, if monotelodisomics of chromosome 4 also show a reduction, we can assume that the recovery of an equal, i f not greater, amount of recombination in the monoisodisomic is an indication that exchange in the centric region of the triplicated arms may be greater than that normally occurring in this region of the monotelodisomic.

It was presumed that recombination frequency in the monoisodisomic would be less and not equal to, or perhaps greater than, the frequency in the monotelo- disomic. The genetic length of the short arm is unknown, but the exchange fre- quency obtained in a heterozygous interchange involving this arm (WILSON and KOHEL 1970) shows that chiasma frequency is at least one, if not greater.

CYTOGENETICS OF ml, st, MUTANTS IN COTTON 985

If the frequency is two and they occur randomly among the three homologous arms, it would be expected that one-ninth (1 1 % ) of the cells would have both chiasmata occurring between the two arms of the isochromosome, resulting in two univalents at MI. This is nearly identical to the 10.5% nonpairing observed in the monoisodisomic, indicating that chiasma frequency in the tripli- cated arms may be close to two. Assuming that chiasmata are random between the three homologous arms and that their frequency in the ml-centromere region is the same as that in the nionotelodisomic, the expected frequency of M1 recombinants would be 10.6%, which is about 5% less than the 16% recorded in the monotelodisomic. On this basis, the 21.7% recombination recovered in the monoisodisomic is twice the expected frequency. Such an increase indicates that crossing over in the ml-centromere region of the monoisodisomic may indeed be greater than that occurring in the same region of the monotelodisomic.

In triploids of Drosophila melanogaster, increase in crossing over in the cen- tromeric regions of chromosomes 2 and 3 was reported by REDFIELD (1930, 1932). She noted a regular gradation in exchange frequency from the center of the chromosome to the end and suggested that crossing over in triploids pro- vides a better measurement of the actual physical distance between genes than does diploid crossing over.

BEADLE (1934, 1935) studied crossover frequency in Drosophila triploid females and triploid intersexes with two of the X chromosomes attached at their centromeric ends (attached-X) ; he noted that crossing over near the centromere is higher between the two attached chromosomes than between either of the attached chromosomes and the normal X chromosome. He also noted that cross- ing over fell off progressively from the spindle attachment to the telomere end of the chromosomes. In attached-X diploid females, however, crossing over in the centric region of the attached-X chromosomes was similar to that of the standard diploid females.

What is of interest in the Drosophila studies is the increase in crossing over in the centromeric region of chromosomes in the presence of extra chromatin. These and similar studies show an interchromosomal mechanism operating in the triploid level that increases chiasma frequency ( SCHULTZ and REDFIELD 1951). Subsequent to the elegant studies in Drosophila, a number of reports demonstrate that extra chromatin has an interchromosomal effect in crossing over.

MATHER (1939) recorded a significant increase in total number of chiasmata in maize plants that were monotelotrisomic for the short arm of chromosome 5. Although not reported, it is possible that chiasma frequency within the mono; telotrisomic was also increased. WARD (1979), however, could find no effect on either the distribution or total chiasmata in maize trisomics for chromosomes 7,8,9and10.

DRISCOLL and DAVEY (1970) studied the frequency with which homologous arms of monoisochromosomes were associated at metaphase I, indicating the for- mation of at least one chiasma. In normal material, the two arms of the mono-

986 J. E. ENDRIZZI A N D R. BRAY

isochromosome were joined by chiasma with about the same frequency as homologous arms o€ normal chromosomes. They concluded that attachment of homologous arms to the same centromere does not normally increase the chance of chiasma formation in the two arms. In colchicine-treated material, however, the monoisochromosome did not behave like normal chromosomes. Here, normal chromosomes exhibited a high degree of asynapsis; whereas, the monoisochro- mosome showed normal chiasma formation. It was concluded that under the effects of colchicine, attachment to a common centromere affords a greater chance of homologous arms entering into chiasma formation. They pointed out that the data established at least two separable stages or processes in pairing of chromosomes. The first involves the alignment or spatial relationship of homol- ogous parts, which is susceptible to colchicine. The second involves synapsis and chiasma formation, which is immune to colchicine. MOENS (1 968) and STERN and HOTTA (1973) also divided chromosome pairing into two main processes, matching or alignment of homologs and stabilization through subsequent synap- tonemal complex formation. DRISCOLL and DAVEY (1970) stated that “immu- nity of the homologous arms of the isochromosome to colchicine is due to the fact that the arms bear a close spatial relationship to each other, at least in the regions close to the centromere. Once the requirement is satisfied, colchicine does not affect the remaining stages of chromosome pairing.”

A colchicine-binding protein has been identified by HOTTA and SHEPARD (1973) in the nuclear envelope of lily meiocytes at early prophase, which was evidence that the nuclear envelope plays a specific role in initiating chromosome pairing.

It is generally assumed that at zygotene the ends of homologs, attached to the nuclear envelope, start moving together to initiate synapsis; and when the ends are at a critical distance of 2000 to 3000 A, synaptonemal complex formation is initiated (MOENS 1969, 1973; WELSCH 1973; GILLIES 1975a). Later, pairing initiation may occur at sites along the bivalent, resulting in a “buttoning-up” rather than a “zippering” process (MOENS 1968; GILLIES 1975a,b).

In wheat monoisochromosomes, the homologous telomeric ends of the dupli- cated arms, perhaps due in part to attachment to a common centromere, are normally at the critical distance in their attachment to the nuclear envelope for the initiation of pairing and synaptonemal complex formation. Consequently, the first stage of pairing between the arms of the isochromosome would not be affected by colchicine. The second stage, which results in synapsis and the for- mation of the synaptonemal complex and chiasmata, would proceed normally.

If this is characteristic of isochromosomes in general, the increase in recombi- nation in the centromeric region of the cotton monoisodisomic could be the result of the closer alignment of the ends of the homologous arms of the isochromosome to each other than to the end of the normal chromosome. Synapsis and chiasma formation would occur initially between the two is0 arms, followed by synapsis and chiasma in the proximal regions, most frequently between the normal chro- mosome and one of the arms of the isochromosome, resulting in a higher frequency of crossing over than normally expected.

CYTOGENETICS OF ml, s t , MUTANTS IN COTTON 987

SUMMARY

Telocentric chromosomes for the long and short arms and an isochromosome for the short arm of chromosome 4 have been identified by cytological tests with T4-5 IV, translocation and in genetic tests with the m2, st, mutants.

Recombination analysis of disomic F17s in coupling and repulsion showed that ml, st, loci segregated independently. In crosses of the monotelodisomics with the ml, and stl mutants, results showed that the ml, locus is in the short arm and 16.2 CM from the centromere and that the st , locus is in the long arm and 48 or more CM from the centromere.

A monoisodisomic for the short arm had 21.7% recombination in the m2,- to-centromere region, which is 5.5% higher than in monotelodisomic 4s. The 21.7% was not significantly different from the 16.2% recorded for the mono- telodisomic; however, it is twice the expected frequency assuming random exchange between the three homologous arms. It was concluded that the increase could be due to a closer alignment of the ends of the homologous arms of the isochromosome to each other than to the end of the normal chromosome. Conse- quently, synapsis and chiasma formation would begin between the two is0 arms, followed by nonrandom synapsis and exchange between the triplicated arms in their proximal regions.

LITERATURE CITED

BEADLE, G. W., 1934 Crossing over in attached-X triploids of Drosophila melanogaster. J. Genet. 29: 277-309. - , 1935 Crossing over near the spindle attachment of the X chromosome in attached-X triploids of Drosophila melanogaster. Genetics 20 : 179-191.

DRISCOLL, C. J. and N. L. DAVEY, 1970 Chromosome pairing: Effect of colchicine on an iso- chromosome. Science 169 : 2%291.

ENDRIZZI, J. E., 1974 Alternate-1 and alternate -2 disjunctions in heterozygous reciprocal trans- locations. Genetics 77: 55-60. - 1975 Monosomic analysis of 23 mutant loci in cotton. 1. Heredity 66: 163-165.

ENDRIZZI, J. E. and R. J. KOHEL, 1966 Use of telosomes in mapping three chromosomes in cotton. Genetics 54: 535-550.

GILLIES, C. B., 1975a An ultrastructural analysis of chromosome pairing in maize. C. R. Trav. Lab. Carlsberg 40: 135-161. --, 1975b Synaptonemal complex and chromosome structure. Ann. Rev. Genet. 9: 91-109.

HOTTA, Y. and J. SHEPARD, 1973 Biochemical aspects of colchicine action on meiotic cells. Mol. Gen. Genet. 122 : 243-260.

LEWIS, C. F., 1958 Genetic studies of a mosaic leaf mutant causing somatic instability in cotton. J. Heredity 49: 267-271.

MATHER, K., 1939 Competition for chiasmata in diploid and trisomic maize. Chromosoma 1: 119-129.

MCMICHAEL, S. C., 1965 Inheritance of aberrant stigmas in the flowers of Upland cotton. J. Heredity 56: 21-22.

MOENS, P. B., 1968 The structure and function of the synaptonemal complex in Lilium longi- florum sporocytes. Chromosoma (Berl.) 23: 4184511. - , 1969 Thefinestructureof meiotic chromosome polarization and pairing in Locusfa migratoria spermatocytes. Chromo- soma (Berl.) 28: 1-25. - 1973 Quantitative electron microscopy of chromosome organization at meiotic prophase. Cold Spring Harbor Symp. Qua t . Biol. 38: 99-107.

988 J. E. ENDRIZZI AND R. BRAY

REDFIELD, H., 1930 Crossing over in the third chromosome of triploids of Drosophila melano- gaster. Genetics 15: 205-252. -, 1932 A comparison of triploid and diploid crossing over for chromosome I1 of Drosophila melanogaster. Genetics 17: 137-152.

Interchromosomal effects on crossing over in Drosophila. Cold Spring Harbor Symp. Quant. Biol. 16: 175-197.

SCHULTZ, J. and H. REDFIELD. 1951

SEARS. E. R., 1974 STEPHENS, S. G., 1955 STERN, H. and Y. HOTTA, 1973 WARD, E. J., 1979

WELSCH, B., 1973

WHITE, T. G. and J. E. ENDRIZZI, 1965

WILSON, F. D. and R. J. KOHEL, 1970

Telocentric chromosomes in wheat and their uses. Genetics 77: s59. Linkage in Upland cotton. Genetics 40: 903-917.

Biochemical controls of meiosis. Ann. Rev. Genet. 7: 37-66. Chiasma frequency and distribution in a maize family segregating for KIO

Synaptonemal complex un chromosomenstruktur in der achiasmatischen

Tests for the association of marker loci with chromo-

and trisomy IO. Genetics 92: 223-230.

spermatogenese yon Panorpa communis (Mercoptera) . Chromosoma (Berl.) 43 : 19-74.

somes of Gossypium hirsutum by the use of aneupoids. Genetics 51: 605-612. Linkage of green-lint and okra-leaf genes in a reciprocal

Corresponding editor: R. L. PHILLIPS translocation stock in Upland cotton. Can. J. Genet. Cytol. 12 : 100-104.