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The Relationship Between Chiasmata and Crossing Over in Triticum Aestivumi

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The Relationship Between Chiasmata and Crossing Over in Triticum Aestivumi THE RELATIONSHIP BETWEEN CHIASMATA AND CROSSING OVER IN TRITICUM AESTIVUMI T. K. FU* AND E. R. SEARSS Department of Agronomy, University of Missouri, Columbia, Missouri 65201 Manuscript received March 19, 1973 Revised copy received June 8, 1973 ABSTRACT Telocentrics for the p arm of chromosome 4A and the long arm of 6B were used as cytological markers for the determination of chiasma frequency. In concomitant studies of recombination, terminal segments of rye and T. um- bellulatum chromatin carrying Hp (Hairy peduncle) and Lr9 (Leaf-rust re- sistance), respectively, marked 4A and 6B. Two temperatures, 21" and 32", were used for bgth the 4A and 6B experiments.--Only one chiasma was observed in each heteromorphic bivalent. Because there was a substantial re- duction in pairing between diakinesis and metaphase I, all determinations of chiasma frequency were made at diakinesis. In the 21" experiments, agree- ment was good between genetic recombination and cytological prediction on the basis of the partial chiasmatypy hypothesis that each chiasma represents a crossover. At 32" both chiasma frequency and crossing over, but particu- larly the latter, were strongly reduced. The fewer crossovers than expected are explained in part hy stickiness of chromosomes at the high temperature, some- times resultkg in adjacent chromosomes being wrongly scored as having a chiasma, and in part by premetaphase disjunction of some recombined bivalents and subsequent independent behavior of the two resulting univalents.-Male transmission of the 4A telocentric from the heteromorphic bivalent was un- usually high: 51 % at 21" and 31% at 32". 0 explain chiasma formation and the pairing relations of the chromatids, two hypotheses have been advanced: the partial chiasmatypy hypothesis (JANS- SENS 1909,1924), which holds that each chiasma is the consequence of a previous crossover between nonsister chromatids; and the classical hypothesis (MCCLUNG 1927; SAX1930), which maintains that chiasmata are mere changes of partner resulting from alternate opening out of sister and nonsister chromatids from the pachytene bundle of four. With the partial chiasmatypy hypothesis, there is a one-to-one correspondence between chiasmata and crossovers; whereas with the classical hypothesis there is no such relation, for crossovers occur only when two of the four chromatids at a chiasma break and rejoin. Cooperative investigations of the Department of Agronomy, Missouri Agriculmal Experiment Station, and the Agri- cultural Research Service, US. Department of Agriculture. Paper No. 6591 of the Journal Series of the Missouri Station. The material herein represents part of a dissertation presented by the senior author in partial fulfillment of the require- ments for the degree of Doctor of Philosophy at the University of Missouri, Columbia. * Presently Geneticist, VA Hospital, Wilshire and Sawtelle Blvds., Los Angeles, California 90073. Geneticist, Agricultural Research Service, U.S. Department of Agriculture. Genetics 75 : 231-246 October, 1973 232 T. K. FU AND E. R. SEARS Although the partial chiasmatypy hypothesis has come to be widely accepted, the evidence favoring it over the classical hypothesis is not entirely conclusive. It is supported by studies on heteromorphic bivalents (DARLINGTON1936; BROWN and ZOHARY1955; DRISCOLLand SEARS1965), reciprocal translocations ( NODA 1960, 1967). and a dicentric chromosome (STEINITZ-SEARSand SEARS1953), but not by observations on male Drosophila (KAUFMAN1934; COOPER1949). Con- flicting evidence has been reported from double-interlocking bivalents (favoring partial chiasmatypy: MATHER1933; favoring the classical hypothesis: MAT- SUURA 1944), paracentric inversions (favoring partial chiamatypy : DARLINGTON 1936 and BROWNand ZOHARY1955; favoring classical: MATSUURA1950 and HAGA1953), and autoradiography (favoring partial chiasmatypy: CHURCHand WIMBER1969 and PEACOCK1970, 1971; favoring classical: TAYLOR1965 and MOENS1966). Almost all previous studies have been of two types, neither of which is capable of giving an unequivocal answer with the materials available. The more direct approach has been to compare the total amount of crossing over in all the chromo- somes of an organism with the total number of chiasmata observed at meiosis. As HENDERSON(1970) points out, “this approach is unlikely to have the precision necessary to test unambiguously the equivalence of chiasmata and cross-overs, for a great deal depends upon the positions in which chiasmata form, in relation to the positions of the genes known within each linkage group.” The other approach has been the purely cytological one of trying to demonstrate a corre- spondence between chiasma formation and exchange of segments. As HENDERSON again says, this is not really a test for correspondence between chiasmata and genetic crossing over. for it is possible that genetic reconibination does not always lead to segmental exchange. A study by DRISCOLLand SEARS(1965) made use of a cytologically marked (heteromorphic) bivalent in which an alien segment in one chromosome consti- tuted a terminal marker. They were thus theoretically able to detect a direct relation between chiasmata and crossing over in this one chromosome arm. Unfortunately their results were inconclusive, perhaps because environmental conditions were not controlled and chiasmata were scored at a stage when some chiasmata had already disappeared. DRISCOLLand BIELIG’S(1968) similar study involving an alien segment very tightly linked to the centromere resulted in only a single crossover, and thus yielded a crossover value with too little reliability for critical comparison with chiasma frequency. In this report, evidence strongly supporting the partial chiasmatypy hypothesis is provided by the use of two genetically and cytologically marked lines of com- mon wheat (Triticum aestiuum L.) tested at two temperatures. One of these lines was the same as that of DRISCOLLand SEARS. MATERIALS AND METHODS In experiments involving chromosome 4A, ‘Chinese Spring’ wheat and the following de- rivatives of Chinese Spring were used: 1. Homozygous Hp translocation line. Carries a gene Hairy peduncle (usually called Hairy neck) in a rye-chromosome segment that replaces the terminal portion of the so-called p arm CHIASMATA AND CROSSING OVER IN WHEAT 233 of chromosome 4A (DRISCOLLand SEARS1965). Hp behaves as a simple dominant to the absence of the locus. Because no crossing over occurs between the rye segment and the corresponding part of the unmodified 4A chromosome, Hp constitutes a terminal marker. 2. A line ditelocentric for the /3 arm of chromosome 4A and monotelocentric for the 01 arm of the same chromosome. In experiments involving chromosome 6B, the following derivatives of Chinese Spring were used: 1. 'Transfer.' Homozygous for a dominant gene Lr9 that conditions resistance to the leaf-rust fungus Puccinia recondita Rob. ex Desm. f. sp. tritici. Lr9 (henceforth shortened to Lr) lies on a Triticum umbellulatum (Zhuk.) Bowden (Aegilops umbellulata) chromosome segment that replaces the terminal region of the long arm of chromosome 6B (SEARS1956, 1966) and always remains unpaired in heterozygotes. On the same arm of the chromosome, located very close to the centromere, is another gene b2 (henceforth designated simply b) whose dominant allele suppresses awns. The short arm of the chromosome is marked by CO,whose recessive allele when homozygous or hemizygous causes necrotic patches in the leaves. 2. A line ditelocentric for the long arm of chromosome 6B carrying B and lacking the Lr segment. 3. A multiple-recessive line. Homozygous for b and CO, lacking the Lr segment. Crosses were made (Figures 1, 2) so as to give rise to plants heterozygous for both the termi- nal genetic marker and the cytological marker (absence of one arm). The plants were trans- ferred to a growth chamber at a constant 21" (optimum temperature) or 32" (high temperature) and 15-hour-per-day illumination about a week before they reached the meiotic stage, providing H" NON CROSSOVER CROSSOVER FIGURE1.-Diagram sh3wing the crossing scheme and the progeny in the chromosome-4A exporiment. 234 T. K. FU AND E. R. SEARS B - X bBL / LOb X -CO -//\\- Lr9 6 119 b CO -- d I b LO b CO b CO -b NONCROSSOVER CROSS OVER FIGURE&.-Diagram showing the crossing scheme and the progeny in the chromosome-6B experiment. ample time for chromosome pairing to reach a stable level. From each plant, two spikes were collected for the determination of chiasma frequency in pollen-mother cells. At least three spikes from each were used for pollinations, as males in crosses to Chinese Spring in the 4A experi- ments and as females in crosses to the multiple-recessive line in the 6B experiments. Offspring of the 4A crosses were checked in root tips for chromosome number and as mature plants for liairi- ness of peduncle. The 6B offspring were inoculated with urediospores of Puccinia recondita tritici and scored at the appropriate times for leaf-rust reaction, awnedness, and corrodedness. For somatic chromosome counts, root tips from germinating seeds were treated with mono- Eromonaphthalene for five hours, fixed in glacial acetic acid, stained according to the Feulgen procedure, and squashed in aceto-carmine. For the meiotic studies, anthers were fixed in acetic- alcohol (1:3), stained in Feulgen, an2 squashed in aceto-carmine. RESULTS Comparisoln of metaphase I and diakinesis When the telocentric chromosome paired with its homolog and a chiasma was formed between them, the resulting heteromorphic
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