Meiotic metaphase I pairing behavior of a 5BL recombinant isochromosomein wheatLiLi Qi, Bernd Friebe* & Bikram S. GillWheat Genetics Resource Center, Department of Plant Pathology, Throckmorton Plant Sciences Center,Kansas State University, Manhattan, KS 66506-5502, USA; Tel: (  1) 785 5322364; Fax: (  1) 7855325692; E-mail: [email protected]* CorrespondenceReceived 19 July 2000; accepted for publication by H. Macgregor 14 August 2000Key words: meiotic metaphase I pairing, recomb inant isochromosome, Triticum aestivum, TriticumdicoccoidesAbstractA recombinant isochromosome i5BLrecof wheat was developed with one arm and the proximal 36% ofthe other arm of Chinese Spring (CS) origin and the distal 64% of the recombined arm of Triticumturgidum subsp. dicoccoides origin. The i5BLrecprovides an unusu al op portunity to analyze the roleof the centromere or arm heterozygosity in chromosome prealignment and synapsis during meiosis.In monosomic condition, the i5BLrecformed a ring univalent in 86.8% of the pollen mother cells(PMCs) at meiotic metaphase I. In the disomic condition, the two i5BLrecpreferentially paired asa normal bivalent in 74.8% of the PMCs, which differed signi¢cantly (p < 0.01) from the normalbivalent pairing of 51% observed in diisosomic 5BL c hromosomes of the CS (Di5BLCS) control plants.In plants with one i5BLrecand a normal 5BCS,thelongarmof5BCSpaired with the homologous armof i5BLrecin 54.4% of the PMCs, and 40.4% of the PMCs had a 5BCSunivalent and a i5BLrecringunivalent. The implications of the i5BLrecpairingdataonthemechanismofPh1 gene action arediscussed.Introductio nIn allohexaploid bread wheat, Triticum aestivumL. (2n  6X  42, genomically AABBDD), thePh1 gene is lo cated on the long arm of chromo-some 5B and ensures that only homologous butnot homoeologous chromosomes pair and rec-ombine at meiosis (Sears & Okamoto 1958, Riley& Chapman 1958, Riley 1968). Therefore, com-mon wheat is a diploid-like allopolyploid in whichonly bivalents between homologous chromosomesare formed at metaphase I of meiosis (for review,see Sears 1976).The mechanism of the Ph1 gene action has beenstudied intensively by analyzing pairing behaviorof isochromosomes with or without colchicinetreatment (Feldman 1966, Driscoll & Darvey1970, Kato & Yamagata 1982, Vega & Feldman1998a, 1998b). Three main hypotheses have beenput forward in light of the information collectedChromosome Research 8: 671^676, 2000. 671# 2000 Kluwer Academic Publishers. Printed in the Netherlandsfrom these studies: (1) Ph1 affects the time avail-able for synapsis; thus, only homologues havethe opportunity to pair (Riley 1968); (2) Ph1regulates strict diploid-like pairing at theprealignment phase by acting on the centromeres(for review, see Feldman 1993, Vega & Feldman1998a); and (3) Ph1 processes homology alongtheentirelengthofthechromosomeattheDNA heteroduplex level d uring synap sis (Ho lm& Wang 1988, Dubcovsky et al. 1995, Luo etal. 1996).Recently, we isolated a recombinant (rec)isochromosome f or the long arm of chromosome5B (i5BLrec)ofcommonwheat.Thei5BLrecisheterogenetic for the distal 64% of the long armand provides an unusual opportunity to analyzethe role of centromere and arm heterozygosityin chromosome prealignment and synapsis duringmeiosis. The meiotic pairing in plants w ithmonoisosomic, diisosomic 5BLrec, and monoiso-somic 5BLrecand a normal chromosome 5B, isreported here. The implications o f the i5BLrecpairing data on the mechanism of Ph1 gene actionare discussed.Materials and methodsA plant monosomic for a recombinant iso-chromosome i5BLrecand trisomic for ch romo-some 5D (Mi5BLrecTri5D) was available from aprevious study (Qi et al. 2000). One arm andthe proximal 36% of the other arm of the i5BLrecis of Chinese Sprin g (CS) origin, and the distal64% of the recombined arm is of T. turgidum sub-sp. dicoccoides origin (Figure 1). Other geneticstocks used in the study were 5BL mono- anddiisosomics of T. aestivum cv. Chinese Spring(CS), a nullisomic 5B^tetrasomic 5D (N5BT5D)line, and a ph1a ph2b line (Sears 1954). The5BL chromoso mes of CS and T. dicoccoides aredesignated as 5BLCSand 5BLT:dic,andthe5BLisochromosomes of CS, and the C S^T. dicoccoidesrecombinant as i5BLCSand i5BLrec, respectively.Cytogenetic analysisChromosome identi¢cation and N- and C-bandinganalyses were as described by Gill et al. (1991).ResultsMeiotic pairing of the i5BLrecThree genotypes, one monosomic for i5BLrec,onedisomic for i5BLrec, and one monosomic for bothi5BLrecand a normal chromosome 5BCS, wereselected in the self-progeny of a Mi5BLrecTri5Dplant and from the progeny of the crossMi5BLrecTri5D/CSph1aph2b. The meiotic meta-phase I pairin g of the i5BLrecwas compared withcontrol plants that were monosomic o r disomicfor i 5BLCS.In m on oso mic i5BLrecplants, a ring univalentresulting from intrachromosomal pairing betweenthe t wo arms of i5BLrecwas observed in 86.8%of the 174 PMCs analyzed (Figure 2b). The i5BLrecremained unpaired and formed a rod univalent inthe remaining PMCs (13.2%). The i5BLCScontrolformedaringunivalentin95%ofthePMCs(Table1). The frequency of ring univalents formed byi5BLrecwas not signi¢cantly different from thecontrol i5BLCS(p  0.05).In Di5BLrecplants, pairing occurredpreferentially between homologous arms. Thetwo i5BLrecchromosomes paired as n ormalbivalents in 74.8%, as two ring univalents in 14.6%(Figure 2d), as one rod and one ring univalent in4.8%, and as two rod univalents in 5.8% of the103 PMCs analyzed (Table 2). In D i5BLCSplants,the two i5BLCSpaired as normal bivalents andas two ring univalents in 51.1% and 37.8% ofthe PMCs, respectively. A rod and a ring univalentwere formed in 11.1% PMCs (Table 2). The fre-Figure 1. N-banding patterns of chromosomes: (a)5BCS,(b)5BT:dic,(c) i5BLCS,(d)i5BLrec.672 L. Qi et al.Figure 2. N-banded mitotic metaphases (a, c, e) and C- banded meiotic me taphase I PMCs (b, d, f) of plants with different chromosomeconstitutions: (a&b)Mi5BLrecTri5D paired as a ri ng univalent; (c&d) Di5BLrecpaired as two ring univalents; (e&f)Mi5BLrecplus5BCSpaired as a ring (top i5BLrec) and a rod (bottom 5BCS) unival ent. Arrows point to the isochromosomes and 5BCS.Scalebar  20 mm.Meiotic metaphase I pairing of an i5BLrecin wheat 673quency of i5BLrecbivalent pairing in Di5BLrecplants was signi¢cantly different (p<0.01) fromthat of the control Di5BLCS(Table 2).In plants with one normal 5BCSand one i5BLrecchromosome, the long arm of 5BCSpaired withthe homologous arm of