Halftetradanalysis inalfalfa usingmultiple restriction fragment length polymorphism markers

A maximumlikelihood approach ofhalf tetrad analysis (HTA)basedonmultiple restriction frag- mentlength polymorphism (RFLP)markers wasdeveloped. Thisprocedure estimates therelative frequencies of2n gametes produced bymechanisms genetically equivalent to first division restitution (FDR)orsecond division restitu- tionandsimultaneously locates thecentromere within a linkage groupofRFLPmarkerloci. Themethod wasapplied tothediploid alfalfa clone PG-F9(2n= 2x= 16)previously selected because ofitshighfrequency of2neggproduction. HTA wasbasedonfourRFLPlociforwhichPG-F9was heterozygous withcodominant alleles thatwereabsent in thetetraploid tester. Modelsincluding three linked andone unlinked RFLPloci weredeveloped andtested. Results ofthe HTAshowed thatPG-F9produced 6%FDRand94%second division restitution 2neggs. Information froma marker locusbelonging toonelinkage groupwasusedtomore precisely locate thecentromere onadifferent linkage group. HTA,together withprevious cytological analysis, indicated thatinPG-F9, FDR2neggsarelikely produced bydiplos- pory, amechanism commonamongapomictic species. The occurrence ofFDR 2neggsinplantspecies andtheir importance forcropevolution andbreeding isdiscussed together withthepotential applicability ofmultilocus HTA inthestudy ofreproductive mutants. Indiploid species, half tetrad analysis (HTA)canbecarried outwhentwoofthefourchromatids ofameiotic tetrad canbe recovered andscored forgenetic markers heterozygous inthe diploid parent. Halftetrads havebeenusedtoanalyze recom- bination forattached X chromosomes ofDrosophila melano- gaster (1)andautosomal trisomies ofhumans(2). First polar bodies andsecondary oocytes havebeenusedtoanalyze first meiotic division products inmammals(3,4),andmutants producing gametes withsomatic chromosome number(2n gametes) havebeenusedinplant species suchasZeamaysL. (5,6),Solanum tuberosum L.(7), andMedicago sativa L.(8). Theseplant mutants havebeenextensively studied because of their importance intheevolution ofpolyploid series (9, 10)and their potential useinadvanced breeding programs ofpoly- somicpolyploids, suchaspotato (11) andalfalfa. There aretwomajor modesfor2ngamete formation, based onthegenetic constitution ofthe2ngametes: first division restitution (FDR)andsecond division restitution (SDR)(12). Thedifferent cytological mechanisms that canbeincluded in eachmodeweresummarized byVeilleux (10). Nonrecombi- nant2ngametes duetotheabsence ofcrossing-over followed byFDR type2ngamete formation (FDR-NCO2ngametes) wouldmaintain theparental genotype andrepresent another interesting class of2ngametes, especially forbreeding pur- poses (12).