TffE THEO_RY AND APPLICATION OF TKE ~BACKCP~OSS 52tO(,KNIQ~ E IN COTTON BREEDING Br R, L, K• !!i.Sc.(Acmzc.), P.E.D., A.I.O.T.A. CoN:rNNTS 76 Theory of backcvossing 77 Applicalffou of baokorosaing in co~to~ breeding 78 1, Police parent 78 2. _b'em~le parent , 78 .3. Progeny size 80 ,i, Basis of seleoMon in hybrids 81 5. The eud-puint . 81. C. AddiMve factors.. 8.3 - 7. Linked factors . 8. 7gleudh~g inherit~mee . 8t 9. JJuR~ propagatlon 8'1- ii). Pm'il.y of b~uekcross parent 85 Seminary 85 lleferences . 86 LWTI~ 013 UOTI ON The essential v.'-~lueof backcrossing is that it, provides a means of limiting the hetero- geneity which we uld resuIt from 'strMght' crosses between two ~ypes, making it possible to produce a hybrid simila~ to whichev'er of the two varieties has the more valuable genetic constitution, yet containing desirable characters transferred from the other parent. Backerossing obviates the necessity for rigid selection generation after generation, in _~'=, Fa, F~, etc., by progJ:essivdy and automatieatl.y, rendering the hybrid more and more homogeneous. In many ways it is, to the plant breeder, the equivalent of line breeding to the stock breeder, with the added advantage that many plants are nob harmed even by the closest izlbreeding. t{arland (1934) gives a list of some of the gone transferenees which have been effeeted" in i=~erspeeis crosses in cotton. I{e was the re'st {o realize the value of baekerossing as a tool for the cot,ton breeder, and his work remains a tallest, one in the development of the genetical approach to cotton breeding. The otass.[cal example (in cotton) of baekcrossing with a commercial aim is probably the transference, by Harland and Evelyn, of fed plant body-weak spot-from Trinidad Red Ki&my to the Sea Island strMn, V135, and, on the genetical side, the transference, by ~arlaud (1935), of the gene R.,as from the diploid coSton Go~s.ypiw~7, e~*5ore~.~..mb. to the allotetraploid American Uphmd ( g. h.irs~t.u.~ L.) Ud. The fact that these, and all other, a~tempts a.t gone transference failed to produce com~ mercially suceessfhl t~)es, in.diea~es fauRs in the technique employed, since it is u~likely {hat, in every case, the genes which R was desb:ed to transfer had unbreakable deleterious linkages. This la.ek of success had made cotton breeders sceptical as to the valu.e of bach crossing althoagh the technique hes been successf~d in other crops, and, in consequence, its possibilities have not been adequately ex]?Ioited. R. L. K~GxT 77 The foH0wi~g notes record the writer's experience of inter- and intrasped~ic back- crossing over a number of years, dm:ing which period four distinct genes fi'om tlu-ee different spades of cotton :'~ have been suocessfulJy ~ra,nsferred to two commercial strains of Q. barbade~se L., whilst several intraspecifie transferences have been made in ft. hirsu~um L. THEOPoY OF ]SACKOI~OSSIlVC- The usual obiect in baekerossing is gene transference, but the technique has also been employed to add genetic variability to m~dsting cotton t3~pes in the 5ope that it might be possible, later, to isoh~te entirely new va.rietie# of commercial value. Notable examples of this latter use are the assortmelR of crosses sent out to the African cotton experiment stations in t932 by Hal'land and Evelyn ([Evelyn & Harland, 1934). These hybrids were very heterogeneous and were dm'ived from early backerosses of accepted American Upland varieties and Jamaica Xerophytic, Gambia-Na~ive, or Galapagos Native. the ba.ckcross parent~ being the American Upland type. The object ~ in this case., was to 'increase the genetic variability of Uzt so that selection for any particular environment wo~fld be facilitated' (Evelyn & Harland, i934), but, at the same time: ~o avoid the. complete heterogeneity which would have resuRed from straight crosses between these widely divergent types. Some of this material did not involve interspecffic hybridization, and Ducker & Miller (194t2) appear to be breeding successful varieties from (U~ x Cambodia) x U4 hybrids sent, out ])y ~-Iar]and and [Evelyn at the same time as their interspecifio crosses. Hutchinson (1938) suggests that 'an impm%ant reason for the lack ofsuccess of Harland's U4 hybrids' is that 'U~ itself was improved by selection so rapiclly that the products of selection in the hybrid material usually failed, to semi?ere ~ T]is use of inter- specific baokc.rossin~ to increase variability has yet t ~ prove, its value in. cotton breeding and it is not proposed to oleo] with it in greater detail here. In gone trs.~ference, the object is to move a single, dominant, or partially dominant, gene{: (or a small number of such genes) from one cotton type to a~].other, without deleteri- ously affecting the other qualitative or quantitati~;e characters of the backcross ]?areat varieb', A genei, al example would ]:,e the trausferen,ce of a genoa ~'om a donor parentw variety 1" to a l)ackcross parent variety Z. The proeedm'e would, be to cross 1:' • Z and cross the F I back to Z, to Droduce a baokoross proge.uy of Aa and aa ])tarts. An Aa plant selected from this first backcross progeny would agaifl be crossed with Z, giving the second backcross. This process should be repeated until the Aa ]?]ants in the baekcross progeny * TransfermJee of the blaekarm resistance genes B~ and B= fi'om G. ldrs~f.~f.m L., of the blaeksrm resisga.nee gone B:, from <~'. p~wgaftcm Sob, & Then. and of tire 6/. (~rborcl~#t.L. geue 767;8 from ]is.cloud's ~U~. (i~h'sld'~n~. x (~,r~csgen~c • Iu a.ddit,Jon, other gone transferences are head3: c<uu?letad. ] t is, ]i.erhaps: premn.t.ure to e~tl t:he 2~z/~'5'~ra.i~sf,-rence suceessNl, since spiraling t,est~ ca the fina.] ba.ckeross material a.re uot ye~ a.vaila.ble. t /B~ekcrossDo.'re~.~(or reeum'm~tparent): tha,t, pa.reat era hybrid with which it is again crossed or with M~ic}: it is repeatedly crossed. ;t It is also possible to fro.haler recessh-c geaas by baekcro~ing hut, the procedure i8 laborimls, slice it is necessary to self after "the fils~ baekcross in order, to o bgai'n homozygotes for l'urtBer b0.ellcrossing.. T.hird ~nd fifth btmkeross selecbions would similarly be sel[~d. An alternative method is to select several pla.nbs 5i each boo,i-zeroes }2rogeny for ibr~ber bn,ckerossiug a.Dd to obt~aJn self-bred seed from each. The selfed .l)l'oge~deswould rtemons~rt~te whether or not any pt~rb~ctda.rparent Dossessed the requited recessive gone. ]3ackm'ossprogenies of those lacking tkls gent wmdd be d.isoarded. w ])crier i~arent: that parent from which, by backcro~si.ng, one or more genes are transfen'ed ~o the backoross parent. 78 Ba.ekcro.s,s teeh)~iq~e in, cotton breediw are indistingrdslmble qualitatively and[ quantitatively from their backoross parch% except for the presence of A. Tlds process of elimination is shown in TabTe t (below). Table 1.. Rate of elim.Dl.atio~ of do'~or ye~.ot,y~ge by ~e~ol,;e'ross@,cd ('n.eg&et'i~N. the e.[/L,cts q[ sdectio~, c~,~d li.rUcage) Donor Baekcros~ Donor 13&ckcross paren~.... paren~% i) a,ren~o, par~n6% /0 /0 2?~ 50.0 50.0 6bh backcrosz 0-8 99,2 ls~ backcross 25'0 75-0 7~h backcross 0"~ 99.6 2ud bacJecrosa 12,5 87-5 8t'll baekcross 0-$ ,~J9.S 3rd backm'o.ss 6'3 93'7 9~h b~ckcrosu 0,1 99-fll :l:l,]t baekcross 3.t 91]-9 10gh baekoross 0,05 99-95 5~b, bt~ckot'o~s 1.6 .98-I, in the endTroduo~ of f:kis hybridization ~ number of Aa plants would be soiled and ~he balk self-bred seed sown. This would give a ]?rogeny consisting of 25 % AA : 50 c~j As: 25 % aa pla~ts. Assuming the gone A to sDow only partial dominance, then the three genotypes would be phenotypieally dis{inc~. The Aa an<[ aa plants would be pulled up before s and the remaining plan% (AA) would bg bulked t.oge~her to form the new strain, tIad die gone A been fn!ly domb nan% instead of only partially dominan% gte Aa and AA plants would have been pheno- typieahy ide.ntioa! and a short progeny row Dora each plant of the A phenotype wou]d have to be grown. All progemes producing a proportion of aa plants would be eliminated and the remaining rows would be balked as the new strain. The foregoing account gives the broad basis of tee theory of baekdrossing, bn~ success or failm'e is largely a matter of attention to the finer points of ~eohnique drab with below. APPLICATION OF ]3ACI~CR.OSSIN~ I2r COTTON BREEDING If possible, the hybrid should be the mate parent and the strain to which it is being back- crossed, the .female. The objects of Qis are threefold: (@ Any pollen from-ehe baekcross parent accidentally left on the stigma would produce self-bred plants which would automatically be eliminaeed from the backeross proge W since they wotfld not contain the gone being transferred.