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WILD SPECIES IN4BROP IMPROVEMENT

J. P. MOSS

Wild relatives are useful sources of characters denired to improve crop. Many of these apecies are resistant to pests and diseases or adapted to adverse environmente. Some have higher yields than their cultivated relatives. Genes from some vild relative. can undergo meiotic recombinatlnn. producing segregante from which the plant breeder can select improved type*: rhese gencfi can usunl ly function ef fertively in cultivars. The limit to utilization of vild relatives depends on the breeder's ability to produce hybrida. but hybrids may be sterile. Ploidy differences between crops nnd vild relatives are frequent, but manipulatins ploidy level can improve gene transfer. Triploids are often sterile. but progeny from triploids lnclude recombinants that may not occur at other ploidy levels. The Croundnut Cytogenetics Unit at ICRISAT ha6 transferred desirable genes from diploid vild specie. into tatraplnid liner crossable vith groundnot. cnabling breeders to incorporate vild species genes into locally adapted material. Triploids, hexaploidm. amphiploids and autotetraploids have been produced and backcrossed vith the cultivated groundnut. Some line8 have disease resistance and good agronomic traits, including high seed yield. and have been entered in A11 India Coordinated Oilfieedfi Project trials.

The elm of the plant breeder is to change one or more genes in a cultivar to better adapt it to its environment or to its intended use. These genes come from a ranue of sources. Moat plant breeders use other cultivated grrarplaaa. Feu attempt to transfer genes from more distantly related gemplasm even though w i l d relatives have many desirable attribmtes. Wild species have often survived pests and diseases. and have many resistance genes. may grow in a vider range of ermironments, including harsh one.. There is now evidence that they can also contribute to yield increasem.

All this can also be claimed for more exotic germplasm, but vild relatives have advantages over unrelated material. There m y

Principal cytopaneticin, Internetions1 Cram Raraarch lnnitute for the SemiArid Ttap*. IICRISAT). Patmcheru P. 0.. 502 324. A P.. India.

--

BIOTECHNOLOGY IN INTERNATIONAL AGRICULTURAL RESURCH WILD SPECIES IN CROP IMPROVEMENT

be gcnomes or parts of genomes common to cultrited and wild species, facilitating gene transfer. Wild genes may function in the genetic background of the cultivar; exotic genes may not.

T h e m have been a number of recent revievs and books on wild species utelization ( 5 . 27. 32). but relatively fev people currently transfer genes from Wild species.

This conference looks at new technology to make genes available to the breeder. This paper focuses on transferring genes of wild relatives to crops, emphasizing the work at ICRISAT using wild Arachis species to improve groundnut.

LIMITS TO THE GENE POOL

The difference betveen cultivated and wild is arbitrary. Many wild species are freely crossable with cultivars; indeed, nome wild species scarcely differ from cultivated, but survive in the wild because of a single gene difference such as rachis fragility. One wild Arachis species. A. mnnticola. freely crofisable vith A. hypogrea, h8R been used to d-he cultivar Spancross (4). The next section of the gene pool is not readily accessible. Itc limits generally follow taxcnomic divisions; it may be restricted to one species, or span even several genera. The gene pool for wheat has increased greatly in recent years, and the breeder has a wide range of potential gene donors. Isolation mny be due to crossability barrlers and the inability to produce hybrids, or to hybrid sterility, preventing further gene flow. When fertile hybrids have been produced, lack of recombination may mean that undesirable characters are alvays expressed in the desired selections (28). When the gene has been transferred, gene interaction or pleiotropy may prevent selection of the desired new phenotypes. Alternatively. transferred genes may produce effects not present in the vild species (15). Only one of these barriers may be effective. but in some cases many may occur, and the more distantly related the speclefi, the more problems are likely. Where a gene has been introduced and has undergone recombination within the cultivated genome, it can then recombine and be used by any breeder. A gene introduced by other means may not be amenable to traditional plant breeding.

This section of the gene pool comprises many apecies. Table I shows crop diverslty in ploidy levels and numbers of wild relatives. Some wild species are represented by few accessions; others have been extensively collected.

SELECTION OF PARENTS

Having decided on the gene pool, assemble all knowledge of those species. Ueually desirable genes will have first priority (31); if poseible, the knwledge should extend to the nature and number of the genes in each species. and whether the gene is the same in all species. This knowledge can come from genetic studies, but they take time and assume easy crossability and full hybrid fertility; genetic ratios from interspecific hybrids vill not be valid if meiosia is not normal. Another approach is to study mechanisms and components of resistance (30). Even where

T.bh 1. Major cto 7.d the IARCs and rh-ir vild r.latlrrs.

cultivncd -is* ~n - 'loidv lev* Wlld ~l l t i v n

A. Yru.lly rspmdumd -Ir 0. 22 21 Mmv, diploids . m m y ~ltirstd -ia,

d wild forms of cultlrnd p c i m v i p ~ "",,"iC"ht. 22 Zr Manv;diploids Gjw w.m 22 2s Fsw; diploids Vici. qr. l? 2x Verv many. dinennt bnic 6,--

numban. no inlrnprific hvbra in- volving V. hb.

Glycim m u 40 4 R Faw: diploids,led~loid~ C i m aristinum 16 Zx Fcw:diploi& Orvr. sat;". 14 Zr Many; mmllv diploids, romm trtrlplold. rrr;wm 42 ex Many wild ralativm;diplaas, tct,gioar

T. &rum 28 4x and hsx.plmdr. Widely ermable. Elymm. S~lanton. Havmidi..hilopr.Aglopyron. Seeale, Hordeum now part of m e paol

Zea mays 20 2x Related genera. with d~fbrmt bsic numberr and different plaidv laal,

Sorphvm w l p m 20 4x Mmny;diploidr,tetrlploih P.nn;oetumamcritmtm 14 Zx Faw: x-7 diploids, tetrqldb

Many; r-0 diploids. tetrolaia Arrhir h y m a 40 4x Mny;dipioids. Few; tatr~loih

8. Vaptmtirslv npmdumd: m y be halv polvploid. tripldd, ruuplmd. also Mrile md produrn C w weds & i prminate pmdy. SoIuwm tubemsum 48 4x Very mmy, n diffemnt ploidv 1.r.l~ Iwmoea Ml#*r 00 Ex M.ny.Zx.4~ Xanthmam w. ?g 2x F w : (terrmloids ocar) Lbloca*mwba 28 2r - Mmihl erulcnta 38 4 F m ; mrwlaids. Intermifit hybrid.

h ~ e been produad

resistance occurs in the cultivated species, resistance in the vild species may be controlled by different genes (23) that can be combined to give a more stahle resistanre. There is a very wide choice of cultivated germplasm (13 ) ; cultivars to be used an parents should be selected for crossabilfty with the wild species as well as for local adaptation.

THE WILD SPECIES

A wild species consists of a number of plants in their natural habitat, but for the plant breeder it is the gene pool at hi* disposal: the number of accessions he can collect from the Wild or acquire from gene banks. These accessions may differ from another. The breeder should screen a11 of them for the desired character. bearing In mind that resjstant selections u Y differ in genotype, ar in resistonce components ( 3 0 ) . ~rossability u Y differ. as it often does in the cultivated species (8. 9 , 17. 24). A study of ellozymes indicates the degree of variability in species (16). and karyotype studies will indicate similaritic6 and differences between genomes (20). The variability may lead to

WILD SPECIES IN CAW IMPtlOVEMENl IDI

differences in chromosome behnvior and hybrid f e r e t y . which can ,result in changes from homologous to homocologous pairing (2. 3) or differences in chiasma distribution.

About 1.000 collections from the wild have been made in Ar.chif, and a eollcction program supported by the International Board for Plant Genetic Resources is still active. Although not all collections have been maintained. ICRISAT has I81 accessions of wild Arachis species (14) . and there is an active program to assemble-ving accessions at ICRISAT. All the accessionn are being screened for resistance to major pests. diseases, and drought (1. 29. 31).

Effective screening techniques are essential to wild species utilization, not only to screen accessions of wild species. but also to identify segregants with the desired characters. The infector row technique han been valuable in screening for resistance to rust (Puccinia erachidis) and late leafspot (Cercosporidium personaturn).

The genus Arachia

There are seven sections in the eenus. Amchis hypogaea. the cultivated groundnut, is in section Arachis. It is a tetraploid; A. monticola is also a tetrilploid freely crossable with. and - - considered by some a subspecies of. A. hypouaea. All the other species in section Arachis are dieloid. and crossable with A. hypogsea, but A. hypogaea will not G o a s Lith species in any other section. unless hormone treatment and embryo culture are used (18. 19). Diploid species in section ~ r a c h i s can be crossed with species in other sections, but gene transfer by bridge crossing has not been possible (28).

ICRISAT has therefore emphasized work to overcome barriers to intersectional hybridization (18) and to utilize dinloid wild species in section Arachia, especially those with bisees. resistance, by producing large populations of hybrids and backcrosses.

A knowledge of crossability and genomic constitution of the species is necessary in planning its utilization; most of this knowledge comes from hybrid production and analysis but knowledge of genome* can come from karyotype studies, DNA studies, and taxonomic studies.

Barriers to hybridization

The available gene pool is limited by the capability and resources of the breeder. Many species can be crossed by conventional means, but the use of mentor pollen. hormone treatment. and embryo culture can extend the pool (19).

Ploidy differences

Many crop species are polyploid, with diploid or polyploid vild relatives, or both. Exiating ploidy differences impede gene transfer, as hybrids are often sterile, but induced ploidy can be used to overcome crossability barriers and to manipulate genome ratios to alter eerie dosea. and tn r f f e r t srnr r r m n f - r I ? ? ) .

A A B B . & A '

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Ploidy level differences between species can be adjusted before or after crossing (Fig. 1). A diploid wild species can be rendered tetraploid before crossinp, with the tetraploid crop (autotetraploid route) ( 6 ) . or the tetraploid crop esn be rendered dihaploid before crossing at the diploid level. Two diploid relatives, where evailahle, can be crossed and the hybrid can be rendered tetraploid before crossing with the cultivated tetraploid (am~hiuloid route). The second diploid species may contribute . . another resistance (leafspot resistant A. cardenasiilrust resistant A. batizocoi)lA. hy o aca. or confer cytol0giCal stability 2nd fertility on thPeB tetraplod hybrids. thereby facilitating transfer of the desired gene. In -. We h.va concentrated on eight diploid species. most of them with A- h po aea as female parent; of the 72 ~ossible species copbinatl0~ Wz htve ~roduced 56: we have also crossed diploid hybrids with a third wiid species and produced complex hybrids.

Often direct hybrids without ploidy manipulation are ntcrile. Uany triploid hybrids in Arechls are totally or partially sterile 1 2 ) Colchicine treatment produces fertile hexaploids (26). Wild and cultivated genmes are combined, but after treatment each genome is homologous, and there may be no pairing between wild and cultivated chromosome^ If the genome6 are related, intergenomic pairing may Occur. resultin8 in multivalents (25). Backcrossing alters genome ratios. removes

BIOTECHNOLOGV IN INTERNATIONAL AGRICULNRAL RESE*RCU WILD SPECIES IN CROP IMPROVEMENT a06

.tomolopous partners, and enrourarPr wild and cultlvarea cnromosomr pairing; but thr reromhinants mny bc relenced hlowly. ne~ding rnanv generations for backcrossing and screenin@. After nn initial backcross, chromosome numherfi mav continue to decllnr in eelfed progenies, but there may he strong ficlection for balanced gametes, and pcncaploids may glvc mostly tetraploids and hexaploids on selfing. Recombination may have occurred at the pentsploid level: pentaploid-derived tetrsplolds and hexaploids would then contain chromosomes with both wild and cultivated segmcntn.

Though triploids are usually sterile. some seeds can be produced from anaphase distribution. giving balanced, vlnhlr gametes, or from restltutinn to produce triploid gametes. Chromosome numbers in progeny from such partiallv fertile triploids range from diploid to hexaploid 1 7 . 2 1 ) : tetraploid or near-tetrnploid progcny are dnuhly valuable as restorinp chromosome numbers to cultivnted level requires little time and no colchicinc or backcrossing, and rrcomblnation betveen wild and rultivated chromosomes, which may not occur at other ploidy levels (22). hnc bccn poaslhle.

The key to successful utilirntion of wild species in recombination; induced transfer of chromosome segments can be used. but producing a lnrpe nsnher of hybrids in which meiotic recombination leads to large segregnting pnpuletfonfi giver more opportunity for the selection of desirable plants, llsually maximum recombination occurs in thc first-generation hybrids. The difficulty in producing hybrids often results in small populations: a common report is that " n hybrid was produced; of the feu F plants obtained, only one was fertile." Vegetative reproductiin of F plants is essential in such cases to ensure acceptable populniion sizes. Tiesue and cell culture at this stage may be valuable to introduce romnclonal variation (10). Where ploidy manipulation and the elimination of wlld sprc lrh cheracters are planncd. rhanges in chromosome numher or the occurrence of deletions, dopllcstions. or translocntions that affect chromosome pairing, chiasma frequency, or position (and hence the number and nature of recomhinantfi in future generations) may be more important than the genetic rhnngrfi that occur.

Backcrossing

It is hoped that the injtial hybrids contain the desired gene. Usually because they combine both vild and cultivated genomes, they contain a numher of undesirable wild characters also. Such characters can be eliminated by backcrossing to the cultivated

species, although beckcrossins to the wild species may be desireblc. Hexaploids can be backcrossed to parental diploid wlld species to regain tetraploidy and also increase the dosage of wild genes ( 4 ) .

Backcrossing to the cultivated species restores cultivated chromosome number, where necessary. and also restores the genotype of the cultivated parent. Backcroseing to a nonparentsl cultiver

can introduce a m e n genes: it vill also ~ i v r rise to variation in the hnrkp,ro~nnd KrnotYpe in Which the introduced gpnc ir t o express itself. T h i s hn* hern used ext~nnlvelv in thc croundnut CytoRenPtirR crasfilnp, program at ICRISAT. 1.onp-smron hexapioids with genome6 from perenninl Ccrcosporldittm-rehistant wild species and from Nigerian cultivers were backrrossed to short-season Indian cultivars; and renifitnnt ahort-fieason selcctjons were backcroused to a Groundnut Rofiptte Virua-reniatsnt cvltiver fro. H~lawi.

Cytological ccreenlng

The need for large popelntlons prerludefi CYtolopical screcnlng of all plant.. Ur currently grow nhout 7 he of vild specice drrivntivea ear11 rainy seanun at ICRISAT. nnd have grown I? ha in one senson. Clmromosome counting and checking for rrpular mclonl. is restrleccd to key hybrids and any plants vith unrxpected morphology or reduced fertility.

Cytological stability

Ploldy diffcrenecs end intergenomic pairing ere useful in the early gcncrations, but not in the flnished product. Uany generations of backcrossing may he nereahary to select stable lines with the desired character and no drlrterinua wild specie8 genes. Selceted plants ran be selfed, and progeny rows g r a m for s~veral generetions until uniform lines are produced. Chromosome numherc are counted and meiosis is checked at this stage. Lincm for release as new germpla~m mu$t he checked for crossability with the cultivars and for reRular rnciosis in the hybrids produced.

Tmbls 2. Dnrm*, rertion 11.8 u l . ) d yield ol u1.n.d wiM tpecier deriwmti.n 1" A r ~ h a I

8 triple Imice, plm xire 16 m'l. IcRlsAT C w t n n d Bh.v.nis.wr. 1982 re ly m a o n

P d Esttrnared 0 ' 8 1 Ert,rnmed Hwlm Entry no. ,,stm, yoeld kcmel yreld -tam ail veW view

( k~h .1 Ikphal (961 %&.I (kgh.1

WILD SPECIES IN CROP IMPAOMMENT OI

ACIIIFVEMENTS

Many fertile, cytologically tatable derivatives with desirshle characters from vild species have been selected ( 7 ) . The numbers of segregants produced from crosses have permitted selection for agronomic traits (grovth. habit, yield, earliness) as well as for diaeasc resistance. The most advanced lines have been grown in replicated trials for a number of seasons at ICRlSAT center and ocher locations'(Tab1e 2). The best lines have been entered In All India Coordinated Oilseeds Research Project rrials. h o foliar disease-resistant lines have been selected for a second pear of testing, and one high-yielding line haa been advanced to regional trials.

REFERENCES CITED

I. Amin, P.W. 1984. Resistance of vild specles of groundnut to lnsect and mite pestfi. ICRlSAT (Internstlonal Crops Research lnstltute for thc Semi-Arid Tropics). Proceedings of the internatlonal workshop on cytogenetics of Arsrhis. 31 Oct-2 Nov 1983. India.

2. Dover. G.A. 1973. ~ G t i c s and interactions of 'A' and '0 ' chromosomes controlling meiotlc chromosome pairing in the Tritlcinae. Proc. 4th lnt. Wheat Genetics Symposium. Uissouri Agric. Exp. Srn.. Columbia, Mo.. USA.

3. Dover. G.A.. and R. Riley. 1972. Prevention of airi inn of honocologous meiotic chromosomes of wheat by'an acrivity of supernumary chromosomes of Aegilops. Nature 240:159-161.

4. Aamons. R.0. 1970. Regifitration of Spancross peanuts. Crop Sci. 10:459.

5. Harlan. J.R. 1976. Genetic resources in vild relatlvea nf . . crop plants. Crop Sci. 16:329-333.

6. ICRISAT (International Croph Reseerch Institute for the Semi-Arid Tropics). 1980. Annual report 1978-79. India.

7. ICRISAT (International Crops Refiearch Institute for the Semi-Arid Tropics). 1983. Annual report 1982. India.

8. Jalani. B.S.. and J.P. Moss. 1980. The sire of action of the crossability genes (Krl and Kr2) betveen Triticum and Seeale I. Pollen germination. pollen tubegrovth and number of pollen tubes in c o m o n wheat. Euphytica 29:571-579.

9. Jalani. B.S.. and J.P. Uoss. 1981. The sire of action of the crossability genes (Krl and Kr2) between Triticum and Secale 11. Proportion of different p i s t i ~ n t a i n i n g pollen tubes in common vheat. Euphytica 30:105-112.

10. Larkin. P.J., and W.R. Scovcroft. 1981. Somaclonal variation -- a novel source of variability from cell cultures. Theor. Appl. Genet. 60:197-214.

11. Mosa, J.P. 1980. Wild species in the improve~nent of groundnuta. Pages 525-535 fi Advances in legume

srlenrr. Vol. I. .I. Summerfield end A.H. Buntlng, =ds. Prnc. Int. 1.egume Conf. Kev. England 1978.

12. Mocs. .I.P.. end 1.V. Splelmnn. 1916. Intcraperjftc hybridisatlon In =I>. Fror. hcrican Peanut Rcccarch and Education A s s o r l ~ t l o ~ ~ R:88. ( ~ b ~ t r )

13. Plucknrtt. D.L.. N.J.H. Smith. J.T. Wllllnm~. and N. nurchj Anishetty. 1983. Crop gcrmplosm conservation .ad developing countries. Sctance 2?0:163-169.

14. Rao. V.R.. and A.K. Sadnsivnn. 1984. Wlld Araehis Cenetlc Rerources at ICRISAT. ICRISAT ( l n t z n a l Crop. Research Institute for the Seml-Arid Tropics). Proceedings of the internatlonal worknhop on cytogenetics of w. 31 Oct-2 Nov 1983. Indim.

15. Rirk, C.M. 1967. ExploitlnR npedes hyhrids for vegetable improvement. Proc. XVll Int. Ilortlc. Congr. 3:Zll-269.

16. Rirk, C.M.. J.F. Fohe*. and S . D . Tanksley. 1979. Evolution of mating systems in 1.ycopersicon hlrsutum as deduced from variation in electrophoretic and marpttolo,~ical cherart~-rs. I'lnnt Syst. Evol. I32 (4):279-298.

17. Riley. R.. and V. Chnpmon. 1967. The inheritance in v h ~ a t of crofisebllity vith rye.. Genet. Reb. Camb. 9:259-267.

18. Snfitri. U.C.. and N. Malliharjuna. 1984. Techniquen for overcoming incompetibillty in wlde crosse*. Inter-Center Seminar on IARCs and Biotechnology. 23-27 April. IRRI, Manila.

19. Sastri. D.C., and J.P. Moss. 1982, Effects of grovth regulators on incompatible crosses in the genus Arachi. I.. J . Exp. Rot. 33:1293-1301.

20. Singh, A.K., and J.P. MOPS. 1982. Utilization of wild relatives in genetlc improvement of lrechis hypogaea L. IT. Chromosome complement of species of oeetlao ARACBIS. Theor. Appl. Genet. 6!:305-314.

21. Singh, A.K., and J.P. Moss. 1983. Utilizntton of vild species tn g~netic improvement of 6. hypogaea L. V I . Fertility in triploids. Cytological hnsffi and brerdiN3 implicntions. Pennut Sci. (in prefis)

22. Slngh. A.K. . and J.P. Mosa. 1984. Interspecific breeding through ploidy mnnipulatjons. Banglad~fih J. Bot. (in press)

23. S3n.h. A.K.. P. Subrahmanvam. and 3.D. Moss. 1983. - - - , . Utilization or vild. species in genetic impravrmcnt of Arachis hypogaea V1I. A note on the dominant nature 0f - resistance to Purcinie arachldis ln vild Ar.chi. species. Peanut Sci. (in press)

24. Snape. J.U.. V. Chapmen. J. Moss. C.E. ~lanchard. and T.E. Miller. 1979. Thc crossabilitje~ of wheat varieties ~ ~ . . with Hordeum hulhosum. Hercdlty 42(3):291-298

25. Splelmsn. I.v.. A.P. R u r ~ e , snd J.P. Moss. 1979. Chromosome lose and meiotic behaviour in inte~s~eciflr hybrids In the genus Arachifi L. end their implications In brrrd1ng for diseaaeresistance. 2. Pflanzenzuchtuns. 83:236-250.

26. Spielman. I.V., and J.P. Moss. 1976. Techniques far chromosome doubljn~ in inrerspecific hybrids in Arachio.

BIOTECHNOLOGY IN INTERNATIONAL AGRICULTURAL R F C U

27. Stalker. 11.T. 19RO. Utilization of vlld apecies'for crop improvement., Adv. Agron. 33:111-147.

2R. Stalker, H.T., J.C. Wynne, and M. Company. 1979. Variation in progenies of an Arnchls hypogaea x diploid wild apecies hybrid. Euphytica 28(3):675.

29. Subrahmanyam, P.. A.M. Ghanekar. B.L. Nolt. D.V. Reddy. and D. McDoneld. 1984. Resistance to groundnut disease in wfld Arachis species. ICRlSAT (International Crops Resear-stitute for the Semi-Arid Tropics). proceedings of the international vorkshop on cytogenetlcn of Arachis. 31 Oct-2 Nov 1981, India.

30. Subrahmanyam. P.. D. McDonald. R.W. Gibbons. and P.V. Subba Rao. 1983. Components of resistance to Puceinia arechidis in peanuts. Phytopathology 73(2):253-256.

31. Subrahmanyam. P.. J.P. Moss. and V.R. Rao. 1983. Resistance to peanut runt in wild Arachis species. Plant Dis. 67(2):209-212.

32. Zeven. A.C.. and A.M. van Hartcn. 1979. Proceedings of the conference on broadening the genetic base of crops. Wageningen. Netherlands 3-7 July 1978. Pudoc. Vageningen.

OVEWOMING INCOMPATIBILITY IN WIDE CROSSES

D. C. SASTRI and M. MALLIKARJUNA

Wild apecies of crop plnnts attract much attention as valueble gene pool. A few successes in improving"crop plants have resulted from crossing wild taxe vith cultivated ones. Several other taxa are not crossable with their cultivated relatives and are therefore mi.allable fnr sexual gene transfers. Methods for

'hreakinp, these b~rrierfi to interspecific hybridization and hybrid production have hcen developed.

The literature on exotic germplasm abounds vith examples genetic introgression from wild taxa conventionally crossed vl their cultivated relatives. But there is a vealth of germpla that cannot be crossed with cultivated taxa.

The barriers to hybridization were known even before t present range of germplasm became available. During the la five decades. interest in incompatibility han increased encompafis phylogenetic-taxonomic purposes and the genet improvement of crop plants. While emerging somatic methods sht promise, sexual manipulation is still the first choice. a sexual methods hnve contrihuted significantly to improving crop

The ~ermplnsm amenable to sexual manipulation is difficu to quantify. It may be an insipnfficant proportion of existi gemplasm. Even in the well-worked gcnun Nicotiana. of vhi about 65 species are known, only a little mo-300 hybri havc been realized. About 90% of rhr cronses in this genus ha not. The situation is similar for most other crops. A number reviev articles and books on incompatibility and methods to bre it are available (9. IS. 16. 17. 18. 41. 49. 51).

CAUSES OF SEED FAILURE IN INCOMPATIBLE CROSSES

The characteristic prefertillration barriers I postfertilization breakdmm of zygote or embryo in incampati! crosses havc been extensively described. inhibition of pol germination on stigma and pollen tube ~rowth through the sty

Cylogeneticirt and rewarh -coale. Internst~onal Crops Research lnstltule for the %mi Tropio IICRISAT). Palancheru P. 0. 502 324.A. P .Ind!a.