Euphytica 39: 123-132 (1988)@ Kluwer Academic Publishers, Dordrecht -Printed in the Netherlands

The breeding potential of wild potato species resistant to the potato tubermoth, Phthorimaea operculella (Zeller)

R. Chavez!, P .E. Schmiediche2, M. T. JacksonI & K.V. Raman21 Department of Plant Biology, University of Birmingham, U. K. (present address of senior author:

Centro Internacional de Agricultura Tropical (CIAT), Cali, Colombia),' 2 International Potato Center (CIP),

Lima, Peru

Received 13 May 1987; accepted in revised form 5 August 1987

Key words: Solanum spp. wild potato species, Phthorimaea operculella, potato tuber moth, interspecifichybridisation, resistance studies.

Summary

The potato tuber moth, Phthorimaea operculella (Zeller) is an important pest of potatoes in the field and instores in warm environments throughout the world. In this study genetic resistance to potato tuber moth wasidentified in clones of Solanum sparsipilum (coded MBN) originally developed for resistance to bacterial wiltand root-knot nematode. Resistance to this pest in S. sucrense and S. tarijense as well as S. sparsipilum wasexploited in wide crosses with diploid and tetraploid cultivated potatoes, and haploids derived fromS.tuberosum,. hybrid progenies were produced. Crosses between resistant S. pinnatisectum or S. commerso-nii and cultivated potatoes failed completely, although S. commersonii did hybridise with two bridgingspecies S. lignicaule and S. capsicibaccatum which are slightly compatible with cultivated potatoes. Resist-ance to potato tuber moth was transferred to all progenies except those in which S. tarijense was the resistantparent. The development of potatoes resistant to potato tuber moth is discussed in the context of populationbreeding for the lowland tropics.

Introduction

The potato tuber moth (PTM), Phthorimaea oper-culella (Zeller), family Gelechiidae, is one of themost damaging potato pests in the world. In manytropical regions it is a major problem in potatostores for seed and ware or consumer potatoes(Couto et al., 1983; Haines, 1977), but severe in-festations have also been reported in cooler areassuch as in highland areas of Peru, Colombia, Nepaland Kenya. Potato tuber moth is now found inevery developing country where potatoes aregrown (International Potato Center, 1984). Expe-rience with control measures has shown that it isunrealistic to rely solely on control with insecticides

as resistance to these compounds develops quicklyin any tuber moth population. Furthermore, chem-ical control of this pest on ware potatoes carriesconsiderable dangers for consumers, and overusecontaminates the environment.

The female moths lay eggs on developing tubersin the field, or on tubers in the store. Althoughtuber damage at harvest can be severe in somesituations, where conditions have been favourablefor the spread of the insect, or where farmers haveallowed tubers to become uncovered as they devel-op, thus rendering such tubers unmarketable, themajor problem with potato tuber moth lies in pota-to stores. Larval damage results in both weight andquality loss of the affected tubers, as wounds cause

124Materials

and methods

The species used in this study are listed in Table 1.The Mexican diploid wild species S. pinnatisectum(Series Pinnatisecta) and the South American di-ploid wild species S. commersonii and S. tarijense(Series Commersonii), as well as the tetraploidwild species S. sucrense (Series Tuberosa) wereused as sources of resistance to the potato tubermoth in the wide crossing programme. Additional-ly, 61 selected clones of the South American di-ploid weedy species S. sparsipilum (Series Tuber-osa) with combined resistance to bacterial wilt(caused by Pseudomonas solanacearum) and root-knot nematode (Meloidogyne incognita) werescreened for resistance to potato tuber moth. Theclones with this combined resistance to bacterialwilt and root-knot nematode were coded MBN.

All the species listed above were utilised in acrossing programme to transfer resistance to pota-to tuber moth into cultivated potatoes. The culti-vated crossing partners from Series Tuberosa wereclones from the diploid species S. goniocalyx, S.phureja

and S. stenotomum, haploids of S. tuber-osum, and from tetraploid S. tuberosum ssp. andi-gena. Furthermore, the S. sparsipilum MBN cloneswere crossed with other MBN clones which hadbeen derived from crosses between S. sparsipilum,S.

phureja and the wild diploid S. chacoe1JSe (SeriesCommersoniana) in various combinations.

The programme of hybridisations

shrinkage through increased transpiration, and al-so provide entrance for micro-organisms which inturn cause secondary infections (Raman & Pala-cios, 1982). In the field, culled tubers and volunteerplants provide an abundant food resource for P.

operculella during non-cropping periods (Shelton& Wyman, 1980).

Research on alternative integrated control mea-sures has been initiated (Raman & Palacios, 1983).This came as a response to the increased awarenessof environmental problems caused by the excessiveuse of pesticides and as a recognition of the fact thatno single control measure was able to overcome theproblem of tuber moth infestations in fields andstores (Foot, 1976). Current measures of integrat-ed pest management consist mainly of the com-bined use of insecticides, sex pheromone traps forattracting male months, and insect species that par-asitise P. operculella (Foot, 1976; Raman & Pala-cios, 1983; Sankaran & Girling, 1980).

No serious attempt has been made to utilise po-tato populations with genetic resistance to potatotuber moth, and in the germplasm of cultivatedpotatoes only some resistance of any consequencehas been identified in S. tuberosum ssp. andigena(Raman & Palacios, 1982). Screening work involv-ing wild potato species has been carried out at theInternational Potato Center (CIP), and has re-vealed the presence of a low frequency of moder-ately resistant genotypes. Out of 452 accessionstested, 27 showed significantly less larval damagethan the controls, and pupation was only slight incomparison with other accessions. The reduced pu-pation was attributed to a factor of antibiosis effec-tive in the tubers of the resistant accessions (Ra-man & Palacios, 1982). The observed resistancewas not correlated however with total tuber gly-co alkaloid content in the species accessions inquestion (Schmiediche, 1977; Osman et al., 1978;Raman & Palacios, 1982).

The identification of wild potato germplasm withresistance to potato tuber moth led to this study inwhich this resistance was transferred from wild tocultivated potatoes in a programme of wide cross-es.

The genetic materials were maintained in an insect-proof glasshouse, and crosses between individualclones were carried out. Only when tuberosum ha-ploids were used as male parents was pollen fromabout 10 clones bulked in order to overcome pollenfertility constraints. The cut stem technique(McLean & Stevenson, 1952) was used for the ma-jority of crosses in order to promote a high fre-quency of berry set.

The parameters used to assess interspecificcrossability were berry set, the number of plumpseeds per berry, seed germination and the degreeof Fl fertility. Pollen stainability was used as a

125

measure of male fertility, following the method ofMarks (1954), and 2n pollen production was as-sessed using the method of Quinn et al. (1974), inboth hybrid progenies and parental genotypes.

sion. These tubers were kept in individual smallplastic containers with tightly fitting lids, in orderto prevent escape of larvae. The infested tuberswere stored in an incubator at 240 C, and tuberdamage as well as pupation were scored after 10days. Tubers which did not show damage after 10days were again scored three weeks later.Screening for resistance to potato tuber moth

(a) Assessment of damage. Female tuber moths laytheir eggs on or near the 'eye' buds, and on hatch-ing, larvae bore into the tubers via the 'eyes' andfeed while boring tunnels or irregular galleriesdeep inside or just below the skin of the tubers. Anaccession was considered resistant when none ofthe tubers in any of the replications had more thanone hole. Conversely, accessions were consideredsusceptible when just one of the tubers in any of thereplications had more than one hole.

(c) In a PTM-infested potato store. Single tubersfrom each accession were placed in open paperbags which were put into wooden crates inside aPTM-infested potato store. Each crate contained24 different clones plus a susceptible control. Theexperiment was replicated four times in a com-pletely randomised design, each crate representi~ga single replication. The potato store was located inSan Ramon, a mid-elevation site (800 m) in thetropical and humid environment on the easternslopes of the Andes in central Peru, where CIP hasone of its field stations. Temperatures ranged from18-260 C during the course of the experiments. Thepotato tuber moth population was kept artificiallyhigh inside the store, by placing crates containingheavily infested tubers from a highly susceptiblepotato variety. An increase in the tuber moth pop-ulation was fastest when the temperature rangedbetween 20 and 250 C. The life cycle of the insectwas completed within three to four weeks (Haines,

(b) In an incubator. Viable eggs of P. operculellawere stored on filter paper inside a plastic petridish, which was kept in a refrigerator at 5-60 C,until required for resistance testing. For each ex-periment, eggs were put in an incubator at 260 C forabout two days after which hatching commenced.Four instar larvae were then transferred to wholetubers from each of the accessions to be evaluated.Six tubers as replicates were used from each acces-

Table 1. Tuber-bearing Solanum species used in wide crosses to transfer resistance to potato tuber moth, P. aperculella, to hybridprogenies.

Species Abbreviation Series Ploidy Status

S. capsicibaccatumS. chacoenseS. commersoniiS. tarijenseS. pinnatisectumS. lignicauleS. sparsipilumS. sucrenseS. goniocalyxS. phurejaS. stenotomumS. tuberosum

spp. andigenassp. tuberosum

CircaeifoliaCommersonianaCommersonianaCommersonianaPinnatisectaTuberosaTuberosaTuberosaTuberosaTuberosaTuberosa

2x2x2x2x2x2x2x4x2x2x2x

capchccmmtar

pnt19isplscr

gonphustn

wildwildwildwildwildwildwildwildcultivatedcultivatedcultivated

adgtbr

TuberosaTuberosa

4x4x/2x

cultivatedcultivated

126

1977; Raman, 1980). Tuber damage was evaluatedthree months after starting the experiment, and thesusceptible controls were evaluated first in eachcrate.

Results

The results from the interspecific hybridisationstudies and resistance testing are closely linked.The former provided progenies for resistance test-ing, and through the latter, resistant genotypeswere identified for use in wide crosses. However, itis convenient to consider these two aspects sep-arately, and cross reference is made where appro-

priate.

well as with tuberosum haploids. The hybrid fam-ilies were labelledPTM.12 toPTM.33, and the dataare given in Table 3. In general crosses between S.sparsipilum and diploid cultigens were successful,although the number of seeds per pollination didvary between the different crosses. The lowest seedset was in crosses with S. phureja. With the othertwo diploid cultigens, there were reciprocal differ-ences in seed set, which was lower when S. sparsipi-lum was the female parent. Seed set was extremelylow (0.2 seeds per pollination) when tuberosumhaploids were the female parent, but much higherin the reciprocal, even though this was still lowerthan in the other crosses.

Crosses of S. tarijense with S. phureja, S. stenoto-mum and tuberosum haploids which represent hy-bridisation between different taxonomic series,yielded normal berries with viable seeds (PTM.40-45), although the cross with haploids failed when S.tarijense was the female parent. All attempts tohybridise S. pinnatisectum and S. commersonii withcultivated diploids and tuberosum haploids failedcompletely. S. commersonii was however hybrid-ised as female with S. capsicibaccatum and S. ligni-caule both of which were also resistant to potatocyst nematode (Chavez et al., 1987a). Hybrid fam-ilies were labelled PTM.38 and 39 (Table 3).

S. sucrense, as pollen parent, was successfullycrossed with S. tuberosum ssp. andigena (PTM.34-37), but it did not function as a female (Table 3).Self-pollinations of S. sucrense produced an aver-

Interspecific hybridisation for the production ofPTM resistant families

The six S. sparsipilum MBN clones resistant topotato tuber moth (Table 5) were successfullycrossed with the other MBN clones of interspecificorigin between S. sparsipilum, S. chacoense and S.phureja (Table 2). The 11 combinations resulted in3375 viable plump seeds, although the number ofpollinations was not recorded. The resulting fam-ilies were labelled PTM.l to PTM.11. The same sixclones were further intercrossed reciprocally withS. goniocalyx, S.. phureja and S. stenotomum, as

Table 2. The seed production from crosses between S. sparsipilum MBN clones resistant to potato tuber moth, P. operculella, andinterspecific hybrid MBN clones of S. sparsipilum, S. chacoense, S. phureja and S. tuberosum.

Cross Pedigree

No.

seeds obtained Family code

MBN 4.188 x MBN 5.2MBN 4.69 x MBN 11.44MBN 4.41 x MBN 9.67MBN 4.188 x MBN 8.29MBN 4.41 x MBN 9.75MBN 4.69 x MBN 8.29MBN 4.90 x MBN 9.75MBN 4.41 x MBN 10.50MBN 4.188 x MBN 5.10MBN 4.41 x MBN 10.25MBN 4.188x MBN 9.63

270180276360260321410252259267520

PTM.lPTM.2PTM.3PTM.4PTM.5PTM.6PTM.7PTM.8PTM.9PTM.I0PTM.ll

spl x (spl x chc)spl x [(spl x phu) x tbr]spl x [(phu x spl) x spl]spl x [(spl x phu) x spl]spl x [(spl x phu) x spl]spl x (phu x spl)spl x [(spl x phu) x spl]spl x [(spl x phu) x spl]spl x (spl x chc)spl x [(spl x phu) x spl]spl x (phu x spl)

127

age of 32 seeds per pollination, giving PTM families46 and 47.

Resistance of S. sparsipilum to potato tuber moth

Six of the 61 MBN clones of S. sparsipilum tested inthe potato store in San Ramon were significantlyless damaged than the control varieties, and wereselected as resistant. Their mean tuber damageranged from 0.25 to 1.0 holes per tuber (Table 5).This level of damage was approximately the sameas shown by the resistant species S. sucrense, S.commersonii and S. tarijense in the same test. Theremaining 55 S. sparsipilum clones showed var-iation in tuber damage, from 1.25 to 10 holes pertuber, whereas the susceptible control varietiesDTO-33 and Desiree had a mean tuber damage of7.50 and 7.75 holes per tuber respectively.

Pollen stainability and production of 2n pollen

Pollen stainability was determined for 2559 clonesfrom 47 PTM families. A total of 2420 clonesshowed stainable pollen, whereas 139 clones wereapparently male sterile (Table 4). Some 23 clonesonly (less than 1%) had 2n pollen. A high fre-quency of male sterility was found in the inter-series hybrids between S. commersonii and S. cap-sicibaccatum or S. lignicaule. Furthermore, thesehybrids did not set berries under field conditions.

Table 3. Berry and seed production in wide crosses between diploid and tetraploid cultivated potatoes and wild species resistant topotato tuber moth, P. aperculella.

Cross No. pollinations % Berry set Seeds/berry Seeds/ pollination Family code (PTM)

2725

12191

1611012092001532283945513532251836162174

108582924923628

89.076.065.552.577.669.877.565.180.047.093.079.073.532.500000

94.062.068.052.234.068.497.00

64.00

47.029.017.522.046.842.50.3

11.134.014.010.415.23.3000000000

38.70

45.521.50

15.50

41.722.011.411.536.430.30.27.3

27.26.59.612.02.400000

161714,3015,2912, 20, 25, 3213,21,24,311819,22,23,26,27,28,334140454442,43

gon x splspl x gonphu x splspl x phustn x splspl x stn2x-tbr x splspl x 2x-tbrphu x tartar x phustnx tartar x stn2x-tbr x tartar x 2x-tbrphu x pntpnt x phustn x pntpnt x stnphu x cmmcmm x phu2x-tbr x cmmcmm x 2x-tbradg x scrscr x adgscr seligcmm x capcap x cmmcmm x IgcIgc x cmm

0

019.60

32.0

18.0

09.90

34, 35, 36, 37

46,4738

39

128

The S. sparsipilum clones found to be resistant inSan Ramon were tested in the incubator test (Table5). Their resistance was confirmed, but damage inthe incubator test was less than under conditions ofnatural infestation. The level of damage in S. com-mersonii and S. tarijense was higher than the S.sparsipilum clones in this test, but S. sucrense againshowed no damage whatsoever. Tuber damage wasalso considerably lower on the susceptible controlsin the incubator test. The type of resistance wasidentified as feeding resistance or antibiosis.

Resistance of hybrid progenies of S. sparsipilumwith MBN clones and cultivated diploids

ance. These results indicates the importance oftesting under conditions of natural infestation.Eleven of the clones which had shown resistance inthe potato store test proved to be susceptible in theincubator test, and must be regarded as escapes inthe former test. Consequently only 160 clones(32%) showed resistance in both tests. The propor-tion of resistant clones in the different familiesvaried between 13 and 86%. Two of the highestlevels were produced in progenies involving S.sparsipilurn MBN 4.41 (PTM.3 and PTM.10),whereas MBN 4.69 gave the lowest proportion(PTM.2). The pedigrees of these PTM families aregiven in Table 2.

Fifteen hybrid families with a total of 1184clones, resulting from crosses between S. sparsipi-lurn and cultivated diploids or tuberosurn haploids(PTM.12-15, 15C, 16-19, 22, 23, 26-28 and 33),were tested in the potato store, and 115 clones wereidentified as resistant (Table 7). Resistance rangedbetween 1% and 30% of clones between families.Four resistant clones (8.8%) were found in a chro-mosome-doubled hybrid family derived from a S.

A total of 501 clones from the families PTM.1 toPTM.ll was evaluated for resistance in both theincubator and potato store tests (Table 6). In theinfested store, both DTO-33 and Desiree werecompletely destroyed by the larvae, but 171 clones(34%) resistant to P. operculella were selected. Inthe incubator test, 78% of the clones showed resist-

Table 4. The occurrence of stainable pollen and 2n pollen in hybrid families (PTM) with resistance to the potato tuber moth, P.

operculella.

PTM families Cross No. clones studied % clones withstainable pollen

No. clones with2n pollen

27261970

3

722

96

92

100

93

100

98

000107

spl x (spl x chc)spl x (phu x spl)spl x [(phu x spl) x spl]spl x [(spl x phu) x spl]spl x [(spl x phu) x tbr]spl x stn*

1,96,1134, 5, 7, 8, 10212, 13,20, 2124,25,31, 3214, 15,29, 3016,1718, 19, 22, 2326-28, 3340,4142,4344,4534-37

46,473839

spl x phu*spl x gon*spl x 2x-tbr*

306184341

889397

13

tar x phu*2x-tbr x tartar x stn*adg x scrscr seligcmm x capcmm x 19l

1513469

369

786523

9910010099994365

0110125

* Includes reciprocal pollinations.

129

sparsipilum x S. phureja cross (PTM.15C), whichwas only about one third that which had been foundamong the diploids of the same family (PTM.15). Itis interesting to note that the highest proportion ofresistant clones from these 15 families was found inthose in which S. sparsipilum MBN 4.69 was theresistant parent, whereas in families PTM.1 toPTM.11, this clone gave progeny with a low fre-quency of resistant genotypes.

There was no resistance in two of the families de-rived from crosses between tuberosum haploidsand S. traijense (PTM.42 and 43). Only five clonesout of 98 from family PTM.36 (S. tuberosum ssp.andigena X S. sucrense) showed no tuber damage.

Agronomic value of hybrids

Long stolons were commonly observed in hybridsbetween wild species and cultivated diploids, but inthe majority of hybrids that had a tuberosum ha-ploid as one of the parents, the stolons were short.Hybrids with a cultivated tetraploid or haploid par-ent showed, in general, good performance in termsof plant vigour, tuber yield and tuber uniformity.Hybrid vigour and uniformity of aerial parts wereobserved in the diploid families derived from cross-es between wild species and tuberosum haploids.Plants displayed broad tuberosum-like leaves, andsemi-erect growth habit also characteristics of S.tuberosum ssp. tuberosum. Early senescence andtuberisation were observed in hybrids from crossesof S. sparsipilum and tuberosum haploids. How-ever, medium to late tuberisation as well as high

Resistance in interspecific hybrids

A total of 287 clones from families PTM.36, 38, 39,42, 43 and 47) were screened in the potato store,but only 20 clones(7%) were resistant (Table 7).Thirteen of the clones were from a selfed S. su-creme family (PTM.47). Less than 2% resistantclones were recorded in hybrid families derivedfrom crosses between S. commersonii and S. capsi-cibaccatum (PTM.38) or S. lignicaule (PTM.39).

TableS. Mean tuber damage, expressed as holes per tuber, of sixS. sparsipilum MBN clones and resistant and susceptible pota-toes after exposure to the potato tuber moth, P. operculella, inan infested potato store and in an incubator test.

Table 6. The occurrence of resistance to the potato tuber moth,P. operculella, in 11 hybrid families derived from crosses be-tween S. sparsipilum, S. chacoense, S. phureja and diploid S.tuberosum, after exposure in an infested tuber store and in anincubator test.

Clone Mean tuber damage (holes per tuber)

Infested potato store Incubator test

spl MBN 4.41spl MBN 4.69spl MBN 4.90spl MBN 4.48spl MBN 4.97spl MBN 4.188scr (resistant)crnm (resistant)tar (resistant)tbr DTO.33

(susceptible)tbr cv. Desiree

(susceptible)

0.50 a0.25 a1.00a0.50a1.00a0.50aOa0.50a0.50a7.50 b

o.o.0

o.o.o.0

0..

0.1

2..

PTM.lPTM.2PTM.3PTM.4PTM.5PTM.6PTM.7PTM.8PTM.9PTM.I0PTM.ll

Totals

4024423749507431243694

501

175

2317118

157

133124

171

31143523394163221836

69

391

143

2116108

157

133122

7.75 b 2.50b

Values are the means of four replicates in the infested storagetest, and means of six replicates in the incubator test. Means inthe same column followed by the same letter are not significant-ly different at the 5% level, according to Duncan's MultipleRange Test.

160

168

168

a

168338168

a

SO 8

66833 b

130

berry set were recorded in hybrids of S. sparsipilumwith S. goniocalyx, S. phureja or S. stenotomum,and among clones derived from crosses of S. tuber-osum ssp. andigena and S. sucrense.

Flowering was extremely sparse in diploid hy-brids that had a tuberosum haploid as a parent. Incontrast, flowering was profuse in tetraploid hy-brids derived from S. tuberosum ssp. andigena andS. sucrense. Likewise flowering was abundant indiploid hybrids which had resulted from crossesbetween resistant diploid clones and cultivated di-

ploids.

The wild species S. sparsipilum and S. sucrensecould be hybridised directly with clones from thediploid and tetraploid cultivated species, to pro-duce fertile and tuber moth resistant Fl hybrids.However, resistance genes in S. pinnatisectum andS. commersonii were not introduced into the culti-vated genepool due to the cross incompatibility ofthese species. Although it was possible to produceFl hybrids between S. tarijense and cultivated pota-toes, all progenies were susceptible to potato tubermoth.

The success of the crosses involving S. sparsipi-lum and S. sucrense and cultivated potatoes can beexplained by their close genomic relationship(Hawkes, 1978). S. sparsipilum has been regardedas one of the putative ancestors of tetraploid S.tuberosum ssp. andigena (Cribb & Hawkes, 1986).S. sucrense originated by natural hybridisation be-tween the wild species S. oplocense and S. tuber-osum ssp. andigena (Astley & Hawkes, 1979).

Discussion

This work has demonstrated the feasibility of trans-ferring genes for resistance to the potato tubermoth, P. apercu/ella, from wild species to culti-vated potatoes through interspecific hybridisation.

Table 7. The resistance of hybrid families derived from crosses between wild and cultivated potato species to the potato tuber moth, P.operculella, after exposure in an infested potato store.

PTM family Cross Resistant parent No. clonesevaluated

No. clones % resistant

Resistant Susceptible

PTM.12PTM.13PTM.14PTM.15PTM.15C.PTM.16PTM.17PTM.18PTM.19PTM.22PTM.23PTM.26PTM.27PTM.28PTM.33PTM.36PTM.38PTM.39PTM.42PTM.43PTM.47

stn x splspl x stnphu X splspl x phuspl x phugon x splspl x gon2x-tbr x splspl x 2x-tbrspl x 2x-tbrsplx 2x-tbrspl x 2x-tbrspl x 2x-tbrspl x 2x-tbrspl x 2x-tbradg x scrcmm x capcmm x 19l2x-tbr x tar2x-tbr x tarscr self

spl MBN 4.69spl MBN 4.69spl MBN 4.69spl MBN 4.69spl MBN 4.69spl MBN 4.69spl MBN 4.69spl MBN 4.69spl MBN 4.69spl MBN 4.188spl MBN 4.188spl MBN 4.41spl MBN 4.41spl MBN 4.188spl MBN 4.90scr HHC 4596.3cmm OKA 507/4cmm OKA 507/4tar OKA 5886.2tar OKA 5886.2scr HHC 4596.1

1007572544599831596

100100897086

100986456241332

13117

1646

123

2114925

876465384193711275999680688199936355241319

13.014.69.7

29.68.86.0

14.620.021.81.04.0

10.12.85.81.05.11.51.700

40.6

51100

13

.Hybrid clones from PTM.15 were treated with colchicine for chromosome doubling.

131

were sterile. Further investigation of this incom-patibility is merited, with a view to overcomingfertility problems in the progeny.

The high pollen stainability of many of the pro-genies is encouraging, and should lead to theirefficient utilisation in breeding. Resistance to thepotato tuber moth can be used easily at the diploidlevel, but must eventually be transferred into thetetraploid genepool. The low level of 2n pollenencountered in the progenies developed in thisstudy is somewhat surprising, since studies at theUniversity of Wisconsin by Quinn et al. (1974) andothers have shown that the phenomenon of 2npollen is widespread in the tuber-bearing sola-nums.

The identification of progenies resistant to pota-to tuber moth is important for the development ofpotatoes adapted to warm environments where P.operculella is a major problem. Progenies resistantto P. operculella are now available for inclusion inthe populations for the lowland tropics through thepopulation breeding strategy which is used at theInternational Potato Center (Mendoza, 1980a,1980b; International Potato Center, 1984). Thetransfer of tuber moth resistance into hybrid proge-nies with cultivated potatoes is an important steptowards the production of potato clones with com-bined resistance to potato tuber moth, bacterialwilt and root-knot nematode, adapted to such envi-ronments.

Crosses between S. sucrense and cultivated tetra-ploids produced vigorous and fully fertile Fl hy-brids, but only when the wild species was the pollenparent. Although genes for resistance to potatotuber moth are easily expressed in hybrids withtetraploid potatoes, as demonstrated in this work,the Fl progenies do have some negative character-istics such as late maturity, long stolons and consid-erable inter-clonal variation with regard to tubershape. The use of tuberosum haploids is worthconsidering in this context, since they would beexpected to have fewer undesirable agronomiccharacteristics. Chavez (1984) has reported the suc-cessful hybridisation of F 1 hybrids resistant to pota-to tuber moth and tuberosum haploids, producingover 5000 seeds. This material has not beenscreened however for tuber moth resistance.

Despite being taxonomically distinct from thecultivated potatoes, S. tarijense shows some geneticaffinities with these through successful crosses withtuberosum haploids and S. phureja, setting viableseeds and giving fertile Fl hybrids. The differentia-tion between S. tarijense and cultivated potatoeswould seem therefore to be due only to crypticgenic structural differences. The fact that resist-ance to potato tuber moth was not manifested inhybrid progenies of S. tarijense is difficult to ex-plain, but could be due some aspects of the inher-itance of resistance to this pest. Furthermore, chro-mosome studies were not made of the hybridplants, so there is no indication of meiotic abnor-malities.

The breeding potential of S. commersonii wasdisappointing since all attempts to hybridise thisspecies with cultivated diploids and tuberosum ha-ploids failed completely indicating strong bilateralincompatibility barriers between them. Neverthe-less hybridisations between species from differenttaxonomic series was possible with the help of twobridging species, namely S. lignicaule and S. capsi-cibaccatum which both crossed with S. commerso-nii, and which have been shown to be slightly com-patible with cultivated potatoes (Chavez et al.,1987b). However, these crosses with S. capsicibac-carum and S. lignicaule were possible only in onedirection, indicating the presence of unilateralcross incompatibility, and many hybrid genotypes

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