Tolerance to low temperatures and tuber soft rot in hybrids between Solanum commersonii and Solanum tuberosum obtained through manipulation of ploidy and endosperm balance number "EBN#

The objectives of this study were to evaluate the tolerance to low temperatures and tuber soft rot in hybrids between Solanum com! mersonii and Solanum tuberosum[ The experimental materials consisted of F0 triploid\ BC0 pentaploidÐnear pentaploid and BC1 tetraploidÐ near tetraploid hybrids[ The F0 triploids had a freezing tolerance and acclimatization capacity closest to S[ commersonii[ This indicated that the endosperm barriers which prevent the introgression of 0EBN S[ commersonii into 3EBN S[ tuberosum had been overcome[ Indeed\ the triploids produced 1n eggs\ thus giving a compatible maternal to paternal EBN ratio in the hybrid endosperm generated by the 2x"1EBN# × 3x"3EBN# crosses[ The tolerance to low temperatures of BC0 and BC1 hybrids was lower than that of the F0[ However\ a number of genotypes were identi_ed which were able to withstand temperatures down to Ð4>C[ Some BC1 hybrids were also tested for their tolerance to tuber soft rot\ and some resistant hybrids were detected[ A number of them combined the capacity for cold acclimatization with tolerance to tuber soft rot[ These hybrids have an EBN of 3^ they are fertile and have been used in backcrosses with 3EBN S[ tuberosum[ Key words] Solanum commersonii * Solanum tuberosum * Erwinia carotovora* 1n gametes * acclimatization capacity * endosperm balance number "EBN# * germplasm introgression It is well known that while the cultivated potato Solanum tub! erosum Group Tuberosum "1n 3x 37# "tbr# has narrow gen! etic bases\ its wild relatives provide a rich\ unique and diverse source of genetic variation "Hanneman 0878#[ Genes for resist! ance to biotic and abiotic stresses\ for characteristics such as high solid content\ good chipping quality and tuber colour can easily be found in these species[ Wild Solanum species vary in ploidy from diploid "1n 1x 13# to hexaploid "1n 5x 61#[ Over 69) are diploid and thus can easily be crossed with haploids "1n 1x 13# extracted from tbr varieties[ However\ a number of diploid species are sexually isolated from tbr haploids and from other diploid species because of endosperm abortion in the hybrid seed[ To explain these cross! ing barriers Johnston et al[ "0879# postulated that each Solanum species has a hypothetical endosperm balance number "EBN#\ ranging from 0 to 3\ and that a 1 ] 0 maternal to paternal EBN ratio in the hybrid endosperm is a necessary condition for its normal development[ Thus\ only cross combinations with the same EBN result in normal endosperm development[ Sexually U[ S[ Copyright Clearance Center Code Statement] 9068Ð8430:1999:0891Ð9016 , 04[99:9 isolated diploid species have an EBN of 0 "Johnston and Han! neman 0871#\ making crosses with tbr haploids and most of the wild species "1EBN# very di.cult because the EBN ratio requirement is not satis_ed[ Among 0EBN wild species\ S[ commersonii "cmm# has several useful traits\ including resistances to Verticillium wilt\ potato virus X and tuber soft rot "Hanneman and Bamberg 0875\ Sirianni 0886#[ This species is also very resistant to low tem! peratures and\ among wild Solanum species\ has the best capacity for cold acclimatization after exposure to low tem! peratures "Palta and Simon 0882#[ These traits are very impor! tant given that frost is often a major factor in reducing tuber yield\ and that the cultivated potato is frost!sensitive and unable to cold acclimatize "Chen and Li 0879#[ One simple and e.cient way to overcome the EBN barriers of Solanum species is to consider the EBN as their e}ective ploidy level\ which can be manipulated by raising and lowering chromosome sets[ With this in mind\ the chromosome number and EBN of cmm were doubled "Cardi et al[ 0882# and 3x"1EBN# cmm was crossed with 1x"1EBN# hybrids[ Through the function of 1n eggs\ the F0 triploids obtained were used in 2x × 3x crosses with tbr to produce fertile BC0 hybrids\ which were further backcrossed to tbr to generate the BC1 progeny[ The ploidy and the cytological and molecular characterization of F0\ BC0 and BC1 hybrids have been reported previously "Carputo et al[ 0884\ 0886a\ Barone et al[ 0888#[ In this paper we summarize the evaluation of resistance traits found in these hybrids obtained through the manipulation of e}ective ploidy levels[ Materials and Methods Plant materials] F0 triploid\ BC0 pentaploidÐnear pentaploid and BC1 tetraploidÐnear tetraploid cmmÐphuÐtbr sexual hybrids were used[ The triploid hybrids were obtained from crosses between a tetraploid "1EBN# clone of PI 132492 of cmm obtained by in vitro tissue culture "Cardi et al[ 0882# and 1EBN diploid hybrids between S[ tuberosum Group Phureja "phu# and S[ tuberosum Group Tuberosum haploids "Carputo et al[ 0884#[ BC0 hybrids were produced through 2x × 3x crosses between triploid hybrids and tbr varieties[ BC1 hybrids were obtained by backcrossing two pentaploids with the varieties {Blondy|\ {Carmine| and {Tollocan| and the breeding line Wis 371 "Carputo et al[