Natural Hybridization Between Sorbus Pohuashanensis Hedl. And S.discolor Maxim.

Sorbus pohuashanensis Hedl.and Sorbus discolor Maxim.belong to Sorbus L of rose family(Rosaceae).Currently,most Sorbus resource are in wild state in China.Studies on Sorbus are mainly focused on geographical distribution and habitats,propagation and cultivation techniques,biological characteristics and genetic diversity assessment,while there are few reports of interspecific hybridization and breeding in Sorbus in China.Sorbus individuals with pink fruits,which were morphological intermediately between S.pohuashanenesis and S.discolor in their sympatric area,were found by members of our research group during their investigation in Mount Tuoliang,Heibei province.The origin of Sorbus with pink fruits and the evolutionary relationships of three Sorbus taxa couldn't be elucidated by current evidence.Therefore,the altidude distribution pattern of S.pohuashanenesis,S.discolor and Sorbus with pink fruits investigation were investigated in Mount Baihua nature reserve,Mount Tuoliang nature reserve and Mount Xiling,and comparative morphology and molecular biology methods(chloroplast and ribosomal DNA internal transcribed spacer)were used to explore the origin of Sorbus with pink fruits found in Beijing Municipality and Hebei Province.The main conclusions are as follows: 1.Altitude distribution pattern of SorbusIn addition to Mount Tuoliang,Sorbus with pink fruits were also found in Mount Baihua,and Mount Xiling.S.discolor was distributed at an altitude of 1 300-2 000 m.S.pohuashanenesis had an altitude of 1 500-2 200 m.The minimum altitude and the maximium altitude of S.pohuashanenesis were both higher than those of S.discolor.Sorbus with pink fruits grow within the overlapping areas were between 1 500-2 000 m of S.pohuashanenesis and S.discolor,with a discontinuous distribution.Three Sorbus had common habitat,they grew on shady slope,semi shady slope and streams.2.Morphological analysisSorbus with pink fruits displayed a stable and distinct intermediate transition types between S.pohuashanenesis and S.discolor in morphology.Nine quantitative characters were observed: compound leaf length(including petiole)(cm)and compound leaf width(cm),ratio of compound leaf length and width,leaflet length(cm),leaflet width(cm)and ratio of leaflet length and width,longitudinal diameter of fruit length(mm),ransverse diameter width of fruit(mm),fruit vertical diameter ratio of length to width.It is easy to distinguish three Sorbus by fruit color.S.discolor had white and yellow fruits,S.pohuashanensis had red or orange fruits,while Sorbus with pink fruits had pink fruits.Pubscence was a character to distinguish S.discolor.Hibernaculum,branch and leaves were labrous in S.discolor,while S.pohuashanensis and Sorbus with pink fruits were covered with white-grayish tomentose.Quantitative characteristics of compound leaf and leaflet of Sorbus with pink fruits were intermediate between S.pohuashanensis and S.discolor,and the fruit size of Sorbus with pink fruits was slightly larger than that of both.Stepwise discriminate analysis indicated that compound leaf length(including petiole)and longitudinal diameter of fruit length were main characteristics to distinguish three Sorbus taxa.The discriminant accuracy rate of S.discolor was 94.3%,and Sorbus with pink fruits was 75%,however,S.pohuashanenesis was only 63.3%.Total discriminant accuracy rate was 78.8%.3.ITS sequence of Sorbus with pink fruitsIt was shown that S.discolor had a single and uniform ITS sequence without heterozygous sites.S.pohuashanenesis displayed three types of ITS sequences.The first one was homozygous and each base in variant sites was quiet different from that of S.discolor.The second was also homozygous with S.discolor except one mutation in 130 th site.The third was heterozygous sequence.Sorbus with pink fruits also had two kinds of ITS sequences.One was same with that of S.discolor.Another was heterozygous.There are 12 ITS haplotypes.S.discolor had only one haplotype H1.S.pohuashanenesis had 7 haplotypes(H6,H7,H8,H9,H10,H11,H12).Sorbus with pink fruits were shown 5 haplotypes(H1,H2,H3,H4,H5)and had four unique haplotypes H2,H3,H4,H5.They shared haplotype H1 with S.discolor.4.Maternal origin of Sorbus with pink fruitsSorbus with pink fruits had bi-maternal origins:S.pohuashanenesis and S.discolor.There were 21 mutation sites in cpDNA sequences including mutation,insertion and deletion of bases.There were 7 cpDNA haplotypes.S.discolor had 1 haplotypes(H6).S.pohuashanenesis had 5 haplotypes(H1,H2,H3,H4 and H5)and haplotype H1 and H3 were its special haplotypes.Sorbus with pink fruits had 5 haplotypes(H2,H4,H5,H6 and H7)and haplotype H7 was its unique haplotype,they shared haplotypes H6 with S.discolor.S.discolor was its maternal parents.They also shared three haplotypes(H2,H4 and H5)with S.pohuashanenesis.S.pohuashanenesis was its maternal parents.5.Origin of some Sorbus with pink fruitsThree were 9 Sorbus with pink fruits were interspecific hybrids between S.pohuashanensis and S.discolor.Results of cpDNA phylogeny indicated that Sorbus was divided into two categories.9 Sorbus with pink fruits were clustered into S.pohuashanensis.8 Sorbus with pink fruits were clustered into S.discolor.Results of ITS phylogeny also indicated that Sorbus was divided into two categories.The first category contained S.discolor and Sorbus with pink fruits.The second category only consisted of S.pohuashanensis.It was concluded that the maternal parent of 9 Sorbus with pink fruits parent was S.pohuashanensis,and paternal parent was S.discolor.Sorbus with pink fruits were interspecific hybrids between S.pohuashanensis and S.discolor.