Comparative Mapping Of 5S And 45s RDNA In Fragaria Species

There are about 25 Fragaria species distributed in the world,and most species are diploids(2n = 2x = 14).Polyploids mainly include tetraploid,hexaploid and octoploid,and a small proportion are pentaploid,nonuploid and decaploid.Fruits of most of these species are eatable,particularly,the cultispecies strawberry has important economic value.The knowledge of phylogenetic relationships and the genetic affinities among Fragaria species will help to choose parents from various species for introgression of wild genes into cultivars.Karyotype analysis is an effective method to explore the biological evolution relationship among different species.However,the Fragaria plants have small chromosomes,which are close in morphology,as well as great chromosome number in polyploid Fragaria species.Therefore,it is almost impossible for accurate karyotype analysis by traditional methods.This study showed that using the chromosome specific oligonucleotide library(oligo)as the probe could identify the single chromosomes in the Fragaria species.The oligonucleotide libraries of 6 chromosomes from Fragaria vesca were synthesized in this study,which were used as the fluorescence in situ hybridization(FISH)probes for the following research.(1)Two rounds of multicolor FISH analysis were conducted on a same metaphase cells from Fragaria vesca,and 3 probe libraries were used in each round of hybridization.Meanwhile,all chromosomes in the Fragaria vesca were identified.The karyotype of Fragaria vesca chromosome at mitosis metaphase was constructed,which was the first FISH-based molecular cytogenetic karyotype of Fragaria vesca.(2)Three times of hybridization were performed on a same slide using 6 oligonucleotide libraries,45 S and 5S rDNA probes.After determining the distribution location and number of 45 S and 5S rDNA in Fragaria vesca,the distribution characteristics of 45 S r DNA and 5S rDNA were determined in all Fragaria species searched so far.It was found that,among the 7 diploid Fragaria species,45 S rDNA located in the terminal of short arm of chromosomes 1,2 and 7,while the 5S rDNA located in the medial chromosome 7 near the 45 S rDNA.However,in Fragaria nilgerrensis,the 45 S rDNA only located on chromosomes 1 and 7;in comparison,45 S rDNA was only distributed on one chromosome in one pair of homologous chromosome in chromosome 1 of Fragaria nipponica.Meanwhile,the 45 S rDNA signal intensity was greatly different on the homologous chromosomes in several diploids,suggesting that the 45 S rDNA site in the diploid Fragaria species was at a dynamic change process.(3)The distribution of 45 S and 5S rDNA were also determined in the 9 polyploid Fragaria species(including 4 tetraploid species,1 hexaploid species,3 octoploid and 1 decaploid).Moreover,the nature(homologous or heterologous)and origin of polyploid Fragaria species genome were also explored according to the intensity of oligonucleotide probe signal,chromosome length,as well as the distribution of 45 S and 5S rDNA in polyploid and diploid.This study has provided an effective method for the identification and reasonable utilization of wild strawberry resources.Besides,it also provides theoretical evidence for modifying and cultivating new strawberry species through technologies,such as interspecies cross and synthetic allopolyploid.