Insight into octoploid strawberry (Fragaria) subgenome composition revealed by GISH analysis of pentaploid hybrids

As the product of interspecific hybridization between its two ancestral octoploid (2n = 8x = 56) species (Fragaria chiloensis and F. virginiana), the cultivated strawberry (F. xananassa) is among the most genomically complex of crop plants, harboring subgenomic components derived from as many as four different diploid ancestors. To physically visualize the octoploids' subgenome composition(s), we launched molecular cytogenetic studies using genomic in situ hybridization (GISH), comparative GISH (cGISH), and rDNA-FISH techniques. First, GISH resolution in Fragaria was tested by using diploid and triploid hybrids with predetermined genome compositions. Then, observation of an octoploid genome was implemented by hybridizing chromosomes of pentaploid (2n = 5x = 35) hybrids from F. vesca x F. virginiana with genomic DNA probes derived from diploids (2n = 2x = 14) F. vesca and F. iinumae, which have been proposed by phylogenetic studies to be closely related to the octoploids yet highly divergent from each other. GISH and cGISH results indicated that octoploid-derived gametes (n = 4x = 28) carried seven chromosomes with hybridization affinities to F. vesca, while the remaining 21 chromosomes displayed varying affinities to F. iinumae, indicating differing degrees of subgenomic contribution to the octoploids by these two putatively ancestral diploids. Combined rDNA-FISH revealed severe 25S rDNA loss in both the F. vesca-and F. iinumae-like chromosome groups, while only the prior group retained its 5S loci.