Genomic origin and organization of the allopolyploid Primula egaliksensis investigated by in situ hybridization

Background and Aims: Earlier studies have suggested that the tetraploid Primula egaliksensis(2n = 40) originated from hybridization between the diploidsP. mistassinica (2n = 18) and P. nutans (2n = 22), which werehypothesized to be the maternal and paternal parent, respectively.The present paper is aimed at verifying the hybrid nature ofP. egaliksensis using cytogenetic tools, and to investigatethe extent to which the parental genomes have undergone genomicreorganization. Methods: Genomic in situ hybridization (GISH) and fluorescent in situhybridization (FISH) with ribosomal DNA (rDNA) probes, togetherwith sequencing of the internal transcribed spacer (ITS) regionof the rDNA, were used to identify the origin of P. egaliksensisand to explore its genomic organization, particularly at rDNAloci. Key Results: GISH showed that P. egaliksensis inherited all chromosomes fromP. mistassinica and P. nutans and did not reveal major intergenomicrearrangements between the parental genomes (e.g. interchromosomaltranslocations). However, karyological comparisons and FISHexperiments suggested small-scale rearrangements, particularlyat rDNA sites. Primula egaliksensis lacked the ITS-bearing heterochromaticknobs characteristic of the maternal parent P. mistassinicaand maintained only the rDNA loci of P. nutans. These resultscorroborated sequence data indicating that most ITS sequencesof P. egaliksensis were of the paternal repeat type. Conclusions: The lack of major rearrangements may be a consequence of theconsiderable genetic divergence between the putative parents,while the rapid elimination of the ITS repeats from the maternalprogenitor may be explained by the subterminal location of ITSloci or a potential role of nucleolar dominance in chromosomestabilization. These small-scale rearrangements may be indicativeof genome diploidization, but further investigations are neededto confirm this assumption.