Annals of Botany

Lost loci in Primula parentage probe

I have often heard it said that speciation is a slow process, taking place over many generations, even when populations are separated. There is certainly much truth in that statement but equally it ignores genetic events that create new species more or less instantly. One of those events is polyploidization. A glance at chromosome numbers in almost any angiosperm genus reveals that polyploidization has played a major role in angiosperm evolution. Indeed, Guggisberg et al. from Zurich, Switzerland (pp. 919–927) cite a review by Soltis (2005. Ancient and recent polyploidy in angiosperms. New Phytologist 166: 5–8), which states that at least 70 % of angiosperms have polyploid origins. Modern molecular and cytological techniques help investigate putative parental origins of polyploids as has been used here by the Zurich group for a North American Primula species, P. egaliksensis (2n = 40). This is believed to be an allopolyploid derived from a hybrid between the relatively widely diverged P. mistassinica (2n = 18) and P. nutans (2n = 22). Hybridization of either P. mistassinica or P. nutans DNA to chromosome spreads of P. egaliksensis revealed that both putative parental chromosome sets are present in P. egaliksensis, with no evidence for major chromosomal re-arrangements between the two genomes. However, focus on the genes encoding the major rRNAs (‘rDNA’) revealed some more local changes, in particular the loss in the hybrid of the maternal parent’s 45S rDNA sequences. Further, heterochromatic knobs carrying the rDNA internal transcribed spacer (ITS), present in the maternal parent P. mistassinica, are absent in the hybrid, while sequencing of 44 clones of the ITS of the hybrid revealed that only 5 % were of P. mistassinica origin. The loss of the maternal rDNA sequences may have resulted from nucleolar dominance while the more general lack of chromosome rearrangement and genome homogenization may be a result of the taxonomic distance between the two parents.