The effect of molecular, morphological and spectral reflectance evolution on nicotiana polyploids of different ages

Abstract

Interspecific hybridisation accompanied by multiplication of chromosome number, or allopolyploidy, results in rapid genetic, epigenetic, chromosomal and morphological changes. Evidence of ancient polyploidisation is found in most if not all angiosperms, suggesting that polyploidisation may have played a role in the success of angiosperm species. Here, I examine the fate of duplicate gene copies in the floral development gene NICOTIANA FLO/LFY (NFL) and the evolution of floral form and colour in Nicotiana (Solanaceae) allotetraploids of different ages. Both NFL copies are retained in all allotetraploids examined, even those ~10 million years old (myo). There are no nonsense or frame-shift mutations, suggesting that all copies are still functional. Both copies are expressed in all allotetraploids examined, even those ~4.5 myo. The evolution of floral form and colour was examined using geometric morphometrics of floral limb shape, corolla tube length and width metrics, and spectral reflectance measurements of floral colour. In floral limb shape, younger polyploids tend to be intermediate in shape between those of their diploid progenitors, whereas older polyploids have more divergent forms; however, divergence in floral limb shape can occur rapidly following polyploidisation. In corolla tube length and width, the majority of polyploids have wider and shorter corolla tubes, suggesting more generalist pollination after polyploidisation. In floral colour, polyploids can either be intermediate between their progenitors, like one or other progenitor, or divergent. The floral colour of N. tabacum is divergent and seems to have resulted from the inheritance of floral plastids that lack chlorophyll from its maternal progenitor and the inheritance of anthocyanin pigmentation from its paternal progenitor. Evidence for convergent evolution of floral form in green/yellow-flowered Nicotiana seems to be linked to hummingbird pollination. Overall, rapid molecular and morphological changes following polyploidisation may be advantageous and may partially explain why polyploids have been so successful in angiosperms.