Differential expression of flavonoid biosynthetic pathway genes drives the production of transgressive pigments in Nicotiana section Repandae allotetraploids

Premise: Hybrids commonly display transgressive characters, which fall outside the range exhibited by progenitors and can provide an adaptive hybrid advantage. Transgressive characters have often been documented, but the molecular bases underlying them have rarely been determined. Here, we investigated the molecular basis of transgressive delphinidin and transgressively high flavonol levels in flowers of Nicotiana section Repandae allotetraploids to determine whether these pigments are driven by differential expression and/or coding sequence evolution and, if coding sequence evolution is involved, whether there is evidence of complementation of the progenitors or resurrection of gene function in allotetraploids. Methods: We analyzed transcriptomes of corolla tissue from buds at 60%, 85%, and 95% of the mean corolla length at anthesis from Nicotiana section Repandae allotetraploids and their diploid progenitors. We examined differential expression of flavonoid biosynthetic pathway (FBP) genes and correlated transcript levels with pigment composition. Results: Diploid progenitors lacked anthocyanins due to a nonfunctional F3′5′H and a retained intron that leads to a premature stop codon in ANS in N. sylvestris and to premature stop codons in ANS in N. obtusifolia. Differential expression of FBP genes sufficiently explains floral pigment composition in Nicotiana section Repandae allotetraploids. Conclusions: Differential expression drives the production of transgressive delphinidin and transgressive flavonol levels in Nicotiana section Repandae allotetraploids, and there is no evidence for complementation nor gene resurrection. Transgressive delphinidin may provide an adaptive advantage to increase pollinator attraction in N. repanda and protect against UV radiation in N. nudicaulis.