The unique morphological basis and parallel evolutionary history of personate flowers in Penstemon.

Adaptive radiation in ecologically and morphologically diverse plant lineages presents an opportunity to investigate the rapid evolution of novel floral traits. While some types of floral traits, such as flower color, are well-characterized, other types of complex morphologies remain understudied. One example is occluded personate flowers, dorso-ventrally compressed flowers with obstructed floral passageways, which have evolved in multiple genera, but have only been characterized from snapdragon. Our study examined the morphological basis and evolutionary history of personate flowers in a clade of Penstemon species that includes three personate-flowered species. We characterized floral morphology and inferred phylogenomic relationships for 13 species in this group in order to examine the evolutionary history of personate flowers. We used phylogenomic tests for introgression to examine whether personate-flowered lineages have a history of introgression. Unlike the personate flowers of snapdragon, personate flowers in Penstemon are produced by deep pleats in the ventral petal tissue that curve the ventral petal surface upwards, obstructing the floral tube opening. Our phylogenetic tree suggests that personate flowers evolved in two separate lineages. Phylogenomic analyses indicate incomplete lineage sorting and introgression between certain taxa have contributed to phylogenomic discordance, however we found little evidence of recent introgression between the two personate-flowered lineages. The evolution of personate flowers in Penstemon involves a distinct morphological basis than snapdragon. Personate flowers have evolved multiple times in Penstemon on a rapid evolutionary timescale. The source of genetic variation for repeated shifts may be de novo mutations or pre-existing variants.