Evolution of the floral nectaries in the Primulaceae sensus lato (Ericales)

The Primuloid clade or Primulaceae sensus lato is well supported and contains several pollination syndromes, including wind pollination and insect pollination with pollen, nectar, or oil as a reward. Previous studies have recognized two types of nectaries (floral and extrafloral) with different distributions (e.g. ovary, stigma, style, pedicel). This study aimed to reassess the diversity of nectary types, and their distribution and evolution within the clade. Flowers were selected to represent the four subfamilies Maesoideae, Theophrastoideae, Primuloideae, and Myrsinoideae. The flowers were investigated for nectary tissue with electron microscopy, light microscopy, and sugar tests. Nectar is mainly exuded by gynoecial nectarostomata (in Maeseoideae, Theophrastoideae, Primuloideae, and a few basal Myrsinoideae, such as Stimpsonia and Coris) distributed at the base, middle, and top part of the ovary. Myrsinoideae is the exception with an absence of a nectary linked to buzz-pollination, or presenting nectariferous or oil-producing trichomes on the perianth and stamens. The evolution of nectaries is variable in the Primuloid clade, with Primuloideae apparently the starting point of an evolutionary reversal in terms of nectary evolution within this clade, and a high diversity of nectaries within the Myrsinoideae representing various trends that are currently under-investigated. The evolution of floral nectaries in Primulaceae presents an exciting shift from nectar-producing flowers towards a loss of nectaries, and an evolutionary reversal with a shift to the co-option of either oil-producing trichomes or nectar-producing trichomes.