Sexually-trimorphic interactions with colour polymorphism determine nectar quality in a herbaceous perennial

Abstract

Amongst gynodioecious plant breeding systems, there can exist intermediate morphs with a reduction in their male function (i.e. reduced number of functional anthers). Along with this sexual trimorphism, plants can also show floral colour polymorphism. Such intricate mixtures of phenotypes within a species may have complex effects on floral rewards. Floral rewards are known to vary between sexually dimorphic species and to a lesser extent between colour morphs. However, the interactive effect of sexual trimorphism and colour polymorphism is unexplored. We measured nectar’s sugar content in the sexually trimorphic Geranium sylvaticum, a gynodioecious plant with a light/dark floral polymorphism. We found that nectar reward differed across genders and colour morphs. Results were not however consistent within the three genders; dark female and hermaphrodite flowers had higher sugar content than light morphs, whereas intermediate flowers did not. As expected, females and hermaphrodites had different nectar reward, with intermediate morphs being midway between the other genders. In intermediates, the sugar content was not related to the number of functional stamens. We show for the first time the existence of sex-specific differences between flower gender and colour morphs in nectar rewards. Our results demonstrate the importance of considering multiple and conflicting selection pressures to explain rewards.

Intraspecific relationships between floral signals and rewards with implications for plant fitness

“…In some cases, both flower size and either flower shape or scent are related to reward production ( Møller 1995 ; Gómez et al 2008a ; Gómez and Perfectti 2010 ; Knauer and Schiestl 2015 ; Gervasi and Schiestl 2017 ). Occasionally, flower colour polymorphisms are also associated with reward variation ( Carlson and Holsinger 2013 ; Varga and Soulsbury 2017 ).…”