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.
Highlights
Amongst gynodioecious plant breeding systems, there can exist intermediate morphs with a reduction in their male function
In species where more than one colour morph exists, directional selection by pollinators alone or in combination with random genetic drift should lead to the loss of floral colour polymorphism especially if different colours differed in their rewards
In female (β±SE =−0.50 ± 0.14, t-ratio =−3.43, P < 0.001) and hermaphrodite (β±SE =−0.17 ± 0.08, t-ratio =−2.01, P = 0.040) flowers, dark morphs had higher sugar content in their nectar, whereas in intermediate flowers there was no difference between the two colour morphs (β±SE =− 0.06 ± 0.12, t-ratio = 0.55, P = 0.58; Fig. 2)
Summary
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 show floral colour polymorphism. We show for the first time the existence of sex-specific differences between flower gender and colour morphs in nectar rewards. Many gynodiecious species contain a third distinct phenotype[15] This phenotype is made up of individuals with an intermediate, or partially male-sterile, phenotype (i.e. individuals with a mixture of pistillate and perfect flowers or with mixed flower types). This may be problematic, as it ignores the possible cost of a male-fertility restoration and how this impacts floral traits
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