Pollen colour morphs take different paths to fitness

Area of Science:

• Plant reproductive biology
• Evolutionary ecology
• Biochemistry

Background:

• Petal color diversity in plants is maintained by various selection pressures.
• Pollen color variation is common but poorly understood.
• In Campanula americana, pollen color ranges from white to dark purple, with dark morphs attracting more pollinators and performing better under heat stress.

Purpose of the Study:

• Investigate the maintenance of pollen color variation in Campanula americana.
• Determine the biochemical basis of pollen color differences.
• Evaluate the fitness consequences of different pollen color morphs.

Main Methods:

• Generated an F2 population segregating for pollen color.
• Measured correlations between pollen color, floral traits, pollen attributes, and plant fitness.
• Analyzed pigment biochemistry of color variants.
• Assessed maternal and paternal fitness through controlled crosses.

Main Results:

• Pollen color was largely independent of floral traits.
• Darker pollen grains were larger, contained more anthocyanins (cyanidin and peonidin), and showed superior performance under heat stress.
• Dark pollen exhibited higher pollen-related fitness and sired seeds with better germination.
• Light pollen plants produced more flowers and seeds per fruit, indicating an ovule advantage.

Conclusions:

• Pollen color variation in Campanula americana is maintained by a balance of different fitness advantages.
• Dark pollen morphs possess a pollen advantage (e.g., heat tolerance, pollinator attraction).
• Light pollen morphs have an ovule advantage (e.g., higher seed production), contributing to the maintenance of color diversity.