Female and hermaphroditic blue cardinal flowers (Lobelia siphilitica)
January 4, 2019
Photo of blue cardinal flower (cropped version shown) by: Agnieszka Kwiecien, Nova (CC BY-SA 03)
Plants exhibit a wide variety of reproductive systems. A relatively small proportion of species are gynodioecious, meaning individual plants can produce either female or hermaphroditic flowers. In this system, females require pollen from a hermaphrodite to reproduce while the hermaphrodite can pollinate and reproduce itself. Some researchers believe that gynodioecy is an evolutionary intermediate step to a reproductive system where female and male organs are on separate plants without there being hermaphrodites (called dioecious plants). One question about gynodioecy that has stirred curiosity from an evolutionary perspective is - why would a plant species that can grow and reproduce as a hermaphrodite also produce individuals that are female?
Studies have demonstrated that under stressful environmental conditions, the proportion of female plants is higher, therefore stress could be maintaining gynodioecy. Bailey et al. (2017) looked at whether two evolutionary mechanisms, reproductive compensation and the cost of male fertility restoration, are at play in increasing the proportion of females under stressful conditions. Reproductive compensation is where females occur in a population because they can produce more and/or greater quality seed compared to hermaphrodites whereas the cost of male fertility restoration is where, due to genetic factors, there is a fitness cost (such as fewer offspring) associated with producing males.
In their study, Bailey et al. (2017) used the blue cardinal flower (Lobelia siphilitica) a perennial wildflower native to eastern North America. They collected seeds from two populations in the southern part of the species’ range (Indiana and Ohio, USA) and two populations in the northern part of their range (Ontario, Canada). Seeds were germinated and grown in incubator chambers under two temperature treatments - one reflecting the higher temperature of the southern population’s range which may be considered stressful since females are more prevalent, and the other reflecting the cooler temperature of the northern population’s range.
To confirm that the higher temperature was in fact stressful to the blue cardinal flowers, Bailey et al. (2017) took measurements on three factors that indicate stress in plants: leaf chlorophyll concentration, efficiency of photosystem II (a protein complex involved in photosynthesis), and the growing season in which plants first flowered. They then measured rosette sizes, a proxy for flower number, and compared these between female and hermaphroditic flowers. If reproductive compensation is a mechanism for maintaining gynodioecy, it would be expected that the higher temperature would lead to larger rosette sizes in females. The second possible evolutionary mechanism, cost of male fertility restoration, was evaluated by determining the viability of pollen produced by the hermaphrodites, with viability predicted to decrease at the higher temperature.
Interestingly, the results ran contrary to expectations. At the higher and presumably stressful temperature, leaf chlorophyll concentration was increased, more plants initiated flowering in the first growing season, and the efficiency of photosystem II was unaffected. Females and hermaphrodites and plants originating from either the southern or northern populations did not differ on any of these measures. This evidence indicates that environmentally-relevant high temperatures were not stressful to the blue cardinal flower.
Reproductive compensation and the cost of restoration were similarly unaffected by the high temperature as evidenced by a lack of effect on rosette sizes between females and hermaphrodites and on pollen viability of hermaphrodites. Bailey et al. (2017) concluded that further research is needed to better understand the relationship between environmental stressors and the maintenance of gynodioecy in plants.
Read full article here (open access):
Bailey MF, Case AL, Caruso CM. 2017. Physiological effects of temperature do not explain prevalence of females in populations of gynodioecious Lobelia siphilitica growing in warmer climates. American Journal of Botany, 104(3):411-418.