‘Two households, both alike …’? How differences between sexes varies with time in flowering plants

‘Two households, both alike in dignity’ is famously the opening line of Romeo & Juliet. How alike houses Montague and Capulet actually are is debatable, but what is clear is that plants with two households can be very different. Plants? Two households? What? Well, around 6% of plant species are dioecious (from the Greek for two houses or two households). This is to say that ‘male’ and ‘female’ sexual organs are split between different individuals, and there are therefore separate male and female plants. The comparative rarity of this in plants is in stark contrast to animals, the vast majority of which are dioecious. An often-recorded property of dioecious organisms is sexual dimorphism, meaning differences in physical characteristics of different sexes. In plants, sexual dimorphism manifests particularly in differences between the flowers of male vs. female individuals. Characteristics of male and female flowers in dioecious plant species are also known to vary with time across a flowering season. Combined understanding of how differences between sexes may vary over time is much less certain, but this may be important for ensuring successful reproduction in dioecious plant species. In their recent paper in Annals of Botany, Moquet and colleagues investigate variation over time of sexual dimorphism using the dioecious plant species Silene dioica, which is native to Northern and central Europe.

Moquet and colleagues measured various features of S.dioica flowers and how these varied with time across two flowering seasons. Male plants consistently had a higher number of flowers compared to female plants, but the number of flowers varied with time substantially more in male plants than in female plants. Female plants had a consistent low number of flowers across the flowering season, whilst male plants had a peak in flower number in the middle of the flowering season (April to June/July). Exactly why this happens is not clear. However, the authors speculate that is likely has something to do with interaction of male flowers with pollinating insects. All measured flower size dimensions were significantly larger for males than for females, but the authors also note that there was large variation in floral dimensions within both males and females. Flower size dimensions were also larger in both males and females earlier in the growing season. From this, the authors speculate that plants likely divert resources away from producing new flowers later in the growing season to supporting seed development in earlier flowers, resulting in decreased flower size as the season progresses.

In male plants, the earliest flowers produced the most pollen but the proportion of pollen grains that were viable stayed the same throughout the season, even as pollen production seemed to decline. Again, this could be explained by resource diversion away from late flowers once earlier flowers had begun to produce seed. Possibly the most interesting question that arises from this work is how the temporal variations in sexual dimorphism described here possibly relate to interactions with pollinating insects. As the authors speculate, the temporal variation in flower sex prevalence in S.dioica across the season could conceivably have either of two opposing effects. It could, as Moquet and colleagues discuss, result in decreased pollination of female flowers over time and presumably allow more resources to be diverted to support developing seed from earlier flowers. Alternatively, it is also possible that it actually increases the average pollen amounts deposited on female flowers and promotes genetic diversity.  

Moquet and colleagues conclude with: ‘Although the consequences in terms of pollinator behaviour and pollen dispersal now need to be dissected, our results underline the interest of investigating temporal variation in floral traits, as it likely affects pollination patterns across time’. Time will tell!