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Can pollinators generate differences between female & male conebushes?

By Asa Hemborg, Botany Dept, UCT

Why do male and female Conebushes (Leucadendron) often look different from each other? What roles do the pollinators play in creating these differences? These are some of the questions I am asking in a post-doc research project in collaboration with Prof. William Bond.

In unisexual plants, i.e. those with separate sexes, pollen has to be transferred from males to females for seeds to develop. There is no other way that these plants can secure their seed production and the recruitment of new plants for future generations. In some cases, pollen is carried by the wind to the female stigmas. Because of the risk for the pollen to be lost on the way, the male plants of species relying on wind for pollen transport often produce an excessive amount of pollen and a large amount of tiny flowerheads. In wind-pollinated unisexual plants, males and females therefore often look very different from each other. One such example is Ld rubrum, where the size of the male flowerhead is only a fraction of the female’s.

If insects play the role of pollen carriers between males and females, it gets a bit more complicated. Insects generally visit the flowers in search of something. This is usually food, for example nectar or pollen, but it can also be to find mates and shelter. If insects systematically visit flowers of a similar kind, and thereby transfer pollen between flowers of the same species, we call them pollinators. The food they find in the flowers is then also referred to as a reward, or payment, for their roles as messengers in the sexual communication between plants. The pollinators usually act in response to plant signals and to environmental factors, to find their rewards. Then, how does this type of communication between plants work if the male and the female plants look different from each other? How do the pollen carrying insects recognize them as the same species?

In my studies, I examine how a tiny (1.5 mm) beetle pollinator (Conebush Pollen Beetle Pria cinerascens, Family Nitidulidae - Sap Beetles) visits male and female plants in the Sickle-leaf Sunshine Conebush Ld xanthoconus), and what plant signals and rewards are important for attracting the beetles. In this Conebush, male plants in the age group of six to eight years produce on average six times more flowerheads than the females. Because the number of flowerheads per plant increases exponentially with age in males, but not in females, this difference between the two sexes will increase as the plants grow older.

I found that the relationship between the Pria beetles and their host plant Ld xanthoconus is very intimate. Pria belongs to a group of beetles sometimes called Pollen Beetles, because their main food source is pollen. At the onset of flowering, adult Pria arrive on the male flowerheads to feed, mate and to place their eggs either at the base of the bracts or in old open flowers at the base of the flowerhead. This happens when the male flowerheads still have a circle of undeveloped flowers at the top. Within two to three weeks, the larvae will develop and (probably) pupate. The larvae can easily be spotted on the flowerheads, especially on sunny days! They have a brown head, white or yellowish body (0.5 - 1.5 mm long) and six legs. Nobody really knows what happens next. Where do the beetles go when the flowering time of Ld xanthoconus is over? What other plants do they feed on, if they are still active? We know that they also visit and probably also pollinate some of the other species of Conebushes such as Ld laureolum and Ld salignum.

Because the life-history of Pria is so closely connected to the plants they feed on, it is easy to understand why the beetles visit the male plants. The question is then; why do they bother with the females? Female flowerheads do not offer any food reward in terms of pollen (or nectar). Last year, we studied how adult Pria beetles visited male and female plants in response to flowerhead number, weather (sunny and dry vs rainy and overcast) and time of day (morning vs evening). For all the plants we examined (170 plants for each sex), we found at least one Pria on 95% of the male and on 43% of the female flowerheads. However, as could be expected, the number of beetles were always higher on the males. On sunny and dry days, male flowerheads had on average 15 times more beetles than the female flowerheads. But we discovered that for female flowerheads, the chances of receiving a visit from a Pria beetle doubled if the weather was colder, i.e. rainy and overcast. On those days, females had on average 1.2 beetles per flowerhead, as opposed to 0.6 on sunny and dry days. We found slightly more beetles on the flowerheads in the evening than in the morning, but this was the same for both sexes.

It appears from our studies that the beetles visit male and female plants for different reasons. Pria uses the male flowerheads as a food source and mating ground. We think that Pria uses the female flowerheads for protection, heat, and perhaps also mating opportunities during bad weather. After all, these plants flower during a very wet and cold period of the year! During the coldest and wettest days, we found no Pria on the males, but on some of the females. We guess that the Pria then search for the females. In female flowerheads, the bracts are often also less open than in the male plants and form a cup around the cone – a nice cozy shelter.

During this coming flowering season, we will focus on the following questions:

• How did a pollination system like this evolve?
• Do the beetles go actively to the female flowerheads or do they have problem finding males on rainy days?

If Pria actively visit males and females for different reasons, do they use different visual cues for finding their respective flowerheads? If that is the case, it may explain why sexual dimorphism in floral characters can be so exaggerated in this species. There would be no need for the male and female plants to look similar to each other. They can have different number of flowerheads, different colour flowerheads and still get the visits they need for effective pollination. They would only have to look attractive enough among the group of males and females, respectively, to make sure that Pria would come and visit them. The future, and this field season, will hopefully let us know more.

Acknowledgements - I am very grateful to the
Cape Peninsula National Park and the
Western Cape Nature Conservation Board for being able to perform my studies in the Silvermine and Kogelberg Nature Reserves, respectively. Nolan van Wyk and Erna Davidse provided valuable assistance in the field.

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