Emily Collins – Conservation Student Intern
When you come across a field of livestock, a dog on a lead or a bird up a tree, guessing at the animal’s gender comes almost instinctively. How often though, do you try to do the same with flowers?
6-10% of plant species, including holly, nettle and the non-native invasive Japanese Knotweed, are dioecious, meaning that they have male and female parts on separate plants. Whilst this increases genetic diversity and therefore reduces the harmful effects of inbreeding in these plants, their dependency on cross-pollination has led to something just as nasty. Say hello to ‘anther smut’ – the STI of the plant world.
I first heard about the phenomenon a few weeks ago when I accompanied a naturalist friend on an education day at the Yorkshire Arboretum. We were heading back to the car after an enjoyable but unseasonably cold day of pond-dipping, when he pointed out a patch of bright pink flowers on the verge of the car park.
“This is Red Campion and it has separate male and female plants,” he told me.
He then explained that a fungus, known as anther smut (Microbotryum violaceum), grows on some of the flowers and is ‘transmitted’ between plants via pollinators such as bees and wasps. If the smut is distributed on the flowers of the female plant, it induces an involuntary sex change in the flower, causing it to grow smut-filled male anthers. These then burst open, releasing fungal spores in the place of pollen.
So how does the smut do it? The sex of Red Campion (Silene dioica), like that of higher animals, is determined by x and y chromosomes. These contain the genetic instruction manual for the plant, with an xx combination coding for female plants and xy coding for male. The y chromosome contains genes which prevent the growth of female parts on the flower and it’s this which the smut mimics. By partially-substituting the x chromosome with instructions for the growth of anthers, the smut creates a sterile, transgender plant.
To understand how this affects individual populations of Red Campion, it’s useful to liken the population to a sweet shop. Just as children choose the tastiest looking sweets in the shop, pollinators, which carry the spores from the infected plants, seek the most attractive flowers which can capture the most pollen. The sweets get eaten, the flowers are sterilised and only the not so nice looking sweets/flowers remain. This is what is called a ‘positive selection pressure’ for the less attractive flowers. However just as a sweet shop might get a delivery in from elsewhere, pollinators can bring pollen in from other populations (which aren’t infected with smut) and which can generate the reproduction of healthy, attractive individuals. Overall then, there is never a very strong selection pressure in either direction.
Red Campion and other members of the Caryophyllaceae family can be found throughout the North York Moors. Next time you spot one, have a look to see whether the plant is male or female. In Red Campion, female plants feature 5 styles whereas males have 10 round white anthers. And look out for dark colouring of the anthers which indicates a plant infested with smut!