Scientists in Japan have discovered that ferns later in development release chemicals in the soil which determines the sex of younger ferns, which maintains a sex ratio in the population that favours cross-fertilisation.
Ferns have a very different reproductive system to the plants that we are used to. A lot of the plants in our gardens are flowering plants: they often have male and female parts in the same flower and reproduce using seeds. Ferns reproduce using spores, not seeds. Ferns have male and female individuals a bit like humans; however in ferns, the community decides on the sex of plants, instead of their genetics.
Mature fern plants can be male, female or hermaphrodite. If there are no mature ferns around then an individual will become a hermaphrodite, and then self-fertilise to produce spores which grow into fern plants. Self-fertilisation in the plant world is not ideal; cross-fertilisation between plant individuals brings more genetic variation into the plant population which means the species are more likely to be better able to cope with disease outbreaks.
A team of scientists at Nagoya University in Japan led by Makoto Matsuoka have developed a two-part model for the determination of sex in the Japanese climbing fern, Lygodium japonicum. Early-maturing female plants add a chemical group to a plant hormone called gibberellin to produce a pheromone which is then released into the soil. This pheromone is then taken up by younger plants; they then take off this chemical group and convert the compound to the active form of the hormone gibberellin. Gibberellin is a type of plant hormone that plays a part in plant growth. This hormone then leads to male organs being formed, producing male plants.
Why don’t the female plants just secrete the active gibberellin? The modifications that the female plants make to the hormone mean that it is better at travelling in a moist environment, like the soil. The hormone released is also more easily taken up by younger plants.
The older plants can’t use the modified form of the hormone and develop male organs. The researchers found that the plants produce different enzymes (proteins that speed up biological reactions) at different stages of development. Even if a mature plant takes up the pheromone, it can’t use it, this ensures that mature female plants don’t change into males. This system means that there are often several females in a population surrounded by many males, this sex ratio prevents self-fertilisation and promotes genetic variation.
In humans, pheromones would be secreted in the bloodstream, not the soil. It seems hard to believe that there would be a high enough amount of this pheromone in the soil to be absorbed by plants and cause a change in sex. This question was addressed by the researchers. They discovered that the enzymes involved in this process are specially adapted to work with really low amounts of the pheromone. The sex determination process in ferns is a very sensitive process.
This study is important in understanding how sex is determined in non-seed plants, in particular how molecules control plant sex, which is not very well known. The hormone gibberellin also plays a part in sex determination in some seed plants such as maize and cucumber. Further studies in this field may show whether sex determination regulated by gibberellin is seen in other plant families.
In some South-Eastern parts of the US, the Japanese climbing fern is an invasive weed species. Currently the methods to control the fern population and spreading are very limited. It is hoped that the knowledge given by this study could be used for further research into developing methods to control the growth of the fern.
It has been known since the 1950s that early-maturing ferns determine the sex of later-maturing ferns; however, this study contributes to the understanding of plant sex determination at a molecular basis. “We know much more about sex in animals than we do in plants”, says Jody Banks, a plant geneticist at a university in Indiana, “this is really the first study to put a molecular face to it”.
Original research article: Tanaka, J. et al (2014). Antheridiogen determines sex in ferns via a spatiotemproally split gibberellin synthesis pathway. Science. 346, pp 469-473. Accessed from: http://science.sciencemag.org/content/346/6208/469
Image taken from a Perspectives article: Sun, T. (2014). Sex and the single fern. Science. 346, pp 423-424. last accessed 26/06/2016 from http://science.sciencemag.org/content/346/6208/423