I’m embarrassed to reveal that only this year have I twigged that that the scientific name of the Red Campion which seems to flower everywhere in summer, Silene dioica, indicates that the plant is dioecious; it has individuals of two separate sexes.
Female flower with curly stigmas (left) and male flowers with anthers protruding (right)
One of the exercises in my BSBI Identiplant course this year was to estimate the ratio of male to female plants and in May, when I did this, I found roughly four male plants in flower for every female one; in order to ensure good rates of fertilization there has to be an excess of pollen. My tutor pointed out that later in the season, when many female flowers have set seed, the ratio of male to female plants in flower is even higher.
Having spent some time this summer collecting wild flower seeds, including red campion, to scatter in my ‘lawn’, I started wondering about what determines the gender of dioecious plants? Am I going to end up with lots of male plants which can’t produce seed or will I have enough female plants to establish a population in my garden? According to a review by Deborah Charlesworth (Charlesworth, 2016) some plants, including Silene species, have visibly different or heteromorphic sex chromosomes, like the X and Y chromosomes in humans, which carry the traits which determine the plant’s sex. Others have smaller sex-linked regions on their chromosomes which are too small to see with a microscope but which can now be visualised using molecular genetic techniques. These regions are often rather young, in evolutionary terms, which means plants can be used to study the evolution of separate sexes. The ancestral condition is believed to be monoecy – plants with both male and female parts. That way, plants can fall back on self-fertilisation if they fail to cross with another plant; some potential for variation is sacrificed but continuation of the species is guaranteed.
The theory goes that plants evolved distinctive sex chromosomes as a result of two, separate mutations; the first creating female plants and the second male plants. This seems to have occurred many times in different species so some species have been dioecious for much longer than others. Natural selection will only allow a mutation to spread in a population if it confers some sort of advantage. The suggestion is that a male-sterile (female) plant might benefit in a population which consists largely of hermaphrodite plants because it will avoid the effects of a reduced gene pool (inbreeding depression) which can follow from self-pollination and so will produce ‘fitter’ offspring than other individuals of the species. In addition, reallocating resources from the production of male parts to female parts may allow the plant to pour more resources into its offspring producing more, or larger, seeds. For a species to become fully dioecious it requires a second mutation to produce male only plants. This is, of course, a simplification – becoming a ‘female’ plant could occur as a result of mutations in many different genes as the development of anthers and pollen is a complex process. In Silene dioica this is believed to have taken place about 10 million years ago, relatively recently in evolutionary terms when you consider that flowering plants first appeared around 130 million years ago.
The evolution of distinct sex chromosomes depends on suppression of the recombination (exchange of genetic material) which normally occurs between homologous (paired) chromosomes, at least in the area of the genome responsible for determining sex. If that didn’t happen, some plants would end up with both male and female sterility mutations, which would disadvantage the population as a whole.
But back to Silene dioica – how exactly is the sex of an individual plant determined? Female plants have two X chromosomes whereas males have one X and one Y chromosome. In humans, the Y chromosome is reduced in size and has lost many of the genes carried on the X chromosome but in S. dioica the Y chromosome is actually the largest in the genome and contains many functional genes, perhaps because of the relatively recent evolution of separate sexes. When egg cells form after meiosis in female plants they will each carry a single X chromosome whereas meiosis in male plants will lead to pollen (the male gamete) which carries either an X or a Y chromosome. When an egg and pollen combine at fertilisation, the resulting embryo will either be female (XX chromosomes) or male (XY chromosomes). Any given egg cell has a 50:50 chance of being fertilised by each type of pollen so the seeds I scattered in my garden will hopefully be enough of a mixture to ensure a long-lasting population.
Charlesworth D (2016) Plant sex chromosomes. Annual Review of Plant Biology, 67, 397-420. https://doi.org/10.1146/annurev-arplant-043015-111911