Anther Smut Disease Causes Transgender Transformations

This post is the latest in a recurring series called the Difference Between Boys and Girls, which talks about gender inequality and the battle of the sexes in biology.

Arctic ecology students hard at work in Svalbard, Norway. Photo by Marie Davey.

Arctic ecology students hard at work in Svalbard, Norway. Photo by Marie Davey.

A group of students in an arctic biology class are lying on the tundra surrounded by beautiful mounds covered in tiny, bright pink flowers. They stare intently at each plant, and calls of “37-Female-5-6” and “38-Male-9-3” ring out like an indecipherable secret code as they measure and sex each plant. There is an easy rhythm to it and a girl sits with a clipboard, madly scratching away. “39-Female….” The pencil pauses expectantly and the girl looks up at the interruption in flow. Her partner starts again, “No, 39-Male….wait. 39-HeShe?? Is that even possible? Half the plant is male and half the plant is female?” The students have unwittingly stumbled into the middle of a sex-reassignment underway in the wild.

The moss campion, Silene acaulis, produces bright pink flowers that are either female or hermaphroditic and produce both male and female parts. Typically, a single plant will only produce one type of flower and can be sexed as being female or hermaphroditic. However, infection by a pathogen called anther smut, or Microbotryum violaceum, can cause involuntary sex-reassignment in the moss campion.

Moss campion (Silene acaulis) infected with anther smut. Anthers are completely full of dark fungal spores, and fallen spores are causing dark smudges on the petals.

Moss campion (Silene acaulis) infected with anther smut. Anthers are completely full of dark fungal spores, and fallen spores are causing dark smudges on the petals. Photo by Marie Davey.

Anther smut is a fungal disease that can infect the leaves, stems, and flowers of campion plants, with plants remaining infected for years, and often their entire life spans. The disease is spread by insects that pollinate the flowers of Silene. Fungal spores, along with pollen, are carried on the body of bees and flies, and deposited on those flowers the insect visits. When the Microbotryum spores are carried to a hermaphroditic campion flower and infect it, they specifically attack the male sexual organs, called anthers, and replace the pollen they contain with fungal spores. The dark spores leave telltale soot-like smudges on the pink petals of the flowers, earning the fungus its ‘smut’ name. When an insect visits the infected flower, it gets covered in these fungal spores and carries them to the next flower it visits, and the cycle repeats.

Healthy female (left) and male (right) moss campion flowers. Each male flower has ten round, white anthers protruding from the flower.

Healthy female (left) and male (right) moss campion flowers. Each male flower has ten round, white anthers protruding from the flower. Female flowers have three white styles that protrude from each flower. Photos by Marie Davey.

However, when a female campion plant is infected, it undergoes an involuntary sex change and begins producing hermaphroditic flowers with male anthers full of fungal spores. Researchers have ruled out the possibility that the fungus is producing a hormone or morphogenic chemical that causes the female plants to become hermaphroditic. Instead, it appears that the anther smut fungus has the ability to flip the genetic switches of its host. In Silene, the sex of a plant is determined by a pair of sex chromosomes, much like in humans. Two X chromosomes yield a female flower, while an X and Y chromosome result in the flower developing both female parts and anthers, the male floral organ. The anther smut fungus can act as a partial genetic substitute for a Y chromosome in female plants, resulting in flowers that are hermaphroditic rather than female and better suited to disperse the fungus’ spores. Although it is not clear how the fungus has developed the ability to instigate this floral sex change, it seems that a sex-change in the wild is a simple matter!

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