Fungi reproduce by releasing spores that lie dormant in the environment until conditions are right for them to grow. The spores of fungi that infect plants are often released from fungal structures that develop on the surface of their hosts’ leaves or stems. However, the anther smut fungus (Micobrotryum lychnidis-dioicae) employs a more unusual strategy. Its spores are displayed on its host’s flowers so that they can be carried to other plants by insect pollinators.
Anther smut fungus can infect a small flowering plant called white campion (Silene latifolia). White campion is dioecious, meaning that each plant can only produce either male or female flowers. In the flowers of the male plants, pollen is produced by structures called stamens. When the fungus infects a male plant, it manipulates the plant so that the stamens no longer produce pollen and display fungal spores instead.
The flowers of the female plants do not have stamens, so infecting a female would appear to be a dead end for the fungus. However, the fungus has another trick up its sleeve: it induces a partial sex change in female white campion plants so that they do produce stamens (albeit primitive ones).
Like us, white campion has sex chromosomes (X and Y) that determine whether a plant will be male or female. Some of the genes on the sex chromosomes regulate the activity (or expression) of genes on other chromosomes. Therefore, certain genes are more active in a male plant than a female plant, and vice versa. This sex-biased gene expression contributes to the physical differences between the males and females. To better understand how anther smut fungus causes the female plants to develop male characteristics, Niklaus Zemp and colleagues used a technique called RNA-seq to study gene expression in white campion (Zemp et al., 2015).
They found that the fungus causes different changes in gene activity in the male and female plants. The biggest differences were in genes that are more highly expressed in healthy male plants than healthy female plants (male-biased genes). Zemp et al. show that fungal infection decreases the expression of many of these genes in male plants, but has the opposite effect on these genes in female plants. The fungus also altered the expression of female-biased genes differently in males and females, but to a lesser extent.
The up-shot to these changes in gene expression is that the male plants become a bit more feminine when anther smut fungus infects, while the female plants become more masculine. How the fungus achieves this is still a mystery, but it is not the only microbe to cause sex changes in its host. Wolbachia bacteria can cause some male insects to become more female, and a parasite called Nosema granulosis also feminizes some crustaceans. So, in the natural world, gender is a more fluid concept than you might think.
Anther smut fungus is the Organism of the Month here at Plant Scientist.
Zemp N, Tavares R, Widmer A (2015) Fungal Infection Induces Sex-Specific Transcriptional Changes and Alters Sexual Dimorphism in the Dioecious Plant Silene latifolia. PLoS Genet 11(10): e1005536. doi:10.1371/journal.pgen.1005536