10/12/2025
What do dwarf males and antibiotics have in common? Windswept Broom Moss! Another dive into the wonderful world of mosses, with Dr. Sue Studlar!
Dicranum scoparium or Windswept Broom Moss, is a moderately robust moss (c. 5-10 cm high) with distinctive sickle-shaped leaves turned to one side. It looks windswept or like an old broom. It can form large cushions and turfs on acidic moist substrates such as rotting logs and soil over sandstone. Often bright green, it is a beautiful component of the mossy understory in the boreal forest and in northern temperate forests, especially in cool moist habitats. Attractive wet or dry, D. scoparium is a favorite for moss gardens, and is sold as Mood Moss. The typical windswept form is easy to recognize, but bog plants may display straight ovate leaves; a microscope is then required for positive identification.
Like most mosses, Dicranum scoparium is unisexual, and getting the sexes together can be problematic. Why? First, moss s***m are weak swimmers and soon run out of steam, though rain may carry them along. Second, males are often uncommon in moss populations; this “female bias” is due to species-dependent genetic and environmental causes. However, D. scoparium has a way of circumventing these problems: dwarf males. Researchers were surprised to find that dwarf males are widely distributed in dioicous (unisexual) mosses, found in 25 moss families (out of over 100 families) though none are known in liverworts.
The dwarf males of D. scoparium are tiny plants which develop from spores that happen to land on the fuzzy (rhizoid-cloaked) stems of the much larger females. Resembling buds, each dwarf male is simply an antheridium (s***m sac) enclosed by several leaves on a short stem bearing a tuft of anchoring rhizoids (hairs). These little gametophytes are short-lived and typically die after the spring breeding season. Thus, dwarf males are spring annuals though the females are perennial.
Remarkably, however, a male spore of D. scoparium that lands away from the female (or falls off her) can develop into a full-size perennial green male as large as the female. The female plant perhaps releases a chemical that induces male dwarfism in spores at close range. Exactly how the female induces male dwarfism in D. scoparium is unknown, but it is clearly “facultative” depending on the interaction of genes and environment. Depending on the species, dwarf males in other mosses are either facultative or obligate – wherein spores develop into dwarf males whether they land on the female or not. In moss colonies with facultative dwarfism such as D. scoparium, there are thus two kinds of potential fathers for spore capsules (sporophytes, which develop from fertilized eggs): full-sized males living independent of females and short-lived dwarf males that grow upon the female as epiphytes.
For a broader perspective, dwarf males are unique to mosses among land plants, but they are also found in animals. Many small animals have long-lived females that nourish the next generation and short-lived males whose chief contribution is genetic variability (s***m). Dwarfed males, whether mosses or micro-arthropods, play critical evolutionary roles by increasing chances of long-term survival in a changing environment.
Large mosses such as Windswept Broom Moss form miniature forests that provide homes for micro-arthropods (such as moss mites), fungi, bacteria, and algae. Growing opportunistically year-round, they defend themselves against attack with hard-to-digest cell walls and a remarkable array of unique chemicals (phenols and terpenoids) with antibiotic properties. Recently researchers extracted antibiotics from D. scoparium that are effective against a wide range of bacteria causing diseases in humans. They also isolated an antibiotic effective against bacteria contributing to honeybee colony collapse.
Future novel uses of Windswept Broom Moss could thus include antibiotics that facilitate long-term survival of both humans and honeybees. However, developing synthetic versions of these antibiotics is essential. Harvesting D. scoparium or other mosses for their antibiotics is not advisable since mosses grow relatively slowly and moss harvest of mosses is generally unsustainable. We should do all we can to protect the Windswept Broom Moss in nature where it is ecologically important, beautiful and of potential medicinal value.
Sources
Chen Ko-Husan et al. 2022. Comparative transcriptomics of fungal endophytes in co-culture with their moss host Dicranum scoparium reveals fungal trophic lability and moss unchanged to slightly increased growth rates. New Phytologist 234(5):1832-1847.
Ekwealor, Jenna T. and Kirsten M. Fisher. 2024. Reproduction and Population Dynamics in Autonomous Gametophytes. International Journal of Plant Sciences 185 (4).
Ireland, Robert R. 2007. Dicranum scoparium. In Flora North America, V. 27: 402-404, Flora of North America Editorial Committee. Oxford University Press. New York.
Ishak, Sarah et al. 2024. Boreal moss-microbe interactions are revealed through metagenome assembly of novel bacterial species. Scientific Reports 27: 14(1):22168. PubMed.
Karaoğlu, Şengül Alpay et al. 2022. Biological activity and phytochemical analysis of Dicranum scoparium against the bacterial disease for honey bee. Chemical Biodiversity 19 (7) . Wiley Online Library.
Lang, Annick S., Thies Gehrmann, and Nils Cronberg. 2021. Genetic Diversity and Population Structure in Bryophyte With Facultative Nannandry. Frontiers in Plant Science 12
Pinchonet, Amélie and S. Robbert Gradstein. 2012. Male dwarfism in the genus Dicranum (Dicranaceae) – a review. Cryptogamie, Bryologie, 2012, 33 (3): 299-311.
Studlar, Susan Moyle and JeriLynn Peck. 2007. Commercial moss harvest in the Appalachian Mountains of West Virginia: Targetted Species and Incidental Take. The Bryologist 110 (4): 752-765
