Hermetospheres

Discover how I learned with one of my hermetospheres what an independent fern gametophyte is.

In a recent post I reported three different bryophytes, that had appeared unintended in the same container. Today, I know I was wrong back then. What I had identified as a liverwort turned out to be an independently propagating fern gametophyte, a thing I didn’t even know exists. Let me explain.

All ‘land plants’, i.e. bryophytes (mosses, liverworts, hornworts), pteridophytes (ferns, lycophytes) and seed plants (gymnosperms, angiosperms), undergo an alternation of generations. This means that their life cycle alternates between a gametophyte and a sporophyte generation. With the exception of bryophytes, the sporophyte generation dominates in all land plants. This is also true of ferns (see the illustration below): what we intuitively identify as a fern plant is the sporophyte. The gametophyte is generally small, short-lived and lacks morphological characters that would allow for species identification (Yoneoka e.a. 2024), and is therefore frequently overlooked. Even in botanical science, the study of diversity of gametophytes has been neglected for many years (Yoneoka e.a. 2024).

The fern gametophyte is also called prothallium or prothallus. Pinson e.a. (2017: fig. 1) give a nice overview on how fern prothalli can look like (see the Figure below). Particularly among tropical, epiphytic fern species, gametophytes with more complex structures and longer lifespans have evolved. These traits are thought to increase the likelihood of two individuals coming into close enough proximity for outcrossing to occur. In three families (Hymenophyllaceae, Polypodiaceae, Pteridaceae), several species have further independently evolved the ability to reproduce asexually via small vegetative propagules called gemmae. These are thought to enhance the likelihood of establishment of fern populations in the canopy, as only one gemma needs be dispersed to give rise to an entirely new population. (Pinson e.a. 2017: 2)

One of the most famous examples of an independent fern gametophyte is Vittaria appalachiana. The sporophytes of this species are thought to be extinct due to global cooling during the last glacial period, whereas the associated gametophytes (see the two pictures below) with stronger cold tolerance survived with gemma reproduction around the Appalachian Mountains of North America for more than 10,000 years (Farrar and Mickel 1991, cited in Yoneoka e.a. 2024: 816)

Back to my own observation: What I now know is a fern gametophyte appeared ‘out of nowhere’ in one of my closed containers (see picture below).

A few small patches were transferred into a sealed glass tray with transparent lid on substrate similar to that I use for my hermetospheres. After 16 weeks, the gametophyte had spread over half of the substrate surface (see picture below).

I began to closely examine and document what had been grown. The thallus looked like a single layer of cells, and certain regions were full of growing or mature propagules (see the pictures below). This, I thought, should be enough to identify the plant. A photograph of a product sold by some retailers specialising in plants for terrariums put me on the right track.

Independent fern gametophytes are sold by some traders under the name of Antrophyum cf. plantagineum as a ‘miniature fern’. A quick check in the GBIF database reveals that the sporophyte of the true A. plantagineum is far from miniature. How the identification of the traded material was established and whether all offerings stem from the same clone, I cannot tell. It is possible that the clone in trade originates from a collection on the Ogasawara Islands, Japan. Murakami e.a. (2021:7) describe occurrences of A. plantagineum as an independent gametophyte from this location. These oceanic islands lie approximately 1,000 km south of central Tokyo and have never been connected to any large landmass. Interestingly, the corresponding sporophyte is not known either from these islands nor from the Japanese mainland. Instead. It is found from India to Sumatera and Jawa to Northwestern Australia. The fact that this product is available in the terrarium trade – and thus grows in the associated greenhouses – could well explain how it managed to find its way into my jar unnoticed. All it takes is a single one of the six-cell propagules pictured above.

Identifying a fern gametophyte without an associated sporophyte can be difficult or even impossible, even for a specialist, particularly when, as in my case, its geographical origin is unknown. In recent years, a new method has led to significant progress in this area: “DNA barcoding, the use of a standard, short region of DNA for species identification (Hebert et al., 2003a), is a powerful method to enable species identification of field‐collected pteridophyte gametophytes. Since this method became available, it has revolutionized the study of pteridophyte ecology by enabling accurate identification of this cryptic sexual life stage over a wide taxonomic range.” (Nitta and Chambers 2022: 2)

Maybe, when in a near future DNA barcoding for fern gametophytes will be affordable for hobbyists, I might want to find out the true identity of my gametophyte. For now, I am happy with what I have learnt so far from my mysterious hitchhiker.