Creature Feature. Dual identity

The longer I consider ferns, the more they seem like a collection of contrasts. Just a few weeks ago, before I talked to fern researcher Joel Nitta, I found ferns uncomplicated. A fern’s feathery leaf, in my mistaken imagination, was something delicate, something fragile. Unscientifically, I projected from my mother’s many unsuccessful attempts to cultivate ferns in our Delhi garden to the conclusion that a fern’s life itself was unusually fragile. But in learning about the biology of ferns, a group of species spread thousands of miles across the globe and millions of years into the past, I have come to find that ferns also embody resilience. And this resilience stems from the contrasts they contain.

Consider, for example, how a fern’s life is neatly, equally divided into two halves: the sporophyte and the gametophyte. What you think of as a fern — with large, green, feathery fronds and wispy brown roots — is a sporophyte. But a fern spends only part of its life this way. The rest of the time, it lives as a gametophyte — a tiny plant made of almost nothing but a few oddly shaped leaves. “It’s really like having two completely different plants in one organism, in one package,” Nitta described.

In terms of DNA, a sporophyte has two copies of each of its genes. The gametophyte, however, has just one copy, half of the full set. This difference in DNA content, by itself, is not unusual. An egg, after all, has half the DNA as the hen that laid it. Recall that for an egg to grow into a chicken, it must be fertilised by fusing with a sperm cell. Because the sperm cell also has half the DNA of the rooster that produced it, the fusion of egg and sperm brings the total DNA count in a fertilised egg up to the requisite two copies of each gene.

What’s unusual about ferns is that the gametophytes, the fern-equivalents of eggs, live completely independently of the sporophytes that birthed them. It’s like an unfertilised egg growing into an entirely different creature, one that looks nothing like a hen. This egg-derived creature (the gametophyte) then has to mate with another one of its kind to produce a hen-like creature (the sporophyte). The strangeness of this metaphor should convince you that the duality of the fern’s existence is complicated.

Like with eggs becoming chickens, the crucial step in a gametophyte becoming a sporophyte is the fusion of two cells with half the DNA into one cell with a full set of DNA. For this to happen, fern gametophytes must come in two types: male and female. And this is where some ferns become really bizarre. A growing female fern gametophyte, by exuding the right chemicals into the water around her, can cause nearby gametophytes to become male.

How might ferns have evolved this odd behaviour? Like with many aspects of a gametophyte’s biology, a possible answer lies in its size. From a human vantage point, two gametophytes may seem like they’re growing right next to each other. But for a gametophyte that’s just a few millimetres long, this distance to its nearest neighbour may seem insurmountable. The task at hand — fusion of egg and sperm — is made even tougher by the fact that the sperm from the male must get to the female by swimming through a film of water that extends continuously between the two gametophytes. So by producing a chemical that induces her neighbours to be male, a female gametophyte is taking her future into her own hands, ensuring that the gametophyte closest to her is one she can mate with.

Though being tiny makes fern gametophytes fascinating, it also makes them tremendously difficult to study. Gametophytes, Nitta explains, have “been largely overlooked because, literally, we overlook them.” When you’re out walking in the woods, marvelling at the lush fern sporophytes, the gametophytes are “beneath your feet somewhere.” As a result, Nitta continues, “a lot of our understanding of fern ecology comes from sporophytes, and we’re missing an important part of the picture.” And even once you find a gametophyte, figuring out what species it belongs to, what its corresponding sporophyte looks like, is no easy task. But technology is helping this effort. Researchers like Nitta can identify gametophytes by spelling out the code of their DNA. We’re at the cusp, therefore, of beginning to understand the hidden halves of ferns.

But there’s one group of ferns for which we know much more about gametophytes than sporophytes, for the simple reason that these ferns have no sporophytes at all. These ferns live deep inside caves at the base of sandstone cliffs in North America, and nowhere else. They moved there millions of years ago from the balmy tropics, and in their caves, sheltered from extreme heat or cold, these gametophytes thrive.

In living solely as gametophytes, and thus abandoning the ability to mate, these gametophytes must persist by replicating exactly. By shedding bits of themselves, each part growing into a new, identical gametophyte, they become a population of clones. So these ferns persist, unchanged in an unchanging environment. But being identical puts them at risk of extinction when change — a change in temperature or moisture, disease — arrives. Resilience and fragility, wrapped up into a single, contradictory organism.

A mbika Kamathstudies organismic and evolutionary biology at Harvard University; ambikamath@gmail.com

COMMENT NOW