Gametophytes are the stage which produces sex cells in plants and algae that undergo alternation of generations. Among land plants, these sex cells may be referred to as “sperm” and “eggs,” with “male” and “female” sex cells combining to produce offspring.
Unlike animals and other organisms that use sexual reproduction, gametophytes do not produce their sex cells through meiosis.
Instead, all cells within a gametophyte organism are haploid – that is, possessing only one copy of each chromosome – and these haploid organisms produce gametes through mitosis.
This is in contrast to animals and other organisms who are diploid – having two copies of each chromosome – and who must cut their number of chromosomes in half before they can create sex cells that have the right number of chromosomes to produce healthy diploid offspring.
Gametophytes’ offspring are indeed diploid plants, and these diploid plants will use meiosis to reproduce. But in a surprising twist, the diploid offspring of gametophytes are called sporophytes.
Instead of using meiosis to produce sex cells, they use meiosis to produce spores – which then undergo mitosis to grow into whole new haploid organisms, a.k.a. gametophytes!
This alternation of generations is a survival strategy in which a plant or algae alternates between different reproductive techniques.
The gametophyte – usually considered to be the first stage of the cycle – reproduces sexually, combining reproductive cells from two different organisms to produce genetically diverse offspring.
This allows the plant population to mix and match traits during gametophyte reproduction, which makes the populations more immune to disease and more adaptable to changing environmental conditions.
The sporophyte offspring of gametophytes, on the other hand, can spread rapidly and do not need partners to reproduce.
This allows a single sporophyte to found a whole new population, which can then mix genes with neighboring populations in the gametophyte generation. Spores can also survive for many years in hostile conditions, while sperm and egg cells cannot.
This alternation of generations allows the parent plant to take advantage of both the benefits of sexual reproduction – such as genetic recombination which promotes genetic diversity – and the benefits of asexual reproduction, such as speed and rapid growth.
Common plants which use alternation of generations include mosses, ferns, and pine trees. In a strange evolutionary reversal, seed plants which use alternation of generations, such as conifers and other pine trees, develop their whole gametophyte life cycle stage inside of an enclosed cone.
By contrast, in some other species, the alternation of generations is quite visible. Among ferns, for example, the sporophyte is the familiar large, leafed plant often seen on forest floors.
The gametophyte, on the other hand, is a tiny heart-shaped plant that may be easily mistaken for a totally different species from the sporophyte generation.
Function of Gametophytes
Gametophyte plants produce sex cells – referred to as “sperm” and “eggs” in land plants – in order to allow their lineage to undergo sexual reproduction.
There are many known benefits to sexual reproduction, as the ability to combine genetic traits from two individuals results in a variety of different combinations of traits within the population.
This diversity is extremely beneficial for disease resistance and the ability to respond to environmental change.
For a practical example of the benefits of sexual reproduction, look no further than the Irish Potato Famine.
In Ireland at the time of the famine, potato crops which reproduced asexually had been grown for many years and had spread across the islands. That meant that the crops feeding the Irish people were all genetically identical to each other, having been asexually produced from a tiny parent population.
When a new disease hit Ireland’s potatoes, because there was no genetic exchange or diversity, virtually all potatoes on the island died. The result was one of the worst humanitarian disasters in history, because all of the potato crops people were relying on for food died off.
As a result, millions of Irish people were forced to leave Ireland in search of better lives – and millions more died.
A die-off of ferns would not be disastrous for the human population, but it would be disastrous for the fern population – so ferns and other plants that practice alternation of generations use gametophytes to perform sexual reproduction, and keep their populations genetically diverse.
Interestingly, whether gametophytes or sporophytes are “dominant” varies depending on the plant. Among ferns, for example, the gametophyte generation takes up more of the life cycle’s resources, growing into a large, visible plant that we recognize.
Among conifer trees, by contrast, it’s the sporophyte generation that is “dominant” – growing into huge, long-lived trees, while the gametophyte generation is restricted to a tiny organism growing inside of a cone.
Examples of Gametophytes
The fern you imagine when you think of Jurassic Park or a forest floor is a gametophyte. The graceful, fringed leaves are haploid – meaning they have only one set of chromosomes and produce sex cells through mitosis, like all gametophyte plants.
If you ever see a fern with what appear to be brown dots covering its leaves, look closer. Those dots are actually separate plants: the tiny sporophyte generation, growing right up from the leaves of its parent gametophytes.
The tiny sporophyte plants are diploid – meaning that they have two pairs of chromosomes, and will undergo meiosis in order to produce spores.
These spores can be seen as a fine powder coming off of the brown dots on the fern leaves when the time comes. A single fern spore can be carried by the wind, land in a new place, and grow into a gametophyte plant. That single gametophyte plant can then self-fertilize and produce a generation of new sporophytes!
The moss you think of when you imagine a carpet of rough, green plant material is a gametophyte. The gametophyte stage of the moss is more long-lived, while sporophytes appear more briefly as long stalks that rise up to release spores into the wind.
Moss sporophytes may easily be mistaken for part of the moss gametophyte plant, because they often grow up right from the gametophyte carpet.
However, the sporophyte stalks are actually independent organisms with different genes from the moss carpet below them.
The sporophytes are created by the merging of gametophyte sex cells. As a result, they have twice the number of chromosomes compared to the gametophyte generation, and contain a unique mixture of genetic traits as a result.
Though not as glamorous in name as ferns or mosses, hornworts are in fact a pretty woodland plant whose gametophyte stage consists of small, emerald green leaves that grow in moist soils.
As with mosses, the gametophyte form of hornwort is green, long-lived, and low to the ground; while the sporophyte generation forms a long, thin stalk from which spores are released to scatter to the wind.
As with mosses, hornwort gametophytes and sporophytes tend to grow right on top of each other and maybe mistaken for different parts of the same plant. But they are actually independent organisms with different chromosome numbers.