Post by @johncarlosbaez@mathstodon.xyz

@johncarlosbaez@mathstodon.xyz 3 hr. ago

While you have 46 chromosomes, certain ferns carry huge numbers - the record is 1,400! Their genomes can be gigantic too: a little fern called Tmesipteris oblanceolata packs about 160 billion base pairs of DNA into each cell, the largest genome ever found in any living thing on Earth.

People call these ferns 'chromosome hoarders'. But why all the hoarding? One classic idea from 1966 blames the ferns' sex lives.

Many ferns can fertilize themselves. A single spore grows into a plant called a gametophyte that makes both egg and sperm. The egg from one plant can be fertilized by the sperm of another plant... or a sperm from the same plant! But when *that* happens, the offspring end up with two identical copies of every gene, losing the genetic variety that keeps a population healthy. The fix? Keep extra whole sets of chromosomes! This restores some redundancy and diversity.

It's a nice theory - but newer genetic data have complicated it, and scientists are still arguing about whether it really explain chromosome hoarding among ferns.

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http://www.biorxiv.org/content/10.1101/2024.09.23.614530

bioRxiv

Ancient polyploidy and low rate of chromosome loss explain the high chromosome numbers of homosporous ferns

A longstanding question in plant evolution is why ferns have many more chromosomes than angiosperms. The leading hypothesis proposes that ferns have ancient polyploidy without chromosome loss or gene deletion to explain the high chromosome numbers of ferns. Here, we test this hypothesis by estimating ancient polyploidy frequency, chromosome evolution, protein evolution in meiosis genes, and patterns of gene retention in ferns. We found similar rates of paleopolyploidy in ferns and angiosperms from independent phylogenomic and chromosome number evolution analyses, but lower rates of chromosome loss in ferns. We found elevated evolutionary rates in meiosis genes in angiosperms, but not in ferns. Finally, we found some evidence of parallel and biased gene retention in ferns, but this was comparatively weak to patterns in angiosperms. This work provides genomic evidence supporting a decades-old hypothesis on fern genome evolution and provides a foundation for future work on plant genome structure. ### Competing Interest Statement The authors have declared no competing interest.

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John Carlos Baez

@johncarlosbaez@mathstodon.xyz 2 hr. ago

Ferns live a double life, alternating between two completely separate plants each generation. The one you'd recognize as a fern - the leafy thing with fronds - is called the 'sporophyte'. Its cells are 'diploid', meaning they carry two copies of every chromosome, one from each parent, just like your own cells do.

On the undersides of its fronds, the sporophyte produces spores, and it makes them through a special kind of cell division that halves the chromosome count. So while the sporophyte itself is diploid, its spores are 'haploid': they carry just a single copy of each chromosome.

Here's where ferns diverge from the animals we're used to: those haploid spores don't grow into new leafy ferns! Instead, each spore grows into a tiny, separate plant - often a green heart-shaped flap just a few millimeters across - called a 'gametophyte'.

Being haploid, this has only one copy of each chromosome, and it's this modest little plant, not the showy frond, that produces the eggs and sperm. When a sperm swimming through a film of water reaches an egg and fertilizes it, the two haploid cells combine their single chromosome sets into one cell with a double set: a diploid cell again! That grows into a new leafy sporophyte, and the cycle begins again.

So a fern is really two alternating plants: a big diploid sporophyte that makes spores, and a small haploid gametophyte that makes eggs and sperm.

Ferns can spread quickly because they produce vast numbers of spores, which can be blown by the wind for enormous distances. Then, when a lone spore grows into a little gametophyte, this can fertilize itself!

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Illustration showing the reproductive lifecycle of a fern, from
https://botanicalgarden.berkeley.edu/learn/garden-stories/fern-reproduction/

They write:

Ferns come from an ancient lineage of non-flowering plants that reproduce by spores. The fern reproduction cycle begins with the asexual spores, which are contained in sporangia (spore cases) usually located on the underside of the fronds. Sporangia may appear as green, brown or black dots or even stripes and are often arranged in distinctive patterns. More unusual is the pattern of tiny, stalked growths along the edge of Tectaria moorei, pictured below.

If conditions are right, the spores usually get ejected into the air to land somewhere else (hopefully a moist, suitable medium) to germinate. The germinating spore grows into a tiny prothallus, the gametophyte phase in the life of a fern. The prothallus is a short-lived heart-shaped structure typically 2–5 mm wide, with a number of rhizoids (root-like hairs) growing underneath. These rhizoids bear the sex organs—archegonia (female) and antheridia (male). Sperm are released from the antheridium when water is present and swim to an archegonium to fertilize the egg. The resulting cell, the zygote, develops into an embryo with stem, roots, and fronds, eventually becoming a plant we see as a fern. — Illustration showing the reproductive lifecycle of a fern, from https://botanicalgarden.berkeley.edu/learn/garden-stories/fern-reproduction/ They write: Ferns come from an ancient lineage of non-flowering plants that reproduce by spores. The fern reproduction cycle begins with the asexual spores, which are contained in sporangia (spore cases) usually located on the underside of the fronds. Sporangia may appear as green, brown or black dots or even stripes and are often arranged in distinctive patterns. More unusual is the pattern of tiny, stalked growths along the edge of Tectaria moorei, pictured below. If conditions are right, the spores usually get ejected into the air to land somewhere else (hopefully a moist, suitable medium) to germinate. The germinating spore grows into a tiny prothallus, the gametophyte phase in the life of a fern. The prothallus is a short-lived heart-shaped structure typically 2–5 mm wide, with a number of rhizoids (root-like hairs) growing underneath. These rhizoids bear the sex organs—archegonia (female) and antheridia (male). Sperm are released from the antheridium when water is present and swim to an archegonium to fertilize the egg. The resulting cell, the zygote, develops into an embryo with stem, roots, and fronds, eventually becoming a plant we see as a fern.

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