oddbiology

March 26, 2026 · View on GitHub

This repo covers weird biology in general — things that don't entirely fit into oddgenes (which focuses on gene annotations and bioinformatics assumptions).

Please feel free to make PRs for more stuff!

Evolution

Odd evolutionary relationships

Try our game https://phyloguessr.com to test your knowledge of these!

Mangoes, cashews, and pistachio are related to poison ivy (https://www.scientificamerican.com/article/what-do-cashews-mangoes-and-poison-ivy-have-in-common/)
Humans are more closely related to mushrooms than plants (https://gizmodo.com/why-are-mushrooms-more-like-humans-than-they-are-like-p-5940434)
Humans are more closely related to a sea urchin than a fly or worm (https://www.abc.net.au/science/articles/2006/11/10/1785449.htm)
Comb jellies are likely the sibling group to all other animals, not sponges (https://www.mbari.org/news/genetic-research-offers-new-perspective-on-the-early-evolution-of-animals/)
Oak trees are more related to pumpkins than to pine trees (https://www.youtube.com/watch?v=ONVpFtiD-fo)
Horses are more closely related to rhinos than antelope (even vs odd toed ungulates) (https://positivepeerpressure.blog/quirky-evolution-5-unlikely-animal-relatives-hiding-in-plain-sight-6e08cfd299a7)
Hyraxes (rodent-looking mammals) are more related to manatees and elephants than to any rodent (https://en.wikipedia.org/wiki/Hyrax)
Bats are more closely related to cows, whales, and pumas compared to flying squirrels https://www.batcon.org/surprising-bat-relatives/
Aardvarks are more related to manatees than to armadillos https://www.livescience.com/55241-aardvark-facts.html (bonus: Aardvarks are the only living species of their evolutionary branch)
New world vultures and old world vultures are not very closely related, it is convergent evolution https://en.wikipedia.org/wiki/New_World_vulture
Seals are more closely related to dogs than to cats, and even more closely related to bears https://www.youtube.com/watch?v=aAOsf004FqQ
Shrews are more closely related to cats than to mice (shrews and hedgehogs are in the same order) https://www.youtube.com/shorts/jRPiiXKmZIA
The malaria parasite shares a surprising common ancestor with kelp and other algae — it even keeps a non-functional remnant of a chloroplast (the apicoplast), a relic from an ancient ancestor that engulfed an alga https://www.youtube.com/watch?v=4ejoVBcLP4U https://en.wikipedia.org/wiki/Apicoplast
Falcons are more closely related to parrots than to hawks or eagles https://en.wikipedia.org/wiki/Falcon#Systematics_and_evolution
Crocodiles are more closely related to birds than to lizards (both are archosaurs) https://en.wikipedia.org/wiki/Archosaur
Termites are actually a family of cockroaches https://en.wikipedia.org/wiki/Termite#Taxonomy
Red pandas are not closely related to giant pandas — they're in the weasel/raccoon superfamily https://en.wikipedia.org/wiki/Red_panda
Whales evolved from hoofed land mammals and are closely related to hippos https://en.wikipedia.org/wiki/Evolution_of_cetaceans

No such thing as a...

"There is no such thing as a shark" (sharks closer to rays) https://www.nature.com/articles/d41586-026-00594-w
"There is no such thing as a tree" (woody trees are not monophyletic) (https://eukaryotewritesblog.com/2021/05/02/theres-no-such-thing-as-a-tree/) see also https://www.youtube.com/watch?v=j1EXBeBA89w
"There is no such thing as a fish" (similar to above) (https://www.businessinsider.com/fish-do-not-exist-2016-8)

Unusual evolutionary transitions

Horses used to be smaller and have multiple toes, similar to a tapir, but it was reduced to where they walk on their "middle finger" https://www.sci.news/paleontology/modern-horse-toes-12022.html https://en.wikipedia.org/wiki/Evolution_of_the_horse

Evolutionary hypotheses

Eukaryotes evolved from giant viruses? https://en.wikipedia.org/wiki/Viral_eukaryogenesis

Parent-of-origin effects in citrus

Nearly all commercial citrus are ancient hybrids of just a few progenitor species (pummelo, citron, mandarin, C. micrantha). These hybrid genomes were "frozen" because citrus seeds grow clonal embryos from maternal tissue instead of through normal sexual reproduction. Most seedlings from a citrus seed are clones of the mother.

Adding confusion: chloroplast DNA is maternally inherited, but citrus unusually shows paternal inheritance of mitochondrial DNA, so chloroplast, mitochondrial, and nuclear phylogenies all give different trees.

Convergent evolution examples

https://en.wikipedia.org/wiki/List_of_examples_of_convergent_evolution

Parenting

The concept of 'parental care' evolved independently in many lineages through convergent evolution

https://en.wikipedia.org/wiki/Parental_care

Gynandromorphs

Some animals are split male on one side and female on the other — literally half and half. This happens when sex chromosomes are distributed unevenly during early cell division, so one half of the body develops as male and the other as female. It's been documented in birds, butterflies, crustaceans, and insects, and is sometimes visible as a striking bilateral split in coloring.

https://en.wikipedia.org/wiki/Gynandromorph https://nautil.us/half-male-half-female-total-animal-234910/

Dosage compensation and mosaicism

Calico cats and X inactivation mosaicism

Mammalian females are genetic mosaics: each cell randomly inactivates one X chromosome early in development, and all descendant cells keep the same X silenced. In calico cats, X-linked coat color genes on different parental X chromosomes produce patches of orange and black fur, making the mosaicism directly visible. This is why calico cats are almost always female — males (XY) have only one X, so no mosaicism occurs.

https://en.wikipedia.org/wiki/Calico_cat https://en.wikipedia.org/wiki/X-inactivation

Dosage compensation solved differently across species

The problem: males and females have different numbers of X chromosomes, so how do you keep gene output balanced? Each lineage evolved a completely different solution:

Mammals: silence one female X entirely
Drosophila: double the output of the single male X
C. elegans: halve the output of both hermaphrodite X chromosomes
Marsupials: always silence the paternal X (not random like placental mammals)
Birds: no known chromosome-wide compensation — males may just tolerate higher expression

https://en.wikipedia.org/wiki/Dosage_compensation

Genomes

Large numbers of chromosomes in a butterfly

The Atlas blue butterfly has 448-452 chromosomes — the highest number among organisms that haven't simply duplicated their whole genome.

https://twitter.com/Jente_O/status/1653469755569782808

Large number of sex chromosomes in platypus

Male platypus has five X and five Y chromosomes. Some of those sex chromosomes are more similar to bird chromosomes than to other mammal sex chromosomes. The platypus also lacks the usual mammalian sex-determining gene (SRY), using a different gene (AMH) instead.

https://en.wikipedia.org/wiki/Platypus#Evolution

https://genomebiology.biomedcentral.com/articles/10.1186/gb-2007-8-11-r243

More complete genome sequencing was done 2021 https://pmc.ncbi.nlm.nih.gov/articles/PMC8081666/

Organisms without mitochondria

Almost all eukaryotes have mitochondria, so finding one without them was a big deal. Monocercomonoides exilis is a gut parasite that completely lost its mitochondria, replacing their essential functions with a system borrowed from bacteria. Other organisms like Giardia have heavily reduced remnants, but Monocercomonoides appears to have none at all.

https://en.wikipedia.org/wiki/Monocercomonoides https://www.cell.com/current-biology/fulltext/S0960-9822(16)30263-9

Seven different "genomes" in a single celled organism

Most cells have two genomes — one in the nucleus, one in the mitochondria. This single-celled alga has 7

https://phys.org/news/2023-04-single-celled-alga-harbor-genomes.html

Horizontal gene transfer

Bdelloid rotifers — animals full of foreign DNA

Bdelloid rotifers are tiny freshwater animals that haven't had sex in millions of years. Instead of swapping genes through mating, their genomes are packed with DNA from bacteria, fungi, and plants — acquired through horizontal gene transfer. They may pick up foreign genes when they desiccate and rehydrate, temporarily opening up their DNA to outside fragments. Some of these foreign genes are functional and help them survive.

https://en.wikipedia.org/wiki/Bdelloid_rotifer https://www.nature.com/articles/nature07817

Photosynthetic animals

Kleptoplasty

Some animals eat algae, digest everything except the chloroplasts, and keep those chloroplasts functional inside their own cells — literally retaining another organism's photosynthetic machinery. The sea slug Elysia chlorotica can survive for months on sunlight after a single algal meal this way. Some acoels (e.g. Symsagittifera reesei) similarly retain algal chloroplasts and can live partly on sunlight. Acoels are themselves an oddity — long lumped in with flatworms, they're actually one of the earliest branching groups of bilaterally symmetric animals.

https://en.wikipedia.org/wiki/Kleptoplasty https://en.wikipedia.org/wiki/Elysia_chlorotica https://en.wikipedia.org/wiki/Acoela

Photosynthetic salamander

Spotted salamander embryos have algae living inside their cells — the only known vertebrate with photosynthetic organisms inside its cells. The algae provide oxygen and the embryo provides CO2 and nutrients.

https://en.wikipedia.org/wiki/Spotted_salamander#Algal_symbiont https://www.nature.com/articles/news.2010.384

Myxozoa — animals that became microscopic parasites

Myxozoans are cnidarians (jellyfish relatives) that evolved into tiny parasites, losing nearly everything that makes an animal recognizable — no gut, no nervous system, no muscles. Some are just a handful of cells. They were classified as protists for over a century before molecular evidence revealed they were animals all along.

https://en.wikipedia.org/wiki/Myxozoa https://www.smithsonianmag.com/smart-news/parasite-really-micro-jellyfish-180957326/

Octopus RNA editing

Most animals rely on DNA mutations for genetic variation. Cephalopods (octopuses, squid, cuttlefish) instead extensively edit their RNA after transcription, rewriting genetic instructions on the fly. This may help them adapt to temperature changes rapidly but appears to come at the cost of slower DNA-level evolution.

https://en.wikipedia.org/wiki/RNA_editing#In_cephalopods https://www.cell.com/cell/fulltext/S0092-8674(17)30340-6 https://www.nature.com/articles/d41586-017-00612-y

Wolbachia — the most successful parasite on earth

Wolbachia is a bacterium that infects an estimated 40-60% of all insect species. It manipulates host reproduction in wild ways: killing males, turning genetic males into females, or enabling females to reproduce without mating. It spreads by being passed from mother to offspring and has become so integrated that some species can no longer survive without it.

https://en.wikipedia.org/wiki/Wolbachia https://www.nationalgeographic.com/science/article/one-parasite-to-rule-them-all-wolbachia-protects-against-mosquito-borne-diseases

Immortal jellyfish

Turritopsis dohrnii can revert from its adult medusa form back to its juvenile polyp stage, essentially restarting its life cycle. It does this by reprogramming its own cells (transdifferentiation) — adult cells revert and transform into the cell types needed to rebuild a juvenile body.

https://en.wikipedia.org/wiki/Turritopsis_dohrnii https://www.nhm.ac.uk/discover/immortal-jellyfish-secret-to-cheating-death.html

Cellular

Multiple-fission cell division

Dental plaque bacteria elongate and then split into 3-14 cells in a single cell division

https://www.sciencealert.com/bacteria-in-your-mouth-reproduce-in-a-strange-rare-way-scientists-discover

More info https://en.wikipedia.org/wiki/Fission_(biology)#Multiple_fission

Sex and reproduction

Parthenogenesis

Parthenogenesis is reproduction where an embryo grows from unfertilized eggs (https://en.wikipedia.org/wiki/Parthenogenesis)

Some species (e.g. whiptail lizards) are entirely female (https://www.scienceabc.com/nature/animals/are-there-any-all-female-species-in-the-wild.html)

There are other types of asexual reproduction as well https://en.wikipedia.org/wiki/Asexual_reproduction

Species with "more than two sexes"?

https://biology.stackexchange.com/questions/77371/are-there-lifeforms-that-have-more-than-2-sexes

Male pregnancy

Male seahorses carry and give birth to the young

https://en.wikipedia.org/wiki/Male_pregnancy

Sexual cannibalism

Sexual cannibalism is common across the tree of life https://en.wikipedia.org/wiki/Sexual_cannibalism

There are other types of cannibalism also https://en.wikipedia.org/wiki/Cannibalism

See also: trophic eggs (eggs as food) https://en.wikipedia.org/wiki/Trophic_egg

Traumatic insemination

A number of examples listed here https://en.wikipedia.org/wiki/Sexual_conflict

Odd genitalia

Kangaroos have "three vaginas" https://grist.org/animals/kangaroo-genitals-are-weirder-than-you-ever-thought-possible-2/

Non-random segregation of chromosomes

Normally we assume each chromosome has a 50/50 chance of ending up in either daughter cell. Non-random segregation means some chromosomes "cheat" — they are preferentially passed to certain cells, spreading through a population faster than expected.

https://en.wikipedia.org/wiki/Non-random_segregation_of_chromosomes

Examples:

Aphids: the X chromosome consistently moves to the larger daughter cell during sperm production (spermatogenesis), ensuring all offspring are female
Butterflies (ZZ/Z0 type, e.g. Taleporia tubulosa): Z chromosome segregation is influenced by temperature and maternal age
Butterflies (ZZ/ZW type): the W chromosome always enters the egg cell, resulting exclusively in female offspring
Flowering plants (e.g. maize K10 chromosome): K10 is an abnormal version of chromosome 10 that cheats during meiosis — it hijacks the cell division machinery so it ends up in the egg cell ~70% of the time instead of 50%
Scale insects: males are parahaploid — they have both parents' chromosomes, but only the mother's are switched on and passed to offspring
B chromosomes in plants (e.g. Lilium callosum): B chromosomes are extra "selfish" chromosomes with no known function that accumulate across generations by biasing their own transmission
Drosophila segregation distorters: selfish chromosomes in fruit flies hijack a sperm quality-control checkpoint to destroy sperm that lack the selfish chromosome — independently evolved in multiple species using the same trick

https://www.nature.com/articles/s41467-025-68254-7

This is closely related to meiotic drive / gene drive, where selfish genetic elements bias their own transmission. Engineered gene drives (e.g. using CRISPR) exploit this principle to spread genes through wild populations.

https://en.wikipedia.org/wiki/Intragenomic_conflict#Segregation_distortion https://en.wikipedia.org/wiki/Gene_drive

Inheritance without DNA

Prions

Prions are heritable elements based on protein shape, not DNA. A misfolded protein can force other copies of itself to misfold too — causing brain-wasting diseases like mad cow disease and scrapie in mammals. In yeast, prions like [PSI+] and [URE3] are genuinely inherited across generations through cell division, altering traits with no DNA change involved.

https://en.wikipedia.org/wiki/Prion https://en.wikipedia.org/wiki/Yeast_prion

Transgenerational epigenetic inheritance

Chemical modifications on top of DNA can be passed down across generations without changing the DNA sequence itself. The idea that experiences like famine can affect descendants garners much attention. Evidence exists in plants, C. elegans, and some mammalian contexts, though the extent in humans remains debated.

https://en.wikipedia.org/wiki/Transgenerational_epigenetic_inheritance

There are skeptics also http://www.wiringthebrain.com/2018/07/calibrating-scientific-skepticism-wider.html

Virus genes repurposed by animals

Arc — a virus gene co-opted for neuron communication

Arc is a brain gene essential for memory that evolved from an ancient virus. Its protein assembles into virus-like shells, packages its own RNA, and ships it between neurons. In fruit flies, a related gene (dArc1) does the same thing — your neurons are literally sending each other virus-like packages to communicate.

https://www.cell.com/cell/fulltext/S0092-8674(17)31504-0 https://www.cell.com/cell/fulltext/S0092-8674(17)31502-7

Syncytins — retroviral genes essential for the placenta

Syncytins are genes required for building the placenta in mammals, and they come from ancient retroviruses that infected our ancestors. Different mammalian lineages independently captured different retroviruses for this purpose.

https://en.wikipedia.org/wiki/Syncytin