The sparrow with four sexes (2016) | Hacker News Reader

The sparrow with four sexes (2016)

nature.com

76 points0DHm2CxO7Lb331 comments

(2016)

Discussion at the time: https://news.ycombinator.com/item?id=13027132

Can someone explain what prevents birds of the same subtype from mating? The article mentions that the chromosome 2 cannot cross over in meiosis, but I just fail to understand the mechanics of the process and how that leads to disassortativity.

Having 2 copies of the mutated gene is probably deadly.

They are saying an individual can only mate with a quarter of the population, but it's obviously incorrect, as the pre-mutation birds can mate with both mutated and unmutated opposite sex.

The reason cross-morph pairs are observed more often as the mutated birds are more sexually aggressive and quickly round up unmutated opposite sex.

The article seemed to suggest that non-cross pairs are not observed at all, but they needed more genetic testing to determine if tan/tan never happens.

Even if the tan/tan is still physiologically possible it may be the case that they still won't mate even if there are no white present at all due to required mating signals being missing.

From a different article:

So are females most strongly attracted to the tough, macho, white-striped males? Actually, no. Lab studies have found that females of either morph prefer the tan-striped males. White-striped females, more pushy than their tan-striped sisters, grab the tan-striped bachelors right away, so these pairs form more quickly than the opposite combination. Males of both morphs tend to prefer the white-striped females, but those females quickly hook up with tan-striped males if they can, so eventually the leftover birds will form pairs consisting of white-striped males and tan-striped females.

https://www.audubon.org/news/the-fascinating-and-complicated...

Granted, the question of homo-zygotic white offspring viability is not even touched.

Edit: yet another article tentatively says the double-white sparrows to exist, but in far lesser number than expected, they suspect some genetic disadvantage.

https://www.cell.com/current-biology/fulltext/S0960-9822(15)...

Thanks! Very interesting!

Crazy-complicated sexes are extremely common in the plant world, probably an adaptation to sessile life and difficult seed propagation.

I.e., I would guess that plants that have easy seed propagation have less complicated sexes, but I don't know.

Example?

It's a common idea that evolution propagates useful mutations, and the non-useful disappear. How does it help a bird to have four sexes?

Or maybe it's just irrelevant for birds. Birds appear to break many laws of nature other animals have to abide by. They fly, they move where they want. They stay close to humans, observe them, and easily avoid them. They're amazingly energetic for how big they are. They migrate extremely far. They don't have to commit to anything except when rearing young.

> It's a common idea that evolution propagates useful mutations, and the non-useful disappear.

Common ideas are commonly wrong. Non-useful ones only disappear reliably if they're selected against. Otherwise, you get goose bumps, or piloerection in humans: We don't have enough hair over most of our bodies for erecting it to do any good, but we still have all the structures to erect it anyway, because none of our ancestors lost any of them even as they became useless.

"Useful" is also context-dependent in two ways: it is relative to the environment of adaption; and also! relative to the existing genotype.

ie., evolution is starting with a bike and turning it into a car

it is not "starting with raw materials" and shaping them

So pretty much every life form is highly non-optimal wrt their environments.

My point is that, as an optimizer, evolution is a very simplistic one. People try to explain why it does or doesn't do things, often falling into "just-so stories" as a result, and fail to grasp just how inhuman the process is.

Also:

> ie., evolution is starting with a bike and turning it into a car

> it is not "starting with raw materials" and shaping them

I'm not sure if you're drawing a distinction between evolution and abiogenesis here. You're right, in that technically evolution doesn't come into play until you have self-replicators of some form, but in colloquial terminology "evolution" is used to encompass the entire process all the way back to whatever nonliving organic chemicals formed the basis for the first life.

I understood them to mean that evolution evolves things on an ongoing basis, and doesn't start from abiogenesis each time. So the current form of a bird species is very much constrained by the form of what it evolved from.

I'd also add that it seems evolution isn't even turning bicycles to cars bolt by bolt, but is adding whole subsystems at a time. It's as if each bike was made of a set of standard ACME parts, and came with a set of blueprints for many other kinds of ACME parts - and evolution, at the large animal level, is just replacing one standardized part for another, or adding new ones where they didn't exist, or altering the manufacturing timings, etc.

I'm of course referring to the findings of evolutionary developmental biology - https://en.wikipedia.org/wiki/Evolutionary_developmental_bio....

> How does it help a bird to have four sexes?

By making procreation harder to achieve, you intensify the selection effect, since only the fittest individuals will be able to achieve it. So maybe it can be thought of as a kind of extra layer of "culling of the weakest" built into the genetics of the species?

Then there's mushrooms...

More specifically, certain kinds of fungi have outrageous numbers of sexes, with a high degree of mating compatibility between them. https://www.nature.com/articles/6800035

It sounds like they're evolving into separate species.

Indeed it's one definition of a species that it's a population that can mate within it's group. So if they can no longer breed with each other, are they same species or not?

Ring species are a weird and fascinating corner case of that definition: https://en.wikipedia.org/wiki/Ring_species

Before the invention of radio this was a property of language as well. Famously you could walk from Paris to Rome and every village could speak comfortably and naturally with its neighbors, yet by the time you got to Rome they spoke a language essentially incomprehensible to Parisians.

30 yrs ago, I rode my bike from East Berlin to Amsterdam, hopping from tiny village to tiny village. I kept out of cities so I could sleep in the forest at night and it seemed as if the the local dialect drifted town by town from deutsch to dutch

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

And heading North: https://en.wikipedia.org/wiki/Frisian_languages

That’s a good example! I remember being on a train where a woman was reading a book in Dutch to her kids and my then 5 year old son (a German speaker) walked over to sit behind her and listen.

They do breed with each other: a-males mate with b-females, and vice versa, where a/b is the new sex gene.

If I'm reading right, the two groups can only breed with each other. That is, white-striped males and tan-striped females can breed, as can white-striped females and tan-striped males. (And the offspring are about half white-striped and half tan-striped.)

So it's not speciation, any more than sexes themselves are speciation.

> white-striped males and tan-striped females can breed, as can white-striped females and tan-striped males

This alone doesn't contradict the two-species theory. It could be that white-M + tan-F is species #1, and white-F + tan-M is species #2. Sure, it sounds weird, but not nearly as weird as the idea of 4 sexes, IMO.

> (And the offspring are about half white-striped and half tan-striped.)

Now, this is the important part. As I understand it, the offspring of "species #1" could be "species #2" or the other way round. This indicates that #1 and #2 are the same species after all.

Just wait until you learn about the sex life of hexaploid species... an offspring needs parents of at least 3 different sexes.

a good book which discusses species with over a dozen sexes read Matt Ridley The Red Queen

What species ist that?

I thought so at first, but no... The two different "species" only mate with each other.

If the tan ones could only mate with the tan ones, that would work. As it is, though, no. A white male and tan female can have a tan chick that can go on to mate with a white bird (indeed must, if it mates at all). That’s not speciation.

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