Have you ever wondered why your chickens gonads are lopsided?

Weird science fact of the day:

Girl birds only have a left ovary. The right one kinda develops, and then regresses. Left ovary is the only one that works. All birds.

Wait... what?

What the hell?

What kind of evolutionary weirdness led to this absurdity?

I bet you guessed an ERV is involved :D

OVEX1, a novel chicken endogenous retrovirus with sex-specific and left-right asymmetrical expression in gonads.

For real, you all should download and read over this one. Its in obnoxious 'pre-print' format (ie almost unreadable format. figures, figure legends, and text about figure all on different pages-- double spaced-- it sucks), but I swear its worth it. This paper is wonderfully descriptive, refreshingly down-to-earth, and touches on just about everything you guys already 'get' about ERVs and evolution.

So heres the deal: birds have lopsided gonads. WTF. So there have got to be some gene expression differences between left/right gonad to tell the left to keep going, and right to shut down. So these folks played some RNA tricks to see what message RNA was being made in chicken gonads during development, left vs right (its easier to look for unknown RNA than it is to look for unknown proteins, which would actually be more relevant, but this is a good place to start).

So what was an interesting mRNA difference between left/right gonads? A previously 'undiscovered' ERV, Ovex1. Ovex1s closest relative in the real world? Another ERV in tuataras. Lizards. WTF LOL! RNA from Ovex1 could be found in both gonads initially, but it trails off in the right, and spikes in the left, along with follicular granulosa differentiation and development.

Ovex1 expression highlights striking desquamation process that leads to profound cortical remodeling associated with follicle morphogenesis.

Now, these researchers are not IDiots like Casey Luskin. They understand that sometimes, transcripts are just junk. So they make this important disclaimer:

To speculate further about the function of Ovex1 (and FET-1) in the ovary, it would be necessary first to establish whether the encoded proteins are actually translated...

... The future objective will be to determine if it (Ovex1) is only witness or if it is an actor.

Just because RNA is made, doesnt mean proteins are made. Sometimes RNA just happens. And cells have lots of tricks for shutting down unwanted RNA before they can make protein. All biologists know this, which kinda explains why Casey doesnt. But before you laugh too hard at Casey, remember that there are many scientists who would have left out these statements, instead choosing to declare that transcription of this ERV is why chickens have lopsided gonads.

So kudos to these folks!

But even though they readily admit that their detection of RNA from this ERV at this particular stage in development could be nothing, they dont think it is.

However, conservation of the ORFs suggests the existence of selection pressure acting to retain not only the RNA but also the encoded proteins.

Even though they found Ovex1 in chickens, they also find it in Zebra Finches, which, according to The TimeTree of Life, diverged from chickens about 100 million years ago.

These species diverged ~100 mya, and yet this ERV has ~83% sequence similarity between the two, and it is located on the same chromosome (#4), between the same two genes (CD8 and SMYD1).

They also found Ovex1 in turkeys and duckies and guinea fowl. If you make a phylogenetic tree with everyones Ovex1, and compare it to standard birdie phylogenies, they match up perfectly. lol.

And while the env genes have gone to hell (OMG JUNK DNA! JUUUNK DNAAAAAA!), parts of the gag gene are pretty damn well conserved between all these species. Like, 100% amino acid homology. Ya. As are parts of pol.

I think its funny that so many RNA fishing expedition papers-- I dont believe their declarations until they show me some proteins and some basic gain-of-function/loss-of-function experiments. The folks on this paper didnt make any extravagant claims, but I believe them without protein data because, well, evolution kinda made their case for them. You dont keep an ERV around with 100% aa homology for over 100 million years just cause you like having eye candy around.

Ovex1 isnt the reason for lopsided bird ovaries, but theyre playing a role in it... Like winning a role in a John Waters movie...


More like this

For each symmetrical organ, many snakes have only one.

Probably unrelated. Just sayin'

They should use, in their next set of experiments, an interesting difference between two extremely closely related species:

In chickens, if you remove the left ovary, the right one grows and develops fully.

In quail, if you remove the left ovary, the right one never regrows back.

Just watch the expression patterns...in a side-by-side comparative experiment.

This is one of the best weird-science-facts I've heard in quite a while.

Pedantic comment: replace "100% aa homology" with "100% aa identity".

Otherwise, interesting. I'll wait for the formatted .pdf, but have that one on my "to read" list.

Greg: Uhh... what? I just checked, snakes have two gonads and two kidneys. Not to mention all the paired structures of the head (eyes, ears, nostrils).


They only have one lung (or rather only one functional lung, the other is sometimes present as a vestigial organ); but if I remember those grow embryologically as a single outgrowth that bifurcates. Kidneys and gonads form completely separately from two distinct patches of tissue (intermediate mesoderm iirc). Interestingly, these tissues have left-right asymmetry from very early on (the left hand side expresses Nodal, the right doesn't) so there's definitely scope for one gonad/kidney to behave differently to the other.

I can't offhand think of any other bilaterally paired organs.

To pick up where OP left off: what's the reason for dudes being lopsided?

Sili - because being squashed is bad for them. One's slightly lower than the other so that they're not squashed together. Same reason people with sit-down jobs (like truck drivers) tend to have lower sperm counts than people who walk around all day. There are very strong selective pressures on fertility, for obvious reasons.

Or did you mean how they get that way?

Mah mind was blown

Growing up, I never would have thought I would be considering the coolness factor of Chicken ovaries, and yet it happened. Thanks Abbie for writing about this.

There were a number of things I wondered about my chickens, but this was not one of them. Nonetheless, very cool!

I can't offhand think of any other bilaterally paired organs.

Well, if you're talkin' snakes, males have a pair of hemipenes.

By Calli Arcale (not verified) on 23 Jun 2009 #permalink

They don't urinate. No foolin', they don't.

Well, they kind of urinate. They do produce urine, but it's solid rather than liquid (being made mostly of uric acid, whereas mammalian urine is urea dissolved in water). And they discharge it from their cloaca, mixed with their feces, rather than firing it out a separate hole like mammals do.

Scientific papers often refer to this as "urination"; lay people don't seem to have arrived at a consensus on whether the word is appropriate for birds.

By Anton Mates (not verified) on 23 Jun 2009 #permalink

Thanks, confused,

I didn't realise there was a benefit to it. Evolutionarily, I mean.

Does that mean that we're all equally lopsided? Since that would make the HIH question rather superfluous. Sorta like "did you breathe any air today?"

Does seem useful. If there were two full ovaries, could they sometimes produce two eggs at the same time, which might collide and break inside the body?

I remember when pluto was demoted from planethood. Astrolgers were up in arms and threatening all sorts of litigation. I wonder what the entrails nutters will make of this.

Confused: Instead of "each" I should have said "one"

I was wondering about the gonads, thanks for looking that up.

Quick point. With mammals the male is heterozygous (xy) and the female homozygous(xx). However with birds it is the female that is heterozygous (wz) and the male that is homozygous (zz). The result? Retroviruses tend to enter the germline through the males in mammals but females in birds.

The error-correcting mechanisms present in meiosis work best when there are two copies -- as there are of all genes in a given homozygous genome. If one copy is "damaged" (where the damage may consist of a provirus taking up residence), what is damaged in that copy may simply be replaced by the undamaged copy.

This works with all of our genes except the y chromosome in male mammals and correspondingly the w chromosome in female birds. Thus retroviruses show a preference for the heterozygous gene. Likewise, early on in their endogenization retroviruses are preferentially expressed in the testes of mammals, so it doesn't surprise me that this particular endogenous virus is expressed in the ovaries of chickens.

By Timothy Chase (not verified) on 25 Jun 2009 #permalink

That is so weirdly awesome, erv. Time for "stupid five year old question time": I could only get the abstract, but is Ovex1 found in reptiles too? And if so, an interesting though: maybe dinosaurs had lop-sided ovaries? Maybe Ovex1 exists in humans, if we can trace it that far back?

By TheEngima32 (not verified) on 27 Jun 2009 #permalink

I, too, had no plans to read about chicken ovaries.Can't help but wonder why the chicken did cross the road. Was it so that she could read how all about it. If chickens lay their eggs, which we eat, would they produce double the amount of eggs if both ovaries worked?

TheEngima32 ,

And if so, an interesting though: maybe dinosaurs had lop-sided ovaries?

I'm not an expert, but AFAIK the evidence is against it. Sinosauropteryx was found with paired eggs inside its body, and egg distribution in Troodon and Oviraptor nests suggests that they were laid in pairs too--one from each oviduct. All of these are maniraptoran theropods, fairly closely related to birds.

The prevailing theory--I think--is that the primitive dinosaur condition was to lay a big mess of eggs at once, like most other reptiles. The maniraptoran theropods evolved a synchronized delay between ovulations, so that they produced one egg from each ovary, laid those two eggs as a pair, and then began again. Among other things, this allowed their eggs to be bigger, since they were only carrying two at a time.

Finally, birds in particular shut down one of their ovaries, so that they only produced one egg at a time. Again, this allowed that egg to be bigger; it probably also cut down on the total weight in eggs they ever had to carry, which is obviously a big plus for a flying animal.

By Anton Mates (not verified) on 27 Jul 2009 #permalink

Someone should poke Darren Naish and get him to blog about dino-bird laying habits, actually.

By Anton Mates (not verified) on 27 Jul 2009 #permalink