What Animal Always Gives Birth To Same Sex Twins

Scientists aren't certain why identical twins differ. Armadillo quads offering an answer

O ne of the armadillo quadruplets runs away when she hears Amanda Withnell approaching; her three siblings calmly go on their faces in their food bowls. Some other, seemingly unable to summon the bravado you'd think would be standard equipment on a little oblong tank with anxiety, is a neurotic mess compared to his 3 mellow siblings, regularly running panicked into walls or jumping straight up similar a jack-in-the-box — a patented armadillo move.

Withnell can keep track of which nine-banded armadillo quadruplet is the shy one or the neurotic one because the siblings often differ in appearance as much as they exercise in beliefs. One has a touch of white on its left ear while the other 3 accept it on the right; one has a symmetric blaze just in a higher place the nose while another's bonfire volition tip left as if drunk.

"Amongst siblings, there are frequently differences in facial pigmentation and sometimes even in the number of vertebrae," said biologist Frank Knight of the University of the Ozarks, ane of the earth's leading experts on the ix-banded armadillo and Withnell'southward husband. Differences from one quadruplet to some other become across advent. "Every yr we see variations in personality," said Withnell, an animal technician who, with Knight, raises armadillos for a government-run research center in Louisiana.

Information technology's no surprise that siblings, armadillo or human or otherwise, differ. But armadillo quadruplets are genetically identical, the result of a single fertilized egg splitting in half, and the two halves splitting in half again, before implanting in the uterine wall months later, a reproductive strategy unique in the fauna kingdom.

So while biologists accept long used armadillos in research aimed at detecting, preventing, and treating Hansen's affliction (leprosy) — they're ane of the only mammals other than humans who can harbor the bacteria that cause information technology — scientists at Cold Spring Harbor Laboratory had a very different idea: The long-nosed, armored creatures would be perfect for studies with a potentially greater reach, namely, probing the power of Deoxyribonucleic acid.

"Uniquely amidst mammals, armadillos always have genetically identical quadruplets," said CSHL computational biologist Jesse Gillis. "Mice are bred to be genetically identical, but armadillos arrive that way naturally." Studying details of armadillo genetics across the basic sequence of A's, T'southward, C's, and 1000's that quadruplets share, he figured, might reveal something about whether genes are equally fateful as the current infatuation with genetic testing, Deoxyribonucleic acid-based personalized medicine, and personal genome sequencing seems to presume.

"I do remember our research has bearing on the tendency for seeing genetic processes equally deterministic," Gillis said. "Really, our genes have a potent incentive to set things up flexibly."

Researchers who study 9-banded armadillos (Dasypus novemcinctus) had long noticed that genetically identical quadruplets oftentimes look and act differently from one some other. Behavioral and personality differences, they figured, reflected different life experiences; maybe one sibling had a scare as a newborn and so was jumpy forever after. Such environmental influences also explain many of the differences betwixt identical human twins.

Physical differences, between "identical" human twins or "identical" armadillo quadruplets, were harder to explain away. They might reflect slightly different conditions in the womb, merely since the quadruplets share a placenta, such differences should exist slight.

Whenever scientists calculate how much of a trait is heritable and how much is environmental, they end upward with a full less than 100%. That is, also genetic and ecology influences, there is "unexplained, not-heritable 'noise'," Gillis said. He and his colleagues therefore gear up off on the trail of that noise.

Identifying that racket could prove relevant for people. Identical human being twins ofttimes differ in many strongly genetic traits, such as bipolar disorder, schizophrenia, and breast cancer. For blazon 1 diabetes, 61% of twin pairs are "discordant" (one has it, the other doesn't); for autism it's lx%, for schizophrenia 58%.

Yet identical twins (from a single fertilized egg) have identical genomes. If one has a genetic variant linked to a illness then and so does the other, and if 1 escaped being dealt a bad genetic hand so the other did, also. A few small studies had hinted that differences in which of their identical genes were activated and which were silenced acquired those discordances, but the evidence was sketchy.

Enter nine-banded armadillos. "They nowadays a unique opportunity," Gillis said. "In people, twin studies are the gilt standard for measuring heritability [of traits]. Considering armadillos always produce litters of identical quadruplets, it gives you fifty-fifty greater statistical power" to estimate the furnishings of genetics, environment, and the mystery "noise" component.

Scientists at the Hansen's illness center in Billy Rouge shipped samples of blood and genetic fabric from five sets of quadruplets, taken at three time points, to Gillis' lab. He and his colleagues sequenced the animals' DNA and its downstream cousin RNA — a first for a full set of quadruplets — and ran a series of molecular analyses, focused on changes to gene expression over time, comparison siblings to each other.

What they found is that differences in gene expression commencement early on, when an armadillo embryo consists of only well-nigh 25 cells. That'south when females, who conduct 2 10 chromosomes in every cell, begin silencing an X in each 1. Whether the X inherited from mom or the X inherited from dad is silenced is completely random — a money flip — but determines whether maternal or paternal 10-linked traits are expressed. Ten inactivation is therefore the showtime source of trait differences in genetically identical siblings, Gillis said: "Information technology creates permanent variability between individuals."

In their other 62 chromosomes (armadillos take 32 pairs, compared to humans' 23 pairs), quadruplets had an boilerplate of 700 genes that differed in whether they were expressed or not, Gillis and his colleagues report in a preprint posted to bioRxiv. The source of that variance is random silencing of genes, or epigenetic changes. Past gamble solitary, some genes become clobbered by clusters of atoms called methyl groups, which sit atop the DNA similar a soundproof ceiling and keep it silenced — the trait it codes for unexpressed.

Past tracing back prison cell lineages, the scientists estimated that the epigenetic silencers hopscotched around the genome when the embryos each consisted of a few hundred cells.

To put those 700 genes in perspective, armadillos have almost twenty,000 genes, as humans do. For identical siblings to differ in 700 is approximately equivalent to differing by fully half an X chromosome's worth of genes. The set up of 700 that differed betwixt siblings was unique to each ready of quadruplets, underlining the randomness of the silencing.

Information technology's certainly possible that armadillos accept different private experiences growing upwardly, merely for the most part, those in captivity experience the same environment. Their genomes are identical, as well, of course. The epigenetic differences therefore stand out as an of import source of individuality, Gillis said. "We estimate that individuality is encoded by a pocket-size fraction of the genome," he and his colleagues wrote.

All told, they estimate, the randomness with which genes are silenced or not at the very dawn of embryonic development accounts for about 10% of individual variability.

Similar calculations have been washed for animals much more than distantly related to humans, including crayfish and fruit flies, where scientists estimate that as many as 25% of genes in genetically identical animals raised in identical (lab) environments are differentially expressed. If not the genome and non environment, then the source of variation must be randomness.

That'south a crafty evolutionary play, said evolutionary biologist Benjamin de Bivort of Harvard University. "Introducing random [genomic] changes tin exist strategic," he said. "Information technology's another mechanism by which individuals tin can differ, increasing the odds that some will survive even if threats like predators and pathogens, or environmental conditions, change. When you don't know what the future has in store, it's adaptive to have a way to survive [equally a species] under all contingencies."

In 'dillos, some of the traits that differ due to random gene expression or repression are visible to the naked center, while others are detectable only through molecular testing. One armadillo sibling ofttimes differs from another in size as much as it does from an unrelated 'dillo, for instance, while other siblings differ in their immune system, cardiac muscle growth, and number of scales.

Random genetic events can even shape the animals' eponymous 9 bands. In ane sibling, ii bands might fuse, Knight said, giving a 9-banded armadillo viii bands.

The widespread genetic randomness present from nascency in armadillos, a swain mammal, should spur more enquiry on whether the same is true in people, Gillis said. A few studies take found Dna-expression differences between identical twins, which can be equally high as xx%, including in regions that affect the risk of developing lupus, bipolar disorder, and schizophrenia.

That underlines how having a particular genetic variant is not necessarily synonymous with having a trait, including a disease take a chance. Dna testing, even genome sequencing, can't tell which variants are silenced and which are expressed. Withal many DNA-testing customers believe that their fate lies in their genes. "Genetic testing is the virtually prominent current instance of genetic determinism," Gillis said.

Scientists who written report armadillos every bit something more than collections of genes have wondered when molecular biologists would go around to appreciating all 'dillos have to offer. "Information technology's an obvious model for [studying] heritability, and underutilized," Knight said. "It seems like there'due south a lot more" geneticists could acquire from these four-footed tanks.