Such anomalies are quite rare, so the new case was awarded a separate scientific article in the Journal of Fish Biology. Researchers at Michigan State University explain that more often scientists come across two-headed lizards and snakes, but this may not reflect the true picture.
"These animals," says Michael Wagner, assistant professor and co-author of the discovery, "are often bred in captivity, so the occurrence of anomalies is easier to detect."
Sharks, unlike snakes and lizards, very rarely breed in front of humans, and two-headed individuals usually live very shortly and do not grow up to adulthood.
The described specimen was retrieved by one of the fishermen who caught a female bull shark and opened her uterus. These large fish are viviparous, they have up to 13 fry at a time and the length of the fish at birth is about 60 cm (an adult is 3.5 meters). The two-headed specimen was delivered to the researchers and they did not even dare to open the unique find: to keep it intact and safe, biologists used a magnetic resonance imaging scanner.
The tomography showed the presence of two different hearts, stomachs and, naturally, all organs above. But the tail of this individual (or individuals?) Was common - a detailed study of such rare specimens can help to understand the general patterns of embryo development and shed light on not only abnormal, but also completely normal physiological processes. Genetic studies of fruit flies with extra legs instead of eyes at one time made it possible to discover genes responsible for defining both the head-tail axis and finer divisions in embryos - the Hox genes are responsible, for example, for the development of fingers on the limbs.
Ecology has nothing to do with it
Separately, Wagner warns that you should not associate the find with environmental pollution. Yes, the fish was caught in the Gulf of Mexico, where the oil spill occurred earlier. But “after” does not mean “due to” - as already mentioned, many animals regularly give birth to such two-headed cubs and in the most favorable conditions. Moreover, even in humans, about a third of all fertilized eggs leave the uterus without even integrating into its wall - and the reason for this is, among other things, developmental anomalies; all animals, including you and me, have a built-in natural defense mechanism against critical developmental errors.
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By the way, congenital anomalies such as two-headedness or irregular limbs are not necessarily mutations. Moreover, mutations are not identical to hereditary disorders, therefore, above in the text you will not see a single sentence about "mutant shark caught by scientists." Let's start with mutation as the most popular concept.
Who are mutants? We are with you
Mutations are DNA changes that are not at all as rare as it is commonly thought - any newborn child has about fifty of them by 3.5 billion base pairs; most mutations are harmless and do not manifest themselves in any way. Some other part is harmful or lethal, but some, on the contrary, are beneficial. Or, which is also quite common, it can be beneficial in certain conditions. Africans, for example, often carry a gene that increases resistance to malaria - but also increases the risk of sickle cell disease.
Each gene exists in several varieties (alleles, as geneticists say) and it is often impossible to say which one is "better". Malaria risk versus anemia risk - which is worse? High chance of melanoma or vitamin D deficiency? Some now unfortunate genes that determine, say, susceptibility to Alzheimer's disease, could save a thousand years ago from a long-forgotten disease, and natural selection, acting on the inhabitants of South Africa, hardly suggested their resettlement to cloudy northern countries.
Where do ugliness come from?
A two-headed fish could appear without any genetic abnormalities - just at the right time, the cells of the embryo divided once again or moved to the wrong place. Such failures are rare, but not impossible; their likelihood is increased by exposure to certain chemicals - the notorious example is the thalidomide disaster. It is very important to understand that such violations cannot be inherited!
Inheritance without mutations
More recently, geneticists have distinguished hereditary and genetic diseases. All our cells have exactly the same set of genes, but in each specific tissue they include only those genes that are needed for specific tasks - heart cells do not need enzymes of liver cells at all, or even more so the hard shell of osteocytes, bone tissue cells. Each gene can be turned on or off due to reversible (as opposed to mutations!) Chemical modification of DNA or rearrangement of chromosomal proteins.
These "switches" are inherited and it has been proven that in some cases such inheritance leads to an increased risk of certain diseases. Maternal smoking during pregnancy can increase the risk of asthma not only in the baby, but also in the grandchildren - at least a similar effect has been demonstrated in rats.