(ORDO NEWS) — In nature, same-sex reproduction – parthenogenesis, when females produce offspring without the participation of males – is not uncommon. It typically occurs among tiny invertebrates, insects, and arachnids. This happens only in 70 species of vertebrates, that is, 0.1 percent. But including mammals.
An unexpected gift for Christmas
In December 2001, a baby hammerhead shark (Sphyrna tiburo) was born at the Nebraska Zoo (USA). These viviparous fish produce offspring once a year and, as a rule, immediately from 12 to 15 sharks. However, there was only one cub that day. The zoo workers, who did not expect an addition, did not manage to get him out of the aquarium – almost immediately the shark was killed by the electric ray living there.
This story would not be much different from other cases of fish breeding in captivity, if not for one caveat: for the past three years, only female hammerhead sharks have lived in the aquarium.
Experts caring for the animals decided that the unlucky mother had sex with a male while still in the wild and kept his sperm in reserve. In the wild, this sometimes happens. However, there was no evidence that sperm retains fertility for such a long time.
The body of the dead calf was sent to the Pew Institute of Oceanology, part of the University of Miami. There, the researchers, after conducting a series of genetic tests, found out that the shark did not have a father at all, and his mother, apparently, conceived through parthenogenesis.
This is the name of the method of reproduction, in which the embryo develops from the female reproductive cell without fertilization. Usually, this is inherent in invertebrates, but there are exceptions – for example, scaly reptiles. And for the hammerhead shark, parthenogenesis could be the last resort to save its species from extinction, biologists suggest.
The virgin female waited too long for the male to continue the genus, and the body considered this a threat to the entire population. As a result, the mechanism of maintaining a minimum number of individuals was activated.
When all means are good
Fifteen years later, Australian scientists recorded the second case of parthenogenesis in fish – and again in captivity. Leoni’s zebra shark (Stegostoma fasciatum), which had not communicated with males for four years, laid 41 eggs. Of the three, healthy cubs hatched.
The first thing the researchers thought about was the incredible vitality of the sperm. The fact is that until 2012, Leonie lived in the same aquarium with a male, from which she brought offspring several times. Biologists suggested that she stored his sperm for four years and, as soon as the opportunity presented itself, used it to fertilize the eggs.
However, genetic analysis showed that all cubs carried only maternal DNA. So, Leonie, in the absence of males, switched to same-sex reproduction. As scientists note, in the process of maturation of the sex cells in the body of the fish, polocytes – polar bodies – were formed. These cells contain a copy of DNA but are usually unable to fertilize. Sometimes, for unknown reasons, they begin to behave like sperm: they fertilize an egg and turn it into an embryo.
According to some works, this method of fish breeding can be used in the wild. At least biologists from the State University of New York at Stony Brook (USA), studying the genetic diversity of sawfish off the southwest coast of Florida, found seven individuals born as a result of parthenogenesis.
The researchers believe that the animals used this method of breeding due to the too low population density. In recent years, the number of individuals has been steadily decreasing and it is becoming increasingly difficult for females to find males for mating. This means that parthenogenesis is quite possible among species close to extinction, scientists say.
Exclusively male offspring
In addition to sharks, biologists have recorded isolated cases of same-sex reproduction in the spotted eagle – this is a species of stingrays – and the common boa constrictor. Moreover, the female of the latter decided to reproduce herself, even having the opportunity to mate with the male. Although intercourse occurred, the two pups in the litter were the result of parthenogenesis. This was confirmed by DNA analysis.
Mammals are capable of same-sex reproduction, albeit artificial. Back in 2004, Japanese biologists received mice from two mothers without a father. For this, immature oocytes were used, in the genomes of which several important regions were “turned off”. One female, born as a result of parthenogenesis, lived to maturity, and gave birth to her own young in the usual way.
Fourteen years later, these experiments were repeated by Chinese scientists. True, they went a little further and got offspring not only from two single females but also from two males (that is, the mice had only fathers). For this, embryonic stem cells were used, in which the DNA of one of the parents was preserved. It blocked the activity of genes that work differently depending on who passed them on – male or female.
Scientists injected such stem cells with corrected DNA into immature eggs. The resulting embryos were transplanted into surrogate mothers. As a result, viable mice were born, which did not have a father. True, the animals had developmental defects. They moved more slowly and got tired faster. But they lived longer.
To obtain offspring from two fathers, the prepared embryonic stem cells were injected into non-nuclear eggs. Only 12 out of a thousand embryos survived. The experimental mice weighed twice as much as normal ones, suffered from dropsy, could not breathe normally, suck milk, and quickly died.
The authors of the work note that developmental defects can only be suppressed in embryos obtained from two mothers. But male parthenogenesis is not very viable. This explains why same-sex reproduction in the wild tends to occur in females.
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