Scientists have discovered a fish carrying genes only from its father in the nucleus of its cells. Found in a type of fish called Squalius alburnoides , which normally inhabits rivers in Portugal or Spain, this is the first documented instance in vertebrates of a father producing a near clone of itself through sexual reproduction — a rare phenomenon called androgenesis, the researchers reported in the journal Royal Society Open Science .
The possibility of androgenesis is just one of many mysteries about Squalius alburnoides . It’s not a species in the usual sense but rather something called a hybrid complex, a group of organisms with multiple parental combinations that can mate with one another.
The group is thought to have arisen from hybridisation between females of one species, Squalius pyrenaicus , and males of another species, now extinct, that belonged to a group of fish called Anaecypris . To sustain its population, Squalius alburnoides mates with several other closely related species belonging to the Squalius lineage.
That it can reproduce at all is unusual enough. Most hybrids, like mules, are sterile because the chromosomes from their parents of different species have trouble combining, swapping DNA and dividing — steps required for egg or sperm production.
Squalius alburnoides males circumvent this problem by producing sperm cells that do not divide and therefore contain more than one chromosome set. This is important because most animals, Squalius alburnoides included, need at least two chromosome sets to survive.
Many chromosome sets
Mostly, animals have sex cells containing only one chromosome set, which means the egg and sperm are both required for reproduction. But in Squalius alburnoides , sperm with multiple chromosome sets can provide all the nuclear genetic material needed for a viable offspring.
The details in Squalius alburnoides are still unknown but in general androgenesis is thought to occur in a couple of ways, said Miguel Morgado-Santos, a graduate student at the University of Lisbon and an author of the study: Sperm could fertilise an egg that contains no chromosomes, or it could destroy the genetic content from the nucleus of the egg after fertilisation.
Mr. Morgado-Santos’ group found this instance of androgenesis by accident, while studying the mating patterns of Squalius alburnoides .
The researchers put male and female Squalius alburnoides with males and females of another Squalius species in an artificial pond, let the fish reproduce and then genetically analysed 100 randomly selected offspring. One of these offspring had only paternal chromosomes.
“We weren’t expecting to find that,” Mr. Morgado-Santos said. Although he acknowledges 1 in 100 fish is a rare occurrence, Mr. Morgado-Santos thinks this instance of androgenesis could represent a “snapshot” of a population moving toward becoming its own species. Put another way, androgenesis may help this fish become independent from the other Squalius species it relies on to reproduce.
Accident or not, it happened and shows that reproduction can vary in all sorts of “weird and wonderful” ways across the natural world, said Benjamin Oldroyd, a professor of genetics at the University of Sydney.
New York Times News Service