They use alien sperm to reproduce asexually: Researchers have gained insight into the genetics of an invasive fish with bizarre reproductive abilities. Apparently, the peculiarities of the pediment are connected with its sixfold set of chromosomes. According to the genetic traces, this complex genome was created by “strange” crosses. This apparently led to an effect that enabled asexual reproduction via clones. This ability, in turn, favored the spread of this fish species, the researchers say.
It originally swam through waters of Asia – but then the Giebel Carassius gibelio made a career: The fish from the carp family is considered one of the most successful invasive fish species in Europe and it is also beginning to spread in the USA. In our country, it mainly competes with the native Crucian for the same living space and puts them under pressure. One of the secrets of the gable’s success is the ability of parthenogenesis, which is unusual in fish: while crucian carp and co reproduce sexually, the gable is not necessarily dependent on finding a partner: female animals can produce eggs from which fish can grow without fertilization. These are copies – gable females can clone themselves.
You can do without a father too
In order for the eggs to develop, however, they still need the stimulus of a sperm. But that doesn’t have to come from a gable male – the sperm cells of other fish from the order of the carp family do the same. In addition, gable females mingle with foreign fish in the “love frenzy” and let their eggs be fertilized at the same time. The “stolen” sperm then stimulate the egg cells of the gable to divide. However, the genetic material of the foreign male is then broken down. The developing offspring are therefore clones of the gable female. “Unisexual, i.e. purely female, reproduction enables rapid colonization of new habitats and offers invasive species a great advantage over their originally occurring competitors,” explains Dunja Lamatsch from the University of Innsbruck.
In order to track down the characteristics of the gable, Lamatsch and her international colleagues have now dedicated a genetic study to the fish. The basis for this was the complete sequencing of his genome. As the team reports, it is a particularly complex genome: While most animals have a double (diploid) set of chromosomes, the gable is hexaploid – it has a sixfold set of carriers of genetic information. And that’s not all: the genome consists of a total of 150 chromosomes – it has more than three times as many as humans.
Insights into an interesting genome
The results of the analysis of the genome so far now also provide clues to the background of the unusual reproductive abilities of the fish, say the researchers. As they fundamentally explain, the genomes of animals that reproduce sexually usually have a double set of chromosomes for practical reasons. For reproduction in females and males, the chromosomes in the germ cells are divided during the process of meiosis and only a single (haploid) set of chromosomes is passed on in each case. The fusion of haploid egg cell and haploid sperm then creates a diploid organism again. Accidents during meiosis or when related species are crossed can sometimes result in creatures that have more than two sets of chromosomes. This is quite common in plants, but rare in animals. But when it does, special effects can arise. This is exactly what is evident in the gable, say the researchers.
The traces in the genome show that its six sets of chromosomes arose in a complex way: four of them came together by crossing distantly related fish species – the other two were added by crossing with a closely related fish. “Probably, at some point in these crossings, problems arose with the formation of the germ cells. That could be one of the triggers for unisexual reproduction,” explains Lamatsch. “In species that reproduce purely as females, meiosis fails and it is no longer necessary to fuse the germ cells,” explains the scientist.
As the researchers finally point out, in the case of the gable they have now, for the first time, described the entire genetic information of a hexaploid animal. They were also able to show aspects of its complicated genesis: The analyzes provide information about how these six sets of chromosomes can exist side by side and work together. Many other research approaches in genetics can now result from this.
Source: University of Innsbruck, specialist article: Nature Communications, doi: 10.1038/s41467-022-31515-w