“The evolutionary significance of androgenesis is not clear and it is a puzzle that scientists do not have yet an answer for.”
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
A male freshwater fish does something that’s unknown in the vertebrate world: it can clone itself.
Females of snakes, sharks and komodo dragons can reproduce genetically identical copies of themselves without any aid from males. But it’s much harder for males to perform the same trick. How can they replicate themselves since they cannot lay eggs or give birth? Unlike parthenogenesis, where females produce clones without having sex, males need to mate to produce genetic copies of themselves.
Typically, gametes – sexual cells like sperm and ova – carry only half the genetic information. Humans have 46 chromosomes in our body cells but our spermatozoa and ova are reduced, i.e. they contain only 23 each. When a sperm fertilises the ovum, the two halves combine to create a full set of chromosomes.
In insects such as bees and ants, individuals can develop even if they inherit only one half of the chromosomal set from either father or mother. Among them, androgenesis, as the phenomenon of males producing genetically identical offspring is called, is a known fact.
Vertebrates can’t produce viable young with only half the genetic material, although researchers have created them in the laboratory. Males with reduced sperm can clone themselves if two spermatozoa fertilise the oocyte, providing a full chromosomal set. Or if the sperm makes a copy of itself after fertilisation.
Alternatively, the sperm has to contain all the chromosomes that an embryo needs. But no matter the method, the sperm has to somehow remove the females’ genes from the egg and insert its own. This is opted by hermaphrodite clams with androgenetic lineage. In other words, they become sexual parasites of females. Or, to begin with, the ovum may not contain any genetic material.
A ‘super-male’ fish in the wild
Squalius alburnoides, related to minnows and carps, lives in southwestern Portugal and Spain. Although technically considered a species, it’s really a hybrid. In prehistoric times, female Squalius pyrenaicus crossed with males of a now-extinct species to produce fertile progeny. This hybrid complex, Squalius alburnoides, has a diversity of genomes that may be diploid (inheriting one set of chromosomes from each parent), triploid (three sets of chromosomes), or tetraploid (four sets). The offspring of all such unions are fertile.
The hybrids’ unusual ability to transcend species boundaries grants them more reproductive tools. They have reduced and unreduced gametes, i.e., sex cells with half and full genetic information respectively, and the unreduced ones seem to be best-placed to clone themselves.
Miguel Morgado-Santos, a doctor student at Universidade de Lisboa, Portugal, gathered a random group of 52 S. alburnoides and the parent species S. pyrenaicus from the Ocreza River during their breeding season. He released them into a pond that mimicked natural conditions to study the dynamics of the hybrid complex. From samples he collected, he examined their genotypes in the laboratory. Ten months later, he collected the fish from the pond.
All the 261 fingerlings belonged to the S. alburnoides complex, and all of them except one had three sets of chromosomes or triploid. The exceptional one was a diploid hybrid.
Morgado-Santos selected 100 of these offspring to test their gender and paternity. The unusual diploid youngster was a male and a copy of one of the adult male S. lburnoides.
“My first thought when we looked at the paternity data was that I had made a mistake and accidentally identified the father itself as a descendant,” Morgado-Santos told The Wire. “However, the descendant was smaller than the father was at the beginning of the project. Assuming he did not shrink during the reproductive season, they had to be two different individuals.”
The spermatozoa can only replace nuclear DNA, not mitochondrial DNA inherited from the mother. The S. alburnoides fingerling’s mitochondrial DNA pointed to a triploid female. “This was our final evidence: father and son did not share the same mitochondrial DNA,” says Morgado-Santos. “The progenitor fathered a nuclear clone through androgenesis.”
Most of the young were born of the same union between the male and female S. alburnoides. The male fathered 77% of the analysed offspring, leading Morgado-Santos to call him a ‘super-male’. The cloned fingerling would have inherited its father’s “stunningly high fitness”. Since only one out of 100 youngsters tested inherited its genes entirely from its father, it indicates how rare it must be in the wild. But if that one individual was the Genghiz Khan of S. alburnoides, it could be immensely successful in passing on its genes. In another study, Morgado-Santos says the ‘super-male’ also produced more young with other females than any of the males.
“This shows that single individuals may play major roles in the reproductive dynamics of wild populations, and, together with androgenesis, may lead to the emergence of highly successful lineages of males, even if androgenesis occurs at a low frequency,” says Morgado-Santos.
Adelino Canário, a professor at the University of Algarve, Portugal, told The Wire that the study highlights the extraordinary diversity of sexual systems in nature, even among vertebrates. “It will be interesting to know what are the mechanisms involved.” This would tell us if androgenesis is a circumstantial event or if certain conditions or genes favour it. He wasn’t involved in the study.
Evolutionary advantage
Squalius alburnoides is dominated by females that mate with males of other species of Squalius to maintain viability. Androgenesis may not only be an important alternate route to produce males, but it may also enable alburnoides to become independent of other Squalius, says Morgado-Santos. “This could give rise to a new species through a process called ‘hybrid speciation’.”
Hybrid diploids in the northern populations are mostly males while they are all females in the southern populations, while hybrid triploids are almost all females. Morgado-Santos says it is worth investigating if males in the north and females in the south produce genetic copies of themselves.
What is the evolutionary significance of this sexual parasitism? After all, females that spend a lot of their energy to reproduce don’t get anything out of this arrangement.
“The evolutionary significance of androgenesis is not clear and it is a puzzle that scientists do not have yet an answer for,” says Professor Canário. “The fact that it is rare shows it is not an evolutionary stable strategy. At first it appears to be a kind of parasitic form of reproduction by the male which requires female eggs as hosts. The egg retains the mitochondria (which contain a few genes) which will be transmitted down generations through the female germ line. But this seems too little advantage, if any, compared to the males.” He also added that there is a strong possibility androgenesis was more widespread – but these cases would still be rare.
Compared to the fecundity of S. alburnoides, none of the S. pyrenaicus males or females bred. In the rivers of the Iberian peninsula, the latter species is becoming rarer and is listed as endangered. In fact, the researcher got permission to collect the species because he needed only a few and promised to return the fish alive to the river. Female pyrenaicus are finicky about who they mate with, preferring males of their own species to hybrids. During the experiment the 9 females had 10 males to choose from and yet they didn’t. The males could mate with females of their own species or alburnoides. But they didn’t father any progeny. Why did the species perform so badly? Could this fussiness in the face of S. alburnoides‘ sexual profligacy contribute to its dire conservation status?
“Maybe the optimal conditions for their reproduction were lacking,” says Morgado-Santos. “Or maybe S. pyrenaicus avoid reproducing near S. alburnoides to avoid hybrids sexually parasitising their spawning events. We do not know for sure. This low reproduction rate between S. pyrenaicus and S. alburnoides makes us wonder if most (if not all) PA [diploid] individuals found in the studied population are actually a result of androgenesis, but further studies are needed.”
The study was published on May 24, 2017, in the journal Royal Society Open Science.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.