Endemic to the Great Lakes region of North America, the unassuming unisexual Ambystoma salamander has been breaking evolution's rules for millions of years to establish a girls-only lineage, turning the tables on what we know about sex and its role in species survival. Conventional knowledge about reproduction states that these critters should have been extinct by now, but a new study sheds light on the genetic balancing act these lady salamanders perform to survive and thrive.
While the unisexual salamander produces exclusively female offspring, this doesn't mean she has no use for male salamanders. Unlike parthogenetic creatures, such as the American cockroach, which can produce eggs in the absence of males, the unisexual salamander needs the sperm to initiate the development of the eggs – and here's where things get fascinating (and a bit kinky). Instead of restricting her to one particular species, nature has enabled her to mate with males of several species. Consequently, her offspring can end up having up to five genomes. By comparison, humans and most other animals only have two – one from each parent.
"Sometimes sperm from these other species fertilizes the unisexual's eggs and that male's genome is incorporated (i.e. passed on to the next generation), other times the sperm is only used to activate development," Kyle McElroy, corresponding author and a biology doctoral candidate at the University of Iowa, told New Atlas. "However, each copy of their genome is ultimately derived from a different male, going back many generations … What's fascinating is that they can uptake and drop genomes from five different species and harbor more genome copies than most animals."
While this might all sound very impressive, it doesn't explain how unisexual salamanders have been able to thrive for millions of years. With regards to this, the researchers had a hunch that it had something to do with how the salamander was able to maintain a balance of genes from the thousands she had acquired via mating.
"We're hypothesizing the successful individuals have balanced gene expression," said study author and biologist Maurine Neiman in a statement. "This balance might have been a prerequisite for the emergence and continued success of this particular hybrid lineage."
To test this hypothesis, the team analyzed close to 3,000 genes from a unisexual female with three genomes and found that 72 percent of its genetic profile was made up of equal contributions from males of three separate salamander species – the blue-spotted salamander (Ambystoma laterale), small-mouth salamander (Ambystoma texanum), and tiger salamander (Ambystoma tigrinum).
With all the genes available to her, having some kind of a management system to pair the different genomes and reduce the likelihood of a mismatch is a good idea. As it turns out, rather than complicate her life by selecting genes individually, the unisexual salamander appears to have settled on using a balanced ratio of different male genes. Think of it as the biological version of fantasy football. Would you want to field a team of equally skilled players or one filled with players of varying ability and one star player?
"If you have a team that's unbalanced and loses a top player, you won't win," explained McElroy in a statement. "But if every player is equal, then you don't lose as much."
That said, exactly how these unisexual salamanders are able to pull off this whole genomic mix-and-match process remains a mystery and questions abound. For example, given that the blue-spotted salamander's genome is ubiquitous in all unisexuals, does this mean that it might have something to do with determining the unisexual phenotype?
On a different note, what is clear is that given that more than 80 taxa of fish, amphibians, and reptiles reproduce by parthenogenesis, unisexuality isn't quite the evolutionary dead-end that scientists used to think it was. In fact, an earlier study showed that these female salamanders are capable of regenerating their tails – an important predator evasion tool – 1.5 times faster than their heterosexual counterparts under certain conditions, though whether this is due to genomic differences alone is up for debate.
In any case, since this study focuses only on one individual specimen, more research will have to be conducted to see if these findings are also applicable to unisexuals with more than three genomes, as well as to crack its selection code.
"What we'd like to find out is how the choosing and using occurs, and how these genes from different sexual salamander species come together to make a successful hybrid," said McElroy.
The study was published in Genome Biology and Evolution.
Source: University of Iowa