Polyploidy is the condition where an organism possesses more than two sets of chromosomes. Most people probably only associate it with plants, as polyploidy in animals has been relatively understudied, and unisexuals—animals that are entirely female, are typically ignored because they use hybridization and parthenogenesis (though personally I think it may be male refusal to accept that they aren’t as permanent or resilient as they may have hoped—see the degenerate Y chromosome). The most famous female-only species are probably the New Mexico whiptails, Cnemidophorus neomexicanus, or “lesbian lizards,” a hybrid species of lizard that no longer involves males in their reproduction but still often perform courtship rituals to stimulate ovulation.
Both parthenogenesis (when eggs develop with no fertilization) and hybridogenesis (fertilization occurs but paternal DNA isn’t passed on) are pretty common in amphibians. A more intriguing example than the lizards, though one that’s gotten less press until now, is the unisexual Ambystoma hybrid salamanders. This salamander ranges from triploid to pentaploid with Ambystoma nothagenes using genes from males from three different salamander species– Ambystoma laterale, Ambystoma texanum, and Ambystoma tigrinum.
The Ambystoma females always require sperm from a related species to fertilize their eggs and initiate development and generally just discard the sperm genome. Sometimes the unisexual sexually reproduces instead and there is a genome addition or genome replacement event where the maternal genome is discarded or the female acquires the male’s genes and then keeps only some of the genes after mating.
What’s kind of intriguing about this case of “kleptogenesis”, or gene stealing, is that the females basically express genes from the different males at a relatively equal rate. How exactly they choose which genes to use and which to throw away is not known, nor is how these genes come together to make a good hybrid.
Why do it?
How Ambystoma, a six-million-year-old lineage, and how other polyploids/unisexuals/hybrid species survive when in competition with “regular” diploids living in the same spatial niche, is also not understood. It’s generally considered that polyploidy is a short-term strategy evolved in environments that are less stable. Which is why Ambystoma being a polyploid for so long is especially surprising.
While polyploidy can be advantageous, it’s initially unstable before becoming a competitive strategy. The presence of duplicated genes can help fuel diversification and evolutionary success.
Heterosis, gene redundancy, and asexual reproduction can all be considered advantages of polyploidy. Heterosis is essentially the ability to make better use of heterozygosity. Gene redundancy allows you to better diversity and provides a protection from harmful mutations. And asexual reproduction enables you to reproduce without a sexual mate around.
Polyploidy in animals is a case of convergent evolution where many fish and amphibians have acquired it separately. All the polyploids have acquired their genomes differently and in different ways. The unisexuals and males in the salamander group have higher gene exchange than other polyploids which may explain the “balance” in the genome not seen in other polyploids. Instead of gene silencing or dominance evolving, it seems natural selection has favored a more balanced genome because that’s just what worked for these girls. If you lose some gene contribution it’s less dramatic this way than if you’d put almost everything into one male salamander only for him to do something inconvenient like die or not show up to mate.
There’s some serious selective pressures for salamanders to survive injuries. They have the ability to regenerate tissue, so if part of their tails snap off, they can grow back. A study published in the Journal of Zoology showed that these polyploid all-female salamanders regenerate lost tissue 36% faster than other salamander species.
Within 10 weeks, after having had 40% of their tails cut off, the all-female salamanders had full length tails. The diploid sexually-reproducing relatives needed another 5 weeks to finish growing their tails.
The explanation may have to do with more genes meaning more proteins meaning faster regeneration. And if you can’t regenerate you don’t do so well so this could have been an added pressure to explain how this lineage was able to stay polyploid for six million years.
- J. Saccucci, R. D. Denton, M. L. Holding, H. L. Gibbs. Polyploid unisexual salamanders have higher tissue regeneration rates than diploid sexual relatives. Journal of Zoology, 2016;
- Comai, Luca. “The advantages and disadvantages of being polyploid.” Nature Reviews Genetics11 (2005)
- Kyle E. McElroy, Robert D. Denton, Joel Sharbrough, Laura Bankers, Maurine Neiman, H. Lisle Gibbs. Genome Expression Balance in a Triploid Trihybrid Vertebrate. Genome Biology and Evolution, 2017
- Evolutionary Significance of Whole-Genome Duplication. Mcgrath-M. Lynch – Polyploidy and Genome Evolution – 2012