The summer after my sophomore year of college, I spent a semester at the University of Michigan Biological Station (UMBS). While I was there I took an ecology field class, which was one of the most memorable classes that I have ever taken. My teacher was very passionate about a lot of topics (house cats, lawns, dunes), but one subject that caught my attention was unisexual salamanders.
It is common knowledge that vertebrate organisms reproduce sexually; in other words, it takes a male and a female of a species to produce offspring and those offspring are a genetic mix of both parents. However, that is not always true. In fact, there are about 90 different lineages of unisexual vertebrates. There are three primary methods of unisexual reproduction in vertebrates: parthenogenesis, hybridogenesis, and gynogenesis. Parthenogenesis, simply put, is cloning. Barring any random mutations, the offspring are genetically identical to the mother. Hybridogenesis is ‘hemi-clonal’, or half clonal. The mother passes on her genes clonally to the offspring, but the genetic material of a father is also passed on. The father’s genetic material is later discarded and only the mother’s genome continues to be passed on. Finally, gynogenesis is neither asexual nor sexual reproduction. Organisms that use gynogenesis require sperm to trigger cellular division in the egg, but the egg is not fertilized and the father’s genetic material is only incorporated about a quarter of the time. Unlike parthenogenesis, gynogenetic genomes are very flexible and will mix and change. Unisexual salamanders use a unique method of gynogenesis called ‘kleptogenesis’, meaning that female unisexual salamanders will steal the spermatophore (sperm packet) of male salamanders of the same genus to use in reproduction.
Unisexual salamanders come from the genus Ambystoma. Commonly known as ‘mole salamanders’, they are native to North America and are typically terrestrial as adults, with the notable exception of the axolotl (Ambystoma mexicanum). They live in burrows and return to water to lay their eggs. Unisexual Ambystoma salamanders live in the Great Lakes and northeastern regions of the United States and incorporate the genomes of 5 different species: the blue-spotted salamander (Ambystoma laterale), the Jefferson salamander (A. jeffersonianum), the small-mouthed salamander (A. texanum), the streamside salamander (A. barbouri), and the tiger salamander (A. tigrinum).
Scientists think that the lineage of unisexual Ambystoma salamanders originated between 2.4-3.9 million years ago from a hybridization event of a female A. barbouri. Now, the genetic material of A. barbouri is virtually unknown within unisexual salamanders but every individual has a least one genome from A. laterale. This suggests adaptation within the lineage. Also, these salamander populations are composed solely of females, and they are sometimes referred to as ‘Amazons’ after the mythical female warriors. This is an advantage for the salamanders, because it means that every individual can give birth. However, how this happens is not fully understood. In salamanders, sex is determined differently than in humans. In humans, female is the default gender and is represented by ‘XX’, while males are ‘XY’. In salamanders, males are the default gender and are represented by ‘ZZ’, while females are ‘ZW’. Most Ambystoma salamanders are diploid, meaning that they have two sets of genes (typically one from each parent). Unisexual Ambystoma salamanders can be triploid, tetraploid or even pentaploid. During reproduction, the male genome is incorporated about 25% of the time. This could be by replacing an existing genome or by increasing the ploidy level.
The more scientists learn about alternative methods of reproduction, the better we can understand why sexual reproduction came to be so prevalent. It can also shed light on to what it means to be a species. Traditionally, a species is considered to be ‘reproductively isolated’, meaning it can only reproduce with its own species. Unisexual salamanders and other similar lineages remind us that our methods of classification are artificial constructions and do not necessarily reflect the natural world. Also, what are the advantages of unisexual reproduction? Perhaps it allows the salamanders to adapt to minute changes in the environment. Research on the topic of unisexuality in vertebrates has exploded in recent years, but we still have a lot to learn.
To learn more about unisexual salamanders, check out the links below!
Bi, Ke; James P Bogart, Jinzhong Fu (2008). “The prevalence of genome replacement in unisexual salamanders of the genus Ambystoma (Amphibia, Caudata) revealed by nuclear gene genealogy” (Open access). BMC Evolutionary Biology 8: 158.
Bogart, James P; Ke Bi, Jinzong Fu, Daniel W A Noble, John Niedzwiecki (2007). “Unisexual salamanders (genus Ambystoma) present a new reproductive mode for eukaryotes”. Genome / National Research Council Canada = Génome / Conseil National De Recherches Canada 50 (2): 119–136.