Tuesday was Valentine’s Day and what better way to celebrate than talking about lizards!
This is a whiptail lizard (genus: Aspidoscelis).
There are quite a few species of North American whiptail lizards. They are small lizards typically found in the southern U.S. and Mexico. They’re diurnal (active during the day) and mainly eat insects.
But what’s extra cool about whiptail lizards is that some species are entirely female. That’s right: there are no males for 6 species of these lizards.
Instead of traditional sex, where a male fertilizes a female’s egg, these lizards are parthenogenetic. This means that the female’s eggs will develop into embryos without fertilization. It also means that each offspring is a clone of its mother.
Even though they don’t need to, whiptail lizards still have sex. Or, more accurately, they display mating behaviors. When ‘mating,’ one female will take a male role and mount a female. They can switch roles back and forth at different times. This switching seems to be driven by the expression of different hormones in the brain.
Why do whiptail lizards bother with mating behaviors if they can reproduce just fine without them? It turns out that mounting could help promote ovulation. Females mounted by another female end up producing more eggs than those who aren’t.
Finding a loophole
Typically, parthenogenesis is a bad thing. Remember your basic biology: with sexual reproduction, you get half your genetic material from your father and the other half from your mother. Each sexual interaction mixes genes differently; that’s why your siblings have different genes than you, even with the same parents. This constant mixing keeps the overall population healthy since having lots of genetic diversity lets a population adapt to changing conditions.
In contrast, parthenogenetic species like whiptail lizards are clones of each other. If the environment changes or a new disease crops up, there’s no way for unique traits to emerge. If the threat kills one individual, it will kill them all.
But even though they’re clones, whiptail lizards have a surprising genetic diversity. Why? First, the all-female whiptail lizards are the descendants of hybrids or mixtures of two closely-related species. This gives the whiptail species quite a large amount of genetic diversity to begin with.
This hybridization also gave 5 species of whiptail lizards an extra set of chromosomes. Unlike us, who have just two sets of chromosomes and are diploid, these whiptail lizards are triploid and have three sets of chromosomes. Two of their sets came from their original mother, and the third came from their original father. This extra set of chromosomes helps maintain genetic diversity and protects the species from deleterious random mutations.
But perhaps most important for maintaining genetic diversity is how the lizards replicate their chromosomes during reproduction. In sexually reproducing species, each parent gives their offspring half of their genetic material. In other words, egg cells have half the chromosomes that other cells in the body do; they get the other half from sperm. But for whiptail lizards, there is no sperm-giving father!
To avoid having offspring with half the genetic material, whiptail lizards double their chromosomes during meiosis (cell division where eggs are made). The resulting egg will still have a complete set of chromosomes. No males are needed!
On the one hand, being able to reproduce by yourself is advantageous. For instance, if you’re the first one in a new habitat, you don’t need to wait for someone else to arrive to start making babies.
But the flip side is that you lose out on a critical benefit of sexual reproduction: genome mixing. Remember, sexual reproduction is great for maintaining genetic diversity in a population and protecting a species from changes in parasites or predators.
Suppose non-native predators or parasites are introduced to the habitats of all-female whiptail lizards. In that case, they will likely have trouble reacting. The same goes for if climate change starts dramatically influencing their habitat. Any subtle change in their environment can be catastrophic for the entire species.
These whiptail lizards are OK being just females for now, but who knows what the future holds.