Kiley BriggsViews:
When someone uses the word ‘species’, you probably know roughly what they mean. But, if you ask ten biologists to define a species, you might get ten different answers. I think of a species as a group of living organisms made up of similar individuals that can breed and produce fertile offspring. With most land animals, species are straight forward. Although some animals may breed with other similar species and produce hybrids, the offspring usually are unable or unlikely to breed.
For example, when horses mate with donkeys, the resulting offspring is an infertile mule. Or, when a Fowler’s Toad mates with an American Toad, the offspring are technically fertile, but their breeding call will be unattractive to both pure species. The hybrid male toads are less likely to attract mates, helping keep the pure species separate. Determining where one species ends, and another begins, is incredibly complicated. Blue-spotted Salamanders and Jefferson Salamanders are one example of animals blurring the line between species, and they do it in a very weird way.
Blue-Spotted Salamanders and Jefferson Salamanders
Blue-spotted Salamanders (Ambystoma laterale) are a species of mole salamander common in much of the northeastern United States. They get four or five inches long, are black or very dark blue, and have blue spots or flecking on their sides. They have a short, rounded snout, a round tail, and they usually breed in bottomland wetlands.
Jefferson Salamanders (Ambystoma jeffersonianum) are a similar species that get up to 7 inches long. They can be slate gray or blue in color, sometimes brown, and may have faint blue flecking on their sides. Their snouts are long and somewhat rectangular, and they breed in upland vernal pools and shallow wetlands.
Hybrid Unisexual Salamanders
Blue-spotted and Jefferson Salamanders cannot breed with each other; they are distinct species. However, there is a group of hybrids that can breed with either of the pure species. “Hybrid” isn’t quite the right word to describe these salamanders, which I’ll get to in a moment. If you ever took a science class that covered genetics, you may recall that we all have two copies of every chromosome (X or Y-shaped bundles of DNA). When animals mate, offspring get one copy of each chromosome from both parents and the total number of chromosomes stays the same. That is not how the hybrid Jefferson and Blue-spotted Salamanders do it.
Hybrid Jefferson and Blue-spotted Salamanders are weird in several ways. First, they are almost entirely female. Rather than calling them hybrids, biologists usually refer to these salamanders as “unisexual” Ambystoma. When the unisexual females are ready to lay eggs, they must mate with a purebred male of either parent species. That is usually where the male’s contribution ends because his DNA is often discarded. All the male’s sperm needs to do is penetrate the egg, which kickstarts the egg’s development using only the female’s DNA. The resulting offspring is essentially a clone of the mother, but not always. Sometimes, DNA from the male is used, but when it is, an entire set of his chromosomes might be added to those already within the egg instead of replacing a set. As a result, the offspring might have between two and five entire sets of chromosomes. Having extra sets of chromosomes, a condition called polyploidy, is very unusual among vertebrates. On very rare occasions, a male hybrid is produced, but they appear to be sterile.
Unisexual Amybstoma are found in most places where Blue-spotted and Jefferson Salamanders share the landscape. Wherever unisexuals occur, they tend to vastly outnumber purebred individuals. Telling the hybrids apart from parent species can be challenging. The unisexuals tend to be a little larger in size, and sometimes they look like a blend of the two parent species. They can also look nearly identical to a purebred. Because the hybrids are almost all females and outnumber purebreds, determining the sex of individuals can help you determine if they might be a hybrid. Under and behind the hind legs, the cloaca or “vent” of males is swollen during the breeding season. If you see a swollen vent, the individual is a male and it is safe to assume he is also a purebred. If you can examine a group of the salamanders and find a nearly 50/50 blend of males and females, the population might lack hybrids. Beyond that, the only way to really know is to examine tissue samples in a laboratory.
Why does it matter?
There is some concern that populations can become so overrun
with unisexuals that they essentially run out of males. In vast landscapes with
minimal roads and other barriers, that might not be an issue because males from
nearby areas can migrate in and replenish the population. In fragmented landscapes, however, migration is challenging. There is at least one instance in a
fragmented landscape of a site losing all its males. In that case, unisexual
females left their breeding wetlands and returned to the uplands without ever
laying their eggs. Whether populations running out of purebred males is a threat
to salamander conservation at a larger scale is unclear, but with the landscape
becoming increasingly fragmented, it is something to keep an eye on.
Those who are more expert in the field of unisexual Ambystoma salamanders will likely think I oversimplified the matter in my explanation. I did, in part because I don’t fully understand the intricacies myself. What I will add, however, is that in some regions, other species get involved. The Tiger Salamander (A. tigrinum), Small-mouthed Salamander (A. texanum), and Streamside Salamander (A. barbourin) all participate. Among terrestrial vertebrates, this kind of complicated relationship between species is rare. While there isn’t much I expect most people to do with this information, I do find it very interesting and simply wanted to share my limited understanding of it with you.
