Oh, what a misleading common name it is. It implies that the Florida panther is a subspecies of
Puma concolor, and indeed it was once classified as such. At one point it was even elevated to the status of being its own distinct species (
Felis coryi), only to be later reassigned subspecific status. This is not, however, consistent with the genetic history of the species.
Stephen J. O'brien recounts the tale in his book, Tears of the Cheetah (I've only read 4 chapters thus far, but it is a fascinating read, written in an engaging narrative style that is easy to understand, and I highly recommend it). The previous two chapters dealt with inbreeding depression (and the historic bottlenecks that caused them) in cheetahs and Gir lions (an Asiatic subspecies), respectively. Not surprisingly, Florida panthers were also suffering from severe inbreeding depression. Mitochondrial DNA and nuclear microsatellites were used to estimate the time of the historic bottleneck which caused the genetic homogeneity of the panthers. For those not well-versed in evolution, a bottleneck occurs when a population is severely reduced in size, leading to a reduction in genetic diversity (simply due to the fact that the survivors, being only a small subset of the original population, possess only a small subset of the former population's genetic diversity). Individuals are forced to mate with genetically similar individuals (as there is no other choice), often close relatives.
So how can mitochondrial DNA and microsatellites be used estimate how long ago a bottleneck occurred? Quite simply, it takes time for mutations to accumulate and for genetic diversity to re-establish itself. Thus, cheetahs today are genetically very similar because of the bottleneck that they went through 12,000 years ago. However, some genes mutate faster than others. Mitochondrial DNA (mtDNA) does not reside in the nucleus of the cell, but in the mitochondria (organelles which supply energy to a cell, which are descended from single-celled organisms that were engulfed by the (single) cells of our very distant ancestors). Because mtDNA is separate from nuclear DNA, it doesn't benefit from the DNA repair mechanisms that are present in the nuclei of cells. Since mutations are essentially replication mistakes (or damage, but we won't get into that), and the mistakes of mtDNA don't get fixed, mutations accumulate faster than in an organism's nuclear genome. Mutations occur at a relatively fixed rate, so differences in mtDNA can be used to determine how long ago two species split from a common ancestor, or how much time has passed since a bottleneck. The situation for microsatellites is similar. Microsatellites are repeated sequences of a small number of base pairs. They exist in the nuclear genome, but are non-coding (i.e. they aren't a functional gene) and so also mutate at a greater rate than the rest of the genome.
So there's that tangent, and now you understand how the time since a bottleneck occurred can be estimated by using “molecular clocks.” Turns out, North American pumas (not just Florida panthers) experience a bottleneck 10,000-12,000 years ago (about the same time as the cheetah). I group North American pumas together because during the course of this study, the historical scheme of 32 different puma subspecies across North and South America was essentially invalidated. Rather, 6 genetically distinct populations were found, 5 of which were South American. All North American pumas could be traced back to the single, small population that resulted from the bottleneck, and yet puma fossils in North America go back much further than this (in fact, pumas were originally a North American species that migrated to South America when the continents became connected, displacing the existing South American marsupial carnivores). So how can this be explained?
The current hypothesis is that North American pumas went extinct during the same extinction event that killed off the dire wolf, saber-toothed cats, American lion, and American cheetah, as well as other large mammals such as mastodons. As a side note, the African cheetah's bottleneck is thought to have resulted from a small refuge population that persisted in sub-Saharan Africa while the rest of the world's cheetahs—which were wide ranging in North America, Europe, Asia, and Africa—succumbed to the Pleistocene extinction event. But South America's pumas escaped this event (the extinctions were largely restricted to the northern hemisphere from what I understand). A small founder population migrated back to Mexico through Central America (small population=genetic bottleneck). This population was kept small through a combination of a geographic bottleneck (the Isthmus of Panama) and behavior—namely the fact that pumas are territorial and further northward migration was blocked off by the territories of the resident founders. The founders slowly made their way north, maintaining their status as a distinct genetic population by preventing southern cats from migrating across the Isthmus.
So all North American pumas are fairly homogenous genetically, but what about Florida panthers? They existed in a refuge of land largely unsuitable for agriculture: the cypress swamps of Florida. As cougars were extirpated from the rest of the southeast, migration to this small refuge population was (obviously) cut off. This naturally led to inbreeding depression, which was actually the cause of identifying traits used to distinguish the Florida Panther from pumas in the rest of the continental U.S. These traits included a dorsal cowlick halfway down the neck, and a kink at the tip of the tail. It's important to note that these traits were
not present when the Florida panther was first described as a subspecies (before widespread extirpation throughout the east), and are actually the results of severe inbreeding. Another such consequence was cryptorchidism, a condition where one or both testicles never descend in males, leading to reduced fertility or sterility (if neither testicle descends).
A recent management decision resulted in several individuals from a Texas population (which historically overlapped with the Florida panther's range) being released into Florida. Offspring from these Texas females (which had obviously mated with male “pure” Florida panthers) did not exhibit the cowlick or tail kink, and also didn't develop cryptorchidism. The hybrids were also markedly healthier and stronger, often jumping clear over the research team after being treed by dogs (a feat that the weaker, inbred, “pure” Florida panthers were rarely able to accomplish).
I find this tale very interesting because it generates questions on where to draw the line with conservation efforts aimed at preserving the genetic integrity of small populations and/or subspecies. In the case of the Florida panther, prior to the genetic tests nobody wanted to “hybridize” them, tainting the genetic makeup of the “authentic” subspecies' population. After bringing to light that the smallest genetically distinct population was North American pumas in general, the case was made to re-introduce migrants. In the case of the Texas females, migration that existed freely before about 1900 was merely re-established.
References:
O'brien, S.J. 2003.
Tears of the Cheetah: And Other Tales From the Genetic Frontier. St. Martin's Press, New York, NY.
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