Most mammals can trace their origins to a single ancestral species. But in the Caribbean islands of the Lesser Antilles, there is a fruit bat with a far more complex family tree. Artibeus schwartzi’s genome is a hybrid mish-mash of DNA inherited from no less than three separate ancestors. One of these is probably extinct and the other two of which still live on the same island chain. It’s a fusion bat, a sort of fuzzy, winged spork.
The ancestry of A.schwartzi has puzzled scientists for almost three decades, and the idea that it’s a hybrid has been mooted before. Peter Larsen from Texas Tech University confirmed the bat’s unique ancestry by sequencing DNA from 237 individuals belonging to the seven fruit bat species of the Lesser Antilles. He found that A.schwartzi’s main genome is a cross between those of two other fruit bats, A. jamaicensis and A. planirostris, with a tiny minority of sequences that don’t match either genome.
Complicating matters, animal cells also have a separate smaller genome, housed in energy-providing structures called mitochondria. But A.schwartzi’s mitochondrial genome doesn’t resemble that of either of the two species that gave rise to it. These accessory genes must have come from yet another source – a third species of fruit bat that has either since gone extinct or that hasn’t been discovered yet.
A. jamaicensis and A.planirostris must have first hybridised fairly recently, for their ranges only overlapped around 30,000 years ago. Nonetheless, today, A.schwartzi is a distinct species in its own right. It has a stable population that can sustain itself without the need for the two ancestral species to continuously mate with each other.
Larsen thinks that its success stems from events that took place after the last Ice Age. Rising sea levels severely isolated the islands that it now lives on, particularly St Vincent. This separated the new hybrid from its parental species, cutting off the flow of genes that would otherwise dilute this unique lineage. Today, A.schwartzi is St Vincent’s dominant bat.
A.schwartzi is also a perfect example of a phenomenon that’s often seen in hybrids, called ‘transgressive segregation’. You might think that a hybrid would blend the features of its parents, leading to a body that’s half-way between the two. But not always – hybrids often do the opposite, developing extreme and overstated traits well beyond the natural variation of their parents. There are many possible explanations for this, including a clash between genes from the two parents or malfunctions in the way the hybrid develops. Either way, A.schwartzi is living proof of the effect – its skull is much bigger than those of either A.jamaicensis or A.planirostris, which are both roughly the same size.
A.schwartzi’s three-way chimeric genome is a rare find indeed. Some animal hybrids go on to establish new species, but such examples are rare, especially among mammals. Some scientists have suggested that the red wolf is a hybrid of the gray wolf and the coyote, but that’s been disputed of late. A couple of monkeys – the stump-tailed macaque and the kipunji – might also be hybrids, but the evidence for this is still uncertain. A.schwartzi is the clearest case study yet that hybridisation can give rise to new species of mammals.
Reference: PNAS http://dx.doi.org/10.1073/pnas.1000133107
Photo by Tobusaru; depicts A.jamaicensis
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- When bacteria merge – two species are turning into one
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