It might look like not much more than an over-sized shrew, but the solenodons (the Cuban solenodon, Solenodon cubanus, and the Hispaniolan solenodon, Solenodon paradoxus) are a very special group of mammals. Found only on the islands of Cuba and Hispaniola, these strange nocturnal insectivores represent the only remaining members of a particularly ancient lineage: A recent study has provided genetic evidence that they split off from the rest of the mammals around 13 million years before the dinosaurs went extinct.
Researchers from the Carl R. Woese Institute for Genomic Biology (University of Illinois), and the University of Puerto Rico set out to sequence the mitochondrial genome of the Hispaniolan solenodon – marking the final mitochondrial genome of the major groups of mammals to be sequenced.
The mitochondrial genome is the sum of DNA contained in the mitochondria, the ‘batteries of the cell’ that provide chemical energy for vital processes. The difference between mitochondrial DNA and the rest of the DNA in the cell (found in the nucleus, ‘nuclear’ DNA), is that it is only passed on from the female to the offspring via the egg cell at fertilisation. Unlike nuclear DNA, it doesn’t mix with the DNA of the sperm cell, and the DNA sequence changes less as a result.
This makes mitochondrial DNA a useful tool for evolutionary biologists when they want to estimate how old a certain group is. If you know the rough rate at which the DNA accumulates mutations (e.g. however many per 10 million years), then by comparing the number of changes between one group and another, you can estimate roughly how long it has been since they split from each other in the evolutionary tree. These techniques are known as ‘molecular-clock’ methods.
Using two different methods of DNA sequencing, the researchers came up with the same result both times, strongly suggesting solenodons split off from the rest of the mammals in the Mesozoic era – the ‘age of dinosaurs’. The approximate date, 78 million years ago, was shortly before the extinction event (65 million years ago) that wiped out dinosaurs and allowed mammals to flourish.
“It’s just impressive it’s survived this long … It survived the asteroid; it survived human colonization and the rats and mice humans brought with them that wiped out the solenodon’s closest relatives,” remarked co-first author of the paper, Adam Brandt.
The findings also provided genetic evidence for a proposed subspecies classification for the Hispaniolan solenodon – it appears that populations in the South have less genetic diversity than those in the North of the island, and the two groups are genetically distinct, supporting the presence of two subspecies; Solenodon paradoxus paradoxus in the North, and Solenodon paradoxus woodi in the South.
This information is important because both solenodon species are Endangered, due to a combination of habitat loss and the introduction of cats and dogs onto the previously isolated islands. Distinguishing between subspecies, especially if they vary in their levels of genetic diversity, is vital because separate conservation plans may be more effective for some populations than others.
The isolation of the solenodons is perhaps the only thing that has allowed them to persist for so long, evolving in an environment free of predators. Becoming increasingly scarce, the researchers had to resort to more ‘unusual’ sampling methods – lying on their backs at night, waiting for one to crawl over them whilst foraging.
The solenodon is an evolutionary relict, with no close ancestors left. Living in the shadow of larger predators, only emerging from its burrow at night to use its acute senses of smell and hearing (and tactile whiskers) to sniff out prey, it seems to resemble what some of the earliest mammals may have looked and behaved like. This research has paved the way for better understanding both how the solenodon is an evolutionary success story of endurance, and how we can preserve it in the face of what is culminating to be another man-made extinction event.
Brandt, A.L. et al. (2016). Mitogenomic sequences support a north–south subspecies subdivision within Solenodon paradoxus. doi: 10.3109/24701394.2016.1167891
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