Evolution: whale adaptive losses and changes (Evolution)

by David Turell , Friday, September 27, 2019, 00:27 (1667 days ago) @ David Turell

It is quite a list:

https://cosmosmagazine.com/biology/why-don-t-whales-have-saliva?utm_source=Cosmos+-+Mas...

On land, hair is often a necessary part of an organism’s heat regulation strategy, but in water it can cause drag, leading to inefficient swimming and all that comes with it, such as poor predator avoidance. In this case, the loss of genes producing keratin actually helped to adapt to the new environment.

In this study, the scientists discovered 85 genes that had been deactivated by mutations in both extant cetacean clades; odontocetes (toothed whales) and mysticetes (baleen whales).

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Of the 85 genes identified, 62 have never been reported and the researchers have identified eight genes specifically that are likely to have been involved in the adaptation of the stem lineage to a fully aquatic lifestyle. These are implicated in a wide range of traits, from sleep to saliva.

Two of the lost genes, called F12 and KLKB1, were associated with blood coagulation. While these are vital on land, in the water they could lead to dangerous clotting inside the blood vessels, known as thrombosis.

F12, for example, causes clotting when it encounters foreign surfaces in the body. Losing this gene may have been beneficial for cetaceans because “nitrogen microbubbles, which readily form in the blood upon repeated breath-hold diving, may act as foreign F12-activating surfaces entailing harmful thrombus formation.”

Other lost genes help to reduce the chance of genetic mutation caused by DNA repair mechanisms working to rectify damage done by the high oxygen levels in the blood necessary for deep diving.

Loss of the genes MAP3K19 and SEC14L3 might help prevent scarring and the resulting loss of elasticity in cetacean lungs which, unlike humans and other terrestrial mammals, collapse during deep diving and explosively expand upon resurfacing. This elasticity helps cetaceans to renew 90% of the air in their lungs in a single breath.

The gene SLC4A9 is partly responsible for the production of saliva in terrestrial mammals, but your average dolphin or whale has little need of spittle which is why it has been lost in cetaceans.

Saliva helps to lubricate the mouth, break down starch and facilitate taste, all of which are less important in an aquatic environment. Who needs oral lubrication when your meal comes with a mouthful of seawater?

Beyond that, write the authors “the hyperosmotic marine environment necessitates strict housekeeping of freshwater resources in marine species; thus, freshwater loss via saliva secretion may be detrimental.”

Cetacean ancestors, as mammals who need air to breathe, also faced issues with regards to the mammalian sleep cycle. Just as humans tend to avoid napping face-down in puddles, cetaceans can’t just fall asleep in the ocean.

So the creatures of the cetacean stem lineage had to find a way to balance the need for sleep with the restriction of their new aquatic environment. As a result, they have a unique adaptation called ‘unihemispheric sleep’, which “allows one brain hemisphere to sleep while the awake hemisphere coordinates movement for surfacing.”

This adaptation was facilitated by the loss of several genes involved in the production and reception of the sleep hormone melatonin. This helped to “decouple sleep-wake patterns from daytime,” which, argue the researchers, “may have been a precondition to adopt unihemispheric sleep as the exclusive sleep pattern.” (my bold)

Comment: Interesting complex changes, and one can imagine some of these adaptations while learning to become aquatic, but note my bold. How does a whale learn to sleep with half a brain active while living in the water, without drowning during the process of adaptation? For me god prepared them.