Friday, August 18, 2006

Shivering pigs   posted by JP @ 8/18/2006 05:15:00 PM

I started writing a summary of this interesting article on pig evolution, but hell, it's in PLoS, so they've already done the work for me. Here's the synopsis:

Brown adipose tissue (BAT) is unique to mammals. It is rich in mitochondria and generates heat to maintain body temperature during cold stress, referred to as nonshivering thermogenesis. BAT is found in abundance in rodents as well as in newborns of larger mammals, including humans. Uncoupling protein 1 (UCP1) is exclusively expressed in BAT and is localized to the inner membrane of the mitochondria. Its physiological role is to uncouple oxidative phosphorylation so that most of the energy in fat stores is dissipated as heat rather than being converted to ATP.

Piglets are sensitive to cold stress and rely on shivering as the main mechanism for thermoregulation. Furthermore, pigs are the only hoofed mammals that build nests for birth; in modern pig production, heat-producing lamps are used to keep the piglets warm. It is also striking that pigs appear to lack BAT.

Here the authors show that the UCP1 gene is disrupted in domestic pigs and their wild ancestors. The inactivation of UCP1 was estimated to have happened about 20 million years ago. The finding provides an explanation for the poor thermoregulation in piglets that may have led to the evolution of the unique maternal behavior in this species.

My reaction on reading the synopsis was that the reverse explanation for the evolution of the maternal behavior could be possible-- that the selection pressure on UCP1 could be negligible (allowing the mutation to spread) due to the maternal behavior. But no:

The wild boar is the only porcine species that has adapted to temperate climates, whereas all other Suidae live in tropical or subtropical environments. The wild boar has then evolved compensatory mechanisms to adapt to a cold environment. Newborn pigs use shivering for thermogenesis, and the wild boar is the only ungulate that builds a thermoprotective nest for giving birth.

This could be a great example of how the "parameter space" of evolution is limited by the vagaries of history--the ancestor of the wild boar lost the ability to make this protein, reducing the probability of getting it back to nearly zero. So when the animal's range expanded to cooler climates, different pathways were those that faced selection. There's more than one way to skin a cat.