Wang, Z., Chen, Z., Zu, S., Ren, W., Zhou, K., and Yang, G. 2015. ‘Obesity’ is healthy for cetaceans? Evidence from pervasive positive selection in genes related to triacylglycerol metabolism. Nature Scientific Reports doi: 10.1038/srep14187
But wait…obesity isn’t healthy!
When most people think about obesity, they might consider it a disease. But then again most people don’t have to dive hundreds of feet into frigid ocean waters to find food. Certain marine mammals, known in the scientific world as cetaceans, spend their entire lives at sea, and, therefore, have a different perspective on obesity. This diverse group encompasses all of the fully marine mammals, including whales, dolphins (like the one pictured in Figure 1 – killer whales are actually more closely related to dolphins than to whales), and porpoises. These marine creatures share a recent common ancestor with land mammals – the closest living relative to the cetaceans is the hippopotamuses, which means that cetaceans had to transition from living life primarily on the land back to living life underwater (remember that all four-legged creatures, including humans and other mammals, are descended from fish-like ancestors that transitioned out of the water onto the land almost four hundred million years ago). The cetacean transition back to the water most likely occurred about forty million years ago.
Figure 1: The killer whale, an iconic cetacean, breaching near the coastline.
Obesity simply means having too much body fat, and it certainly is a pervasive health issue in human societies, especially in the United States. Being obese increases your risk of heart disease, type II diabetes, certain types of cancer, and arthritis – this increased health risk is supported by ample scientific literature (although the picture is never as simple as increased weight equals increased risk). More than one in three adult Americans is classified as obese, according to the U.S. National Institute of Health. Obesity is an epidemic in our country as well as globally, and combatting it requires a comprehensive understanding of its causes. Cetaceans, on the other hand, need much more body fat than other mammals – they are naturally and beneficially obese. Blubber – the fatty tissue found in all marine mammals – acts as an insulator, keeping the animal warm in cold temperatures (Figure 2). It also helps keep these mammals afloat, which saves the animal valuable energy that might be spent elsewhere.
Figure 2: A schematic of cetacean skin, from the dermis down to the muscle. Blubber acts as an insulator, blocking heat exchange between an organism’s body and its surroundings.
This study was particularly interesting to me because it suggests that whales and dolphins may serve as an “evolutionary mutant model” for a human disease. In traditional evolutionary studies of disease, scientists use chemical agents or ultraviolet radiation to produce random mutations in model organisms such as flies or mice in hopes of generating traits that mimic human diseases. An evolutionary mutant model capitalizes on natural differences found within wild populations to model disease outcomes in humans. The main benefit of this tactic is that both the disease and the natural variation are subject to similar evolutionary processes, while traditional models are artificially created in a laboratory setting. Other proposed evolutionary mutant models include blind cavefish populations as a model for human blindness and extreme facial variation in African cichlids (a diverse group of fish) as a model for human defects such as cleft lip and palate and micrognathia (small jaw). This particular study looked for changes in genes that were associated specifically with the transition from land mammal to cetacean.
Making Fat – Triacylglycerol synthesis
Obesity mainly occurs when more triacylglycerol (TAG) is produced than broken down in the body. TAG primarily acts as an energy storage molecule within adipose tissue, and some TAG storage is important for normal metabolic function. However, excessive TAG storage is associated with obesity in humans. TAG is also the main component of cetacean blubber. TAG is a complex molecule derived from multiple components, and its synthesis relies on many different gene products (Figure 3). The researchers targeted genes in the TAG pathway for the present study because it is implicated in both obesity and blubber formation/maintenance.
Figure 3: A schematic of some of the genes involved in the A. production and B. breakdown of TAG. This is definitely a complex pathway!
Finding the Genes that Code for Blubber
The researchers collected and sequenced DNA from five new species, taking tissue samples from dead individuals found in the wild. Gene sequences were also obtained for other cetaceans and land mammals using online gene databases. A total of 88 TAG related genes were sampled across more than twenty mammalian species (Figure 4). The researchers searched for signs of positive selection in those genes. Positive selection involves gene variants that are beneficial to the organism. Within a larger population, natural selection will rapidly act to increase the frequency of those variants until the population becomes “fixed” for them (all other variants become extinct or extremely rare). This process leaves distinct signatures in the genome that these researchers were able to look for – and find. The genes in this study that were positively selected in the cetacean lineage are therefore specifically the TAG-related genes that changed in a beneficial way during the transition from land to sea.
Figure 4: Cetacean species used in the analyses. Outgroups included representative species from rodent, carnivore, and primate lineages. Boxes encompass genes that showed signs of positive selection.
Strong signals of positive selection were found along cetacean lineages in many of the genes related to TAG synthesis (Figure 4). This study suggests genes that may underlie the evolutionary origins of blubber in marine mammals. Because obesity is closely related to blubber formation through the TAG pathway, these genes are also excellent candidates for future study in health and obesity related research. Who knew that whales and dolphins might help us understand and treat obesity?
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