Five basic types of taste exist: sweet, sour, salty, bitter, and umami. Most people are familiar with all of these except the last, umami, which is best described as a pleasant savory taste. These tastes occur because of receptors that occur on cells in the mouth. Genes dictate the presence and number of these cells and receptors and thus taste has an evolutionary basis. Umami and sweet tastes are associated with protein-rich and nutritious foods. Salt at low concentrations is an attractive taste, as salt in small amounts is needed for proper cell function including neurons. Bitter is associated with aversion and protects from ingesting toxic substances. Sour tastes are unpleasant and can prevent the ingestion of unripe and decayed food resources. Despite these adaptive benefits of taste some animals have lost the ability to taste one or more of the basic categories. Vampire bats cannot taste umami or sweet. Chickens cannot taste sweet. What about whales? They can only taste salty.

In a recent study, a group of scientists from Nanjing Normal University and Harvard Medical Center scanned the genomes for genes related to taste in 12 whale species. Only the genes for salty receptors were found. Why would whales loose four senses of taste? Both toothed and filtering whales swallow food whole without chewing. Furthermore, when living in the oceans everything would just taste salty. No joking, salt would swamp out all the other flavors. Given this the evolutionary pressure to be able to taste all these other flavors just would not be there. Also keep in mind its not just many of the genes for taste that have been lost. Thier tongues have degenerated epithelia and only a few taste buds

But why just keep salt receptors? Like all animals, whales need to osmoregulate, i.e. balance the water and ion concentrations in the body. This is vital for cellular processes like transporting across the cellular membrane. Salt receptors are vital in reabsorption of salt a key feature of osmoregulation. So if you ever have a whale over for dinner make sure to not over salt the fish, they may be little sensitive.

Related articles

• The loss of taste genes in cetaceans

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Yeah sometimes I’m lost too. In metagenomics, researchers collect ocean water or soil samples and sequence random bits of DNA from whatever blob of gunk they collect–they end up with little snippets from all the genomes of the species present in that sample. But understanding metagenomics is like understanding human genomics. It’s a lot more complex and nuanced than you think, even if the overarching concept seems easy to understand. Of COURSE we want to sequence the Human Genome, just like we’re obsessed with peering into genomes of the species that live in seawater (seawater is easy to collect and we know lots of things live there; its the low-hanging fruit of marine genomics). But scientists are often asking very specific questions under these broad banners–and the questions that fall under a category such as “metagenomics” might be surprising.

The Eisen lab is currently hosting Matthew Haggerty, a visiting grad student from Elizabeth Dinsdale’s lab at San Diego State University. Last week he presented his research at lab meeting, discussing different kinds of scientific stories he’s trying to find in marine metagenomic datasets.

Matt is working with public DNA datasets collected as part of the Global Ocean Survey (= scientists who sailed around the world in a yacht and collected buckets of seawater. How can I get this job?).

The Global Ocean Survey (GOS) data has been a keystone dataset for marine science. Collected way, way back in the dark ages of genomics (2007), people have continued to look deeper and re-analyze these datasets to get the low down on ocean biology. Matt is using GOS data to look at some super cool, but super sekrit questions…at least his questions are sekrit until he gets them published. Grad students gots to gets the manuscripts peer-reviewed!

But Matt did present some pretty awesome metagenomics factoids that made me take notes in the form of iPad doodles (thank you, Notability app!).

FACT 1: 

Coral reefs that are stressed or in decline show a particular increase in pathogenic bacteria and virulence genes (Ainsworth et al. 2010). Ainsworth et al. note that metagenomic studies have shown  “a dramatic difference in microbial metabolic function indicated that proximity to human populations and local disturbance significantly influenced microbial diversity and metabolism and as a result influenced coral reef health.”  The coral reef story is complex (involving interactions between corals, algae, bacteria, and viruses) and still being investigated, but metagenomics is now offering an unprecedented look into how coral reefs function and stay healthy (or become not so healthy).

FACT 2:

Metagenomics also lets us look at nutrient cycling in the oceans, and study how the limited availability of certain elements can control the metabolism of microbial communities. For example, Toulza et al. (2012) used that handy Global Ocean Survey data to look at genes for iron uptake in ocean microbes. Why? Because, they say, “despite the lack of knowledge of iron uptake mechanisms in most marine microorganisms, our approach provides insights into how the iron metabolic pathways of microbial communities may vary with seawater iron concentrations.” In other words, metagenomics allows us to understand how nutrients get cycled in the ocean, and how microbial cells react to different levels of nutrient availability–without ever having to know anything about any individual species.

FACT 3:

Yes, metagenomics can even involve the study of oral slime in fish–something that Matt is very interested in (although based on the number of underwater pictures, I think he may be using this question as an excuse to go diving in exotic places. Right, Matt? :P ). Parrotfish are famous for their teeth, but less famous for the green slime that grows on their teeth (see below photo). It’s pretty gross, so now you won’t forget it. I couldn’t find any peer-reviewed information about this disgusting green tooth scum, which means that the field of “parrotfish oral microbiome” needs some attention. How many species live in this slime? Does it affect parrotfish health? Do we see any cool adaptations in these cyanobacterial genomes? These questions are unanswered for now, but anyone has more info please let us know!

References:

Ainsworth TD, Thurber RV, Gates RD (2010) The future of coral reefs: a microbial perspective. Trends in Ecology & Evolution, 25(4):233–40.

Toulza E, Tagliabue A, Blain S, Piganeau G (2012) Analysis of the Global Ocean Sampling (GOS) Project for Trends in Iron Uptake by Surface Ocean Microbes. PLoS ONE, 7(2): e30931. doi:10.1371/journal.pone.0030931

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My opinions are similarly skewed across the Tree of Life. We all know I love nematodes. Protists are really weird, and therefore really cool. Algae can hold my interest. But I really can’t stand plants. I feel so guilty about this, because I’ve learned so many awesome things about plants over the years. Every time I learn a new fact, I reevaluate my reasons for shunning leafy species and commit to do some more reading. And then I get bored. I guess its kind of like reading unabridged Victor Hugo–you know you really should sit down and read classics like Les Miserables and The Hunchback of Notre Dame, but trying to persevere through Hugo’s hundred-page architectural descriptions is just so…tedious.

At my inaugural Eisen lab meeting in UC Davis last month, I had an epiphany: I FINALLY found a plant I liked

Seagrasses are unabashedly awesome.

These marine species are kind of like the nematodes of the plant kingdom: they might not be the most well known group, but they can be used as model organisms to investigate the Big Questions in biology and evolution.

Seagrasses are also the whales of the plant kingdom – extant aquatic species came from terrestrial ancestors that, over time, walked back into the sea (since before that, we think all life originally spawned from a primordial ocean). Like nematodes, it wasn’t just one hipster seagrass group that decided continental life was “too mainstream”. Re-entering marine habitats happened independently in at least three separate branches on the seagrass tree of life.

Moving from the land to the sea doesn’t just happen. If it did, we’d all be Aquaman. But nope, Aquaman’s abilities stem from the fact that he is some weird genetic mutant (Sidenote: The whole story of Aquaman presents some serious ethical concerns, since he appears to be the by-product human experimentation from his own FATHER: “.. By training and a hundred scientific secrets, I became what you see — a human being who lives and thrives under the water“. I don’t even want to know what kind of lab chemicals he was cajoled into ingesting).

In seagrasses, gene changes must also have occurred for these plans to live in marine environments. Think about it – even if you kept your head above water (breathing oxygen), you couldn’t live life with your whole body submerged in the ocean. Your skin cells are dependent on exposure to air, and I imagine that briny bath would get pretty painful after a while.

Water bends light. And seagrasses are one plant group that needs a lot of light. Species must deal with the lower intensity of underwater light, as well as the shift in proportions of different wavelengths that penetrate the ocean surface.

The sea is also salty. At the level of cells and tissues, a huge array of basic molecular processes are controlled by the flow of sodium and potassium ions across membranes. Certain ions like potassium are also a critical ingredient for enzymatic reactions governing protein synthesis and ribosome function. For non-adapted species, seawater can be a poisonous broth, fatally disrupting these basic cellular processes. Marine species must possess specific adaptations to grow and thrive amongst high environmental levels of salt.

Surprise #3, the ocean has waves. Flimsy grasses must be able to hold their ground. Tides and currents also impact reproduction (you don’t want all your gametes to float away) and photosynthesis (a reduced availability of carbon dioxoide).

So life in the sea required seagrasses to address some serious issues that would otherwise be very detrimental to essential biological processes. Remember that three different groups of seagrasses did this on three separate occasions – for us modern day scientists, this has provided a seriously elegant look into the exact genetic modifications that are critical for adapting to life in the ocean.

Strikingly, despite their independent evolutionary routes, seagrasses from the three different lineages have evolved many similar morphologies, life history strategies, and breeding systems [3,18]. This indicates that the aquatic habitat imposes novel selection forces that can lead to parallel evolution. [Wissler et al. 2011]

Which is pretty awesome.

In a recent study, Wisslet et. al (2011) wanted to take things to a whole other level and look for candidate genes that helped seagrasses survive life in a marine environment. They focused on identifying adaptive mutations in conserved (orthologous) protein-coding genes also present within the genomes of their terrestrial plant relatives. [Although it was not applied in this study, environmental adaptation can also be studied in the context of changes to gene expression, e.g. the same set of genes used in different ways across tissues and life stages]

Reverting to a marine life showed a noticeable impact on 51 seagrass genes. Note that these kind of comparative studies are restricted to genes that share a common ancestry; in this case, the seagrass study only looked at 189 gene clusters, equivalent to glimpsing at only ~1% of an entire genome.

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Now the results of this study aren’t particularly surprising–the pathways for marine adaptation are pretty logical–but this study perfectly showcases the POWER OF GENOMICS (please invoke Darth Vader voice there).

…photosynthesis, a few metabolic pathways, and ribosomes have strongly diverged after the split of the common ancestor of seagrasses from terrestrial monocots. Further studies will need to address the following questions: (1) How seagrasses have acquired osmoregulatory capacity to tolerate high salinities, (2) how CO2 is fixated, (3) how their photosynthetic apparatus has evolved for under water light harvesting, and (4) under what conditions anaerobiosis takes place. [Wissler et al. 2011]

The (comparatively) harsh marine environment, coupled with the action of natural selection, has caused seagrass genomes to diverge noticeably from their terrestrial ancestors. By promoting the survival of individuals that thrive in salt water, natural selection has, over time, tinkered with the cellular machinery for nutrient production and ribosome function. Additional tweaking has allowed seagrasses to deal with the different availability of gasses in the environment–for example, low oxygen availability in marine sediments (e.g. that disgusting pond muck smell if you start digging in an estuary) means that seagrasses can uniquely switch to fermentation instead of aerobic respiration if needed.

Seagrasses’ ancestors may have been a sputtering old tractor when they first tried to live in the sea, but modern-day species have evolved into a fine-tuned Lotus Exige. A beautiful, glorious machine that was contstructed on three separate occasions.

Reference: 
Wissler et al. (2011) Back to the sea twice: identifying candidate plant genes for molecular evolution to marine life. BMC Evolutionary Biology 11:8 http://www.biomedcentral.com/1471-2148/11/8

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