Sea stars and mosaics

By Grant Jacobs 29/11/2012

At first you wonder if this sea star is real. It looks more like a kid’s geometric doodle from a distracted afternoon at school than an animal.[1]

Click on image for source; Creative Commons Attribution-Noncommercial-Share Alike 3.0 license

The World Asteriodea Database indicates Iconaster longimanus was first described in 1859. (The photo above was taken by Dieter in the Philippines in February 2007.)

(Source: Wikimeda, pubic domain.)

Among the later descriptions in the Asteriodea Database entries are ones from the famous HMS Challenger voyage.

To most today the name Challenger recalls the space shuttle that horrifically broke up little over a minute into it’s flight and brought a temporary halt to the shuttle program. The shuttle Challenger was named after the HMS Challenger, whose voyage was a grand British scientific survey of it’s day.[2]

Unlike the short life of the ill-fated space-farer, HMS Challenger’s voyage spanned over several years, 1872-1876 and travelled the world.

The Natural History Museum describes this voyage as a ‘journey into the unknown, just as the Apollo mission to the Moon was a new landmark for humans in the 20th century’ and a key founding step of ‘the new science of oceanography.’ Their excellent short documentary (9 min 48 seconds)[3] recounts the journey and it’s mission, including an extraordinary log written in two directions on the same page to make more use of the limited pages available. Over these years HMS Challenger scientists surveyed the ocean depths, sea floor and life, as well as documenting the lands they visited. They passed through New Zealand by way of the Cook Strait during June 28th – July7th 1874.

Photo credit: Dieter (Copyright).

I know very little about sea star biology, beyond that they are famous for being able to regenerate their limbs. Exploring how regeneration happens is interesting to biologists because it might tell us how some animals are able to recreate different kinds of cells, as we might through stem cell transplants.

Mosaics warning visitors of the dog (Cave Canem) were apparently popular features on the thresholds of Roman villas. (Source: Wikimedia Commons.)

My scientific interests include how our genomes work. When I looked at Dieter’s photograph of Iconaster longimanus the irregular pattern of dark and lighter segments (marginal plates) around the edge of the animal’s body drew my attention.

It made me think of mosaics, artistic and biological.

Mosaics, the art, feature small tiles of differently coloured glass, stone or other materials.

Tortoise shell cats are an example of mosaic X-chromosome inactivation (Image source: Wikimedia Commons.)

Biological mosaics are common, too.

Like their artistic namesake they use the variants of same basic material, in this case cells.

Possibly the best-known example of mosaicism is the tortoise shell cat. The ‘choice’ of expressing the black or orange alleles[4] for fur colour are randomly chosen over the cat’s body.[5]

If an animal that has grown from a single fertilised egg cell (zygote) has cells of the same type have slightly different genetics (genotypes) it’s a mosaic. These differences are not always visible, as they are in the tortoise shell cat.


Chimeras are fusions, where the different types of cells come from more than one fertilised egg cell. Cattle breeders, for example, will know that freemartins are female cattle from a fusion of male and female foetuses. Freemartins have been exposed to male hormones and as a result are masculinized. Geeps are another kind of chimera, this time across species – goats and sheep. Germline chimerism in pygmy marmosets can have the startling result that a male monkey’s uncle can be his father – and not through ‘monkey business’.

Research published this month suggest that our cells might commonly be mosaic.[6]

Sometimes are region of our genes are duplicated. When the number of copies vary it’s called copy number variation. One cell might have two copies, another three.

Scientists have been trying to reprogram cells able to become any kind of cell, to be pluripotent[7]. Sea stars are able to pull off this reprogramming trick themselves, in order to ‘re-grow’ the damaged part of their bodies. Reprogramming of cells to be able to become many kinds of cells is an area of research with a lot of promise for medicine. The Nobel Prize for Medicine or Physiology this year was awarded for key research enabling reprogramming of cells.

Researchers have noticed that some of these reprogrammed cells have copy number variations – different numbers of some regions of the genome.

Comparing the variations seen in the reprogrammed cells and the cells they were reprogrammed from, Vaccarino‘s team found that at least a half of the variations found in the reprogrammed cells where already present in a small number of the cells the stem cells were reprogrammed from. The variations in the reprogrammed cells were unlikely to be caused by the reprogramming process. This research used skin cells (fibroblasts) from the upper arm, but the results suggest that many of our other tissues might be mosaic too, a mixture of cells with different genetic variants.

Most genome sequencing uses blood cells. If we’re to use people’s genome sequences for thinking about what might be happening in, say, their brain cells, we might need to check what kind of copy number variations are typical of brain cells. Vaccarino’s group are now testing if this is happening.

Just like the HMS Challenger scientists roaming the earth, we’re exploring our genomes in an inner voyage that charts and catalogues our own seas. It’s an intriguing voyage.


Another account of Iconaster longimanus can be found on invertebrate zoologist/marine biologist ChrisM’s blog, Echinoblog.

There is, incidentally, a Mosaic Sea Star, Plectaster decanus, shown to right.

This post began life as a counter piece to Alison offering a stunning x-ray of a (stingless) stingray. Go check it out.

1. Did any reader not do this?

2. Apparently the voyage got off to a bad start with a sailor falling off the gangplank and drowning before the ship ever left port.

3. I can’t imbed it here so you’ll just have to make the trek to their website if you want to see it.

4. Alleles are different variations of the same gene. Lactose tolerance in adults is an example. As infants most of us get lactose is a sugar from our mother’s milk. Lactose is also found in milk from others species like cows and goats. Lactase (two ‘a’s) is an enzyme used to digest lactose. People with some variants of the MCM6 gene persist in producing lactase as an adult and hence are lactose tolerant. The older variants of the MCM6 gene cause lactase production to stop as a child – people with these alleles are lactose intolerant as adults. There are different alleles for lactose tolerance in different parts of the world.

5. This is from an earlier, slight more technical article, I wrote about how epigenetics controls the structure of our genome, the loops of inactive and active genes.

6. I’m leaving out sex chromosome mosaicism in the interests of brevity.

7. i.e. induced pluripotent stem cells, iPSCs.

Other articles on Code for life:

Deleting a gene can turn an ovary into a testis in adult mammals

Doggie ERVs

Genetic tests and personalised medicine

Monkey business, or is my uncle also my Dad?

One example of why all those genomes from different species are useful to biologists