Isopod Genetics - Colours And Morphs

Genetics is a wonderfully complex mess, and we are discovering things every year. Millions of years of tiny changes at a level much smaller than the cell mean that things are linked, and separate, one change here impacts something all the way over there, two things that you think should be together have nothing to do with each other.

 

DNA

Deoxyribonucleic Acid, sounds complex, and in terms of what it can do it is. But in its structure, it is just two long chains of simple molecules. Twisted around each other and coiled up in discrete units we call chromosomes. Think of all that your computer can do and that’s just 0 or 1, 2 choices, in DNA there are 4 choices, and that means far more storage than a computer.

 

A section of this DNA contains plans that the cells use to build stuff, from hormones to new cells. As the developing embryo in the egg develops the cells use these plans to build the entire animal. An allele, this section of information, contains a single bit of discrete information in terms of this plan.

 

Starting Easy, Inheritance

Inside your body are 26 pairs of chromosomes. You inherited 26 from your mum and 26 from your dad. Now they weren't a straightforward mix of their parents. Each chromosome they handed down was mostly, but not entirely, from one of the other of their parents. That means in theory you could have inherited all the chromosomes from just two grandparents. In theory, in reality, the chance of that is 1 in 1.6 hundred thousand octillion, that’s 16 followed by 52 zeros, it’s a very large number. You have a better chance of winning the UK Lottery main prize 6 times one after the other.

 

So you have a mix of chromosomes from your parents and they in turn from their parents. The vast majority of it is the things that make you human, like having cells, tissues, blood, bones, organs etc. Limbs rather than fins, an upright spine and a pelvis that we inherited from tree-climbing primates. A really small amount is what makes you different from other humans.

 

Take eye colour, you probably learned in school that brown was dominant and blue recessive, that two blue-eyed parents would have a blue-eyed child, but there was a 1 in 4 chance that two brown-eyed parents would have a blue-eyed child. That is because if you inherit one blue and one brown gene, the brown gene will be expressed, and you will get brown eyes. In this case brown is dominant, and blue recessive.

 

Now we know that there are 120 sections of DNA that can influence eye colour, but in most cases the simple model holds true.

 

There is a third classification that it's worth knowing about, co-dominant genes. In effect, these share the job.

 

Finally, you can have a modifier. That means that as well as the gene that controls that feature, you need at least one more gene that modifies the output from the section of code.

 

• Dominant gene: If there is one dominant and one recessive gene then the dominant will be expressed. You will see that trait.
• Recessive. If there is a dominant and a recessive gene you will not see the recessive trait. Both genes need to be recessive for the trait to be expressed.
• Co-dominant. Two co-dominant genes mean that the trait will be a mix of the two.
• Modifier: Something that modifies a separate gene.

 

 How are Colours Expressed in Isopods?

As you can see from above it's not straightforward. Some wild populations seem to remain with their original wild-type colouration in captivity. But others have various different morphs.

The main issue is that not all isopods have the same genes or mechanisms for their colouration.

 

Natural Variation

Within some populations of isopods, there is naturally a variation in their colours, some may be darker than the others, or have wider stripes. If you are trying to breed for something within the natural variation then you can just select the parents that are the closest to your ideal colour. The brightest yellows, or the widest stripes for example. This is likely to be co-dominant genes, but again different species have different genes.

 

Albinos

Pure white or even pink isopods are lacking in colour, this is well documented and studied in lab populations of isopods. There is some variation in what counts as being albino. Some studies refer to albinos as being white with dark eyes and on occasion dark tips to various limbs or other parts of the isopods.

 

Hemilepistus elongatus the Turkish Desert Woodlouse is known to have two types of albinism. Both types are recessive. So if a white parent and a dark parent are put together then all the offspring will be dark. If two of those offspring are crossed then about a quarter will be white. If two white adults are mated then all the offspring are white.

 

Remember punnet squares at school. That's what's going on here. In the parent generation, one adult has two white genes, and one has two dark genes, all the young have one of each. When they mate then their offspring have a 1 in four chance of having two white genes, one from each parent.

 

But here's the really interesting part. There are two types of albinism. If you take one of each type, and let them mate, all the offspring will be dark-coloured.

 

What does this mean for your breeding colony?

 

You get a group of all-white isopods from one source, then another from a different source. Add them to the same enclosure, and suddenly start seeing wild-type colouration. It's probable that the two groups have different types of albinism. Ideally, you want to try and keep the groups separate.

 

What About Other Colours?

 

There have been some studies on Porcellio scaber, they come in a variety of colours, including Moo Cow, and Lava, these are different to the grey-black colour of their wild counterparts.

 

Studies identified that the carapace can be white, dark grey/brown, light orange and variegated, which is orange with areas of pigmentation. The eye colour can be colourless, red, or black.

 

Two places, or loci, in the DNA, were found to control these colours. One has two alleles, and one has three alleles. There are a lot of interactions between these different genes.

 

If you have one of these isopods with a grey carapace and black eyes, and cross that with one with an orange carapace and red eyes, all the offspring will be grey carapace and black eyes. This shows that the grey carapace and black eyes are dominant. But if you cross those offspring then a quarter will be orange with red eyes. They have the recessive gene on both alleles.

 

For these albino is also recessive to grey, but if you cross the offspring of that match you may get some mottled offspring. This mottled gene is passed through the male line. The variegated gene seems to show up in females over males. Although it also may be passed down the male line.

 

The combination of these genes and the variety of colours that can be made means there is still a chance that there are more colours waiting to be discovered in the populations.

 

How Do These Colours Occur?

 

Morphs tend to pop up in inbred populations. One adult has an alteration in their DNA, then passes that on to their offspring, they then mate and that second-generation expresses the recessive genes.

 

When the cells are being copied the DNA is also copied. But this is done by simple proteins, and that means that on occasion mistakes happen. Sometimes somewhere serious and the cells are too damaged to be compatible with life. Sometimes in the so-called junk genes, genes that aren't in use any more, but are still in the chain of code. Occasionally they hit that perfect spot, important enough to create a change, but not important enough to alter things too much. They might turn off a protein that alters how a colour is expressed, or add something to change the shade.

 

In the wild these more noticeable colours mean that predators catch them easier, meaning these genes are not passed down. In caves however often the albino gene will dominate, as there is a cost in terms of energy for generating colour pigments, so an albino creature will have a tiny evolutionary advantage. Since albino is normally recessive a white individual will have both genes, meaning once the population is white, it will stay that way until a mutation brings back the wild type, or creates a new colour.

 

 Lots of Species, Lots of Alleles

 

There are too many species to go into in one article, but there are some things you can look for.

 

A genotype is all the alleles that the isopod carries, a phenotype is the ones that are expressed, the ones that you can see.

 

If you have two parents of the same colour, and all the offspring are the same colour then it’s likely both have that genotype, although one of the parents might carry a recessive allele.

 

If the parents are different colours and the offspring are all one colour, then you know which is dominant.

 

If both parents are the same colour and about a quarter of the offspring are a different colour, then that colour is the recessive one, and half of all the offspring are carrying that recessive colour.

 

Easy Way to Get a Morph

 

Buy them. (www.postpods.co.uk) We have sourced many of the morphs known in the hobby and more are becoming available all the time.

 

There are no secrets to breeding new morphs. You need a population large enough to increase the chances of that random mutation, and small enough to be inbred enough that the mutation will be carried by several members. You can increase the chances by having a number of groups. But there isn’t a magic number.

 

You can take a shortcut and buy morphs that are already on the market. Many of the morphs we know are a combination of one or more alleles, so starting with one of those alleles already mutated means that you have a higher chance of something occurring.

 

Research is the key thing here, or just waiting for that chance occurrence. There are some journal articles below, so if research is your thing start there.

 

There is a certain amount of passion for the hobby that you need, this can take years, maybe even decades, and even then you may never get the morph you are after. But you can have fun getting there.

 

 

Further Reading

Achouri, Mohamed S., and Faouzia Charfi-Cheikhrouha. “Albinism and Polychromatism in Some Species of Woodlice from Mediterranean Regions (Isopoda, Oniscidea).” Crustaceana 82, no. 8 (2009): 1045–56. http://www.jstor.org/stable/27743360.

 

Bhella, Sita, Eleanor Fung, Janet Harrison, Bryant Ing, Ellen W. Larsen, and R. Duncan Selby. “Genetics of Pigmentation in Porcellio Scaber Latreille, 1804 (Isopoda, Oniscidea).” Crustaceana 79, no. 8 (2006): 897–912. http://www.jstor.org/stable/20107717.

 

Johnson, Clifford. (2011). Color-morph genetics in the terrestrial isopods Armadillidium nasatum and Porcellionides virgatus. Canadian Journal of Genetics and Cytology. 26. 167-173. 10.1139/g84-028.

 

Röder, G., and K. E. Linsenmair. “On Heredity of Two Forms of Albinism and on the Fitness of Albinos in the Turkish Desert Woodlouse Hemilepistus Elongatus Budde-Lund, 1885 (Isopoda, Oniscidea).” Crustaceana 71, no. 1 (1998): 57–72. http://www.jstor.org/stable/20105956.