Oniscidea are hyperdiverse when it comes to their color genetics; however, the subject is understudied in academia and is mainly restricted to the isopod hobby. People in the isopod hobby usually do not understand the genetics behind their variations and morphs and many misconceptions have been spread. Here we aim to correct many of these misconceptions and describe the mechanisms behind these variations and morphs using Mendelian inheritance models to aid hobbyists in making calculated crosses and to aid in the identification of species, as color is an extremely valuable diagnostic feature for image-based identifications. Important terms are in the glossary. The information here has been gathered by Nathan Jones and Oonagh Degenhardt.

 The science behind isopod color morphs can be a fascinating subject to explore. Isopod coloration is influenced by a combination of genetics and environmental factors, and understanding these factors can help us better understand the biology and evolution of these animals.  One key factor influencing isopod color morphs is genetics. Like all animals, isopods have a set of genes that determine their physical traits, including their coloration. Different isopod species have different sets of genes, and within each species, there may be genetic variation that leads to different color morphs. For example, some isopod species may have genes that code for different shades of brown, while others may have genes that code for patterns of black and white.  In addition to genetics, environmental factors can also influence isopod coloration. For example, some isopod species are able to change their color in response to changes in their environment, such as changes in temperature, humidity, or the availability of food. These changes may be driven by hormones or other signaling pathways, and they can help isopods better blend in with their surroundings or communicate with other members of their species.  Overall, the science behind isopod color morphs is an interesting and complex topic that can provide insight into the biology and evolution of these fascinating animals.

Glossary 

Chromosome: A contiguous block of DNA that is passed from parent to child after recombination.

Gene: A section of DNA on a chromosome that codes for a protein. Genes are the basic unit of heredity passed from parent to child.

Allele:  An allele is a particular version of a gene sequence which controls differences in phenotype (eg what color an animal will be). A gene can have one or more different alleles that control one or more different traits.

 
Locus: A term used to describe where on a chromosome a gene or allele resides.

Genotype:  The  DNA sequence of alleles that an individual carries.

Phenotype: The visual appearance of an individual. It is the instantiation of the genotype the individual carries.

Co-dominant: refers to a type of inheritance in which two versions (alleles) of the same gene are expressed equally which results in a display of traits from both alleles.

Partial/Incomplete Dominance: Refers to when a dominant allele does not completely mask a recessive allele which results in a blend of the two.

 
Dominant:  Refers to an allele of a gene which fully masks the phenotype of another allele that is recessive to it.

Recessive: Refers to a gene that needs two alleles to express as opposed to one with dominance.

Homozygous: An individual with matching alleles from both parents at a locus.

Heterozygous: An individual with miss-matched alleles from each parent at a locus.

Inheritance: The alleles passed from parent to child.

Cultivar: A selected-for trait that didn't originally come from the wild in its current form.

Morph: A color/phenotypic trait that is shared amongst an entire subgroup of a species or population.

Variation: Individual differences (mutations) in genes that result in phenotypic differences. Variations are not fixed in the entire population or a subgroup. 

Variable trait/Expressivity: A trait that shows high amounts of variation while still remaining genetically the same.

Penetrance: The chance of a phenotype being expressed when an individual carries a particular genotype. Individuals who carry a mutation with incomplete penetrance may appear as wild type even with a dominant allele.

Strain:   A genetic variant of a line with the same mutation (e.g. P. scaber 'Moo Cow' and 'Dalmation' are strains of the same mutation).

Line: A group of animals homozygous for an allele that belong to a lineage, obtained by inbreeding. A line-bred population will have similar genetic traits achieved through the selection of desired phenotypic traits.

Wild Type: A term used to describe the state of a gene in its most common form in that species.

Breed true: Two parents with matching phenotypes produce offspring, all of which have the same phenotype.

Albinism: A trait in which individuals lack or have significantly reduced production of melanin. Resulting in a yellowish/white individual with typically red eyes.

Leucism: A trait in which individuals lack or have significantly reduced production of all pigment cells, unlike albinism which only affects melanin. Resulting in a pure white coloration. 

Piebaldism: Having only two dominant colors in patches. It's form of leucism in which some portions of the animal still produce pigment cells. Piebaldism is often called partial albinism but this isn't correct as albinism prevents all melanin production meaning partial albinism can not exist.

Xanthism: Lack of melanin and pheomelanin resulting in an all/mainly yellow individual. Usually seen in vertebrates.

Axanthism: Lack of melanin and xanthophores resulting in a pink, red, dark red, or blue individual. most commonly seen in frogs which appear blue.

Melanistic: An individual with an excess amount of melanin which usually results in an all-black or much darker individual. The most common example of this is the "black panther" which is just a highly melanistic jaguar or leopard.

Amelanistic: A lack of melanin resulting in an all-yellow individual. In herpetoculture it is described to retain all pigmentation in the eyes but lack melanin in the rest of the body, unlike albinism which reduces melanin throughout the entire body including the eyes but in reality, it is the same mutation as albinism.

 
Pheomenalistic: An individual with excess amounts of pheomelanin resulting in a reddish brown or red individual while still retaining some melanin.

Calico: In isopods is a sex-linked trait expressed by females which typically results in mottled coloration. 

Sex-linked: Refers to characteristics (or traits) that are influenced by genes carried on the sex chromosomes.

Sex-limited: Genes that are present in both sexes but are expressed in only one sex and have no penetrance or are simply 'turned off' in the other.

Sex-influenced: Traits that are influenced by sex hormones usually developing as sexual maturity is reached.

Porcellio scaber as a Model Organism in the Study of Color Genetics

By Nathan Jones

Porcellio scaber is a widely distributed and well-studied isopod species that can be used as a model organism in color genetic studies. One reason for this is its ability to exhibit a range of colors and color patterns, including different shades of brown, tan, and grey, as well as more complex patterns such as mottling and banding. P. scaber is relatively easy to maintain in laboratory settings and has a short generation time, making it practical for genetic studies. Additionally, P. scaber has a well-known and stable genome, allowing for the identification and study of specific genes involved in color variation. This stable genome also makes P. scaber a valuable species for studying the mechanisms of genetic inheritance and expression. Furthermore, the availability of extensive genetic resources for P. scaber, including genomic sequences and genetic maps, allows for more detailed and precise genetic analyses. These factors make P. scaber a valuable model organism for color genetic studies, enabling researchers to better understand the genetic basis of color variation in isopods.

In addition to its use in genetic studies, P. scaber is popular among hobbyists who keep them as pets or clean-up crews. The availability of multiple color morphs allows hobbyists to select and breed isopods with specific color patterns and traits. This diversity in coloration has also led to the development of several distinct morphs within the species, each with its own unique characteristics. Some of the most common and popular morphs include the orange, black, and albino morphs.  In contrast, other isopod species such as Porcellio laevis may not be as valuable as P. scaber for color genetic studies due to a lack of color variation. P. laevis typically only exhibits one color, which is typically dark grey or black. This limited color variation makes it difficult to study the genetic basis of color variation, as there is a lack of variation to compare and analyze. Additionally, P. laevis does not have an as well-studied genome as P. scaber, making it more difficult to identify and study specific genes involved in color variation. Overall, while P. laevis may be a valuable species in its own right, but its limited color variation and lack of genetic resources make it less useful than P. scaber for color genetic studies. 

Examining the genetic basis of color variation in P. scaber could provide insights into the evolution of coloration in other crustaceans or even other animals. Additionally, comparing the genetic basis of color variation in P. scaber with that of other species could help identify common mechanisms or pathways involved in color evolution. This could also provide insight into the adaptive significance of color variation, such as its role in species recognition, mate selection, or predator avoidance. Furthermore, studying the genetic basis of color variation in P. scaber could also have practical applications, such as aiding in the identification of genes involved in color-related diseases or disorders.

Despite its potential value as a model organism, P. scaber has been relatively understudied in the field of color genetics. While a wealth of information is available on its ecology, behavior, and distribution, there has been relatively little research on its genetic basis for color variation. This may be due to a lack of funding or resources for such studies, or perhaps a lack of awareness of its potential value as a model organism. However, given the potential value of P. scaber as a model organism, it is essential to encourage further research on its color genetics. There are several ways in which this could be done, such as providing funding for research on P. scaber, promoting the value of P. scaber as a model organism, and increasing awareness of its potential among researchers and the general public. 

The Role of Wolbachia Infection in Inducing Genetic Hitchhiking and Influencing Phenotype Diversity in Porcellio scaber

By Nathan Jones

Wolbachia is a type of bacterium that can infect a wide range of insects and other arthropods. One interesting aspect of Wolbachia infection is its ability to induce genetic hitchhiking, which is the spread of genetic changes associated with the bacteria's presence (Morrow & Riegler, 2021). This process can result in the evolution of new traits in infected populations, including changes in the expression of phenotypes. Porcellio scaber is a species of terrestrial isopod that is found in gardens, fields, and forests around the world. Like many other arthropods, P. scaber can be infected by Wolbachia, and this infection has been shown to have significant impacts on the host's genetics and biology (Zhao et al., 2013). 

Wolbachia-induced genetic hitchhiking may increase the diversity of phenotypes expressed by P. scaber populations in several ways. For example, Wolbachia infection may alter the expression of host genes and introduce new genetic material into the host genome. If these changes are beneficial to the host, they may become more common in the population through natural selection. Additionally, Wolbachia may modify gene regulation in the host, leading to the suppression or enhancement of certain traits and the emergence of new phenotypes. It is important to note that the effects of Wolbachia infection on host phenotype are likely to be complex and depend on various factors, such as the host's specific genetic background, the strain of Wolbachia, and the environmental conditions. Further research is needed to fully understand how Wolbachia-induced genetic hitchhiking may influence the diversity of phenotypes in P. scaber and other host populations. 

One way in which Wolbachia-induced genetic hitchhiking may increase the complexity and diversity of color genetics in P. scaber is through the spread of new alleles that are associated with coloration. Multiple genes often control coloration in arthropods, and various factors, including the presence of Wolbachia can influence the expression of these genes. For example, Wolbachia infection has been shown to alter the expression of pigment genes in insects, leading to changes in the coloration of the host. In addition to the spread of new alleles, Wolbachia may also modify gene regulation in P. scaber, leading to the suppression or enhancement of certain coloration traits. For example, Wolbachia may alter the expression of pigment genes through mechanisms such as gene silencing and RNA interference, resulting in the suppression or enhancement of certain colors. This could lead to the emergence of new color patterns in the host population. It is worth noting that the impact of Wolbachia on color genetics is likely to be complex and depends on a range of factors, including the specific strain of Wolbachia, the genetic background of the host, and the environmental conditions. Further research is needed to fully understand how Wolbachia-induced genetic hitchhiking may influence the complexity and diversity of color genetics in P. scaber and other host populations. 

Wolbachia infection has the potential to significantly impact the genetics and biology of its host, Porcellio scaber. Through the process of genetic hitchhiking, Wolbachia can alter the expression of host genes and introduce new genetic material, leading to the evolution of new traits and the emergence of new phenotypes. One way in which this process may increase the diversity of phenotypes expressed by P. scaber populations is through the spread of new alleles that are associated with coloration. Additionally, Wolbachia may modify gene regulation in the host, leading to the suppression or enhancement of certain coloration traits and the emergence of new color patterns. The impact of Wolbachia on color genetics is likely complex and depends on various factors, including the specific strain of Wolbachia, the host's genetic background, and environmental conditions. Further research is necessary to fully understand the mechanisms and extent of Wolbachia-induced genetic hitchhiking in P. scaber and other host populations.

References: 

Allam, M., Fangary, H., & Marie, Z. (2021). PHYLOGEOGRAPHIC AND GENETIC DIVERSITY OF PORCELLIONIDES PRUINOSUS AND PORCELLIO LAEVIS BY USING THE MITOCHONDRIAL CYTOCHROME C OXIDASE SUBUNIT 1 SEQUENCE. Egyptian Journal of Zoology, 0(0). https://doi.org/10.21608/ejz.2021.73095.1054

Bi, J., & Wang, Y. (2019). The effect of the endosymbiont Wolbachia on the behavior of insect hosts. Insect Science, 27(5), 846–858. https://doi.org/10.1111/1744-7917.12731

Deng, J., Assandri, G., Chauhan, P., Futahashi, R., Galimberti, A., Hansson, B., Lancaster, L. T., Takahashi, Y., Svensson, E. I., & Duplouy, A. (2021). Wolbachia-driven selective sweep in a range expanding insect species. BMC Ecology and Evolution, 21(1). https://doi.org/10.1186/s12862-021-01906-6

Jones, N. (2022). Culturing Ligidium elrodii: Hydric Soil Method (Isopoda: Oniscoidea: Ligiidae). AIMG Study Notes, 1, 1–9.

Morrow, J. L., & Riegler, M. (2021). Genome analyses of four Wolbachia strains and associated mitochondria of Rhagoletis cerasi expose cumulative modularity of cytoplasmic incompatibility factors and cytoplasmic hitchhiking across host populations. BMC Genomics, 22(1). https://doi.org/10.1186/s12864-021-07906-6

Zhao, D.-X., Zhang, X.-F., Chen, D.-S., Zhang, Y.-K., & Hong, X.-Y. (2013). Wolbachia-Host Interactions: Host Mating Patterns Affect Wolbachia Density Dynamics. PLoS ONE, 8(6), e66373. https://doi.org/10.1371/journal.pone.0066373