Rat Species Has No Y Chromosome!

Amami Spiny Rat

A team of researchers at Hokkaido University in Japan has figured out how a species of rat is able to survive and reproduce despite not having a Y chromosome. The rat species they studied was the Amami spiny rat (Tokudaia osimensis), a rodent native to the island of Amami Ōshima in southern Japan. The species, also called the Ryukyu spiny rat, is unique in that males do not have a Y chromosome and females only have one X chromosome as well. Previous attempts to study the rat have not been able to reveal how males are able to develop within the population with no Y chromosome. Typically in mammals, a gene on the Y chromosome called SRY tells the organism's body to express male genes like the SOX9 gene for testes. Not having the Y chromosome scientists were stumped at how males could develop. Lead researcher, Asato Kuroiwa, and her colleagues at Hokkaido have now discovered the answer! The secret was looking at the autosomes, non-sex chromosomes, in the rat. On chromosome 3 they found that one copy had a duplicated region next to the SOX9 gene. The duplication increases the activity of the SOX9 gene and it is able to code for testes. This duplicated region effectively replaces the SRY gene on the Y chromosome and explains how males of this species are formed. If a rat has the duplication region it acts as a proto-Y and the rat will be a male, and if the duplication region is absent, it acts as a proto-X and the rat will be a female. Further work to explore this is limited as the rat is an endangered species but the research so far has shown an amazing trait inherited by this rat population. The researchers predict the trait appeared about 2 million years ago when the Amami spiny rats diverged from their ancestors with a Y chromosome. Kuroiwa believes that a mixed population existed initially on the island but then a natural disaster like flooding or rising seas left mostly rats without the Y chromosome and over time the rats reproduced and evolved into a new species with this trait. 

Personally, this was my favorite article to read so far about genetics. I find it incredible that a species can exist without a Y chromosome and that another chromosome was able to replace its function to make males. This is a really interesting example of population evolution, genetic drift, and speciation. I hope that these rats can be studied more to learn about this unique and interesting trait. 

A New SRY Gene Fragment has been Discovered

    The SRY gene is a y linked sex determining gene. The SRY gene encodes for male genitalia. The SRY gene is located on one exon. The SRY gene is pleiotropic as it encodes for multiple traits related to male sexual development. If one were to have two x chromosomes but an SRY gene they would have male genitalia (testes). If one were to have the x and y chromosomal pair, but no SRY gene they would have female genitalia (ovaries). Also, if one were to have any sex linked chromosome combination with an SRY gene but the SRY gene is rendered inactive (due to a mutation) they would have female genitalia.

    The SRY gene was discovered in 1985 by studying the DNA sequences of a person with ovaries and a XY gene and 3 people with testis and XX genes. This supported a long held theory that having male genitalia was not a consequence of having a Y chromosome but having a specific gene on the chromosome. The development of human sex charactaristics and it's linkage to specific genes remains largely a mystery.

    Recently researchers in Japan have discovered another piece in mouse SRY. This second piece of mouse SRY, SRY-T, consists of two exons. This SRY-T also seals "instability sequences" on the end of SRY preventing it in degrading, meaning SRY-T is just as crucial for the development of testis as SRY. So the same differences in phenotype when displacing the SRY gene also occur when displacing the SRY-T gene, this was confirmed by researchers at the University of Queensland by using CRISPR. I think this new discovery of a male genitalia linked gene will give us more insight into the development of sex characteristics of both male, female and intergender people.

 Links:

https://www.sciencedirect.com/topics/neuroscience/sry-gene

http://www.embryology.ch/anglais/ugenital/molec02.html#:~:text=It%20was%20only%20thanks%20to,was%20finally%20identified%20in%201985.&text=The%20sequencing%20and%20cloning%20of,is%20responsible%20for%20determining%20gender.

https://theconversation.com/we-discovered-a-missing-gene-fragment-thats-shedding-new-light-on-how-males-develop-147348

Male Sensitivity Written in the Genes

In a study, it was found that the gene responsible for activating male development is surprisingly unstable, leaving inconsistencies in the pathway to male sexuality. Normally, in human development certain genes act as master switches that ensure we are born with similar attributes such as one hand, two lungs and 10 fingers. These genes tend to highly reliable and resistant against environmental factors. However, the SRY gene on the Y chromosome has the job to set off the growth of male sex organs in human embryos but if this gene is unstable and fails to fire, it can leave a genetically male embryo to develop as a sterile female. Researchers found that the SRY is highly vulnerable, allowing any interruption to alter male sexual development, which would ultimately lead to a variety of testosterone related male attributes.

I liked this article because I’ve heard of these cases in the past where the female inherits male attributes as an embryo. To realize that it’s the SRY gene that causes this defect is interesting especially because they say it’s very sensitive to environmental factors. It would be nice to see if they would research what exactly these factors are to help prevent this defect from reoccurring in the future.