incompatible alles of swordtail fish species leave hybrids with Melanoma

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Researchers finally discover the specific genes of different swordtail fish species that interact to produce melanoma in their offspring and identify the loci of these genes in their genomes.

Although the dark spots on the fins of male swordtail fish are successful at attracting mates, they come at a deadly cost, cancer. In fact, these caudal spots are not sexually selected traits, but malignant melanoma. They are the result of two distantly related species of swordtail fish, such as Xiphophorus birchmanni and Xiphophorus malinche, reproducing and incompatible genes from each interacting. Hybrids develop melanoma have decreased relative fitness in the wild. While such reproductive barriers that drive evolution and biological diversity are well studied in the field of genomics, very little is known about the specific gene interactions that create these hybrid incompatibilities. For vertebrates, however, two genes have been identified to play a role. The first, xmrk, is a melanoma receptor tyrosine-protein kinase, and the other, prdm9, regulates mammalian recombination (Mijola et al. 2009). The significance of the xmrk gene in swordtail fish hybrids was disputed until Powell and his colleagues identified it and cd97, as the causes of this hybridization-derived melanoma. Beyond identifying the specific genes, these researchers also defined X. birchmanni as responsible for the spotting phenotype while identifying chromosome 21 as the locus for xmrk, and defined X. malinche as responsible for melanocyte invasion from identifying chromosome 5 to contain prdm9.

After proving melanoma is a hybrid incompatibility due to the presence of melanoma in hybrids, but not the parental species, the researchers performed a genome-wide association study on 392 adult male X. birchmanni fish scanning for allele frequency differences between spotted cases (n= 159) and unspotted controls (n=233). Chromosome 21 was identified as having a strong association between the spotting pattern and allele frequency. Further analysis of this chromosome revealed it as the locus for genes xmrk and myrip.

Next, the researchers performed admixture mapping for spot presence of hybrids and compared the results to the previous findings of X. birchmanni. They found that chromosome 21 was significant for hybrids as well confirming that the genetic basis is the same in X. birchmanni and hybrids. 

Admixture mapping for melanocyte invasion of the body also revealed chromosome 5 to be significant in hybrids and to be associated with X. malinche ancestry. 

Using RNA sequencing, they discovered that only xmrk, not myrip, from chromosome 21 interacts with X. malinche ancestry on chromosome 5 to produce melanoma because it is expressed in caudal tissue and has higher expression in spotted than unspotted tissues. Myrip was not expressed in caudal tissue or in the melanoma itself. Next, they identified two amino acid substitutions in the extracellular domain of xmrk in X. birchmanni that are known to catalyze the oncogenic properties of xmrk in vitro.

Further research into chromosome 5 revealed that only two genes, cd97 and a fatty acid transporter gene, were in their region of interest. Researchers again used RNA sequencing to exclude the transporter gene from playing a role in melanoma development, and credit only cd97 from X. malinche as it is highly expressed in melanotic tissue of hybrids. 

Using a targeted quantitative polymerase chain reaction approach, they compared the differences of cd97 expression between the two species. The found that cd97 was expressed at low levels in fin tissue of X. birchmanni, regardless of spotting phenotype, but at high levels in X. malinche. 

Lastly, the researchers revealed the results of years of observing shifts in the frequency of the spotted caudal trait between juvenile and adult males. In hybrid populations with high incidences of melanoma, juvenile males had a significantly higher frequency of spotting than adult males, but that was not the case for populations with a low incidence of melanoma or X. birchmanni parental populations. They were surprised to see that spotting remains because of the reduced fitness it presents: slower escape responses when startled and greater visibility to predators. It should be noted that their inferences of fitness are not directly measured and future studies may wish to do so and include other reasons that spotting may remain in natural populations.

Although this study was the first to reveal the specific gene interactions between the X. birchmanni and X. malinche genomes that cause melanoma hybrid incompatibility, there is still much to uncover such as which genetic interactions are particularly prone to breakdown in hybrids according to the researchers. Further research that also maps specific gene interactions will provide more insight into the evolution of other hybrid incompatibilities. 

Sommer Holmes is a Biology major at Davidson College in Davidson, NC. Contact her at soholmes@davidson.edu

References

Mihola, O., Trachtulec, Z., Vlcek, C., Schimenti, J. C., Forejt, J. 2009 A mouse speciation gene encodes a meiotic histone H3 methyltransferase. Science. 323(5912):373-5. 

Powell, D. L., García-Olazábal, M., Keegan, M., Reilly, P., Du, K., Díaz-Loyo, A. P., Banerjee, S., Blakkan, D., Reich, D., Andolfatto, P., Rosenthal, G. G., Schartl, M., Schumer, M. 2020 Natural hybridization reveals incompatible alleles that cause melanoma in swordtail fish. Science. 368(6492):731-736.

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