Powel et al. look at the genetic factors that lead to melanoma, a skin cancer in swordtail fish caused by incompatible alleles. It focuses on species of Xiphophorus (swordtail fish) that naturally hybridize within their own population, not represented by the parent population. The study shows that the hybrids have conflicting genetic traits (alleles) linked to an increased risk of developing melanoma and exhibit unique pigmentation patterns. The researchers had a goal of studying how these species breed and face reproductive barriers to gain insight on the individual genes that are responsible for the distance in the related species.
The research was completed using controlled breeding experiments and genomic analyses. The study found that the hybrids had incompatible alleles from both parent species. This causes disruptions in the cellular process related to the development of pigment cells, leading to the uncontrolled and dangerous overgrowth of pigment cells, a characteristic of melanoma. Degradation of the fin was found to play a role in lack of survival for the hybrids. The researchers used the Dobzhansky-Muller model as a comparing framework on why distant related species created hybrids with low viability. However, experimental work to support the model is slim.
Due to divergence of related model species, data to back the model has become unclear. Most of the mapped incompatibilities are now species that no longer hybridize. Powell et al. look at breeding of Xiphophorus birchmanni and Xiophophorus malinche. These two species naturally hybridize and have a subset of viable and fertile offspring. However, results found that in some populations, males were developing melanoma before sexual maturity, interfering with continuing the lineage.
Moreover, further research found the phenotype of a dark blotch on the caudal fin is derived from X. birchmanni. This spot is called spotted caudal and allowed Powell et al. to further genetic mapping from the macromelaocyte cells that made up the spotted caudal. Hybrid populations of X. birchmanni exhibited spotting at different and growing phenotypes than the stable phenotype of the parent. Observation in the lab showed the spots growing over a 6-month period in hybrids, which were linked to the overgrowth of the macromelaocyte.
The results highlight the necessity of understanding genetic compatibility and the effects of hybridization in marine biology, as the parent species X. birchmanni has never been recorded to have melanoma. Researchers noted ultraviolet radiation and natural carcinogens cannot be key contributors to the dramatic difference in melanoma rates between the parent and hybrid species.

    In conclusion, the study serves as a compelling example of how natural hybridization can unveil intricate genetic interactions that contribute to disease, reinforcing the importance of genetic diversity in comprehending health of related species breeding and divergence. The authors advocate for continued research to investigate the byproduct of their findings for conservation strategies and the management of hybrid populations. 

Reference
Daniel L. Powell et al., Natural hybridization reveals incompatible alleles that cause melanoma in swordtail fish. Science 368, 731-736. 10.1126/science.aba5216 (2020).