This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Vividly colored fish dwelling at the bottom of shallow Midwestern streams may help biologists understand the evolutionary mechanisms by which species remain distinct despite their similarities and overlapping ranges. Orangethroat and rainbow darters regularly interbreed where their ranges overlap, yet—millions of years after they first diverged—they remain separate species.
"A big focus of our lab is speciation: the fundamental evolutionary process by which one lineage splits into two distinct species," said Rachel Moran, Ph.D., assistant professor of ecology and evolution at the University of Chicago. "Observing these species that hybridize in the wild was puzzling. How can you have two different species that persistently coexist if they're interbreeding?" A new study published in Nature Communications helps explain that puzzle.
By sequencing whole genomes from multiple darter species, UChicago researchers found that closely related darters use entirely different chromosomes to determine whether offspring develop as male or female. These rapid "sex chromosome turnovers," combined with subtle conflicts between the genes housed in the cell nucleus and those in the mitochondria, appear to create a genetic barrier that keeps hybridizing species from combining into one. The findings offer a glimpse into a mechanism of speciation that evolutionary biologists have long speculated about but rarely documented.
"Speciation is very important for generating biological diversity," Moran said. "Understanding its mechanisms also helps us infer how we can maintain that diversity, which is important in a changing world." The project grew out of an earlier observation in Moran's lab: when orangethroat (Etheostoma spectabile) and rainbow (E. caeruleum) darters were crossed in the lab, their second-generation hybrid offspring were overwhelmingly male. Purebred crosses, by contrast, produced the expected roughly 50-50 sex ratio.
Crucially, hybrid clutches were not smaller. The "missing" females weren't dying as embryos. "It seemed like genetically female individuals were phenotypically male in these hybrid crosses," said Moran, the study's senior author.
"So, we thought there must be something happening with their sex-determining regions." To find out, first author Wynne Radcliffe, a graduate student in Moran's lab, ran genome-wide association studies comparing males and females of multiple darter species. "Fish are weird," Radcliffe explained. "At one point on the ancient tree of life, there was a second whole-genome duplication in bony fishes, so their genomes are already very crazy and full of repeats.
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