Fish DNA in Ozark streams reveals region’s complex geologic history

Unexpected DNA reveals a split in a common fish

My students and I encountered a mystery. We were partnering with the Missouri Department of Conservation to monitor for fish presence and abundance in the Missouri Ozarks. We visited half a dozen rivers and streams across the region and collected a few cups of water from each, as described in an account of the research.

We were testing for environmental DNA to determine which species had left behind their genetic material and were therefore present in the stream, without the need to capture or even see the species themselves. The effort yielded successes: the researchers detected a variety of species in their appropriate habitats that are considered at risk in Missouri, including the lake sturgeon, crystal darter and Alabama shad. In the surface waters emerging from a large underground spring, they detected DNA from a cave-dwelling fish species that would never have been detected by researchers who looked only for fish in the surface waters.

But a problem emerged: the team was certain they would find one particular species in every stream—the bleeding shiner (Luxilus zonatus), one of the most common fish in Ozark streams. The failure of its DNA to appear in every sample spurred a check of sampling techniques, water chemistry, and other aspects of the testing.

A watertight clue

The data itself revealed a clue: streams in which bleeding shiner DNA was detected all drained to the southern Ozarks, while those without it were in the northern Ozarks. The researchers could draw a line through the north-south drainage divide between those areas, as reported in the study accounts.

Follow-up investigation led to a previously unknown fact: northern and southern Ozard bleeding shiners have different DNA histories, despite looking identical and belonging to the same scientific species. The northern populations carry a genetic signature of ancient hybridization, as their ancestors interbred with another shiner species hundreds of thousands of years ago.

The role of glaciers and rivers

According to the researchers, this genetic divergence is linked to the region's geological history. During the Pleistocene epoch, about 2.5 million to 12,000 years ago, glacial advances caused some Ozark rivers to repeatedly change course. Northern drainages in the Ozarks that flow into the Missouri and Mississippi rivers were directly influenced by these glacial advances, while southern drainages, such as the St. Francis, Black, White, and Spring rivers, were less affected.

The result is a richer aquatic ecosystem in the south, which contains more endemic species and genetically unique populations. Examples include the genetically distinct Black River walleye and numerous minnow, sunfish, and darter species.

The findings, described as an ancient artifact and a window into the evolutionary history of an entire watershed, highlight how geological processes can shape genetic diversity over evolutionary time. For the researchers, the investigation transformed an unexpected sampling result into a deeper understanding of the complex interplay between landscape and life in the Missouri Ozarks.