7/ We use ancestry information from mothers & embryos to show that assortative mating by ancestry in a downstream Calnali population is weakened, but still significant.

Through histology, we also see signs of olfactory irritation (cilia loss, goblet cell hyperplasia) in the downstream population.

6/ Repeated water chemistry measurements show differences between rivers, but also a unique signature in the downstream Rio Calnali.

Many chemicals elevated there are known disruptors of olfaction and behavior in fishes!

5/ GIS analysis confirmed that this section of the Rio Calnali has the highest anthropogenic impact (measured as proportion of land area with built infrastructure) of any river reach in our study area.

The Calnali stands out even more in terms of infrastructure built close to the water.

4/ Hybrid population structure varies drastically across rivers.

The coolest case is in the Calnali River, where a structured hybrid zone (two ancestry clusters, Xbir-like and Xmal-like) collapses into an unstructured one (one admixed cluster) as the river passes through the town of Calnali.

3/ Our hypothesis was that human impacts drove hybridization between the swordtails X. birchmanni and X. malinche — specifically, that water pollution might disrupt their preference for the mating pheromones of their own species. We tested this idea across four rivers near Calnali, Hidalgo, Mexico.