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Ben Moran @ben-moran.bsky.social · 25/08/2026

Hybrid zones are increasingly common; to what extent are human impacts on the environment the cause? We used pop gen, water chemistry, GIS, and ecotoxicology to explore the causes of hybridization in swordtails. With @mollyschumer.bsky.social, now out in Current Biology! www.cell.com/current-biol...

cell.com

Increased rates of hybridization in swordtails are associated with water pollution

Anthropogenic disturbance can drive hybridization by altering pre-mating reproductive barriers. Moran et al. pair population genomic surveys with water chemistry and GIS data to show that human develo...

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Ben Moran @ben-moran.bsky.social · 01/05/2025

9/ Many thanks to the collaborators without whom this work wouldn’t have been possible, especially to Chelsea Rochman's lab, @gilrosenthal.bsky.social, and the CICHAZ field station in Calnali, HGO, Mexico, which has supported 20+ years of genetic and environmental measurements.

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Ben Moran @ben-moran.bsky.social · 01/05/2025

8/ Take-home: our results are consistent with human impacts on water chemistry disrupting mate choice in swordtails, weakening reproductive isolation between them. It was encouraging to identify populations where isolation between these species is still relatively strong!

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Ben Moran @ben-moran.bsky.social · 01/05/2025

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.

Results of olfactory histology of fishes from sites upstream (Aguazarca) and downstream (Calnali Low) of the town of Calnali. C) Proportion of each fish’s sensory epithelium with visible cilia, visualized with H&E staining. D) Number of mucus-producing goblet cells per µm of olfactory rosette surface area, visualized by staining with AB/PAS. P-values indicate results of Mann-Whitney U Test for difference between sites. E-F) Representative AB/PAS stains of olfactory rosettes of X. malinche × birchmanni hybrids from E) Aguazarca, and F) Calnali Low. Mucoproteins are stained blue by AB/PAS, eosin counterstain appears as pink. Arrowheads indicate locations of goblet cells.

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Ben Moran @ben-moran.bsky.social · 01/05/2025

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!

Plots of the change in water chemistry over the course of the Río Atlapexco drainage rivers, focusing on parameters that were elevated in sites downstream of the town of Calnali. These include (left to right) fluorescent dissolved organic matter, ammonia, and total copper. The building icon on the X-axis denotes the sites located within the town of Calnali on the Río Calnali transect. X-axis indicates distance downstream from the first site sampled on a river, points indicate individual measurements, and lines show a loess-spline fit of change over the river course. Note that the Y-axis is log-scaled in the case of ammonia measurements. For copper, translucent points below the gray dashed line in represent measured values below the detection limit of the ICP-MS assay. Points below this limit are plotted at half the detection threshold, with jittering to prevent overlap and represent sample size, but do not represent point estimates of the concentration.

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Ben Moran @ben-moran.bsky.social · 01/05/2025

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.

Map of classification results from 2020-2023 hyperspectral imagery. 10 m^2 pixels were classified as developed (buildings, mines, and roads), cleared land (grassland, pasture, or cultivated crops) forest (continuous tree cover, excluding plantation-style agriculture), or water by an SVM algorithm, with training polygons identified from 3 m^2-resolution 2022 imagery from Planet Labs. Rivers are shown as blue lines over classified pixels.

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Ben Moran @ben-moran.bsky.social · 01/05/2025

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.

Genetic ancestry results from X. malinche × X. birchmanni hybrids in the Río Calnali. Histograms show ancestry distributions at sampling locations along the river. Sites are arranged with upstream sites on the left side of the X-axis, and downstream sites on the right. Y-axis corresponds to the proportion of genome-wide ancestry derived from X. malinche among individuals in that populations (1 = pure X. malinche, 0 = pure X. birchmanni), and the width of bars indicates the frequency of individuals in that ancestry bin. Building icon denotes the sites located within the town of Calnali. Bolded X-axis numbers indicate sites with significant deviations from a unimodal ancestry distribution.

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Ben Moran @ben-moran.bsky.social · 01/05/2025

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.

Overview of Río Atlapexco basin, highlighted in orange within the inset Mexican state map, whose area is indicated in yellow on the gray outline of Mexico. Rivers are shown as blue lines, the four major rivers studied are labelled, sites where fish were collected are yellow circles, and sites where water chemistry data were collected are orange diamonds. The black box encloses the Calnali area.

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Ben Moran @ben-moran.bsky.social · 01/05/2025

2/ We were motivated by the growing problem of anthropogenic hybridization: when humans modify the environment in ways that cause species which were previously separate to hybridize and homogenize. We know such hybridization exists, but it’s unclear how common it is, and exactly what can cause it.

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Reposted by Ben Moran

Graham Coop @gcbias.bsky.social · 02/01/2025

Congrats to @jeffgroh.bsky.social et al on publication of "Ancient structural variants control sex-specific flowering time morphs in walnuts and hickories" www.science.org/doi/10.1126/...

Science magazine front cover. Picture of a walnut in its opened shell. The text reads "All about timing. Mating type evolution in walnuts and pecans"

graphical abstract:
"In Juglans (top left) and Carya, two morphs show complementary temporal separation between male and female flowering (heterodichogamy). Mating types are controlled by two nonhomologous single-locus mechanisms that arose in the common ancestor of each genus, respectively. (Bottom left) Simplified schematic of a putative functional regulatory element at the Juglans locus. (Bottom right) Strong genotypic correlations across the Carya locus indicate a lack of recombination between two colinear haplotypes with similar gene content."

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Reposted by Ben Moran

Molly Schumer @mollyschumer.bsky.social · 10/01/2024

So glad our paper on lethal hybrid incompatibilities is out at Nature today: rdcu.be/dvFcM ! @ben-moran.bsky.social moved mountains for this project: we identified two new incompatibilities in swordtail hybrids , characterize mechanisms of lethality, and their evolutionary history.

rdcu.be

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