Male Dispersal Drives Gene Flow in Timber Rattlesnakes (Crotalus horridus)
Threatened across much of their range, Timber Rattlesnakes (Crotalus horridus) exhibit patterns of genetic diversity that are shaped by movement and aspects of their environment in ways that may foster or constrain conservation. We combine movement data with nuclear and mitochondrial population genetic data to understand the population structure and patterns of gene flow for snakes in four overwintering sites (hibernacula) in central Pennsylvania. Our analyses show that hibernacula separated by only a few kilometers can maintain genetic structure along with high gene flow. Female spatial use rarely overlapped with individuals from adjacent hibernacula, whereas male spatial use typically did overlap with individuals from adjacent hibernacula. This study offers new evidence for how asymmetry in gene flow (with males responsible for inter-hibernaculum matings) shapes Timber Rattlesnake population genetics.
Location of each initial capture for snakes used in the genetic analysis. Red stars denote each of the five hibernaculum locations. We did not use any snakes from the northeast hibernaculum for genetic analysis due to insufficient sample size. Different colors represent snakes assigned to different hibernacula (black = hibernaculum A, gray = hibernaculum B, blue = hibernaculum C, and red = hibernaculum D). The two individuals from hibernaculum D that were initially captured to the west were tracked via radio-telemetry to hibernaculum D.
Spatial use for female and male Crotalus horridus. Red circles denote the spatial use for each radio-tagged snake. Black circles and error bars represent means ± SE.
Spatial use for male and female Crotalus horridus as determined by Brownian bridge movement models. The top panel shows spatial use for all females from hibernaculum C (blue shade) and the females from hibernaculum D (red shade). The middle panel shows female spatial use (similar to top panel) but without the one female whose spatial use polygon overlapped with females from the adjacent hibernaculum. The bottom panel shows spatial use for males from hibernaculum B (gray shade), the males from hibernaculum C (blue shade), and one male from the hibernaculum to the northeast (yellow shade). Different line colors distinguish individuals, but the focus of the figure is to show overlap among spatial use of snakes from different hibernacula. Red stars refer to hibernaculum locations.
Nuclear genomic structure for hibernacula. Results of discriminant analysis of principal components, illustrating how clustering by geography relates to clustering by genetic similarity. (A) Plot differentiating the hibernacula along the first two discriminants of nuclear microsatellite variation, with the center for each population indicated by the location of their identifying letters. (B) Heatmap of membership probabilities for each snake. Horizontal lines represent each individual. Columns give probability of belonging in each of four groups identified by DAPC clustering. The hibernaculum the snake is actually associated with is identified with a blue cross. Probability of membership to each cluster indicated by heatmap color (varying continuously from red = 1, white = 0).
Map of hibernaculum locations illustrating gene flow as estimated by FST. Thickness of black lines represents connectivity as measured by FST; numbers associated with each line are pairwise FST estimates with P-values < 0.01 indicated by two asterisks. The unfilled star represents a fifth documented hibernaculum that was not included in this study due to insufficient sample size. Pie charts represent the frequency of mitochondrial (ND2) haplotypes in each hibernaculum (showing variation only in hibernaculum B).
Contributor Notes
Associate Editor: J. Kerby