DOI: 10.5061/dryad.dr7sqv9zt
title: Introgressive hybridization erodes morphological divergence between lentic and lotic habitats in an endangered minnow

description: Introgressive hybridization may erode phenotypic divergence along
 environmental gradients, collapsing locally adapted populations into a
 hybrid swarm. Alternatively, introgression may promote phenotypic
 divergence by providing variation on which natural selection can act. In
 freshwater fishes, water flow often selects for divergent morphological
 traits in lake versus stream habitats. We tested the effects of
 introgression on lake-stream morphological divergence in the minnow Owens
 Tui Chub (Siphateles bicolor snyderi), which has been rendered endangered
 by introgression from the introduced Lahontan Tui Chub (Siphateles bicolor
 obesa). Using geometric morphometric analysis of 457 individual Tui Chub
 from thirteen populations, we found that both the native and introgressing
 parent taxa exhibited divergent body and caudal fin shapes in lake vs.
 stream habitats, but their trajectories of divergence were distinct. In
 contrast, introgressed populations exhibited intermediate body and caudal
 fin shapes that were not differentiated by habitat type, indicating that
 introgression has eroded phenotypic divergence along the lentic-lotic
 gradient throughout the historic range of the Owens Tui Chub. Individuals
 within hybrid populations were less morphologically variable than those
 within parent populations, suggesting hybrid adaptation to selective
 agents other than water flow or loss of variance by drift.
description: Fish Sampling We collected fish in July and August in 2019 and
 2020, except for one site (East Walker River) which we sampled in February
 2021, using beach seines and backpack electrofishing. We sampled one lake
 and one stream population from each of the parent subspecies (Owens and
 Lahontan), and four lake and five stream populations of putatively
 introgressed populations (see Appendix S1 for site descriptions). We
 sampled a total of 457 individual Tui Chub (mean = 35 fish/site). For the
 Lahontan and hybrid populations, we humanely euthanized the fish upon
 collection. For the endangered Owens populations, we took photos and
 measurements of live anesthetized fish, then revived and released them. We
 measured all fish to the nearest millimeter (standard length), weighed to
 the nearest centigram (wet weight), and photographed from three
 perspectives (dorsal, ventral, and lateral) for morphometric
 analysis.  Morphometric
 Analysis For each fish, we placed
 sixteen ventral, eighteen lateral, and seven dorsal homologous landmarks
 (Armbruster 2012) using the R package ‘StereoMorph’ (Olsen &
 Westneat 2015) (Figure 2). A single researcher placed all landmarks to
 avoid potentially confounding effects. We performed generalized procrustes
 analysis (GPA) to align and adjust the raw landmarks, providing centroid
 size and shape coordinates (Gower 1975, Rohlf & Slice 1990) using
 the R package ‘geomorph’ (Adams et al. 2020). Sexual dimorphism has not
 been reported in Tui Chub so we did not perform separate morphometric
 analyses by sex. To calculate caudal fin aspect ratio, we digitally
 measured caudal fin height and surface area using the software ‘ImageJ’
 (Schneider et al. 2012) (Figure 2). We calculated caudal fin aspect ratio
 (AR) as the squared height of the fin divided by the surface area: AR =
 h2/SA (Langerhans & Reznick 2010). The uploaded dataset contains GPA-aligned landmark
 coordinates.  
description: <strong>Column descriptions:</strong>
 id: a unique identifier for each fish (factor)
 location: the collection site of the fish (factor)
 loc.type: whether the site is considered a lake or stream
 (factor) putative.subspecies: whether the population is
 Owens Tui Chub, Lahontan Tui Chub, or Hybrid (introgressed)
 (factor) sl.mm: fish standard length reported in
 millimeters (numeric) weight.g: fish wet weight
 reported in grams (numeric) caudal.AR: caudal fin
 aspect ratio (unitless) (numeric) D_X1 through D_X7 :
 x-coordinates for GPA-aligned dorsal landmarks 1 through 7,
 respectively.  D_Y1 through D_Y7 : y-coordinates for
 GPA-aligned dorsal landmarks 1 through 7, respectively. 
 V_X1 through V_X16 : x-coordinates for GPA-aligned ventral
 landmarks 1 through 16, respectively.  V_Y1 through
 V_Y16 : y-coordinates for GPA-aligned ventral landmarks 1 through 16,
 respectively.  L_X1 through L_X18: x-coordinates for
 GPA-aligned lateral landmarks 1 through 18, respectively. 
 L_Y1 through L_Y18: y-coordinates for GPA-aligned lateral
 landmarks 1 through 18, respectively. 
 <strong>Missing Values:</strong>
 Fish standard length (sl.mm) is missing for 10 fish from White
 Mountain Research Station (WMRS): id #500-509