Introgressive hybridization erodes morphological divergence between lentic and lotic habitats in an endangered minnow

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 f..., 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, a..., Column descriptions:
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-al...

渐渗杂交（introgressive hybridization）可能会侵蚀沿环境梯度的表型分化，使局部适应的种群坍缩为杂交群。与之相对，渐渗也可通过为自然选择提供可作用的遗传变异，促进表型分化。在淡水鱼类中，水流环境通常会对湖泊与溪流生境中的形态性状产生趋异选择压力。本研究针对欧文氏图尾鱼（Siphateles bicolor snyderi）——这一因外来引入的拉洪坦图尾鱼（Siphateles bicolor obesa）的渐渗而濒临灭绝的物种——检验了渐渗对湖泊-溪流生境间形态分化的影响。通过对13个种群的457尾图尾鱼开展几何形态计量分析（geometric morphometric analysis），我们发现本土亲本类群与渐渗亲本类群在湖泊与溪流生境中均表现出躯体与尾鳍形态的趋异，但二者的分化轨迹存在显著差异。与之形成对比的是，渐渗种群则表现出中间型的躯体和尾鳍……

### 鱼类采样
我们于2019年、2020年的7月至8月采集鱼类，仅East Walker River站点于2021年2月完成采样，采样方式采用岸用围网与背包式电渔法。我们从每个亲本亚种（欧文氏与拉洪坦）各采集1个湖泊种群与1个溪流种群，同时采集了4个湖泊种群与5个溪流种群的疑似渐渗种群（采样点详情见附录S1）。本次研究共采集457尾图尾鱼，平均每个站点采样35尾。对于拉洪坦种群与杂交种群，我们在采集后对鱼类实施了人道处死；对于濒危的欧文氏种群，我们对麻醉后的活体个体进行拍照与形态测量，随后将其复苏并放归野外。我们测量了所有个体的标准体长至最近1毫米，体质量至最近0.01克（湿重），并从背侧、腹侧、侧位三个视角拍摄个体照片，用于后续形态计量分析。

### 形态计量分析
针对每尾鱼类，我们设置了16个腹侧地标、18个侧位地标以及……

### 字段说明
- `id`：每尾鱼的唯一标识符（因子型变量）
- `location`：鱼类的采集站点（因子型变量）
- `loc.type`：站点生境类型，分为湖泊或溪流（因子型变量）
- `putative.subspecies`：种群的推定归属类型，包括欧文氏图尾鱼、拉洪坦图尾鱼或杂交（渐渗）类群（因子型变量）
- `sl.mm`：以毫米为单位的鱼类标准体长（数值型变量）
- `weight.g`：以克为单位的鱼类湿体质量（数值型变量）
- `caudal.AR`：尾鳍展弦比（无单位）（数值型变量）
- `D_X1`至`D_X7`：分别对应经广义Procrustes分析（Generalized Procrustes Analysis, GPA）对齐后的背侧地标1至7的x坐标
- `D_Y1`至`D_Y7`：分别对应经广义Procrustes分析（GPA）对齐后的背侧地标1至7的y坐标
- `V_X1`至`V_X16`：分别对应经GPA对齐后的腹侧地标1至16的x坐标
- `V_Y1`至`V_Y16`：分别对应经GPA对齐后的腹侧地标1至16的y坐标
- `L_X1`至`L_X18`：分别对应经GPA对齐后的侧位地标1至18的x坐标
- `L_Y1`至`L_Y18`：分别对应经GPA对齐后的……