Parallel introgression, not recurrent emergence, explains apparent elevational ecotypes of polyploid Himalayan...

Area of Science:

• Evolutionary biology
• Genomics
• Ichthyology

Background:

• Recurrent evolutionary patterns can arise from parallel selection or introgression.
• Drainage-specific snowtrout 'ecotypes' present a case for studying these evolutionary mechanisms.
• Previous hypotheses favored parallel divergence, but introgression offered an alternative explanation.

Purpose of the Study:

• To test competing hypotheses of parallel divergence versus introgression in snowtrout evolution.
• To investigate the genetic basis of morphological 'ecotypes' in snowtrout.
• To understand the role of secondary contact and admixture in shaping snowtrout genomes.

Main Methods:

• Utilized double digest restriction site-associated DNA sequencing (ddRADseq) for SNP discovery.
• Analyzed 35,319 de novo and 10,884 transcriptome-aligned SNPs from Nepali/Bhutanese snowtrout samples.
• Employed Bayesian genotype inference and partitioned phylogenetic and admixture analyses.

Main Results:

• Morphological 'ecotypes' showed consistency in geometric morphometric space but phylogenetic relationships aligned with drainage levels.
• Subsets of loci under selection retained genealogical concordance with morphology.
• Widespread genomic homogenization due to admixture was identified, masking previous isolation.

Conclusions:

• Morphological adaptations in snowtrout are retained despite ongoing hybridization.
• Genomic homogenization driven by admixture effectively obscures historical isolation patterns.
• Varying degrees of admixture across tributaries suggest influence of environmental or anthropogenic factors.