Incomplete lineage sorting and ancient admixture, and speciation without morphological change in ghost-worm cryptic...

Area of Science:

• Evolutionary Biology
• Genomics
• Marine Biology

Background:

• Morphologically similar cryptic species may exhibit stasis due to decelerated evolution.
• Factors causing stasis in cryptic species are poorly understood, though hypotheses exist from paleontology.
• Shared genetic variation, such as incomplete lineage sorting or hybridization, can constrain morphological evolution.

Purpose of the Study:

• To investigate the evolutionary history and population structure of three morphologically similar *Stygocapitella* species.
• To determine the degree of connectivity between populations and species within the complex.
• To identify demographic scenarios explaining the observed genetic patterns and morphological similarity.

Main Methods:

• Employed whole-genome amplification (WGA) for DNA extraction from microscopic organisms.
• Utilized double-digestion restriction-site associated DNA sequencing (ddRAD) for genomic data generation.
• Reconstructed evolutionary history, analyzed population structure, and assessed inter-population connectivity.

Main Results:

• Successfully obtained genomic data from microscopic eukaryotes (<1mm) using WGA and ddRAD.
• Found evidence of shared genetic variance among the three *Stygocapitella* species.
• Demographic scenarios generally rejected recent hybridization, suggesting incomplete lineage sorting and ancient admixture as likely causes for shared variation.

Conclusions:

• The combination of WGA and ddRAD is effective for population genomics and phylogenomics in small-sized eukaryotes.
• Shared genetic variance, likely from incomplete lineage sorting and ancient admixture, may explain the morphological similarity in this Atlantic *Stygocapitella* species complex.
• Further research can leverage these genomic methods to study cryptic species complexes.