Homoploid Hybrid Speciation in a Marine Pelagic Fish

Area of Science:

• Evolutionary biology
• Marine biology
• Genomics

Background:

• Homoploid hybrid speciation (HHS) is an evolutionary process where new species arise from hybridization without chromosome number changes.
• While documented in various organisms, convincing cases in marine fishes have been scarce.
• This study investigates a potential HHS case in the pelagic marine fish Megalaspis cordyla.

Purpose of the Study:

• To present a case of homoploid hybrid speciation in a marine fish.
• To identify and characterize distinct clusters within Megalaspis cordyla.
• To explore the genomic, morphological, and ecological factors contributing to speciation.

Main Methods:

• Population genomics and species tree estimation were used to analyze genetic structure.
• Tests for introgression and admixture identified hybrid origins.
• Morphological and ecological data were analyzed to assess differentiation and niche partitioning.

Main Results:

• Three sympatric clusters of Megalaspis cordyla were identified in the western Pacific, with one admixed cluster.
• Genomic analyses supported a hybrid speciation scenario over simple introgression.
• Morphological and ecological differences suggest reproductive isolation and niche partitioning among clusters.
• The hybrid cluster is geographically restricted, while parental clusters show distinct distributions.

Conclusions:

• The findings present a compelling case for homoploid hybrid speciation in Megalaspis cordyla.
• Pleistocene glacial cycles are proposed as the primary driver of this speciation event.
• An ecogeographic model for marine coastal HHS is proposed, offering insights into its initial stages.