Swordtail fish hybrids reveal that genome evolution is surprisingly predictable after initial hybridization.

Area of Science:

• Evolutionary Biology
• Genomics
• Speciation

Background:

• Hybridization, the exchange of genetic material between distinct lineages, is increasingly recognized as a common evolutionary process across diverse taxa.
• Many species' genomes contain regions inherited from related species, raising questions about the repeatability of genomic outcomes and the role of natural selection.
• Empirical studies on hybridization repeatability have been limited by a lack of suitable model systems.

Purpose of the Study:

• To investigate the repeatability of genomic outcomes in independently formed hybrid populations.
• To determine the extent to which natural selection drives repeatable patterns of local ancestry following hybridization.
• To compare repeatability across hybrid populations with different levels of evolutionary divergence.

Main Methods:

• Utilizing independently formed hybrid populations of swordtail fish (Xiphophorus birchmanni and X. cortezi).
• Applying newly developed methods to analyze local ancestry patterns and quantify repeatability.
• Employing time-series data to assess the stability of selected ancestry regions over evolutionary time.

Main Results:

• Local ancestry patterns in one hybrid population were highly predictive of those in a separate, demographically independent population.
• Strong natural selection in early generations significantly contributed to the observed repeatability of local ancestry.
• Ancestry at selected regions remained stable over approximately 40 generations, as evidenced by time-series data.
• Hybrids between X. birchmanni and X. cortezi exhibited stronger selection and higher repeatability compared to X. birchmanni × X. malinche hybrids, correlating with greater evolutionary divergence.

Conclusions:

• The genomic outcomes of hybridization can be remarkably repeatable, largely driven by strong natural selection.
• Independent hybrid populations can serve as powerful systems for studying the evolutionary consequences of hybridization.
• Greater evolutionary divergence between parental species leads to stronger selection and more predictable patterns of local ancestry in their hybrids.