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Neo-sex chromosome evolution shapes sex-dependent asymmetrical introgression barrier
Silu Wang1, Matthew J Nalley1, Kamalakar Chatla1
1Integrative Biology, University of California, Berkeley, CA 94720.
Summary
Newly evolved sex chromosomes create barriers to gene flow between Drosophila species. These neo-sex chromosomes cause asymmetrical introgression, differing between males and females, impacting species evolution.
Area of Science:
- Evolutionary biology
- Genetics
- Speciation
Background:
- Sex chromosomes are crucial in developing reproductive isolation between species.
- The study investigates hybrid swarms of recently diverged sister species, Drosophila nasuta and Drosophila albomicans, which possess distinct sex chromosome systems.
- D. albomicans features neo-sex chromosomes formed by autosome fusion with ancestral X and Y, while D. nasuta retains unfused chromosomes.
Purpose of the Study:
- To examine the role of nascent sex chromosomes in shaping introgression patterns within an experimental hybrid swarm.
- To understand the asymmetrical and sex-dependent nature of the introgression barrier imposed by neo-sex chromosomes.
- To elucidate the genetic factors contributing to observed sex chromosome genotypes in hybrid females and males.
Main Methods:
- Analysis of introgression patterns in an experimental hybrid swarm of Drosophila nasuta and Drosophila albomicans.
- Utilizing a population genetic model to simulate and analyze the interplay of various genetic factors.
- Investigating factors such as meiotic drive, heterospecific pairing incompatibility, and sex chromosome advantage/disadvantage.
Main Results:
- A significant introgression barrier was identified on the neo-sex chromosome, attributed to overlapping inversions.
- The introgression barrier demonstrated asymmetry and sex-dependence: female hybrids showed D. albomicans-biased introgression (neo-X excess), while males exhibited heterosis with excessive neo-X and D. nasuta Muller CD genotypes.
- Population genetic modeling indicated that moderate neo-Y disadvantage, D. albomicans-specific meiotic drive, pairing incompatibility, and neo-X advantage are necessary to explain the observed sex-specific introgression patterns.
Conclusions:
- Nascent neo-sex chromosomes play a complex, multifaceted role in establishing sex-dependent, asymmetrical introgression barriers at species boundaries.
- The findings highlight the dynamic interplay of genetic factors in shaping reproductive isolation during early speciation.
- This study provides insights into the evolutionary significance of sex chromosome systems in hybrid zones.
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