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Chicken W: a genetically uniform chromosome in a highly variable genome
1Department of Evolutionary Biology, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden.
Summary
Genetic diversity on sex chromosomes is unexpectedly low. In chickens, the W chromosome shows significantly reduced nucleotide diversity, indicating strong selection shapes these non-recombining regions.
Area of Science:
- Evolutionary genetics
- Comparative genomics
- Sex chromosome evolution
Background:
- The Y chromosome in males typically exhibits reduced genetic diversity due to its effective population size.
- Previous studies in various species show Y chromosome diversity is lower than expected, suggesting factors beyond effective population size.
- Hypotheses include skewed male reproductive success or strong selection acting on non-recombining chromosomes.
Purpose of the Study:
- To investigate nucleotide diversity on the W chromosome (female-specific) in domestic chickens.
- To test hypotheses explaining reduced sex chromosome diversity in a system with female heterogamety.
Main Methods:
- Resequencing of 7,643 base pairs across the W chromosome.
- Analysis of 47 domestic chickens from 10 divergent breeds.
- Comparison of W chromosome polymorphism with autosomal sequence diversity.
Main Results:
- Only one single segregating site was found on the W chromosome, contrasting sharply with autosomal sequences.
- W chromosome polymorphism was 28-fold lower for segregating sites and 13-fold lower for nucleotide diversity than expected.
- Observed diversity is significantly lower than expected even after accounting for mutation rates and effective population size.
Conclusions:
- Strong selective pressures are the primary driver of reduced diversity on the W chromosome, regardless of heterogamety.
- The low variability on the W chromosome is not readily explained by sexual selection.
- This finding highlights the significant role of selection in shaping genetic diversity on sex-limited chromosomes.