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In most organisms, sex is determined by the ratio of X and Y chromosomes. However, in some organisms, such as Drosophila and C.elegans, sex is determined by the ratio of the number of X chromosomes to the number of sets of autosomes. The Y chromosome in Drosophila is active but does not determine sex. It contains genes responsible for the production of sperms in adult flies.  
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The Population Genetics of Evolutionary Rescue in Diploids: X Chromosomal versus Autosomal Rescue.

Robert L Unckless, H Allen Orr

    The American Naturalist
    |February 26, 2020
    PubMed
    Summary

    Evolutionary rescue in diploid organisms involves a race between adaptation and extinction. This study finds that the X chromosome is more likely to contribute to adaptation in diploids than in haploids, increasing the chances of evolutionary rescue.

    Keywords:
    adaptationautosomesenvironmental changeevolutionary rescuesex chromosomes

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    Area of Science:

    • Population Genetics
    • Evolutionary Biology
    • Genomics

    Background:

    • Traditional population genetics models often assume stable population sizes during adaptation.
    • Severe environmental changes can lead to population decline and extinction, necessitating 'evolutionary rescue' models.
    • Previous evolutionary rescue research primarily focused on haploid organisms.

    Purpose of the Study:

    • To investigate evolutionary rescue in diploid organisms, considering adaptation on both sex chromosomes (X) and autosomes.
    • To compare the contribution of X chromosomes versus autosomes to adaptation during evolutionary rescue.
    • To explore the impact of dominance effects on the likelihood of evolutionary rescue in diploids.

    Main Methods:

    • Theoretical modeling of evolutionary rescue in diploid populations.
    • Analysis of adaptive evolution on sex chromosomes and autosomes.
    • Comparison of adaptation rates under different dominance scenarios.

    Main Results:

    • In diploids, there is a greater parameter space where the X chromosome contributes more to adaptation during rescue compared to non-rescue models.
    • The relative contribution of X chromosomes versus autosomes to adaptation is influenced by the dominance of beneficial mutations.
    • Subtle dominance effects can enhance the probability of evolutionary rescue when heterozygote fitness is near 1.

    Conclusions:

    • Diploidy introduces unique dynamics to evolutionary rescue, particularly regarding sex chromosome adaptation.
    • The X chromosome plays a potentially more significant role in evolutionary rescue than previously recognized in diploid models.
    • Understanding dominance is crucial for predicting the success of evolutionary rescue in diploid populations.