The Evolution of Sex Chromosomes and Dosage Compensation in Structurally Dynamic Butterfly Genomes

• Dimorphic sex chromosomes have evolved independently throughout evolution from initially homologous autosomes. Recombination suppression between the sex chromosomes causes one sex chromosome to degenerate, which will influence evolution of sex-linked genes. This thesis investigated the evolution of genes and regulatory mechanisms on the sex chromosomes in Leptidea butterflies. Butterflies are female heterogametic, and females carry a dimorphic Z/W pair, while males have two homologous Z chromosomes. Dosage compensation is expected to evolve to equalize gene expression differences which are caused by this imbalance. Our analyses showed that Z-linked gene expression was reduced in males to parity with females, and reduced compared to autosomal levels. This is likely an adaptation to homogenize expression patterns between the sexes. However, the Z chromosome was enriched with male-biased genes, and sex-biased genes were not downregulated, suggesting that specific tuning occur for subsets of genes. An exploration of the Leptidea genomes revealed an unprecedented amount of chromosomal rearrangements, and detection of chromosomal heterozygotes suggests that the process is ongoing. Transposable elements were enriched in fused chromosome regions and are likely promoting rearrangements. The observed restructuring has formed several neo-sex chromosomes, which makes Leptidea useful for studies of evolutionary dynamics of  sex chromosomes. Z-linked genes showed a faster-Z effect, which was strongest for female-biased genes. In addition, the neo-Z chromosomes revealed a temporal dynamic, with younger Z chromosomes diverging faster. This could potentially be caused by an increased adaptive potential compared to the ancestral Z chromosome. Dualistic dosage compensation was found on the neo-Z chromosomes, with some chromosome regions being downregulated in males, and some regions being upregulated in females, in both cases resulting in inter-sexual expression balance. One Leptidea neo-W chromosome is largely intact, and analyses of allele specific expression provided strong evidence for W gametolog expression. Furthermore, male Z-linked expression was reduced for genes that lacked W gametologs, indicating that gene regulation has evolved rapidly to achieve dosage balance. This work has led to several interesting discoveries about gene regulation, chromosome rearrangements and sequence divergence of sex chromosomes, and in general revealed a more complex picture of how these specific chromosomes evolve.

Ämnesord

NATURVETENSKAP  -- Biologi -- Evolutionsbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Evolutionary Biology (hsv//eng)

Nyckelord

Sex chromosome evolution

Dosage compensation

Sex-biased gene expression

Faster-Z

Lepidoptera

Biology with specialization in Evolutionary Genetics

Biologi med inriktning mot evolutionär genetik

Publikations- och innehållstyp

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