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The evolution of sex chromosomes and sex-linked sequences in birds
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics.ORCID iD: 0000-0001-8667-6279
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Identifying the processes involved in the evolution of suppressed recombination between sex chromosomes and understanding their consequences for the evolutionary dynamics of sex-linked loci have been major topics of research during the last century. In this thesis, I used the avian ZW system, where females are the heterogametic sex, to investigate the underlying processes in sex chromosome evolution in birds. I identified the gametologous genes between the largely recombining Z and W chromosomes of ostrich and dated the timing of the cessation of recombination to prior to the split of modern birds. I then constructed a genetic map of the ostrich Z chromosome and corrected its assembly in order to obtain the ancestral organization of the Z chromosome in a basal clade of birds. By analyzing the inversion events across the avian phylogeny, I concluded that a combination of Z- and possibly W-linked inversions might have been responsible for the evolution of suppressed recombination in avian sex chromosomes. To understand the determinants of levels of genetic diversity on Z chromosome compared to autosomes, I calculated Z to autosome (Z:A) genetic diversity across 32 avian species. This revealed a broad range of Z:A genetic diversity, between 0.278 – 1.27. Lineage-specific estimates of the nonsynonymous to synonymous substitution rate ratio (dN:dS) for autosomal and Z-linked genes further revealed a Fast-Z effect in the majority of birds. The lack of a significant correlation between Z:A dN:dS and Z:A genetic diversity indicated that genetic drift might not be sufficient to explain faster evolution of Z-linked genes, suggesting that positive selection might also contribute to the observed values. Finally, I calculated genetic diversity and linkage disequilibrium (LD) along the pseudoautosomal region (PAR) of the Z chromosome using population genomics data of ostrich. In contrast to theoretical expectation, levels of diversity on the PAR were not significantly higher close to the sex-determining region (SDR) compared to autosomal values. Additionally, I observed a lower level of LD on the PAR compared to the average for the Z chromosome and no significant level of LD across the PAR boundary was detected, indicating recombination allows the boundary-proximal region of PAR to behave independently of SDR. Considered together with a higher level of recombination rate in females in the proximity of the SDR, this observation might help explain the maintenance of a long PAR in ostriches and other ratites. Altogether, the results of this thesis make a modest contribution to our understanding of sex chromosome evolution in birds.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 51
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1788
Keywords [en]
sex chromosomes, female heterogamety, recombination suppression, genetic map, inversions, genetic diversity, pseudoautosomal region
National Category
Evolutionary Biology Genetics
Research subject
Biology with specialization in Evolutionary Genetics
Identifiers
URN: urn:nbn:se:uu:diva-379665ISBN: 978-91-513-0612-4 (print)OAI: oai:DiVA.org:uu-379665DiVA, id: diva2:1298940
Public defence
2019-05-20, Lindahlsalen, Evolutionsbiologiskt centrum, Norbyvägen 18A, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2019-04-23 Created: 2019-03-25 Last updated: 2019-06-18
List of papers
1. Old but Not (So) Degenerated-Slow Evolution of Largely Homomorphic Sex Chromosomes in Ratites
Open this publication in new window or tab >>Old but Not (So) Degenerated-Slow Evolution of Largely Homomorphic Sex Chromosomes in Ratites
2014 (English)In: Molecular biology and evolution, ISSN 0737-4038, E-ISSN 1537-1719, Vol. 31, no 6, p. 1444-1453Article in journal (Refereed) Published
Abstract [en]

Degeneration of the nonrecombining chromosome is a common feature of sex chromosome evolution, readily evident by the presence of a pair of largely heteromorphic chromosomes, like in eutherian mammals and birds. However, in ratites (order Palaeognathae, including, e.g., ostrich), the Z and W chromosomes are similar in size and largely undifferentiated, despite avian sex chromosome evolution was initiated > 130 Ma. To better understand what may limit sex chromosome evolution, we performed ostrich transcriptome sequencing and studied genes from the nonrecombining region of the W chromosome. Fourteen gametologous gene pairs present on the W chromosome and Z chromosome were identified, with synonymous sequence divergence of 0.027-0.177. The location of these genes on the Z chromosome was consistent with a sequential increase in divergence, starting 110-157 and ending 24-30 Ma. On the basis of the occurrence of Z-linked genes hemizygous in females, we estimate that about one-third of the Z chromosome does not recombine with the W chromosome in female meiosis. Pairwise d(N)/d(S) between gametologs decreased with age, suggesting strong evolutionary constraint in old gametologs. Lineage-specific d(N)/d(S) was consistently higher in W-linked genes, in accordance with the lower efficacy of selection expected in nonrecombining chromosomes. A higher ratio of GC > AT:AT > GC substitutions in W-linked genes supports a role for GC-biased gene conversion in differentially driving base composition on the two sex chromosomes. A male-to-female (M:F) expression ratio of close to one for recombining genes and close to two for Z-linked genes lacking a W copy show that dosage compensation is essentially absent. Some gametologous genes have retained active expression of the W copy in females (giving a M:F ratio of 1 for the gametologous gene pair), whereas for others W expression has become severely reduced resulting in a M:F ratio of close to 2. These observations resemble the patterns of sex chromosome evolution seen in other avian and mammalian lineages, suggesting similar underlying evolutionary processes, although the rate of sex chromosome differentiation has been atypically low. Lack of dosage compensation may be a factor hindering sex chromosome evolution in this lineage.

Keywords
Z chromosome, W chromosome, evolutionary strata, gametologs, nonrecombining chromosome, biased gene conversion
National Category
Biochemistry and Molecular Biology Evolutionary Biology Genetics
Identifiers
urn:nbn:se:uu:diva-228463 (URN)10.1093/molbev/msu101 (DOI)000337067400013 ()
Available from: 2014-07-16 Created: 2014-07-15 Last updated: 2019-03-25Bibliographically approved
2. A Genetic Map of Ostrich Z Chromosome and the Role of Inversions in Avian Sex Chromosome Evolution
Open this publication in new window or tab >>A Genetic Map of Ostrich Z Chromosome and the Role of Inversions in Avian Sex Chromosome Evolution
2018 (English)In: Genome Biology and Evolution, ISSN 1759-6653, E-ISSN 1759-6653, Vol. 10, no 8, p. 2049-2060Article in journal (Refereed) Published
Abstract [en]

Recombination arrest is a necessary step for the evolution of distinct sex chromosomes. Structural changes, such as inversions, may represent the mechanistic basis for recombination suppression and comparisons of the structural organization of chromosomes as given by chromosome-level assemblies offer the possibility to infer inversions across species at some detail. In birds, deduction of the process of sex chromosome evolution has been hampered by the lack of a validated chromosome-level assembly from a representative of one of the two basal clades of modern birds, Paleognathae. We therefore developed a high-density genetic linkage map of the ostrich Z chromosome and used this to correct an existing assembly, including correction of a large chimeric superscaffold and the order and orientation of other superscaffolds. We identified the pseudoautosomal region as a 52 Mb segment (approximate to 60% of the Z chromosome) where recombination occurred in both sexes. By comparing the order and location of genes on the ostrich Z chromosome with that of six bird species from the other major Glade of birds (Neognathae), and of reptilian outgroup species, 25 Z-linked inversions were inferred in the avian lineages. We defined Z chromosome organization in an early avian ancestor and identified inversions spanning the candidate sex-determining DMRT1 gene in this ancestor, which could potentially have triggered the onset of avian sex chromosome evolution. We conclude that avian sex chromosome evolution has been characterized by a complex process of probably both Z-linked and W-linked inversions (and/or other processes). This study illustrates the need for validated chromosome-level assemblies for inference of genome evolution.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2018
Keywords
sex chromosomes, inversions, assembly correction, linkage analysis
National Category
Genetics
Identifiers
urn:nbn:se:uu:diva-365830 (URN)10.1093/gbe/evy163 (DOI)000444553900017 ()30099482 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2018-11-26 Created: 2018-11-26 Last updated: 2019-03-25Bibliographically approved
3. Variation in the Z Chromosome to Autosomes Ratio of Genetic Diversity across Birds and its Relationship to the Fast-Z effect
Open this publication in new window or tab >>Variation in the Z Chromosome to Autosomes Ratio of Genetic Diversity across Birds and its Relationship to the Fast-Z effect
(English)Manuscript (preprint) (Other academic)
Keywords
sex chromosomes, genetic diversity, Fast-Z evolution, genetic drift, selection
National Category
Evolutionary Biology
Research subject
Biology with specialization in Evolutionary Genetics
Identifiers
urn:nbn:se:uu:diva-380263 (URN)
Available from: 2019-03-25 Created: 2019-03-25 Last updated: 2019-04-09
4. Patterns of Nucleotide Diversity and Linkage Disequilibrium along the Ostrich Pseudoautosomal Region
Open this publication in new window or tab >>Patterns of Nucleotide Diversity and Linkage Disequilibrium along the Ostrich Pseudoautosomal Region
Show others...
(English)Manuscript (preprint) (Other academic)
Keywords
sex chromosomes, pseudoautosomal region, genetic diversity, linkage disequilibrium
National Category
Evolutionary Biology
Research subject
Biology with specialization in Evolutionary Genetics
Identifiers
urn:nbn:se:uu:diva-380265 (URN)
Available from: 2019-03-25 Created: 2019-03-25 Last updated: 2019-03-25

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Yazdi, Homa Papoli

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