Final Report Summary - NEW SEX SAME GENES (Contradictory phenotypic and genetic sex can lead to great insights into sex chromosome evolution)
Genetic sex determination normally involves a pair of morphologically distinguishable sex chromosomes. Many organisms have an XY chromosome system (like mammals) where males are the heterogametic (XY) and females the homogametic (XX) sex. However in some organisms, including birds, some lizards and fish species, females are the heterogametic sex (ZW) while males are homogametic (ZZ). There are many common features of these two systems, in particular the existence of sex-linked genes that are sexually antagonistic, which describes the situation when the expression of a gene is beneficial to one sex but harmful to the opposite sex. The establishment of antagonistic genes is thought to be preeminent on the sex chromosomes, due to the lack of recombination between them, leading to a non-random distribution of sexually antagonistic loci over the genome. However, our knowledge as to how sexual antagonism has evolved is limited.
An interesting direction in research about sex chromosome evolution has therefore been to understand why a pair of chromosomes that contain genetic information to determine sex cease to recombine and start becoming increasingly different from each other. Diverging sex chromosomes are in addition faced with the dilemma of maintaining a balance in gene expression levels between interacting sex-linked and autosomal genes. Following cessation of recombination between the sex chromosomes, and the subsequent degeneration of genes on the heterogametic chromosome, one is left with only half the expression in the gender with the degenerated sex chromosome. To compensate for this change in gene dose, the evolutionary pressure for adjusting expression levels should be strong.
We chose to work with two model organisms, ostrich and mosquitofish. Both have a female heterogametic ZW system but whereas avian sex chromosomes are considered highly conserved, fish sex chromosomes are rather 'young' with XY / ZW swaps even between closely related species. Neither ostrich nor mosquitofish have the typical morphological degenerated W chromosome compared to the Z chromosome. Instead, ostrich ZW chromosomes are homologous and still largely recombining while the mosquitofish W-chromosome seems to have increased in size rather than degenerated.
In mosquitofish we are especially interested in how gender reversals, when females have ZZ chromosomes and males ZW chromosomes, effect gene expression differences. We also aim to link sex-biased gene expression to fitness estimates to pin-point sexual antagonism. This completely novel approach to analyse responsiveness in gene expression to sex turnarounds has immense potential for understanding the mechanisms behind activation of sexually antagonistic genes.
In ostrich, we search for gene expression differences between genders even though the sex chromosomes are mostly recombining. In particular we aim to identify the regions which do not recombine and evaluate general dosage compensation, as chicken surprisingly was shown to have incomplete dosage compensation. We will thus be able to see whether a lack of dosage compensation could hinder a degeneration of the W-chomosome in ostrich.
We have applied high throughput ribonucleic acid sequencing (RNAseq) (Illumina platform) on ostrich brain and liver tissues and mosquitofish testis and ovarian tissue to identify male- and female-biased expression. Both transcriptomes were de novo assembled and gene annotation was done by comparing to closely-related sequenced and annotated genomes from Ensembl. Differences in gene expression between males and females were quantified using Bayesian estimation.
Due to difficulties in microdissecting the W chromosome for sequencing we have not yet performed RNAseq on gender reversed mosquitofish as a sex-chromosome marker is necessary to identify these individuals. We have thus started an initiative to randomly screen for sex-specific markers (i.e. W targeted primers) based on highly expressed female contigs absent in males from the mosquitofish transcriptome, which could be used to identify gender reversed fishes. We have successfully identified one marker and we now have 4 males with female genotype and 3 females with male genotype ready for RNA extraction and RNAseq.
The male-to-female expression ratio of Z-linked genes in ostriches showed a bimodal distribution with peaks corresponding to an equal ratio between sexes and a twofold excess of male expression. We found support that these regions represent recombining and non-recombining regions, respectively. This in turn suggests that, overall, non-recombining Z-chromosome genes in ostrich are not dosage compensated.
We have a priori no socio-economic impacts to report.
Project website: http://www.ebc.uu.se/Research/IEG/evbiol/people/pages/Adolfsson_Sofia/
An interesting direction in research about sex chromosome evolution has therefore been to understand why a pair of chromosomes that contain genetic information to determine sex cease to recombine and start becoming increasingly different from each other. Diverging sex chromosomes are in addition faced with the dilemma of maintaining a balance in gene expression levels between interacting sex-linked and autosomal genes. Following cessation of recombination between the sex chromosomes, and the subsequent degeneration of genes on the heterogametic chromosome, one is left with only half the expression in the gender with the degenerated sex chromosome. To compensate for this change in gene dose, the evolutionary pressure for adjusting expression levels should be strong.
We chose to work with two model organisms, ostrich and mosquitofish. Both have a female heterogametic ZW system but whereas avian sex chromosomes are considered highly conserved, fish sex chromosomes are rather 'young' with XY / ZW swaps even between closely related species. Neither ostrich nor mosquitofish have the typical morphological degenerated W chromosome compared to the Z chromosome. Instead, ostrich ZW chromosomes are homologous and still largely recombining while the mosquitofish W-chromosome seems to have increased in size rather than degenerated.
In mosquitofish we are especially interested in how gender reversals, when females have ZZ chromosomes and males ZW chromosomes, effect gene expression differences. We also aim to link sex-biased gene expression to fitness estimates to pin-point sexual antagonism. This completely novel approach to analyse responsiveness in gene expression to sex turnarounds has immense potential for understanding the mechanisms behind activation of sexually antagonistic genes.
In ostrich, we search for gene expression differences between genders even though the sex chromosomes are mostly recombining. In particular we aim to identify the regions which do not recombine and evaluate general dosage compensation, as chicken surprisingly was shown to have incomplete dosage compensation. We will thus be able to see whether a lack of dosage compensation could hinder a degeneration of the W-chomosome in ostrich.
We have applied high throughput ribonucleic acid sequencing (RNAseq) (Illumina platform) on ostrich brain and liver tissues and mosquitofish testis and ovarian tissue to identify male- and female-biased expression. Both transcriptomes were de novo assembled and gene annotation was done by comparing to closely-related sequenced and annotated genomes from Ensembl. Differences in gene expression between males and females were quantified using Bayesian estimation.
Due to difficulties in microdissecting the W chromosome for sequencing we have not yet performed RNAseq on gender reversed mosquitofish as a sex-chromosome marker is necessary to identify these individuals. We have thus started an initiative to randomly screen for sex-specific markers (i.e. W targeted primers) based on highly expressed female contigs absent in males from the mosquitofish transcriptome, which could be used to identify gender reversed fishes. We have successfully identified one marker and we now have 4 males with female genotype and 3 females with male genotype ready for RNA extraction and RNAseq.
The male-to-female expression ratio of Z-linked genes in ostriches showed a bimodal distribution with peaks corresponding to an equal ratio between sexes and a twofold excess of male expression. We found support that these regions represent recombining and non-recombining regions, respectively. This in turn suggests that, overall, non-recombining Z-chromosome genes in ostrich are not dosage compensated.
We have a priori no socio-economic impacts to report.
Project website: http://www.ebc.uu.se/Research/IEG/evbiol/people/pages/Adolfsson_Sofia/