| Sexual dimorphism is common in most organisms and has an important effect on the survival and reproductive success of individuals.Sexual dimorphisms usually result from genes that are expressed at significantly different levels between the sexes.Insufficient dosage compensation mechanism of the Z-linked genes result in a certain proportion of sex-biased genes on the sex chromosomes,which play an important role in the differentiation of sexual dimorphism.Chinese hwamei(Garrulax canorus)is a good model for studying sex-biased gene expression and dosage compensation mechanism of the Z-linked genes,because the males produce a varied and elegant sound,while the females have a monotonous sound.In this study,we performed transcriptome sequencing on the cerebrum and syrinx samples of male and female Chinese hwamei.Based on gene expression data in cerebrum and syrinx tissue,we analyzed the expression and evolution of sex-biased genes and the relationship between sex chromosome dosage compensation mechanisms and sex-biased genes.Main results are shown as below:(1)Based on de novo assembled Chinese hwamei transcriptome data,a total of227,595 transcripts and 95,962 unigenes were obtained.Functional annotation of all unigenes was performed in seven databases.A total of 5,017(5.22%)unigenes were successfully annotated in all databases,and 58,762(61.23%)unigenes were successfully blasted against at least one of seven databases.In addition,a total of828,037 SNPs were identified in the transcriptome and 24,022 unigenes contained SSRs.(2)Based on expression data,we found that the number of actively expressed genes in the cerebrum and syrinx tissues were 19,926 and 23,753,respectively.The number of sex-biased genes in the cerebrum and syrinx was 501 and 450,respectively.Further analysis revealed that the number of male-biased genes and female-biased genes in the cerebrum were 252 and 249,respectively,while the number of male-biased genes and female-biased genes in the syrinx were and 248,respectively.(3)Based on the expression characteristics of sex-biased genes,we found that in both tissues the number of exclusively actively expressed in females was significantly greater than genes that were actively expressed in males only.This result suggests that most male-biased genes are actively expressed in both sexes,while most female-biased genes are only actively expressed in females.(4)Performing chromosome enrichment analysis on sex-biased genes,we found that both male-biased genes and female bias genes were significantly enriched on the Z chromosome in the two tissues,and they were almost evenly distributed on the Z chromosome.The enrichment of sex-biased genes on chromosome Z result from the susceptibility of sex chromosomes to produce sexually antagonistic loci.Female-biased genes were accumulated on the chromosome 4A and almost all located between 0-9 Mb,especially 3-9 Mb,indicating that Chinese hwamei chromosome 4A may have a sex linkage region.(5)Based on evolutionary analysis of the coding sequences of sex-biased genes,male-biased genes in both tissues and cerebrum female-biased genes showed elevated evolutionary rates,while female-biased genes did not show rapid evolution only in the syrinx.We further analysed the effect of selection pressure on the evolution of sexbiased genes,and we found that codon bias was significantly lower for male-biased genes than unbiased genes.This suggests the rapid evolution of male-biased genes is the result of positive selection.(6)Based on sex chromosome dosage compensation analysis,we found that under different expression data filtering criteria,the Z: A values of both male and female cerebrum and syrinx ranged from 0.5 to 1,and the global expression levels of both male and female Z chromosome genes were significantly lower than the global expression levels of autosomal genes.Meanwhile,the global expression of female Z chromosome was significantly lower than the global expression of male Z chromosome.There exists Z < ZZ < AA,suggesting that there is an insufficient dosage compensation mechanism in the Chinese hwamei Z chromosome genes at different filtering thresholds.It also confirms that the enrichment of male-biased genes on the Z chromosome also result from insufficient dosage compensation on the Z chromosome.In this study,we provide an investigation into the molecular regulation of sexual dimorphism in the song of the Chinese hwamei,which will provide a theoretical basis for further studies on the vocal mechanisms of songbirds and humans.At the same time,our study will provide some guidance for the conservation of Chinese hwamei. |
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