Abstract
Dosage compensation has been thought to be a ubiquitous property of sex chromosomes that are represented differently in males and females. The expression of most X-borne genes is equalized between XX females and XY males in therian mammals (marsupials and “placentals”) by inactivating one X chromosome in female somatic cells. However, compensation seems not to be strictly required to equalize the expression of most Z-borne genes between ZZ male and ZW female birds. Whether dosage compensation operates in the third mammal lineage, the egg-laying monotremes, is of considerable interest, since the platypus has a complex sex chromosome system in which five X and five Y chromosomes share considerable genetic homology with the chicken ZW sex chromosome pair, but not with therian XY chromosomes. The assignment of genes to four platypus X chromosomes allowed us to examine X dosage compensation in this unique species. Quantitative PCR showed a range of compensation, but SNP analysis of several X-borne genes showed that both alleles are transcribed in a heterozygous female. Transcription of 14 BACs representing 19 X-borne genes was examined by RNA-FISH in female and male fibroblasts. An autosomal control gene was expressed from both alleles in nearly all nuclei, and four pseudoautosomal BACs were usually expressed from both alleles in male as well as female nuclei, showing that their Y loci are active. However, nine X-specific BACs were usually transcribed from only one allele. This suggests that while some genes on the platypus X are not dosage compensated, other genes do show some form of compensation via stochastic transcriptional inhibition, perhaps representing an ancestral system that evolved to be more tightly controlled in placental mammals such as human and mouse.
Highlights
Monotremes are unique mammals that exhibit a mix of reptilian and mammalian features, as they lay eggs, yet have fur and produce milk for their young
First we gained an overall assessment of the level of dosage compensation by comparing the amounts of transcript from X-specific, autosomal and pseudoautosomal genes in males and females using quantitative real-time RT-PCR
We identified single-nucleotide polymorphisms (SNPs) within the sequence of X-borne genes to determine if they are expressed from both alleles, or only one, as would be expected from imprinted X inactivation
Summary
Monotremes are unique mammals that exhibit a mix of reptilian and mammalian features, as they lay eggs, yet have fur and produce milk for their young. Monotreme genomes show a curious mixture of reptilian and mammalian characteristics. They have a smaller genome than therian mammals [2], and their karyotype comprises a few large chromosomes, and many small ones, somewhat reminiscent of chicken macro and microchromosomes. Most curious of all is the sex chromosome system of monotremes. Like other mammals, subscribe to an XY system of male heterogamety, they have multiple X and Y chromosomes [3] which form a multivalent translocation chain during meiosis [4]. Platypus (Ornithorhynchus anatinus) have ten sex chromosomes; males have five X chromosomes (X1X2X3X4X5) and five Y chromosomes (Y1Y2Y3Y4Y5), and females five pairs of X chromosomes [5]. X and Y chromosomes pair within terminal pseudoautosomal regions [6], forming a chain of alternating X and Y chromosomes (numbered by their order in the chain X1–Y1–X2–Y2– X3–Y3–X4–Y4–X5–Y5) which segregate into five X-bearing (femaledetermining) and five Y-bearing (male-determining) sperm [7]
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