X-Chromosome inactivation in female mammals starts during early blastocyst stage with expression of the X-inactive specific transcript (XIST), which coats and silences the inactive X chromosome. However, this compensation is not complete in blastocysts, as a large number of X-linked transcripts are more highly expressed in female embryos than in males. Furthermore, the process of X chromosome inactivation is altered in IVF and cloned porcine embryos, possibly explaining problems of embryo survival with these techniques. The aim of this study was to gain more insights into the transcriptional dynamics of the porcine pre-implantation embryo, with a particular focus on sex-specific differences. RNA sequencing (RNA-Seq) was performed for individual blastocysts at 8, 10, and 12 days after ovulation, and the temporal development of sex-specific transcripts was analysed. German Landrace sows were cycle synchronized and inseminated with sperm of the same Pietrain boar. On Days 8, 10, and 12 post-insemination, sows were slaughtered and embryos were removed from the uterus using 10 mL of PBS (pH 7.4) per horn. Single embryos were shock frozen in liquid nitrogen and stored at –80°C until the extraction of RNA and DNA (AllPrep DNA/RNA Micro Kit, Qiagen, Valencia, CA, USA). Using the isolated DNA, the sex of the embryos was determined and 5 female and male embryos, respectively, were analysed per stage. Illumina TruSeq Stranded mRNA libraries (Illumina Inc., San Diego, CA, USA) were sequenced on a HiSEqn 2500 (Illumina Inc.), and 15 to 25 million 100-bp single-end reads were generated per sample. Reads were filtered and processed using Trimmomatic and mapped to the porcine genome assembly Sscrofa10.2 with TopHat2. Mapped reads were counted by the use of QuasR qCount based on the current National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/) GFF3 annotation file. Statistical analysis of count data was performed with the BioConductor R (https://www.bioconductor.org/) package DESEqn 2. At all 3 stages, we found 7 Y-linked transcripts that were highly expressed in male embryos (EIF2S3, EIF1AY, LOC100624590, LOC100625207, LOC100624329, LOC102162178, LOC100624937). On the other hand, 47 X-linked transcripts showed increased expression in female blastocysts, most of them at all 3 time points. However, a small number of genes (DDX3X, LAMP2, and RPS6KA3) were more highly expressed in females at Days 8 and 10 but more highly expressed in males at Day 12. Three X-linked genes (OFD1, KAL1, and LOC100525092) were more highly expressed in male embryos, although only at a low fold change of 1.2 to 1.4. Furthermore, expression of 8 transcripts located on autosomes was higher in females. In conclusion, our study expands the current knowledge of sex-specific gene expression in 8- to 12-day-old porcine blastocysts, a critical time period during pre-implantation embryo development.
Diverse developmental strategies of X chromosome dosage compensation in eutherian mammals
