- Letter to the Editor
- Open Access
Genetic sex determination of mice by simplex PCR
© The Author(s). 2017
- Received: 27 July 2017
- Accepted: 9 October 2017
- Published: 17 October 2017
Investigating fetal development in mice necessitates the determination of fetal sex. However, whilst the sex of adult and juvenile mice can be readily distinguished from anogenital distance, the sex of fetal and neonatal mice cannot be identified visually. Instead, genetic sex must be determined by PCR amplification of X chromosome genes with divergent Y chromosome gametologs. Existing simplex PCR methods are confounded by small size differences between amplicons, amplification of unexpected products, and biased amplification of the shorter amplicon.
Primers were designed flanking an 84 bp deletion of the X-linked Rbm31x gene relative to its Y-linked gametolog Rbm31y. A single product was amplified from XX samples, with two products amplified from XY samples. Amplicons were resolved by gel electrophoresis for 20 min, with unbiased amplification of both products observed in XY samples.
This method achieves rapid and unequivocal genetic sex determination of mice in low volume PCR reactions, reducing reagent usage and simultaneously eliminating shortcomings of previous methods.
- Sex genotyping of mice
- Simplex PCR
A short gestation and large litter size make the laboratory mouse a valuable and widely used research tool for studying fetal and placental development. However, sexual dimorphism in basal gene expression and placental phenotypes necessitates determination of fetal sex in such studies [1, 2]. Whilst the sex of adult and juvenile mice can be readily distinguished by comparing the anogenital distance, accurately identifying neonatal sex is more problematic , whereas fetal sex cannot be identified visually. Instead, genetic sex must be determined by PCR. The first such method amplified the Y chromosome genes Sry or Zfy, with amplification of the X chromosome microsatellite repeat DXNds3 serving as an amplification control . Subsequent variations to the protocol have targeted different regions of Sry or Zfy, or used alternative autosomal or X chromosome genes as amplification controls [5–9]. Such methods rely on successful amplification of the Y chromosome to identify XY animals, with XX animals inferred by the absence of amplification from the Y chromosome. However, these methods necessitate multiplex PCR reactions in which two sets of primers are used to amplify the two targets in the same reaction, presenting difficulties not encountered in simplex PCR methods [10, 11].
Summary of existing simplex PCR methods for determining genetic sex of mice
217 bp (X)
198 bp (Y)
XX 217 bp
XY 198 bp; 217 bp
Forward primer is mismatched to Uba1y.
Small difference (19 bp) in amplicon size necessitates extended gel run time.
331 bp (X)
302 bp (Y)
XX 331 bp
XY 331 bp; 302 bp
Primers are mismatched to Kdm5c.
Small difference (29 bp) in amplicon size necessitates extended gel run time.
685 bp (X)
280 bp (Y)
XX 685 bp, ~ 660 bp, ~ 480 bp
XY 280 bp
Additional products amplified from X chromosome.
Failure to amplify X chromosome in XY samples.
The recently sequenced male-specific region of the mouse Y chromosome identified additional X chromosome genes with acquired and amplified Y chromosome gametologs , providing the opportunity to explore potential novel targets to be utilized in a simplex PCR-based method of sex genotyping. Many of the acquired Y chromosome genes belong to large gene families, with dozens, or even hundreds, of Y chromosome copies, and estimates of up to 25 X-chromosome gametologs . This amplification and subsequent divergence of family members explains the amplification of additional unanticipated products from Xlr . Unique amongst the annotated acquired Y-chromosome genes is the two-copy Y-linked Rbm31y and the single copy X-linked Rbm31x. The low copy number of these gametologs mitigates the risk of divergent family members yielding amplification of additional products. Sequence alignment of Rbm31x and Rbm31y identified a high degree of sequence homology and revealed an 84 bp deletion in Rbm31x compared with Rbm31y. This work reports the development and testing of a new method of sex determination of fetal mice by amplification of the Rbm31x/y genes.
Archived yolk sacs and ear biopsies from 129S2/SvHsd and C57BL/6JOlaHsd strains (Envigo) had been previously lysed by incubation overnight at 55 °C in 450 μl (yolk sacs) or 100 μl (ear biopsies) lysis buffer (50 mM Tris (pH 8), 10 mM EDTA, 20 mM NaCl and 0.031% SDS) supplemented with 400 μg/ml Proteinase K (Promega). Lysates were vortexed, heated at 95 °C for 15 min, cooled for 10 min and 0.6 μl used as template in a 15 μl PCR reaction containing 1X buffer, 0.2 mM each dNTP (ThermoFisher), 0.17 μM each primer (Sigma), and 0.5U DreamTaq HotStart DNA Polymerase (ThermoFisher). Thermocycler conditions were 94 °C for 2 min, followed by 30 cycles of 94 °C for 20 s, 60 °C for 20 s and 72 °C for 30 s, with a final elongation at 72 °C for 5 min. Reactions were run on a MyCycler (Bio-Rad), with a total run time of under 70 min. PCR reactions were mixed with 6X Orange G Gel Loading Dye, loaded on to a 1% (w/v) agarose gel containing 10 μl SafeView (NBS Biologicals), electrophoresed in 1X TAE (40 mM Tris acetate, 2 mM Na2EDTA) at 10 V/cm for 20 min and visualized on a UV transilluminator. Fetal sex was confirmed using Uba1/Ube1y1 primers as described previously .
The Rmb31x/y method yielded the anticipated results from yolk sac samples from both 129S2/SvHsd and C57BL/6JOlaHsd strain backgrounds, with two products of 269 and 353 bp produced in male samples with only the 269 bp product produced in female samples (Fig. 1b, c), with results confirmed using primers for Uba1/Uba1y . The methodology was further validated by performing the PCR assay in a blinded manner on ear biopsies from adult animals from both 129S2/SvHsd and C57BL/6JOlaHsd backgrounds for which sex had been determined visually (Fig. 1d). The visually determined sex was confirmed by the genetically determined sex for all samples tested.
Further to facilitating rapid, unambiguous genetic sex determination of mice, this method also achieves substantial savings in reagent usage. The ability to use lysates directly as template eliminates the need to purify genomic DNA prior to PCR. The low volume (15 μl compared with typical 25 μl reaction volume) PCR reactions also reduce reagent usage including primers, dNTPs, and Taq polymerase. Furthermore, the ability to resolve PCR reactions on 1% gels as opposed to 2% gels further saves on reagents.
In summary, this method facilitates rapid genetic sex determination of mice from lysed samples within 2 h, substantially reducing reagent usage and yielding results that are unambiguous and simple to interpret.
SJT was supported by a Next Generation Fellowship from the Centre for Trophoblast Research, University of Cambridge.
Availability of data and materials
No animals were generated specifically for this study; only archived yolk sac material was used.
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The author declares that he has no competing interests.
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