This protocol describes an accurate, inexpensive, rapid and non-toxic method to determine the sex of Day 30 porcine embryo using PCR method after grinding an embryo into powder without phenol chloroform extraction and DNA column purification.
Research into prenatal programming in the pig has shown that the sex of the developing embryo or fetus can influence the developmental outcome. Therefore, the ability to determine an embryo's sex is necessary in many experiments particularly regarding early development. The present protocol demonstrates an inexpensive, rapid and non-toxic preparation of pig genomic DNA for use with PCR. Day 30 embryos must be humanely collected according to the guidelines established by Institutional Animal Policy and Welfare Committees for the present protocol. The preparation of the whole embryo for this PCR based sexing technique simply involves grinding the frozen embryo to a fine powder using a pre-chilled mortar and pestle. PCR-quality DNA is released from a small amount of embryo powder by applying a hot incubation in an alkaline lysis reagent. Next, the DNA solution is mixed with neutralization buffer and used directly for PCR. Two primer pairs are generated to detect specific sex determining region of the Y- chromosome (SRY) and ZFX region of the X- chromosome with high accuracy and specificity. The same protocol can be applied to other elongated embryos (Day 10 to Day 14) earlier than Day 30. Also, this protocol can be carried with 96-welled plates when screening a large number of embryos, making it feasible for automation and high-throughput sex typing.
The domestic pig has become a fundamental research subject in development, genetics and nutrition in both human and livestock sectors. The potential of pigs as biomedical models for human research can be attributed to their physiological similarities. In livestock, the manipulation of sex ratio can enhance the effectiveness of selection and genetic improvement programs1. Sexing individual embryos is a fundamental tool used in many experimental investigations including but not limited to genotype, epigenetics and X inactivation of sexual dimorphism during early embryo development2.
Studies in mice suggest that maternal diet and other factors may result in gender imbalance3. In pigs, causes of sex ratio imbalance include paternal breed4, uterine capacity5, and the sow's metabolic condition6. Since the differences observed in embryos and litters can be influenced by sexual dimorphism, researchers should be aware of embryo sex and sex ratios before drawing conclusions regarding their research. The development of efficient tools and protocols for sexing pig embryos at Day 30 of development will be discussed here.
Various methods of sex typing have been developed for genetic studies in model organisms and livestock. Particularly in livestock, identifying male and female early embryos is a very common practice to enhance genetic selection for breeding programs. Early embryos karyotyping in pig using Giemsa7 or the intense fluorescence8,9 techniques have been used for sex typing. However, these methods are time consuming and not suitable for screening large numbers of embryos quickly and accurately.
The most effective sex typing method is DNA amplification using a heat stable DNA polymerase and a pair of primers. DNA sexing by PCR method is more specific, rapid and sensitive, only requiring a minute amount of cellular materials. The first PCR-based embryo sexing was performed on humans10, and later in mice11, cattle12, buffalo13 and sheep14 pre-implantation embryos. In the pig, the earliest DNA sex typing method was established for pre-implantation embryos via a single pair of Y-chromosome specific DNA primers15. However, the most common PCR primers for sex determination were selected from the Y-chromosome of male specific SRY gene16 and the non-sex discriminative region of a zinc-finger gene located at both X and Y chromosome17. Subsequently, these primers have been applied to determine the sex of Day 30 embryos in this study with improved specificity of the primers to detect only the X- chromosome of a zinc-finger gene.
Genomic DNA from porcine pre-implantation embryos can be extracted by exposing an intact blastocyst to buffer with proteinase K16 or by taking a biopsy of a few cells from the individual early cleavage embryos15 and using them for direct PCR. However, release of DNA from porcine blood, hair, tissues or a large conceptus over a few centimeters in size is not effectively done using the proteinase K method. DNA extraction methods for these materials have been established using either time consuming phenol/chloroform protocols6 or expensive column based kits18. In order to avoid the use of potentially toxic chemicals, there is a trend to develop inexpensive, easy and phenol-free DNA extraction methods. This type of protocol for the isolation of PCR-quality genomic DNA from mouse19 and zebrafish20 tissues has been established using hot sodium hydroxide and Tris (HotSHOT). This study provides a protocol to obtain DNA with modified HotSHOT and redesigned duplex primer pairs for PCR sex typing directly from cell lysates of Day 30 porcine embryos with high accuracy.
Most of the existing protocols related to porcine embryos DNA sex typing are only suitable for early stage pre-implantation stage15,16. We have successfully developed a protocol suitable for porcine embryo screening during late gestation. Based on studies with similar developmental stages of embryos from previous studies6,18, the present protocol is considered to be safer and low cost.
This protocol is also suitable for a large number of samples for sex typing using PCR b…
The authors have nothing to disclose.
The authors would like to acknowledge the cooperation and financial contributions of the following research funding agency: Alberta Livestock and Meat Agency Ltd., Pork CRC, Alberta Pork, Hypor A Hendrix Genetics Company and NSERC CRSNG.
KAPA HiFi HotStart Ready Mix PCR kit | KAPABiosystems | KR0370 | other Hot Start Taq polymerase can be used after optimization |
SYBR Safe DNA Gel Stain | Life Technologies | S33102 | Ethidium bromide can be substituted for SYBR Safe |
Pig female and male genomic DNA | Zyagen | GP-160-F1 & GP-160-M5 | Postive controls from the tissues of known sex DNA can be used. |
Typhoon FLA 9500 laser scanner | GE Healthcare Life Sciences | 28-9969-43 | other imaging system can be used |
Free Soft Nitrile Examination Gloves | WWR | 89038-270 | any other examination glove can be used |
Sodium hydroxide, solid | Fisher | BP 359 -212 | Molecular Biology Grade |
Eppendorff DNA LoBind Tubes, 1.5 ml, PCR clean | Eppendorff | 0030 108.051 | heat resistant |
ThermoStat plus | Eppendorff | 22670204 | Use as a incubator for 95C, don't need to use the heater |
Toothpick | Bunzl Plc | 75200815 | Any round wooden toothpicks can be used – quality wood |
Microcentrifuge 5417R/5417C | Eppendorff | 22621807 | This model was discontinued. But another newer model can be used |
Microspatula | Fisher | SDI28540115 | Autoclaved before use each time. |