In viviparous mammals, preimplantation embryo development occurs post fertilization and prior to implantation in the uterus.
During this period of time the egg develops into a zygote which activates its own genome, and goes through a series of cell divisions called cleavage divisions to ultimately become a blastocyst ready to implant in the uterus and develop into a new organism.
The preimplantation embryo development (Ped) gene regulates the rate of preimplantation embryonic cleavage division and subsequent embryo survival in mice. In the mouse, the Ped gene product is Qa-2 protein, a non-classical major histocompatibility complex (MHC) class Ib molecule encoded by four tandem genes, Q6/Q7/Q8/Q9. Most inbred strains of mice have all four genes on each allelic chromosome, making a total of eight Qa-2 encoding genes, but there are a few strains that are missing all eight genes, defining a null allele. Mouse embryos possessing the Qa-2 encoding genes express Qa-2 protein on their surface and develop at a faster rate and have a greater chance of survival compared to mouse embryos expressing the null allele.
The B6.K1 and B6.K2 strains of mice are congenic strains that have identical genetic loci with the exception of the genes that encode Qa-2.
The B6.K1 mice have a deletion of the four Qa-2 encoding genes whereas the B6.K2 mice do possess all four Qa-2 encoding genes.
Using these mice as either controls or subjects I have done research towards three specific aims: (1) Test the hypothesis that the number of Qa-2 encoding genes varies in the wild mouse population; (2) Test the hypothesis that male preimplantation embryos develop faster than female preimplantation embryos from the B6.K1 and B6.K2 strains of mice; (3) Test the hypothesis that the Ped gene is influenced by the expression of two microRNAs, miR-125a and miR-125b.
The results from the first specific aim showed that there was great variability in the number of Qa-2 encoding genes in 32 wild mice tested.
The wild mouse with the highest number of Qa-2 encoding genes had 85 such genes, whereas one wild mouse was discovered without any Qa-2 encoding genes.
The results from the second specific aim showed that there was no statistically significant difference in the ratio of male to female preimplantation embryos in either strain.
Therefore, the Ped gene is entirely responsible for mediating the faster development of B6.K2 embryos compared to B6.K1 embryos.
Finally, the results from the third specific aim showed that the absence of one of the Qa-2 encoding genes, Q9, resulted in a 10-fold increase in expression of the developmental timing miRNA miR-125a , but not of its close family member miR-125b.
This finding is significant because miR-125a and its homolog lin-4 have been found to play important roles in development in many species.
Our results suggest that similar to lower organisms, miRNAs play an important role in developmental timing in mice.
Taken together, the findings presented in this dissertation have answered important basic research questions regarding Ped gene control of the rate of preimplantation embryo development and have led to the discovery of an entirely new way of thinking about gene regulation during mouse embryo development, namely the involvement of miRNAs.
