BACKGROUND
although nuclear transfer has been applied to a range of species  <cit> , embryos generated via somatic cell nuclear transfer  generally have low developmental competency and many abnormalities occur in the course of development.
the underlying mechanisms for these limitations are unclear.
many recent studies on nuclear transfer embryos revealed that gene expression patterns in the cloned embryo, fetus and placenta were abnormal  <cit>  and suggested that these commonly observed abnormalities were attributed to inefficient or incomplete "nuclear reprogramming".
it is generally accepted that in nuclear transfer embryos, the reprogramming of gene expression is linked to epigenetic mechanisms and does not involve modification of dna sequences.
epigenetic modifications such as dna methylation of cpg islands, core histone acetylation, phosphorylation, ubiquitination and ribosylation can eventually alter the transcriptional status of individual genes or larger genomic regions.
particularly, for the normal development of embryos, proper modifications of both dna  and histones are necessary.
on one hand, dna methylation can lead to gene silencing and heterochromatin formation  <cit>  such that proper dna methylation patterns are crucial in the early development of postimplantation embryos, especially for some key reprogramming factors such as oct3/ <dig>  sox <dig> and nanog normally activated in the blastocyst stage  <cit> .
on the other hand, histone modifications, which often are directly associated with dna methylation patterns, also play an important role in heterochromatin formation, mitosis, and regulation of genes and regional chromosome structure.
methylation of histone  <dig> lysine  <dig>  and histone  <dig> lysine  <dig>  residues correlate with gene silencing by chromatin condensation  <cit> .
in nuclear transfer embryos, abnormal patterns of methylation for dna and h3k <dig> have been reported  <cit> .
on a more cellular level, the interaction between the donor nucleus and recipient cytoplast may influence a number of important biological functions in scnt during nuclear reprogramming.
when considering the karyoplast-cytoplast interaction, mitochondria are the most abundant organelle in cytoplasm and play an important role in development by supplying energy for normal cellular functions.
mitochondrial dna  is supplied mainly by the recipient oocyte during scnt, but is regulated by genes in the donor nucleus.
under the high oxygen environment, and with limited dna repair ability, mtdna has high rates of heritable polymorphism and de novo mutation which can result in many haplotypes.
in cattle, oocytes with various mtdna haplotypes usually have different atp content and this may affect the efficiency of in vitro production of embryos  <cit> .
however, the relationship and the underlying mechanisms between mtdna haplotypes and scnt efficiency have not been fully investigated.
in this study, we divided bovine mtdna into four haplotypes by pcr-rflp  with six restriction sites.
subsequently, scnt was performed using donor nuclei and recipient oocytes with particular mtdna haplotypes to compare developmental competency for different haplotype combinations.
when the donor cells and oocytes are of the same mtdna haplotype , higher scnt embryo survival rates were observed.
to further explore the underlying mechanisms, the status of homotype and heterotype scnt blastocysts, in vitro fertilization  blastocysts and in vivo cultured  blastocysts were compared using their histone and dna methylation patterns.
genome-wide h3k <dig> dimethylation profiles were detected by immunohistochemistry, and dna methylation status at the promoters of pluripotency genes oct- <dig>  nanog and sox <dig> was also analyzed.
low methylation levels in the promoter regions of these genes is thought to correlate with their high expression levels at the blastocyst stage.
both histone and dna methylation show that homotype scnt blastocysts have an epigenetic status closer to the control ivf embryos than heterotype scnt blastocysts.
this study indicates that homotype scnt has a more complete nuclear reprogramming than heterotype scnt and, therefore, is more competent for further embryonic development.
RESULTS
ovum pick-up
four mtdna haplotypes were identified in research herds of holstein and chinese yellow cattle .
two haplotypes, a and b, were chosen for preparation of donor nuclei and recipient oocytes as they represented 25% and 53%, respectively of several hundred cattle tested previously.
the average number of oocytes recovered per heifer was  <dig>  cumulus-oocyte complexes .
a total of  <dig> cocs were obtained from  <dig> opu procedures.
<dig> high-quality mii oocytes were used for scnt and  <dig> for ivf.
pre- and post-implantation development of embryos from homotype and heterotype scnt
as summarized in table  <dig>  after the 8-cell stage, homotype scnt embryos constructed from enucleated oocytes and donor cells with the same haplotype  had significantly higher developmental potential than heterotype scnt embryos , as well as a higher blastocyst rate  which is similar to rates from ivf embryos   <cit> .
note that the homotype scnt, heterotype scnt and ivf embryos were produced from the same opu session, and the procedures and assays for the three groups were kept as similar as possible.
differences in pregnancy rate at  <dig>   <dig> and  <dig> days of gestation were also observed between homotype and heterotype groups.
one hundred and three homotype scnt blastocysts and  <dig> heterotype scnt were transferred into  <dig> and  <dig> synchronous recipients, and resulted in  <dig> % and  <dig> % 90-day pregnancy rates for homotype and heterotype, respectively.
the percentage of live term birth for homotype and heterotype groups were  <dig> % and  <dig> %, respectively.
overall, the differences in postimplantation development were statistically significant starting at  <dig> days after implantation .
*kcc, karyoplast--cytoplast complexes.
values with different superscript letters a, b, c, d differ with statistical significance .
f: percentage based on number of surviving and fused reconstructed embryos, which were activated and cultured following kcc fusion.
g: percentage based on number of cleaved reconstructed embryos.
a, b, c, d values within same horizontal columns with different superscripts differ significantly .
histone h3k <dig> dimethylation
the patterns of distribution and intensity of dimethylated h3k <dig> were previously shown to closely parallel overall dna methylation  <cit> .
in our study, h3k <dig> methylation patterns of homotype scnt embryos more closely resembled the control ivf embryos than did heterotype scnt embryos .
in the ivf control, homotype a-a and b-b scnt groups,  <dig> % ,  <dig> %  and  <dig>  of blastocysts respectively established epigenetic asymmetry successfully, with a hypomethylated trophectoderm and a hypermethylated icm; whereas only  <dig> %  and  <dig> % heterotype a-b and b-a scnt groups achieved asymmetry .
a, b, c, d, e values in same columns with different superscripts differ significantly  based on fisher's exact test.
methylation status of pluripotency regulators
oct <dig>  nanog and sox <dig> are known to be critical for the maintenance of pluripotency, and the bisulfite conversion sequencing method was used to examine the methylation patterns of promoter regions of these three genes in normal in vivo developed embryos, as well as ivf, homotype scnt and heterotype scnt embryos.
about  <dig> - <dig> % of the total cpg sites examined were found to be methylated, and the overall methylation frequencies were different for embryos from the ivf, ivc and nt groups .
the relative methylation levels at various cpgs for a particular gene seem to be relatively stable within each sample , implying that the methylation of these promoters in embryos is biased for specific cpg positions.
the two scnt embryos showed various degrees of increased promoter methylation compared to the ivf and ivc groups; however, statistical testing confirmed more significantly increased promoter methylation levels in heterotype scnt compared to homotype scnt for oct <dig> and sox <dig>  but not nanog , relative to the control groups.
*: statistically significant difference compared to the homotype group  analysed with chi-square test.
✶ : statistically significant difference compared to the heterotype group  analysed with chi-square test.
discussion
early mammalian embryonic development, spanning fertilization to blastocyst, is a very dynamic process, especially in large animals such as cattle and goats.
numerous studies have reported the development capability of in vitro produced  embryos could be influenced by aberrant gene expression levels and epigenetic reprogramming  <cit> .
six to eight days post fertilization or after reconstruction for scnt embryos is a critical period when various developmentally important events occur, including the first cleavage division which is critical in determining the quality of subsequent embryo development.
in this study, we investigated whether the in vitro development of reconstructed bovine embryos produced by opu-scnt technique could be influenced by mtdna haplotype compatibility between the oocytes and donor cells, and we found that the developmental competency of homotype  scnt appeared to be better than that of heterotype  scnt, especially after the eight-cell stage.
this result is in agreement with our previous finding that autologous scnt embryos have a much improved developmental capacity compared to embryos derived with allogeneic scnt  <cit> .
with the autologous scnt, the donor cells and the recipient oocytes were derived from the same cattle and thus had the same mitochondrial genetic background whereas in the allogeneic group, the mitochondria haplotypes were not matched.
the percentage of pregnant animals at  <dig> days of gestation was also higher in the homologous haplotype scnt embryos compared to the heterogeneous group, indicating a more appropriate microenvironment for further embryonic development, as the nuclear-cytoplasmic compatibility in cloned embryos may prevent abnormality during gestation as previously reported  <cit> .
the birth rates of live offspring for a-a, b-b, a-b, and b-a haplotypes were  <dig> %,  <dig> %,  <dig> % and  <dig> %, respectively.
thus, from the reconstructed embryos to live calves, the survival rate for the homotype scnt group was  <dig> % of reconstructed embryos, and is a little higher than reported in a previous study   <cit> .
during the development of the reconstructed embryos, nuclear-cytoplasm interactions are necessary for successful reprogramming to ensure the proper activation of pluripotency genes.
after the donor nuclei enter the oocytes, the differentiated somatic cell nuclei will be epigenetically rebuilt with an "erase-and-rebuild" strategy to set a new life cycle, and then the genes in dedifferentiated nuclei will be expressed to control embryo development, instead of maternal rnas and proteins after the ega  period  <cit> .
in the present study, we found that the blastocyst formation rates as well as postimplantation development capability for the haplotype of a-a scnt reconstructed embryos were higher than the other types  .
this suggests that the mtdna haplotype of oocytes, in addition to the nuclear-cytoplasmic compatibility of mitochondria, could be an important factor for the development of cloned embryos, possibly related to mtdna copy numbers and atp content in the haplotype a oocyte as previous studies indicated  <cit> .
during development and differentiation, somatic nuclei acquire highly specialized dna and chromatin modifications, i.e.
the epigenetic marks, which are thought to result in cellular memory of the differentiated states  <cit> .
this may involve epigenetic changes similar to those that occur in normal embryos during early development  <cit> .
however, there is accumulating evidence that epigenetic reprogramming is severely deficient in cloned embryos  <cit> .
several reports reveal inefficient demethylation and inappropriate reestablishment of dna de novo methylation in quantitative and qualitative patterns during scnt  <cit> .
given that there are potential variations associated with development for the two types of scnt embryos , we further analyzed epigenetic modifications for dna and histones in these embryos.
using hoechst  <dig> and an antibody that identifies h3k <dig> methylation specifically in the context of constitutive heterochromatin, we double labelled embryonic cells to establish the distribution of h3k <dig> methylation in ivf control embryos and embryos produced by homotype and heterotype scnt.
interestingly the h3k <dig> profile of embryos from homotype scnt resembles more closely that from the control  embryos compared to the heterotype scnt embryos, and only  <dig> out of  <dig> heterotype scnt embryos from b-a and a-b groups showed the correct epigenetic pattern.
this suggests that homotype scnt embryos have fewer h3k <dig> epigenetic defects than the heterotype scnt embryos .
dna methylation/demethylation processes occur during pre-implantation development.
genes such as oct <dig>  klf <dig>  foxd <dig> and nanog, which are required for the maintenance of pluripotency, are inactivated in donor fibroblast cells because of the hypermethylation of their promoters until cloned embryos begin demethylation and development.
in the case of oct <dig>  the down-regulation is triggered by trans-acting repressors and the methyl-cpg-binding proteins mbd <dig> and mbd <dig>  <cit> .
the locus then gradually becomes "heterochromatinised" followed by the methylation of h3k <dig>  the binding of heterochromatin protein  <dig>  beta and dna methylation  <cit> .
hypomethylation of regulatory regions of these genes would generally be expected to result in up-regulated expression.
we analyzed the methylation profiles for three pluripotency-related genes, oct <dig>  sox <dig> and nanog in the homotype and heterotype scnt  using in vitro and in vivo produced blastocysts.
although there are various degrees of increased methylation in the scnt embryos compared to those from ivf or in vivo produced embryos, our data indicated a much more significant change in dna methylation for oct <dig> and sox <dig> for the heterotype compared to homotype scnt embryos.
on the other hand, methylation for nanog was increased for both types of scnt embryos but with no significant differences between the two groups.
previous studies showed that the initiation of transcriptional activity of the embryonic genome occurs around the transition period from 8-cell to 16-cell for nt embryos  <cit> , and the changes in epigenetic modifications occurring during this period  are critical for the appropriate reprogramming to be initiated.
the differences seen in epigenetic status of the scnt and the ivf embryos, as well as between the homotype and the heterotype embryos, reflects the underlying developmental competence of these embryos; future analysis of factors involved in critical functions in preimplantation development, such as the somatic-type dna methyltransferase and the dynamic patterns of methylation in h3k <dig> and pluripotency-related genes, may elucidate the underlying mechanisms governing reprogramming in nt and embryos.
CONCLUSIONS
we report that the compatibility of donor nuclei and recipient oocytes in scnt is associated with epigenetic modifications which result in different responses to reprogramming.
these results may explain why certain mitochondrial dna haplotypes are inherently more efficient for cloning.
therefore, an approach may be available to improve cloning efficiency by using proper donor-recipient populations to control mitochondrial dna haplotype combinations.

 although nuclear transfer has been applied to a range of species, embryos generated via somatic cell nuclear transfer generally have low developmental competency and many abnormalities occur in the course of development. the underlying mechanisms for these limitations are unclear. many recent studies on nuclear transfer embryos revealed that gene expression patterns in the cloned embryo, fetus and placenta were abnormal and suggested that these commonly observed abnormalities were attributed to inefficient or incomplete "nuclear reprogramming". it is generally accepted that in nuclear transfer embryos, the reprogramming of gene expression is linked to epigenetic mechanisms and does not involve modification of dna sequences. epigenetic modifications such as dna methylation of cpg islands, core histone acetylation, phosphorylation, ubiquitination and ribosylation can eventually alter the transcriptional status of individual genes or larger genomic regions. particularly, for the normal development of embryos, proper modifications of both dna and histones are necessary. on one hand, dna methylation can lead to gene silencing and heterochromatin formation such that proper dna methylation patterns are crucial in the early development of postimplantation embryos, especially for some key reprogramming factors such as oct3/ sox and nanog normally activated in the blastocyst stage. on the other hand, histone modifications, which often are directly associated with dna methylation patterns, also play an important role in heterochromatin formation, mitosis, and regulation of genes and regional chromosome structure. methylation of histone lysine and histone lysine residues correlate with gene silencing by chromatin condensation. in nuclear transfer embryos, abnormal patterns of methylation for dna and h3k have been reported. on a more cellular level, the interaction between the donor nucleus and recipient cytoplast may influence a number of important biological functions in scnt during nuclear reprogramming. when considering the karyoplast-cytoplast interaction, mitochondria are the most abundant organelle in cytoplasm and play an important role in development by supplying energy for normal cellular functions. mitochondrial dna is supplied mainly by the recipient oocyte during scnt, but is regulated by genes in the donor nucleus. under the high oxygen environment, and with limited dna repair ability, mtdna has high rates of heritable polymorphism and de novo mutation which can result in many haplotypes. in cattle, oocytes with various mtdna haplotypes usually have different atp content and this may affect the efficiency of in vitro production of embryos. however, the relationship and the underlying mechanisms between mtdna haplotypes and scnt efficiency have not been fully investigated. in this study, we divided bovine mtdna into four haplotypes by pcr-rflp with six restriction sites. subsequently, scnt was performed using donor nuclei and recipient oocytes with particular mtdna haplotypes to compare developmental competency for different haplotype combinations. when the donor cells and oocytes are of the same mtdna haplotype, higher scnt embryo survival rates were observed. to further explore the underlying mechanisms, the status of homotype and heterotype scnt blastocysts, in vitro fertilization blastocysts and in vivo cultured blastocysts were compared using their histone and dna methylation patterns. genome-wide h3k dimethylation profiles were detected by immunohistochemistry, and dna methylation status at the promoters of pluripotency genes oct- nanog and sox was also analyzed. low methylation levels in the promoter regions of these genes is thought to correlate with their high expression levels at the blastocyst stage. both histone and dna methylation show that homotype scnt blastocysts have an epigenetic status closer to the control ivf embryos than heterotype scnt blastocysts. this study indicates that homotype scnt has a more complete nuclear reprogramming than heterotype scnt and, therefore, is more competent for further embryonic development.  ovum pick-up four mtdna haplotypes were identified in research herds of holstein and chinese yellow cattle. two haplotypes, a and b, were chosen for preparation of donor nuclei and recipient oocytes as they represented 25% and 53%, respectively of several hundred cattle tested previously. the average number of oocytes recovered per heifer was cumulus-oocyte complexes. a total of cocs were obtained from opu procedures. high-quality mii oocytes were used for scnt and for ivf. pre- and post-implantation development of embryos from homotype and heterotype scnt as summarized in table after the 8-cell stage, homotype scnt embryos constructed from enucleated oocytes and donor cells with the same haplotype had significantly higher developmental potential than heterotype scnt embryos, as well as a higher blastocyst rate which is similar to rates from ivf embryos. note that the homotype scnt, heterotype scnt and ivf embryos were produced from the same opu session, and the procedures and assays for the three groups were kept as similar as possible. differences in pregnancy rate at and days of gestation were also observed between homotype and heterotype groups. one hundred and three homotype scnt blastocysts and heterotype scnt were transferred into and synchronous recipients, and resulted in % and % 90-day pregnancy rates for homotype and heterotype, respectively. the percentage of live term birth for homotype and heterotype groups were % and %, respectively. overall, the differences in postimplantation development were statistically significant starting at days after implantation. *kcc, karyoplast--cytoplast complexes. values with different superscript letters a, b, c, d differ with statistical significance. f: percentage based on number of surviving and fused reconstructed embryos, which were activated and cultured following kcc fusion. g: percentage based on number of cleaved reconstructed embryos. a, b, c, d values within same horizontal columns with different superscripts differ significantly. histone h3k dimethylation the patterns of distribution and intensity of dimethylated h3k were previously shown to closely parallel overall dna methylation. in our study, h3k methylation patterns of homotype scnt embryos more closely resembled the control ivf embryos than did heterotype scnt embryos. in the ivf control, homotype a-a and b-b scnt groups, %, % and of blastocysts respectively established epigenetic asymmetry successfully, with a hypomethylated trophectoderm and a hypermethylated icm; whereas only % and % heterotype a-b and b-a scnt groups achieved asymmetry. a, b, c, d, e values in same columns with different superscripts differ significantly based on fisher's exact test. methylation status of pluripotency regulators oct nanog and sox are known to be critical for the maintenance of pluripotency, and the bisulfite conversion sequencing method was used to examine the methylation patterns of promoter regions of these three genes in normal in vivo developed embryos, as well as ivf, homotype scnt and heterotype scnt embryos. about - % of the total cpg sites examined were found to be methylated, and the overall methylation frequencies were different for embryos from the ivf, ivc and nt groups. the relative methylation levels at various cpgs for a particular gene seem to be relatively stable within each sample, implying that the methylation of these promoters in embryos is biased for specific cpg positions. the two scnt embryos showed various degrees of increased promoter methylation compared to the ivf and ivc groups; however, statistical testing confirmed more significantly increased promoter methylation levels in heterotype scnt compared to homotype scnt for oct and sox but not nanog, relative to the control groups. *: statistically significant difference compared to the homotype group analysed with chi-square test. ✶: statistically significant difference compared to the heterotype group analysed with chi-square test. discussion early mammalian embryonic development, spanning fertilization to blastocyst, is a very dynamic process, especially in large animals such as cattle and goats. numerous studies have reported the development capability of in vitro produced embryos could be influenced by aberrant gene expression levels and epigenetic reprogramming. six to eight days post fertilization or after reconstruction for scnt embryos is a critical period when various developmentally important events occur, including the first cleavage division which is critical in determining the quality of subsequent embryo development. in this study, we investigated whether the in vitro development of reconstructed bovine embryos produced by opu-scnt technique could be influenced by mtdna haplotype compatibility between the oocytes and donor cells, and we found that the developmental competency of homotype scnt appeared to be better than that of heterotype scnt, especially after the eight-cell stage. this result is in agreement with our previous finding that autologous scnt embryos have a much improved developmental capacity compared to embryos derived with allogeneic scnt. with the autologous scnt, the donor cells and the recipient oocytes were derived from the same cattle and thus had the same mitochondrial genetic background whereas in the allogeneic group, the mitochondria haplotypes were not matched. the percentage of pregnant animals at days of gestation was also higher in the homologous haplotype scnt embryos compared to the heterogeneous group, indicating a more appropriate microenvironment for further embryonic development, as the nuclear-cytoplasmic compatibility in cloned embryos may prevent abnormality during gestation as previously reported. the birth rates of live offspring for a-a, b-b, a-b, and b-a haplotypes were %, %, % and %, respectively. thus, from the reconstructed embryos to live calves, the survival rate for the homotype scnt group was % of reconstructed embryos, and is a little higher than reported in a previous study. during the development of the reconstructed embryos, nuclear-cytoplasm interactions are necessary for successful reprogramming to ensure the proper activation of pluripotency genes. after the donor nuclei enter the oocytes, the differentiated somatic cell nuclei will be epigenetically rebuilt with an "erase-and-rebuild" strategy to set a new life cycle, and then the genes in dedifferentiated nuclei will be expressed to control embryo development, instead of maternal rnas and proteins after the ega period. in the present study, we found that the blastocyst formation rates as well as postimplantation development capability for the haplotype of a-a scnt reconstructed embryos were higher than the other types. this suggests that the mtdna haplotype of oocytes, in addition to the nuclear-cytoplasmic compatibility of mitochondria, could be an important factor for the development of cloned embryos, possibly related to mtdna copy numbers and atp content in the haplotype a oocyte as previous studies indicated. during development and differentiation, somatic nuclei acquire highly specialized dna and chromatin modifications, i.e. the epigenetic marks, which are thought to result in cellular memory of the differentiated states. this may involve epigenetic changes similar to those that occur in normal embryos during early development. however, there is accumulating evidence that epigenetic reprogramming is severely deficient in cloned embryos. several reports reveal inefficient demethylation and inappropriate reestablishment of dna de novo methylation in quantitative and qualitative patterns during scnt. given that there are potential variations associated with development for the two types of scnt embryos, we further analyzed epigenetic modifications for dna and histones in these embryos. using hoechst and an antibody that identifies h3k methylation specifically in the context of constitutive heterochromatin, we double labelled embryonic cells to establish the distribution of h3k methylation in ivf control embryos and embryos produced by homotype and heterotype scnt. interestingly the h3k profile of embryos from homotype scnt resembles more closely that from the control embryos compared to the heterotype scnt embryos, and only out of heterotype scnt embryos from b-a and a-b groups showed the correct epigenetic pattern. this suggests that homotype scnt embryos have fewer h3k epigenetic defects than the heterotype scnt embryos. dna methylation/demethylation processes occur during pre-implantation development. genes such as oct klf foxd and nanog, which are required for the maintenance of pluripotency, are inactivated in donor fibroblast cells because of the hypermethylation of their promoters until cloned embryos begin demethylation and development. in the case of oct the down-regulation is triggered by trans-acting repressors and the methyl-cpg-binding proteins mbd and mbd. the locus then gradually becomes "heterochromatinised" followed by the methylation of h3k the binding of heterochromatin protein beta and dna methylation. hypomethylation of regulatory regions of these genes would generally be expected to result in up-regulated expression. we analyzed the methylation profiles for three pluripotency-related genes, oct sox and nanog in the homotype and heterotype scnt using in vitro and in vivo produced blastocysts. although there are various degrees of increased methylation in the scnt embryos compared to those from ivf or in vivo produced embryos, our data indicated a much more significant change in dna methylation for oct and sox for the heterotype compared to homotype scnt embryos. on the other hand, methylation for nanog was increased for both types of scnt embryos but with no significant differences between the two groups. previous studies showed that the initiation of transcriptional activity of the embryonic genome occurs around the transition period from 8-cell to 16-cell for nt embryos, and the changes in epigenetic modifications occurring during this period are critical for the appropriate reprogramming to be initiated. the differences seen in epigenetic status of the scnt and the ivf embryos, as well as between the homotype and the heterotype embryos, reflects the underlying developmental competence of these embryos; future analysis of factors involved in critical functions in preimplantation development, such as the somatic-type dna methyltransferase and the dynamic patterns of methylation in h3k and pluripotency-related genes, may elucidate the underlying mechanisms governing reprogramming in nt and embryos.  we report that the compatibility of donor nuclei and recipient oocytes in scnt is associated with epigenetic modifications which result in different responses to reprogramming. these results may explain why certain mitochondrial dna haplotypes are inherently more efficient for cloning. therefore, an approach may be available to improve cloning efficiency by using proper donor-recipient populations to control mitochondrial dna haplotype combinations. 
the interaction between the karyoplast and cytoplast plays an important role in the efficiency of somatic cell nuclear transfer, but the underlying mechanism remains unclear. it is generally accepted that in nuclear transfer embryos, the reprogramming of gene expression is induced by epigenetic mechanisms and does not involve modifications of dna sequences. post-implantation development ability, pregnancy rate up to day of gestation, as well as percent of term births were higher in the homotype scnt groups than in the heterotype groups. we investigated whether the in vitro development of reconstructed bovine embryos produced by scnt would be influenced by mtdna haplotype compatibility between the oocytes and donor cells. this suggests that the compatibility of mtdna haplotypes between donor cells and host oocytes can significantly affect the developmental competence of reconstructed embryos in scnt, and may include an epigenetic mechanism. as mtdna comes from the recipient oocyte during scnt and is regulated by genes in the donor nucleus, it is a perfect model to investigate the interaction between donor nuclei and host oocytes in scnt. in addition, homotype and heterotype scnt embryos showed different methylation patterns of histone 3-lysine genome-wide and at pluripotency-related genes. both histone and dna methylation show that homotype scnt blastocysts have a more successful epigenetic asymmetry pattern than heterotype scnt blastocysts, which indicates more complete nuclear reprogramming. in cattle, oocytes with various mitochondrial dna haplotypes usually have different atp content and can further affect the efficiency of in vitro production of embryos. embryos from homotype a-a or b-b showed significantly higher developmental ability at blastocyst stages than the heterotype a-b or b-a combinations.
 the interaction between the karyoplast and cytoplast plays an important role in the efficiency of somatic cell nuclear transfer, but the underlying mechanism remains unclear. it is generally accepted that in nuclear transfer embryos, the reprogramming of gene expression is induced by epigenetic mechanisms and does not involve modifications of dna sequences. in cattle, oocytes with various mitochondrial dna haplotypes usually have different atp content and can further affect the efficiency of in vitro production of embryos. as mtdna comes from the recipient oocyte during scnt and is regulated by genes in the donor nucleus, it is a perfect model to investigate the interaction between donor nuclei and host oocytes in scnt.  we investigated whether the in vitro development of reconstructed bovine embryos produced by scnt would be influenced by mtdna haplotype compatibility between the oocytes and donor cells. embryos from homotype a-a or b-b showed significantly higher developmental ability at blastocyst stages than the heterotype a-b or b-a combinations. post-implantation development ability, pregnancy rate up to day of gestation, as well as percent of term births were higher in the homotype scnt groups than in the heterotype groups. in addition, homotype and heterotype scnt embryos showed different methylation patterns of histone 3-lysine genome-wide and at pluripotency-related genes.  both histone and dna methylation show that homotype scnt blastocysts have a more successful epigenetic asymmetry pattern than heterotype scnt blastocysts, which indicates more complete nuclear reprogramming. this may result from variability in their epigenetic patterns and responses to nuclear reprogramming. this suggests that the compatibility of mtdna haplotypes between donor cells and host oocytes can significantly affect the developmental competence of reconstructed embryos in scnt, and may include an epigenetic mechanism. 