Rapid mitochondrial DNA segregation in primate preimplantation embryos precedes somatic and germline bottleneck

Abstract

The timing and mechanisms of mitochondrial DNA (mtDNA) segregation and transmission in mammals are poorly understood. Genetic bottleneck in female germ cells has been proposed as the main phenomenon responsible for rapid intergenerational segregation of heteroplasmic mtDNA. We demonstrate here that mtDNA segregation occurs during primate preimplantation embryogenesis resulting in partitioning of mtDNA variants between daughter blastomeres. A substantial shift toward homoplasmy occurred in fetuses and embryonic stem cells (ESCs) derived from these heteroplasmic embryos. We also observed a wide range of heteroplasmic mtDNA variants distributed in individual oocytes recovered from these fetuses. Thus, we present here evidence for a previously unknown mtDNA segregation and bottleneck during preimplantation embryo development, suggesting that return to the homoplasmic condition can occur during development of an individual organism from the zygote to birth, without a passage through the germline.

Figure 1. Schematic model demonstrating mtDNA segregation and bottleneck in primates

Heteroplasmic rhesus monkey oocytes with equal mixture of two wild-type mtDNA haplotypes were constructed and mtDNA transmission to preimplantation embryos, fetuses and germ cells was followed. We demonstrate rapid segregation of mtDNA variants between daughter blastomeres in preimplantation embryos. However, fetuses and ESCs derived from these embryos shifted towards homoplasmic conditions. This bottleneck suggests that possibly a few cells within an ICM contribute to the somatic cell lineage of embryo proper. In contrast, individual fetal oocytes (F2) showed a wide range of heteroplasmy. This model also implies that the majority of ICM cells may contribute to the germline.

Figure 2. Segregation of mtDNA in individual blastomeres of the 2-, 4- and 8-cell embryos

Segregation of alien mtDNA between daughter blastomeres of individual (A) 2-cell, (B) 4-cell and (C) 8-cell embryos expressed by range (maximum or minimum alien mtDNA values minus the mean median values). Comparison of mtDNA dispersal between 2-, 4- and 8-cell embryos based on the (D) coefficient of variation and (E) range. Different letters indicates P values <0.05. (F) MtDNA segregation between the ICM and TE in individual blastocysts. The mean alien mtDNA in ICM and TE were 56.8 ± 10.7% and 58.4 ± 9.0%, respectively. Data are represented as mean±s.d. See also Tables S1–S3.