**1.3.4 Logistic advancement on assisted reproduction technologies**

A traditional human *in vitro* fertilization cycle involves hormone induced ovarian stimulation for oocyte retrieval, *in vitro* fertilization, culture of embryos to blastocyst, selection of embryos and transfer of embryos into a recipient in an uninterrupted program. Surplus embryos are discarded. If the first pregnancy failed or another pregnancy is desired, the whole cycle has to be started again. Cryopreserving surplus embryos from the first round of the program can now be a backup for the second or more rounds of pregnancy. It can be done simply by transferring the thawed embryos to the recipients when the endometrial conditions are ready (Parriego et al., 2007).

In addition, for preimplantation genetic diagnosis programs, embryos must be transferred back to the mother at or younger than day-6 blastocysts, and so biopsies for prenatal diagnosis must be taken at the latest at day-6. More genetic materials (i.e. blastomeres or trophoblastic cells) can be obtained if the biopsy is performed later. However, accompanied shortcomings will be less time for genetic tests to be performed (Manipalviratn et al., 2009).

Cryopreservation of biopsied embryos can eliminate such conflicts between the quantity of biopsy material and the quality of genetic tests (as a function of available time; Figure 2). It has been demonstrated recently on mouse and then human embryos that biopsied embryos at various stages can survive cryopreservation well (Krzyminska & O'Neill, 1991; Wilton et al., 1989; Liu et al., 1993; Snabes et al., 1993 ; Zhang et al., 2009; Keskintepe et al., 2009 ).

The continuous passing of live animals in a small population may lead animal strains to accumulate spontaneous mutations and undergo genetic drift. In mouse, as an example of a lower mutation rate because of its relatively long life cycle, about 0.4 mutations are accumulated in each genome in each generation (Drake et al., 1998). Using this estimation, and assuming there are four generations per year, about ten mutations are accumulated in each descendent mouse diploid genome every 6.25 years (Tsang & Chow, 2010). After four years of sibling intercrosses, there is a 90% probability that more than one mutation can be fixed in a particular mouse line (Stevens et al., 2007). To circumvent this problem, the Jackson Laboratory (Bar Harbor, Maine, USA) adopted the Genetic Stability Program to refresh some mouse colonies with cryopreserved embryos once every five generations. This strategy aims at wiping out spontaneous mutations accumulated over time to ensure

Maintaining genetic stability and diversity of wild parasitic nematodes collection is vital for research on parasite-host interactions, drug resistance and their applications. For example, prolonged passage of insecticidal nematodes (i.e. entomopathogenic nematodes) can cause a reduction of traits beneficial to pest control (Shapiro et al., 1996; Stuart & Gaugler, 1996; Wang & Grewal, 2002). Three continuous passages of *Galleria mellonella in vitro* resulted in a significant reduction in reproductive potential, and attenuated tolerance to heat, UV and desiccation (Wang & Grewal, 2002). These observations suggest that a selective pressure had been exerted on an isolated population that experienced continuous passages. The original genetic diversity in an isolated population will thus be largely reduced if cryopreservation is not practiced immediately after collection, before experimental analysis is performed.

A traditional human *in vitro* fertilization cycle involves hormone induced ovarian stimulation for oocyte retrieval, *in vitro* fertilization, culture of embryos to blastocyst, selection of embryos and transfer of embryos into a recipient in an uninterrupted program. Surplus embryos are discarded. If the first pregnancy failed or another pregnancy is desired, the whole cycle has to be started again. Cryopreserving surplus embryos from the first round of the program can now be a backup for the second or more rounds of pregnancy. It can be done simply by transferring the thawed embryos to the recipients when the

In addition, for preimplantation genetic diagnosis programs, embryos must be transferred back to the mother at or younger than day-6 blastocysts, and so biopsies for prenatal diagnosis must be taken at the latest at day-6. More genetic materials (i.e. blastomeres or trophoblastic cells) can be obtained if the biopsy is performed later. However, accompanied shortcomings

Cryopreservation of biopsied embryos can eliminate such conflicts between the quantity of biopsy material and the quality of genetic tests (as a function of available time; Figure 2). It has been demonstrated recently on mouse and then human embryos that biopsied embryos at various stages can survive cryopreservation well (Krzyminska & O'Neill, 1991; Wilton et al., 1989; Liu et al., 1993; Snabes et al., 1993 ; Zhang et al., 2009; Keskintepe et al., 2009 ).

**1.3.4 Logistic advancement on assisted reproduction technologies** 

will be less time for genetic tests to be performed (Manipalviratn et al., 2009).

endometrial conditions are ready (Parriego et al., 2007).

**1.3.2 Genetic stability control** 

consistency of the mouse genome composition.

**1.3.3 Genetic diversity maintenance** 

Fig. 2. Role of cryopreservation in assisted reproduction technology. Conflict between the time allowed for genetic diagnostic tests and the amount of genetic materials available (represented by the numbers of haploid genomes, N) from biopsies at different embryonic stages exists in a continuous pre-implantation genetic diagnosis program (dashed arrow). The release of the time constraint by a cryopreservation cycle after blastocyst biopsy is denoted by the solid arrow.
