Isolation of genes associated with developmental competency of bovine oocytes.

Eggs differ widely in their ability to develop into an embryo. To address this characteristic, the concept of developmental competency has been coined, defined as the ability or potential of an oocyte to undergo maturation, fertilization and development to blastocyst stages or live offspring. Developmental competency is acquired progressively during folliculogenesis and is linked to follicular size. In an effort to understand the molecular changes underlying differences in competency we compared oocytes derived from large follicles (>or=5mm) to those from small follicles (<or=2mm). We used an approach combining suppressive subtraction hybridization with a linear amplification step to identify genes upregulated in the more competent oocytes. Real-time RT-PCR quantification indicated highly significant upregulation for 10 genes. However, the observed changes did not exceed three-fold suggesting that the molecular causes for poor developmental capacity may be reliant on many small changes. In monovulatory species oocyte developmental competency is further modulated in a process termed follicular dominancy, whereby only one of a cohort of developing ovarian follicles continues to grow. In our second approach, we aimed to identify genes that may be involved in the choice of one follicle as becoming dominant and thus restricting the developmental competency to a single oocyte. This approach, focusing on granulosa cells, yielded a small set of five genes that could be verified to be reliable markers for dominant follicles. We have further analyzed one of these involving the activin/inhibin pathway. Lastly, in a third approach we are investigating the feasibility of using nuclear transfer (NT) to interrogate oocyte developmental competency.