Molecular aspects of avian oogenesis and fertilisation.

In this paper, we summarise studies which have been carried out on the metabolism of nucleic acids (maternal RNA, DNA, nucleolytic enzymes) in avian oocytes and embryos (Japanese quail, Coturnix coturnix japonica ) within the last 10 years in the Institute of Genetics and Animal Breeding of the Polish Academy of Sciences. The accumulation of maternal RNA in the quail oocyte during oogenesis is shown and discussed. Several individual transcripts were identified in RNA from the germinal disc and some also in extraembryonic RNA under the perivitelline membrane. The presence of the transcript encoding chick zona pellucida C protein (chZPC) points to the possibility of ZPC synthesis by the oocyte itself. The transcript encoding AA-NAT (arylalkylamine N-acetyltransferase, the penultimate enzyme in melatonin synthesis) was present in 2 forms (with and without an intron) and the ratio of the two forms changes during oogenesis. Melatonin and the two enzymes engaged in its synthesis (AA-NAT and HIOMT) have been found in the egg yolk; their transcripts and the transcripts of the melatonin receptors mel-1a,b and c are present in RNA from the germinal discs. This suggests a possible role for melatonin in early avian development. DNases I and II activity has been detected in the germinal disc and the cytoplasmic layer under the perivitelline membrane. We propose that they participate in degradation of supernumerary sperm entering avian oocytes during polyspermic fertilisation. A hypothesis to explain the selection of a single sperm participating in the formation of the zygotic nucleus is discussed; we propose that sperm entry into the centre of the germinal disc is the essential event underlying the selection mechanism.