What role might lampbrush chromosomes play in maternal gene expression

The biological significance of lampbrush chromosomes from urodelan amphibians is far from being elucidated. Their particularly well developed lateral loops are the site of intense transcriptional activity, which can be visualized in electron microscopy using the Miller spreading procedure. All transcription units functioning in lampbrush loops synthesize RNA at a maximum rate. In situ hybridization has provided evidence for transcription of both unique coding sequences and highly repetitive sequences. The role of lampbrush transcripts in the production of maternal information remains unclear. RNAs transcribed from unique coding sequences are exported to the cytoplasm; there, they contribute either to maintaining the required level of maternal messenger RNA in a basal state during late oogenesis, or to increasing the store of these maternal RNAs throughout oocyte growth, i.e., until stage VI. For repetitive sequences, their intense transcription appears to be non-productive, in that RNAs are not translatable and might be useless products of readthrough transcription. The non-productive transcription of repetitive sequences, the expression of which is directly related to hyperdevelopment of lateral loops, raises the issue of the role of lampbrush chromosome transcription.