The maternal transcriptome of the crustacean Parhyale hawaiensis is inherited asymmetrically to invariant cell lineages of the ectoderm and mesoderm [rest_8cell_embryonic_vs_ectoderm]. Parhyale hawaiensis

The embryo of the crustacean Parhyale hawaiensis has a total, unequal and invariant early cleavage. It specifies cell fates earlier than other arthropods like Drosophila, as individual blastomeres of the 8-cell stage are allocated to the germ layers and the germline. Furthermore, the 8-cell stage is amenable to embryological manipulations. These unique features make Parhyale a suitable system for elucidating germ layer specification in arthropods. Since asymmetric localization of maternally provided RNA is a widespread mechanism to specify early cell fates, we asked whether this is also true for Parhyale. A candidate gene approach did not find RNAs that are asymmetrically distributed at the 8-cell stage. Therefore, we designed a high-density microarray from 9400 recently sequenced ESTs (1) to identify maternally provided RNAs and (2) to find RNAs that are differentially distributed among cells of the 8-cell stage. Maternal-zygotic transition takes place around the 32-cell stage, i.e. after the specification of germ layers. By comparing a pool of RNAs from early embryos without zygotic transcription to zygotic RNAs of the germband, we found that more than 15% of the targets on the array were enriched in the maternal transcript pool. A screen for asymmetrically distributed RNAs at the 8-cell stage revealed 129 transcripts, which were associated with germ-layer-specific GO-terms. Cross-correlation analysis of the microarray experiments indicated that half of the asymmetrically distributed RNAs at the 8-cell stage are degraded before the start of zygotic transcription. Finally, we performed knock-down experiments for two of these genes and observed cell-fate-related defects of embryonic development. Overall design: microarray analysis of the 8-cell stage embryonic transcriptome - ectodermal progenitors vs. rest Institution: Max Planck Institute for Developmental Biology, Tuebingen, Germany Time of experiment: April 2008 to April 2009