The Maternal Transcriptome of the Crustacean <em>Parhyale hawaiensis</em> Is Inherited Asymmetrically to Invariant Cell Lineages of the Ectoderm and Mesoderm

<div><h3>Background</h3><p>The embryo of the crustacean <em>Parhyale hawaiensis</em> has a total, unequal and invariant early cleavage pattern. It specifies cell fates earlier than other arthropods, including <em>Drosophila</em>, 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 <em>Parhyale</em> 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 <em>Parhyale</em>. 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.</p> <h3>Results</h3><p>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 10% 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, from which 50% are predominantly expressed in the early embryonic stages. Finally, we performed knockdown experiments for two of these genes and observed cell-fate-related defects of embryonic development.</p> <h3>Conclusions</h3><p>In contrast to <em>Drosophila</em>, the four primary germ layer cell lineages in <em>Parhyale</em> are specified during the maternal control phase of the embryo. A key step in this process is the asymmetric distribution of a large number of maternal RNAs to the germ layer progenitor cells.</p>