Meeting Abstract
The widely-distributed pelagobenthic life cycle, comprised of a dispersive pelagic larva and a reproductive benthic juvenile/adult form, is an excellent model for one of the most fundamental questions in evolutionary biology: what is the genomic basis for the origin of new phenotypes? Using replicated transcriptome data from two pelagic and two benthic stages of the sponge Amphimedon queenslandica, we examine what gene expression profiles can reveal about how the genome produces two clearly distinct body plans. We focus on the 14,699 (32.9%) genes that are significantly differentially expressed (DE) between at least two of the following stages: larval (precompetent and competent), 3 day old juvenile, and reproductive adult stages. PCA analysis of all genes confirms that the two larval stages are more transcriptionally similar, and the greatest transcriptional differences lie between larval and juvenile/adult samples, consistent with the hypothesis that distinct gene regulatory networks underpin pelagic and benthic body plans. Contrary to this, DE genes in the juveniles show more similarity in gene age to DE genes in larvae than those in the adult. Specifically, phylostratigraphy reveals that genes differentially upregulated (DU) in larval and juvenile stages are predominantly of pre-metazoan and metazoan origin, whereas genes DU in the benthic adult body plan tend to be younger, lineage-specific innovations. This is supported by orthology analyses, which indicate that the well-recognized and conserved metazoan genes are primarily DU in larval and juvenile stages but not the adult. Here, we discuss the implications of these results in the context of larval biology and historical embryological theory and suggest that future comparative analyses of additional life cycle transcriptomes in eumetazoans may help to address some the long-standing debates in the field.
