Developmental dynamics are a proxy for selective pressures on alternatively polyadenylated isoforms

Alternative polyadenylation (APA) leads to multiple transcripts from the same gene, yet their distinct functional attributes remain largely unknown. Here, we introduce APA-seq to detect the expression levels of APA isoforms from 3’-end RNA-Seq data by exploiting both paired-end reads for gene isoform identification and quantification. Applying APA-seq, we detected the expression levels of APA isoforms from RNA-Seq data of single C. elegans embryos, and studied the patterns of 3’ UTR isoform expression throughout embryogenesis. We found that global changes in APA usage demarcate developmental stages, suggesting a requirement for distinct 3’ UTR isoforms throughout embryogenesis. We distinguished two classes of genes, depending upon the correlation between the temporal profiles of their isoforms: those with highly correlated isoforms (HCI) and those with lowly correlated isoforms (LCI) across time. This led us to hypothesize that variants produced with similar expression profiles may be the product of biological noise, while the LCI variants may be under tighter selection and consequently their distinct 3’ UTR isoforms are more likely to have functional consequences. Supporting this notion, we found that LCI genes have significantly more miRNA binding sites, more correlated expression profiles with those of their targeting miRNAs and a relative lack of correspondence between their transcription and protein abundances. Collectively, our results suggest that a lack of coherence among the regulation of 3’ UTR isoforms is a proxy for selective pressures acting upon APA usage and consequently for their functional relevance.