Specialized eRpL22 paralogue-specific ribosomes regulate specific mRNA translation in spermatogenesis in Drosophila...

The functional significance of ribosome heterogeneity in development and differentiation is relatively unexplored. We present the first in vivo evidence of ribosome heterogeneity playing a role in specific mRNA translation in a multicellular eukaryote. Eukaryotic-specific ribosomal protein paralogues eRpL22 and eRpL22-like are essential in development and required for sperm maturation and fertility in Drosophila. eRpL22 and eRpL22-like roles in spermatogenesis are not completely interchangeable. Flies depleted of eRpL22 and rescued by eRpL22-like overexpression have reduced fertility, confirming that eRpL22-like cannot substitute fully for eRpL22 function, and that paralogues have functionally distinct roles, not yet defined. We investigated the hypothesis that specific RNAs differentially associate with eRpL22 or eRpL22-like ribosomes, thereby establishing distinct ribosomal roles. RNA-seq identified 12,051 transcripts (mRNAs/noncoding RNAs) with 50% being enriched on specific polysome types. Analysis of ∼10% of the most abundant mRNAs suggests ribosome specialization for translating groups of mRNAs expressed at specific stages of spermatogenesis. Further, we show enrichment of "model" eRpL22-like polysome-associated testis mRNAs can occur outside the germline within S2 cells transfected with eRpL22-like, indicating that germline-specific factors are not required for selective translation. This study reveals specialized roles in translation for eRpL22 and eRpL22-like ribosomes in germline differentiation.