Contrasting Patterns Of Evolutionary Constraint And Novelty Revealed By Comparative Sperm Proteomic Analysis

BackgroundRapid evolution is a hallmark of reproductive genetic systems and arises through the combined processes of sequence divergence, gene gain and loss, and changes in gene and protein expression. While studies aiming to disentangle the molecular ramifications of these processes are progressing, we still know little about the genetic basis of evolutionary transitions in reproductive systems. Here we conduct the first comparative analysis of sperm proteomes in Lepidoptera, a group that broadly exhibits dichotomous spermatogenesis, in which males simultaneously produce a functional fertilization-competent sperm (eupyrene) and an incompetent sperm morph lacking DNA (apyrene). Through the integrated application of evolutionary proteomics and genomics, we characterize the genomic patterns associated with the origination of this unique spermatogenic process and assess the importance of genetic novelty in Lepidoptera sperm biology.\n\nResultsComparison of the newly characterized Monarch butterfly (Danaus plexippus) sperm proteome to those of the Carolina sphinx moth (Manduca sexta) and the fruit fly (Drosophila melanogaster) demonstrated conservation at the level of protein abundance and post-translational modification within Lepidoptera. In contrast, comparative genomic analyses across insects reveals significant divergence at two levels that differentiate the genetic architecture of sperm in Lepidoptera from other insects. First, a significant reduction in orthology among Monarch sperm genes relative to the remainder of the genome in non-Lepidopteran insect species was observed. Second, a substantial number of sperm proteins were found to be specific to Lepidoptera, in that they lack detectable homology to the genomes of more distantly related insects. Lastly, the functional importance of Lepidoptera specific sperm proteins is broadly supported by their increased abundance relative to proteins conserved across insects.\n\nConclusionsOur results suggest that the origin of heteromorphic spermatogenesis early in Lepidoptera evolution was associated with a burst of genetic novelty. This pattern of genomic diversification is distinct from the remainder of the genome and thus suggests that this transition has had a marked impact on Lepidoptera genome evolution. The identification of abundant sperm proteins unique to Lepidoptera, including proteins distinct between specific lineages, will accelerate future functional studies aiming to understand the developmental origin of dichotomous spermatogenesis and the functional diversification of the fertilization incompetent apyrene sperm morph.