Contrasting patterns of evolutionary constraint and novelty revealed by comparative sperm proteomic analysis in Lepidoptera

Abstract

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 exhibits dichotomous spermatogenesis, in which males produce a functional fertilization-competent sperm (eupyrene) and an incompetent sperm morph lacking nuclear DNA (apyrene). Through the integrated application of evolutionary proteomics and genomics, we characterize the genomic patterns potentially associated with the origination and evolution of this unique spermatogenic process and assess the importance of genetic novelty in Lepidopteran sperm biology.ResultsComparison 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.ConclusionsOur results identify a burst of genetic novelty amongst sperm proteins that may be associated with the origin of heteromorphic spermatogenesis in ancestral Lepidoptera and/or the subsequent evolution of this system. 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 lepidopteran 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.Electronic supplementary materialThe online version of this article (10.1186/s12864-017-4293-2) contains supplementary material, which is available to authorized users.

Nonfertilizing sperm in Lepidoptera show little evidence for recurrent positive selection

“…Recently, characterizations of the proteins found in lepidopteran sperm has opened a new avenue to assess their evolution and function (Whittington et al, 2017;Whittington, Zhao, Borziak, Walters, & Dorus, 2015). Proteomic studies have revealed distinct protein profiles for these two cell types (Whittington, Karr, Mongue, Walters, & Dorus, 2019).…”

Molecular evolution of non-fertilizing sperm in Lepidoptera suggests minimal direct involvement in sperm competition

“…As discussed above, sperm competition should create a distinctive pattern of molecular evolution for involved proteins [16]. We perform the first molecular evolutionary analyses of dimorphic sperm, assessing patterns of both polymorphism and divergence among sperm proteins from both eupyrene and apyrene sperm using proteomic datasets of two species: the monarch butterfly, Danaus plexippus [42], and the Carolina sphinx moth, Manduca sexta [43]. North American monarchs spend time at incredibly high density in overwintering colonies in Mexico [44] and, owing to these unique population dynamics, have some of the highest female remating rates observed in Lepidoptera.…”

“…Next, we leveraged orthology, as established by Whittington et al [42], to test for differences in mating system while controlling for the effects of sperm proteome content. As substantial number of proteins, hereafter referred to as sperm homologs, are found in the sperm proteomes of both species and offer the opportunity to directly assess the selective pressures experienced by the same genes with conserved function but found in species with different levels of postcopulatory selection.…”

“…We used gene sets from the published genomes of each species [81,82] with sperm genes identified from their respective proteomes [42,43]. We inferred selection from patterns of polymorphism and divergence from congeners using whole genome Illumina resequencing data for both species: a previously published dataset for North American monarch butterflies [54] and a new dataset of North Carolinian sphinx moths.…”

Reproductive functions and genetic architecture of the seminal fluid and sperm proteomes of the mosquitoAedes aegypti

“…Previous analyses of insect sperm proteomes have consistently identified proteins 328 generally considered to be SFPs (i.e. highly expressed in the MAG and believed to be 329 secreted molecules transferred to females as non-sperm components [32,62,63]. To 330 identify proteins predominantly produced by the MAG, but identified in both our sperm 331 and ejaculate sample, we used a 2.5-fold greater abundance threshold in the ejaculate 332 relative to sperm.…”