The jewel wasp <i>Nasonia vitripennis</i> utilizes two single-copy, protamine-like sperm nuclear basic proteins

The DNA of sperm is uniquely packaged into an exceptionally condensed chromatin state with sperm nuclear basic proteins (SNBPs). These proteins are diverse, including protamines, protamine-like (PL) proteins, testis-specific (TS) histones, and HMG-box proteins. Usage of SNBP type varies widely among and within phyla. Moreover, SNBPs can be evolutionarily dynamic in copy number and amino acid composition, even among closely related species. Despite the diversity of insects, little is known about SNBP usage across members of this taxonomic group. A previous study biochemically identified SNBP candidates from the male reproductive tissue of the jewel wasp, Nasonia vitripennis, in addition to other insects. Here, we computationally examined existing N. vitripennis tissue transcriptomes to independently identify SNBP candidates. Our analyses uncovered 13 genes encoding proteins with SNBP characteristics, including exclusive gene expression in the testis, short protein length, and a high proportion of basic residues. The two highest-expressed of these genes encode PL proteins, which are evolutionary derivatives of the histone H1 family. Targeted degradation of the transcripts produced by the highest-expressed PL gene resulted in partial male sterility and defective sperm nuclear morphology in fertilized embryos, while RNAi treatment of the other PL gene yielded a subtle fertility effect. Both genes are present across hymenopteran insects but not outside this insect order. Moreover, they are not evolutionarily dynamic like the testis-specific HMG-box SNBPs in Drosophila species, which may reflect the lack of sex chromosomes, and resulting conflict between them, in hymenopteran insects.