The sperm proteins from amphioxus mirror its basal position among chordates and redefine the origin of vertebrate protamines.

The sperm nuclear basic proteins (SNBPs) that participate in chromatin condensation in spermatozoa belong to 3 groups: histone (H), protamine-like (PL), and protamine (P) type. They share a common origin with histone H1 resulting from the segregation of PL components, corresponding to different regions of an H1 precursor molecule (N-terminal, winged-helix, C-terminal domains), becoming independent and following a subsequent process of parallel vertical evolution (H <--> PL <--> P). In the present work, we describe the sequence and primary structure of the main SNBP component in the sperm of the cephalochordate Branchiostoma floridae (amphioxus), revealing that it represents the deuterostome counterpart of the PL-III SNBP component from molluscs corresponding to the H1 N-terminal region. Until now, this has been a missing piece needed to complete the evolutionary history of SNBPs in metazoan genomes. The discovery of this PL lineage in deuterostomes definitively validates the parallel vertical evolution of SNBPs across metazoans, giving further support to the "basal" position of amphioxus among chordates, with respect to tunicates. Sequence analyses suggest that later on in evolution, the appearance of positively selected arginine-rich protamines, derived from the H1 C-terminal region, led to the extinction of this PL lineage in the genomes of early protostomes and deuterostomes. Given that tunicates are now viewed as a sister group of vertebrates, the lysine to arginine transition responsible for the origin of vertebrate protamines must be set a step back from tunicates.