Indirect sexual selection drives rapid sperm protein evolution in abalone

Area of Science:

• * Evolutionary Biology
• * Molecular Biology
• * Marine Biology

Background:

• * Sexual selection is a known driver of rapid fertilization protein evolution.
• * Sperm proteins, even those not directly involved in fertilization, also exhibit rapid evolution.
• * Abalone sperm store large quantities of lysin and sp18 proteins at high concentrations.

Purpose of the Study:

• * To elucidate the mechanism behind the extraordinary packaging and storage of abalone sperm proteins.
• * To investigate the role of Fuzzy Interacting Transient Zwitterion (FITZ) complexes in sperm protein packaging.
• * To understand how sexual selection influences the coevolution of sperm proteins and their binding partners.

Main Methods:

• * Nuclear Magnetic Resonance (NMR) analyses to determine protein structures and interactions.
• * Investigation of protein complex formation at varying ionic strengths (intracellular vs. seawater).
• * Comparative analysis of protein-ligand interactions and species-specificity.

Main Results:

• * Abalone sperm proteins lysin and sp18 form Fuzzy Interacting Transient Zwitterion (FITZ) complexes with FITZ Anionic Partner (FITZAP) for storage.
• * FITZ complexes are stable at intracellular ionic strengths and disperse in seawater, facilitating fertilization.
• * Lysin utilizes a conserved interface to bind both FITZAP and the egg receptor VERL.
• * Coevolution between lysin-VERL and FITZAP-lysin interactions demonstrates a molecular arms race driven by sexual selection.

Conclusions:

• * The FITZ complex mechanism explains the high-concentration storage of rapidly evolving sperm proteins.
• * Sexual selection drives a coevolutionary "arms race" affecting sperm proteins and their binding partners.
• * This mechanism provides a general explanation for the rapid evolution of sperm proteins beyond those directly mediating fertilization.