Sperm-Egg Interactions in Ascidian Fertilization

Ascidian fertilization has long served as a tractable model for understanding fundamental mechanisms of gamete recognition, binding and fusion in chordates. Spermatozoa released into seawater are guided towards the egg by species-specific chemoattractants, triggering activation and reorientation of the flagellum. Upon contact, sperm bind to the vitelline coat—a complex extracellular matrix enveloping the egg—where the acrosome reaction releases a suite of proteolytic enzymes. These enzymes, including astacin-like metalloproteases and trypsin-like proteases, locally digest the vitelline coat, enabling the sperm to penetrate and ultimately fuse with the egg plasma membrane. Simultaneously, hermaphroditic ascidians employ a genetically encoded self-incompatibility system to prevent inbreeding: multi-allelic gene pairs expressed on both sperm and egg surfaces engage in stringent matching interactions that block fertilisation when self-haplotypes coincide. Insights gained from ascidians illuminate general principles of sperm–egg communication, the evolution of reproductive barriers and potential applications in aquaculture and conservation of marine invertebrates.

Research from Nature Portfolio

Recent studies have identified the genetic architecture underlying self-sterility in a widespread ascidian species. Three tightly linked multi-allelic gene pairs on separate chromosomes encode complementary sperm- and egg-surface proteins. When allelic haplotypes on the sperm-expressed and vitelline-coat-expressed partners match, fertilisation is blocked, enforcing outcrossing and maintaining genetic diversity. Targeted disruption of any one gene pair by genome editing bypasses the block and permits self-fertilisation, confirming their roles as S-determinants. Complementing these genetic findings, work on sperm surface proteomes has uncovered a cohort of astacin-type metalloproteases bearing thrombospondin repeats. Inhibition of these enzymes prevents vitelline-coat digestion and sperm penetration, while gene knockouts compromise larval metamorphosis. Together, these studies clarify how coordinated action of surface proteases and self-recognition molecules ensures both successful fertilisation and inbreeding avoidance.

Sperm-Egg Interactions in Ascidian Fertilization publication trend

The graph below shows the total number of articles in sperm-egg interactions in ascidian fertilization across all publications each year (not limited to Nature Index journals).

Technical terms

Acrosome reaction: Calcium-triggered exocytosis of the acrosomal vesicle in sperm, releasing enzymes that digest the egg’s protective coat.

Vitelline coat: The protein-rich extracellular matrix surrounding the ascidian egg, analogous to the zona pellucida of vertebrates.

Chemoattractant: A signalling molecule released by eggs that directs sperm swimming via concentration gradients.

Self-incompatibility: A genetic mechanism preventing fertilisation between gametes sharing identical alleles at specific loci, thus avoiding inbreeding.

Astacin metalloprotease: A zinc-dependent protease subclass with roles in extracellular matrix degradation during fertilisation and development.

References

1. Mechanisms of Sperm–Egg Interactions: What Ascidian Fertilization Research Has Taught Us. Cells (2022).
2. Fertilization of Ascidians: Gamete Interaction, Self/Nonself Recognition and Sperm Penetration of Egg Coat. Frontiers in Cell and Developmental Biology (2022).
3. Three multi-allelic gene pairs are responsible for self-sterility in the ascidian Ciona intestinalis. Scientific Reports (2020).
4. The role of metalloproteases in fertilisation in the ascidian Ciona robusta. Scientific Reports (2019).