Diversity in the fertilization envelopes of echinoderms

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Marine Biology

Background:

• Fertilization triggers essential cell surface changes for zygote development and protection.
• Most eggs form a barrier by modifying their extracellular matrix, often via cortical granule exocytosis.
• In sea urchins, this forms the fertilization envelope (FE), a barrier built on the vitelline layer.

Purpose of the Study:

• To investigate the molecular mechanisms of fertilization envelope formation in sea stars.
• To analyze evolutionary changes in FE structure and function compared to sea urchins.
• To understand the timing of protein and mRNA accumulation for FE formation.

Main Methods:

• Proteomic and transcriptomic analyses were employed.
• Antibodies against sea star SFE9 protein were used.
• Comparative analysis with sea urchin fertilization processes was performed.

Main Results:

• Sea star FEs are significantly more permeable than sea urchin FEs, allowing large molecule diffusion (>2 megadaltons).
• Most sea urchin FE proteins (e.g., SFE9, proteoliaisin, rendezvin) are present in sea stars.
• FE protein mRNAs and cortical granules accumulate early in sea star oocytes, with granules translocating before meiosis.
• Sea urchin FE protein preparation is shifted later in oogenesis, correlating with post-meiotic fertilization.

Conclusions:

• Evolutionary divergence in FE permeability exists between sea stars and sea urchins.
• Differences in meiotic timing at fertilization likely influenced the evolution of FE formation strategies.
• Sea star oogenesis exhibits an earlier preparation for fertilization envelope formation compared to sea urchins.