Cleavage modification did not alter blastomere fates during bryozoan evolution.

Area of Science:

• Developmental Biology
• Evolutionary Developmental Biology
• Embryology

Background:

• Stereotypic cleavage patterns are vital for cell fate determination and embryonic development.
• Evolutionary modifications in cleavage patterns can lead to altered blastomere fates and embryonic defects.
• The transition from spiral to biradial cleavage in bryozoan evolution presents a unique case to study these changes.

Purpose of the Study:

• To investigate how evolutionary changes in cleavage patterns impact blastomere fate specification.
• To analyze the transition from spiral cleavage to biradial cleavage in bryozoans.
• To understand the molecular mechanisms underlying retained cell fates despite altered cleavage geometry.

Main Methods:

• Utilized 3D-live imaging time-lapse microscopy (4D-microscopy) for high-resolution observation.
• Characterized cell lineage, MAPK signaling pathways, and the expression of 16 key developmental genes.
• Focused on the bryozoan species *Membranipora membranacea*.

Main Results:

• Early blastomere molecular identity and fates in bryozoans are conserved and similar to homologous blastomeres in spiral-cleaving embryos.
• Despite evolving biradial cleavage, bryozoans retain key features of spiral development, including the early embryonic fate map.
• MAPK signaling and developmental gene expression patterns were analyzed in the context of cleavage pattern evolution.

Conclusions:

• Bryozoans exhibit retained traits of spiral development, such as the early embryonic fate map, even after evolving a novel biradial cleavage geometry.
• Evolutionary modifications of stereotypic cleavage patterns can occur without significant alterations to the molecular identity and fate of embryonic blastomeres.
• This study provides evidence that developmental pathways can be robust to changes in early embryonic patterning.