Speed vs completeness: a comparative study of solitary and colonial tunicate embryogenesis

Area of Science:

• Developmental Biology
• Marine Biology
• Evolutionary Biology

Background:

• Solitary ascidians like Ciona robusta are established models for embryological studies due to external fertilization and rapid development.
• Colonial ascidians, such as Botryllus schlosseri, exhibit ovoviviparity with internal development, posing challenges for direct embryonic observation.
• Understanding developmental differences between solitary and colonial ascidians is crucial for insights into ascidian diversification.

Purpose of the Study:

• To develop an in vitro culture method for Botryllus schlosseri embryos to enable detailed observation of embryonic development.
• To describe and stage the embryonic development of Botryllus schlosseri from fertilized egg to swimming larva.
• To compare the developmental timeline and heterochrony between colonial Botryllus schlosseri and solitary Ciona robusta.

Main Methods:

• Developed an in vitro culture technique for Botryllus schlosseri embryos.
• Utilized time-lapse and confocal microscopy for observing early embryonic stages.
• Employed histological analysis of serial sections for late-stage embryonic development due to opacity.

Main Results:

• Identified and described 19 distinct stages of Botryllus schlosseri embryonic development, including organogenesis and differentiation.
• Documented rapid larval settlement and metamorphosis in Botryllus schlosseri, occurring much faster than in Ciona robusta.
• Revealed significant heterochrony in developmental timing, particularly in organ appearance and growth rates, when comparing B. schlosseri to C. robusta.

Conclusions:

• The observed heterochrony in Botryllus schlosseri development suggests a shift in key developmental pathway regulation contributing to ascidian diversification.
• Heterochronic evolution in colonial ascidians likely facilitated a significantly shortened metamorphosis period by extending internal development.
• The study provides a detailed developmental staging for Botryllus schlosseri, serving as a foundation for future research in colonial ascidian biology.