Brachyury expression in tailless Molgulid ascidian embryos

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genetics

Background:

• The Brachyury gene, a T-box transcription factor, is essential for notochord differentiation across chordates.
• Molgulid ascidians have independently evolved tailless larvae, presenting a unique model to study the loss of notochord development.

Purpose of the Study:

• To investigate the role and expression patterns of the Brachyury gene (Bra) in the development of notochord in both tailed and tailless molgulid ascidian species.
• To compare Brachyury gene expression dynamics in Molgula oculata (tailed) with two tailless species, Molgula occulta and Molgula tectiformis.

Main Methods:

• Isolation and cDNA sequence analysis of Brachyury genes from molgulid ascidians.
• Comparative analysis of Brachyury gene expression during embryogenesis in tailed (M. oculata) and tailless (M. occulta, M. tectiformis) ascidian species.
• Examination of hybrid embryos (M. occulta x M. oculata) to assess gene expression rescue.

Main Results:

• In tailed M. oculata, Brachyury expression supports normal notochord formation with 40 cells that converge and extend.
• In tailless M. occulta, premature Brachyury expression loss leads to a reduced notochord lineage (20 cells) that fails to develop a tail.
• In tailless M. tectiformis, Brachyury expression is limited to precursor cells, inhibiting notochord division and elongation. Hybrid embryos show rescued notochord development.

Conclusions:

• Neither Brachyury gene function nor the initial establishment of the notochord lineage is fundamentally impaired in tailless ascidian embryos.
• The loss of notochord development in tailless molgulid ascidians involves complex regulatory mechanisms beyond simple Brachyury gene inactivation.