Evidence for segment polarity during regeneration in the Devonian asteropygine trilobite Greenops widderensis

A complete molted exoskeleton of the asteropygine phacopid trilobite Greenops widderensis Lieberman and Kloc, 1997 from the Middle Devonian (Givetian) Widder Formation in southwestern Ontario, Canada that has suffered predatory trauma provides insights into the sequence of regeneration of segments. The molt configuration is such that it is possible to interpret the molting technique used by the trilobite. Predatory trauma affected four areas of the exoskeleton. The pygidium shows loss of the spinose margin on one side and damage to a single spine on the other; one genal spine has been broken and partially regrown; and the posterior of the glabella has been removed. It is thought that the first three traumas occurred during life, as these areas affected show signs of exoskeletal regeneration. The fourth trauma probably occurred to the exuvium. Analysis of the degree of regeneration of the pygidial pleurae indicates that there was an anteroposterior polarity to the regeneration. Other examples in the literature suggest that this regeneration polarity pattern may have been widespread in trilobites. It is suggested that, as in modern arthropods and annelids, this sequential regeneration was under the control of segmentation polarity genes.

Nodal signaling patterns the organizer

Spemann's organizer plays an essential role in patterning the vertebrate embryo. During gastrulation, organizer cells involute and form the prechordal plate anteriorly and the notochord more posteriorly. The fate mapping and gene expression analyses in zebrafish presented in this study reveal that this anteroposterior polarity is already initiated in the organizer before gastrulation. Prechordal plate progenitors reside close to the blastoderm margin and express the homeobox gene goosecoid, whereas notochord precursors are located further from the margin and express the homeobox gene floating head. The nodal-related genes cyclops and squint are expressed at the blastoderm margin and are required for prechordal plate and notochord formation. We show that differential activation of the Nodal signaling pathway is essential in establishing anteroposterior pattern in the organizer. First, overexpression of cyclops and squint at different doses leads to the induction of floating head at low doses and the induction of both goosecoid and floating head at higher doses. Second, decreasing Nodal signaling using different concentrations of the antagonist Antivin inhibits goosecoid expression at low doses and blocks expression of both goosecoid and floating head at higher doses. Third, attenuation of Nodal signaling in zygotic mutants for the EGF-CFC gene one-eyed pinhead, an essential cofactor for Nodal signaling, leads to the loss of goosecoid expression and expansion of floating head expression in the organizer. Concomitantly, cells normally fated to become prechordal plate are transformed into notochord progenitors. Finally, activation of Nodal signaling at different times suggests that prechordal plate specification requires sustained Nodal signaling, whereas transient signaling is sufficient for notochord development. Together, these results indicate that differential Nodal signaling patterns the organizer before gastrulation, with the highest level of activity required for anterior fates and lower activity essential for posterior fates.