Regional specification during embryogenesis in the inarticulate brachiopod Glottidia

A fate map has been constructed for the lingulid brachiopod Glottidia pyramidata. The animal half of the egg forms part of the apical lobe and the dorsal valve of the larva. The vegetal half of the egg forms mesoderm and endoderm and is the site of gastrulation; it also forms part of the apical lobe and the ventral valve of the larva. The plane of the first cleavage goes through the animal-vegetal axis of the egg along the future plane of bilateral symmetry of the larva. The timing of regional specification in these embryos was examined by isolating animal or vegetal, anterior or posterior, or lateral regions at different times from prior to fertilization through gastrulation. Animal halves isolated at all stages formed an epithelial vesicle and did not gastrulate. When these halves were isolated from unfertilized eggs or early cleavage stage embryos, they usually did not form an apical lobe or valve; however, when the halves were isolated at later developmental stages, these structures differentiated in a high frequency of cases. Vegetal halves were isolated at all stages gastrulated and formed a larva; however, when these halves were isolated at gastrulation they frequently lacked a dorsal valve. When lateral cuts were made along the animal-vegetal axis at all developmental stages, both halves gastrulated. When the cut was made perpendicular to the plane of the first cleavage from the four-cell stage on, one-half formed the anterior end and the other half formed the posterior end of the larva. These results suggest that there are localized determinants in the egg that specify the different regions of the larva, but there is also an inductive signal(s) from the vegetal region of the embryo that is necessary in order for cells that inherit a given determinant to differentiate. Embryogenesis in Glottidia is compared with articulate brachiopods and phoronids.