Cathy J. Wedeen

Molluscan engrailed expression, serial organization, and shell evolution

SUMMARY Whether the serial features found in some molluscs are ancestral or derived is considered controversial. Here, in situ hybridization and antibody studies show iterated engrailed‐gene expression in transverse rows of ectodermal cells bounding plate field development and spicule formation in the chiton, Lepidochitona caverna, as well as in cells surrounding the valves and in the early development of the shell hinge in the clam, Transennella tantilla. Ectodermal expression of engrailed is associated with skeletogenesis across a range of bilaterian phyla, suggesting a single evolutionary origin of invertebrate skeletons. The shared ancestry of bilaterian‐invertebrate skeletons may help explain the sudden appearance of shelly fossils in the Cambrian. Our interpretation departs from the consideration of canonical metameres or segments as units of evolutionary analysis. In this interpretation, the shared ancestry of engrailed‐gene function in the terminal/posterior addition of serially repeated elements during development explains the iterative expression of engrailed genes in a range of metazoan body plans.

Segmentally iterated expression of an engrailed-class gene in the embryo of an australian onychophoran.

Abstract We have identified an engrailed-class (en-class) gene and determined the distribution pattern of its protein during embryogenesis in a member of the Onychophora. The results of this work add to our understanding of the evolution of development and in addition, they contribute information toward clarifying the phylogenetic position of this group. We observe transient expression in a portion of each developing segment. By the time limbs have formed, segmental expression of en-class protein is restricted to the mesoderm. This pattern shares important spatio-temporal characteristics with those of Annelida and Arthropoda, both of which have members that express en-class genes segmentally in mesoderm and ectoderm.

Cloning and sequencing of a leech homolog to the Drosophila engrailed gene

We have cloned and sequenced a homolog (ht‐en) to the Drosphila engrailed (en) gene from the glossiphoniid leech, Helobdella triserialis. Amino acid comparisons of the ht‐en homeodomain and C‐terminal residues with the corresponding residues encoded by en‐class genes of other species reveal 75–79% sequence identity. In addition, the ht‐en sequence appears to have u serine‐rich region 16 residues C‐terminal from the homeodomain, which by analogy to Drosophila may be a target site for phosphorylation. The leech gene encodes some amino acid substitutions for residues that are highly conserved in other species. These are found within the second and third of the three putative helices of the homeodomain, and in both of the intervening turn regions.

Cell death in late embryogenesis of the leech Helobdella

Abstract Cell death was characterized during stages 8 and 9 in the leech Helobdella with a modified terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling method. Using confocal analysis, the positions of dying cells were compared to rows of cells expressing the leech engrailed protein ht-en and to fluorescently marked cell lineages. Dying cells were present in diverse tissues. Some dying cells were in no obvious pattern, and others were in segmentally iterated patterns. Particular attention was paid to the ectoderm and mesoderm, where most of the cells examined died over a period equivalent to 1–4 h at 25°C. Segmentally iterated rows of dying cells were observed in the mesoderm just beneath the nf-derived ht-en expressing cell rows at a time when ht-en expressing cells were beginning to disappear. The position of these dying cell rows was consistent with a role in the partial deterioration of the septum.