Pax6 and Six1/2 orthologs in leech ectodermal patterning
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Clitellate annelids display conserved mechanisms of segmental ectodermal and mesodermal patterning. These tissues are generated by asymmetric divisions of large stem cells called teloblasts, elongating the ectoderm and mesoderm of the embryo. Each teloblast-derived lineage makes highly stereotyped contributions to the leech: the N, O, P, and Q contribute specific neurons, epidermis, and other ectodermal tissues along the ventral-to-dorsal axis of the embryo, respectively. The N and Q ectodermal lineages appear to be specified autonomously, but specification of the O and P lineages depends upon interactions with other, neighboring teloblast lineages. Until quite recently, there have been precious few teloblast lineage-specific markers, and virtually no molecular candidates for genes influencing the proper differentiation of any of these lineages. Here, I explore the possibility that members of the Pax-Six-Eyes absent-Dachshund network are involved in leech ectodermal patterning. I show that the leech Helobdella sp. Austin has two Pax6 paralogs, and demonstrate that Hau-Pax6A is expressed early in a subset of N-derived cells and O-derived cells. Next, I demonstrate that an ortholog of the six gene family, Hau-six1/2a, is expressed in the P lineage. I show through a series of cell ablations that Hau-six1/2a expression is regulated by neighboring teloblasts in a manner consistent with P fate induction, hinting that this transcription factor may be involved in P specification. The identification of these genes is a first step towards dissecting the molecular mechanisms of ectodermal teloblast differentiation in the leech embryo. The evolutionary context of the deployment of these genes is also discussed. In the appendices, I present two projects on the evolution of pigment patterns in Danio rerio and its relatives. In the first, I show that the larval melanin-containing pigment cells of Danio nigrofasciatus are uniquely redeployed into the adult pigment pattern, in contrast to seven related fishes. In the second, I show that variation in yellow pigment cell populations in different danio species may be dependent on variable signaling through the receptor tyrosine kinase fms pathway.