Homology and evolutionary novelty in the deployment of extracellular matrix molecules during pigment pattern formation in the salamanders Taricha torosa and T. rivularis (Salamandridae).

Salamander larvae exhibit a diverse array of pigment patterns shortly after hatching. Previous studies have identified roles for the extracellular matrix and lateral line sensory system in promoting the development of a phylogenetically common pattern of horizontal melanophore stripes. In contrast, salamanders in the genus Taricha exhibit evolutionarily derived pigment patterns and pattern-forming mechanisms. Taricha torosa larvae exhibit compact melanophore stripes that develop via redundant, lateral line-independent mechanisms, whereas T. rivularis larvae lack stripes and instead have melanophores uniformly distributed over the flank. In this study, I test roles for candidate patterning molecules of the extracellular matrix in promoting the development of species-specific pigment patterns in Taricha. I show that tenascin deposition is negatively correlated with melanophore distributions both intraspecifically and interspecifically: this matrix molecule is present where melanophores do not localize in T. torosa and is absent from these same regions where melanophores are abundant in T. rivularis. Embryological manipulations further indicate that transient expression of tenascin in a prospective interstripe region of T. torosa reflects a phylogenetically conserved effect of lateral line development. Finally, anti-laminin immunoreactivity is negatively correlated with melanophore distributions in T. torosa, and this species exhibits a general retardation of extracellular matrix development that may allow persistent, evolutionarily novel melanophore motility in this species. Together these findings identify tenascin and laminin, or molecules co-regulated with these matrix components, as candidates for promoting early larval pigment pattern development in Taricha.