Pigment pattern formation in larval ambystomatid salamanders: Ambystoma talpoideum, Ambystoma barbouri, and Ambystoma annulatum

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Abstract
The pigment pattern formation in embryos and larvae of three ambystomatid salamanders was investigated in an evolutionary context. Early neural crest development was studied with scanning electron microscopy. Pigment cell migration and pattern formation were investigated at the light microscopy level with markers that labelled the two pigment cell types specifically before they were fully differentiated. In all three species, the pigment pattern formation started when xanthophores that had first formed aggregates in the crest migrated ventrally. As previously observed in other species, vertical bars always form by a mechanism involving earlier onset of migration in melanophores than in xanthophores and aggregate formation in the crest. In Ambystoma talpoideum and A. annulatum, a pattern of vertical chromatophore bars formed, which was superimposed on a pattern of horizontal stripes. In Ambystoma barbouri, the tendency to form this pattern was obscured by the high density of melanophores. It is suggested that variation among species may be due to differences in the chromatophore density and in the melanophore/xanthophore ratio. Mapping of the evolution of vertical bars onto existing phylogenies for the group was confounded by controversies about how to interpret the phylogenetic data. On the phylogeny that takes all the available evidence into account, there are two equally parsimonious mappings. Vertical bars have either evolved only once and been lost twice, or evolved twice and been lost once. This rather conservative pattern can be explained both as an effect of stabilizing selection and as a result of developmental constraints.