Size-dependent pigmentation-pattern formation in embryos of Alligator mississippiensis : time of initiation of pattern generation mechanism

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Abstract
The pigmentation pattern of Alligator mississippiensis was examined. The number of white stripes on the dorsal side of embryos (stages 21-28) and hatchlings from eggs incubated at 30.degree. C (100% females) and 33.degree. C (100% males) was recorded. Total length, nape-rump length and tail length were recorded for each embryo and hatchling. The number of white stripes was affected by incubation temperature but not sex; hatchlings incubated at 33.degree. C had two more white stripes than those at 30.degree. C, despite being the same length. Five female hatchlings produced at 33.degree. C by manipulation of the temperature, had the same number of stripes as males that developed under the same incubation temperatures. The appearance of the pigmentation was accelerated in embryos incubated at 33.degree. C, occurring eight days earlier than at 30.degree. C. At the time just before the first signs of pigment deposition, embryos from 33.degree. C were longer than those at 30.degree. C. If the stripe formation is size dependent this explains why hatchlings at 33.degree. C have more stripes than hatchlings from 30.degree. C. The mechanism that produces the stripe patterns is unknown. We describe key elements a pattern formation mechanism must possess to produce such stripes and suggest a possible mechanism, based on cell movement driven by chemotaxis. We apply the mathematical model to dorsal patterning on A. mississippiensis. We show how length at pattern formation is the prime factor in determining stripe number and how the pattern can be formed in the observed anterior-posterior sequence. We present numerical simulations and show that the qualitative behaviour is consistent with the experimental results.