Pigmentation in the sensory organs of the ascidian larva is essential for normal behavior
2005
SUMMARY Free-living animals and their larvae utilize light and gravity as cues to
navigate in open space. Detection and response to these environmental stimuli
are important for the dispersal and settlement of ascidian larvae. Two
pigmented structures in the brain of the ascidian larva, the ocellus and the
otolith, have been shown to function as the photoreceptive and gravity
sensitive organs, respectively. Here, we show that pigmentation is essential
for proper phototactic and geotactic behavior in larvae of the ascidian
species Ciona savignyi . Two recessive and complementing mutant lines
of C. savignyi , immaculate and s potless , that
specifically disrupt the pigmentation of the sensory organs during larval
development are described. Homozygous mutant larvae are unable to respond
properly to gravity and illumination cues while settling. Genetic analysis
shows that spotless is caused by a point mutation within the
tyrosinase gene that creates a premature stop codon, while the molecular
nature of immaculate is unknown. Although the role of pigmentation in
the ocellus of C. savignyi is similar to that in vertebrate visual
systems, our results demonstrate a novel use of melanin in geotactic
behavior.
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