Meeting Abstract
Limulus has been a favorite preparation for vision scientists since the 1930’s; therefore much is known about its photosensitivity. More recently, the opsin repertoire and expression pattern in Limulus have been characterized. These data indicate that photosensitivity in Limulus is distributed widely and is complex. Limulus has a pair of image-forming lateral compound eyes (LE) and two types of “simple eyes”: a pair of median ocelli (ME), and three pair of larval eyes (lateral, median and ventral) that persist in the adult. It also has photosensitive cells in its segmental ganglia and tail. A recent assembly of the Limulus genome revealed 18 opsin genes, many more than anticipated, and 17 are expressed. Ten, which are expressed in the eyes, have been characterized in detail. Some of these and additional opsins were detected in PCR screens of cDNA from segmental ganglia and tail, consistent with the presence of photosensitive cells in these tissues. However, in situ hybridization assays of segmental ganglia for opsin transcripts yielded negative results, suggesting opsin transcript levels are low in this tissue and preventing identification of opsin-expressing cells. There is one exception, however. In situ hybridization assays detected transcripts for peropsin1/RGR throughout the CNS, and immunocytochemical assays confirmed the expression of the protein in cells surrounding neurons. Some functions for the simple ME and larval eyes and photosensitive tail in Limulus have been identified. The relevance of photosensitive cells in the CNS of dark-shelled adult animals is unclear. However, for several years, at least, juvenile Limulus are largely transparent, and light stimuli may easily penetrate directly to photoreceptors in its CNS to influence behavior.
