Abstract
IntroductionFrom the large-brained cephalopods to the acephalic bivalves, molluscs show a vast range of nervous system centralization patterns. Despite this diversity, molluscan nervous systems, broadly considered, are organized either as medullary cords, as seen in chitons, or as ganglia, which are typical of gastropods and bivalves. The cephalopod brain is exceptional not just in terms of its size; its relationship to a molluscan cordal or ganglionic plan has not been resolved from the study of its compacted adult structure. One approach to clarifying this puzzle is to investigate the patterns of early cephalopod brain neurogenesis, where molecular markers for cephalopod neural development may be informative.ResultsWe report here on early brain pattern formation in the California two-spot octopus, Octopus bimaculoides. Employing gene expression analysis with the pan-bilaterian neuronal marker ELAV and the atonal-related neuronal differentiation genes NEUROGENIN and NEUROD, as well as immunostaining using a Distalless-like homeoprotein antibody, we found that the octopus central brain forms from concentric cords rather than bilaterally distributed pairs of ganglia.ConclusionWe conclude that the cephalopod brain, despite its great size and elaborate specializations, retains in its development the hypothesized ancestral molluscan nervous system plan of medullary cords, as described for chitons and other aculiferan molluscs.
Highlights
From the large-brained cephalopods to the acephalic bivalves, molluscs show a vast range of nervous system centralization patterns
Employing gene expression analysis with the pan-bilaterian neuronal marker ELAV and the atonal-related neuronal differentiation genes NEUROGENIN and NEUROD, as well as immunostaining using a Distalless-like homeoprotein antibody, we found that the octopus central brain forms from concentric cords rather than bilaterally distributed pairs of ganglia
Staging of O. bimaculoides development The embryos of O. bimaculoides display the direct development mode typical of cephalopods, without the cataclysmic metamorphosis seen in other molluscan larvae [41, 42]
Summary
From the large-brained cephalopods to the acephalic bivalves, molluscs show a vast range of nervous system centralization patterns. Despite this diversity, molluscan nervous systems, broadly considered, are organized either as medullary cords, as seen in chitons, or as ganglia, which are typical of gastropods and bivalves. Molluscs are highly successful animals with a great diversity of body plans. Exceptional even among this group are the cephalopods, with the elaborate sensory systems, flexible learning abilities, and sophisticated motor outputs expected of an accomplished predator (reviewed in [1,2,3]). Aculiferan molluscs, such as the chiton, have a nervous system based on cords, which are longitudinal neuropils surrounded by a rind of
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