Sepia officinalis

WHY CEPHALOPODS?
EVOLUTIONARY INTEREST

Among Bilateralia, many important experimental studies of the nervous system physiology are undertaken with invertebrates mainly because of their simple nervous system and the easiness of accessing to large neurons and fibres. However, several important advances in understanding gene functions in invertebrate nervous system development are issued from studies on the two main models of Ecdysozoans, Caenorhabditis and Drosophila. Beside these traditional invertebrate models, lophotrochozoan models are essential as representatives of the bilaterian diversity for the understanding of molecular mechanisms and pattern evolution.

Cephalopods exhibit major morphological peculiarities among molluscs that are worth being better explored: arms and funnel derived from the foot, a brain, a closed circulatory system, a direct development, each of them accounting for an evolutionary step. The diversity observed in these structures responds to a range of adaptations to different life style.

For example, while nautiloids, with external shell have a rudimentary nervous system, the coleoids (i.e. without external shell) have other protective strategies, either a performing chromatophore system (camouflage), or giant axons (fast escape) or both. Their diversity in morphology attests a high flexibility and adaptability and makes them a relevant biological material for evolutionary studies.

The developmental approach is essential for understanding evolutionary mechanisms leading to these derived traits.

WHY THE CUTTLEFISH?
DEVELOPMENTAL INTEREST

The target species is the cuttlefish Sepia officinalis. Sepia officinalis has been chosen for their morphological characteristics among cephalopods, in relation to its necto-benthic life style, making it ideally suited for comparative developmental and evolutionary studies. Adding to these fundamental interests, Sepia officinalis has an economical importance: it is one of the most important species for cephalopod fisheries in many European countries. This species inhabits European and African Atlantic coasts and Mediterranean. The life cycle of Sepia officinalis is less than two years long and mature females, 18 months years old, lays eggs near the coast in spring and summer. Cuttlefish (male and female) die soon after breeding.

It is easy to obtain eggs and the development is followed in the laboratory. The development is two months long.

Compared to other molluscs, they have a direct development (no larval stage) allowing continuous observation of the territories without changes due to metamorphosis. The hatched juveniles are identical to adults and adopt immediately the adult benthic life style.

WHY CEPHALOPOD NERVOUS SYSTEM ?
FUNCTIONAL AND COMPARATIVE INTERESTS

Cephalopods exhibit a “complex” nervous system both in the Central Nervous System (CNS) and Peripheral Nervous System (PNS). Few lophotrochozoan possess as many convergences with vertebrates as cephalopods : they are used as a comparative model for vertebrates as an important material for neurocytology, electrophysiology and biophysics, directed to understand function of the brain and axons. Data from this model, with vertebrate convergent nervous structures, could bring also response elements on the molecular basis of some degenerative diseases and neuromuscular disorders. In addition, this convergence allows access to differences or similarities which appeared during evolution in the development of structures with functional equivalence (i.e. analogy).

Within the molluscs, and regard their nervous system, cephalopods show the most numbered derived characters : they have ganglia condensed in a brain, developed sensory organs (eyes similar to those of vertebrates), specific stellate ganglia, chromatophore systems, nervously controlled, and giant axons. The numerous peculiarities of cephalopods constitute an original landmark in nervous system evolution in lophotrochozoans but the developmental aspect of such structures remains to be investigated. In spite of these functional interests, nothing is known about the molecular pathways underlying their development.

WHAT DO WE KNOW ABOUT NERVOUS SYSTEM DEVELOPMENT?

The nervous system (CNS and PNS) is positioned before hatching and is formed from typical molluscan ganglion. The ganglia differentiate into lobes and condense in a brain with hierarchical functions. CNS lobes undergo profound changes in relation to external cues after hatching.

The characterization of homologous genes known in other metazoan groups to intervene in the nervous system setting up may help to approach, by their expression patterns, their role in Sepia development. For example Pax6, a very conserved gene, is expressed at early stage in the embryo and could be implied in the setting up of the cerebral and optic ganglia as well as in the eyes.

Although elucidating molecular mechanisms underlying (both in developmental and evolutionary aspects) the nervous system structure and function is crucial to elaborate evolutionary hypotheses, few studies have concerned the characterization of target “neural” genes in a mollusc.