Over 700 species of cephalopods live in the Earth’s waters, occupying almost every marine zone, from the benthic deep to the open ocean to tidal waters. The greatly varied forms and charismatic behaviors of these animals have long fascinated humans. Cephalopods are short-lived, highly mobile predators with sophisticated brains that are the largest among the invertebrates. While cephalopod brains share a similar anatomical organization, the nervous systems of coleoids (octopus, squid, cuttlefish) and nautiloids all display important lineage-specific neural adaptations. The octopus brain, for example, has for its arms a well-developed tactile learning and memory system that is vestigial in, or absent from, that of other cephalopods. The unique anatomy of the squid giant fiber system enables rapid escape in the event of capture. The brain of the nautilus comprises fewer lobes than its coleoid counterparts, but contains olfactory system structures and circuits not yet identified in other cephalopods.
Cephalopod Nervous System Organization
Z. Yan Wang and Clifton W. Ragsdale
Xenacoelomorpha Nervous Systems
Pedro Martínez, Volker Hartenstein, and Simon G. Sprecher
The emergence and diversification of bilateral animals are among the most important transitions in the history of life on our planet. A proper understanding of the evolutionary process will derive from answering such key questions as, how did complex body plans arise in evolutionary time, and how are complex body plans “encoded” in the genome? the first step is focusing on the earliest stages in bilaterian evolution, probing the most elusive organization of the genomes and microscopic anatomy in basally branching taxa, which are currently assembled in a clade named Xenacoelomorpha. This enigmatic phylum is composed of three major taxa: acoel flatworms, nemertodermatids, and xenoturbellids. Interestingly, the constituent species of this clade have an enormously varied set of morphologies; not just the obvious external features but also their tissues present a high degree of constructional variation. This interesting diversity of morphologies (a clear example being the nervous system, with animals showing different degrees of compaction) provides a unique system in which to address outstanding questions regarding the parallel evolution of genomes and the many morphological characters encoded by them. A systematic exploration of the anatomy of members of these three taxa, employing immunohistochemistry, in situ hybridization, and high-throughput transmission electron microscopy, will provide the reference framework necessary to understand the changing roles of genes and gene networks during the evolution of xenacoelomorph morphologies and, in particular, of their nervous systems.
Cephalochordate Nervous System
Simona Candiani and Mario Pestarino
The central and peripheral nervous systems of amphioxus adults and larvae are characterized by morphofunctional features relevant to understanding the origins and evolutionary history of the vertebrate CNS. Classical neuroanatomical studies are mainly on adult amphioxus, but there has been a recent focus, both by TEM and molecular methods, on the larval CNS. The latter is small and remarkably simple, and new data on the localization of glutamatergic, GABAergic/glycinergic, cholinergic, dopaminergic, and serotonergic neurons within the larval CNS are now available. In consequence, it has been possible begin the process of identifying specific neuronal circuits, including those involved in controlling larval locomotion. This is especially useful for the insights it provides into the organization of comparable circuits in the midbrain and hindbrain of vertebrates. A much better understanding of basic chordate CNS organization will eventually be possible when further experimental data will emerge.
Genetics and Evolution of Color Vision in Primates
Gerald H. Jacobs
Color is a central feature of human perceptual experience where it functions as a critical component in the detection, identification, evaluation, placement, and appreciation of objects in the visual world. Its role is significantly enhanced by the fact that humans evolved a dimension of color vision beyond that available to most other mammals. Many fellow primates followed a similar path and in recent years the basic mechanisms that support color vision—the opsin genes, photopigments, cone signals, and central processing—have been the subjects of hundreds of investigations. Because of the tight linkage between opsin gene structure and the spectral sensitivity of cone photopigments, it is possible to trace pathways along which color vision may have evolved in primates. In turn, such information allows the development of hypotheses about the nature of color vision and its utility in nonhuman primates. These hypotheses are being critically evaluated in field studies where primates solve visual problems in the presence of the full panoply of photic cues. The intent of this research is to determine which aspects of these cues are critically linked to color vision and how their presence facilitates, impedes, or fails to influence the solutions. These investigations are challenging undertakings and the emerging literature is replete with contradictory conclusions. But steady progress is being made and it appears that (a) some of the original ideas about there being a restricted number of tasks for which color vision might be optimally utilized by nonhuman primates (e. g., fruit harvest) were too simplistic and (b) depending on circumstances that can include both features of proximate visual stimuli (spectral cues, luminance cues, size cues, motion cues, overall light levels) and situational variables (social cues, developmental status, species-specific traits) the utilization of color vision by nonhuman primates is apt to be complex and varied.
Transcriptomic Architecture of Reproductive Plasticity
Susan C. P. Renn and Nadia Aubin-Horth
Several species show diversity in reproductive patterns that result from phenotypic plasticity. This reproductive plasticity is found for example in mate choice, parental care, reproduction suppression, reproductive tactics, sex role, and sex reversal. Studying the genome-wide changes in transcription that are associated with these plastic phenotypes will help answer several questions, including those regarding which genes are expressed and where they are expressed when an individual is faced with a reproductive choice, as well as those regarding whether males and females have the same brain genomic signature when they express the same behaviors, or if they activate sex-specific molecular pathways to output similar behavioral responses. The comparative approach of studying transcription in a wide array of species allows us to uncover genes, pathways, and biological functions that are repeatedly co-opted (“genetic toolkit”) as well as those that are unique to a particular system (“genomic signature”). Additionally, by quantifying the transcriptome, a labile trait, using time series has the potential to uncover the causes and consequences of expressing one plastic phenotype or another. There are of course gaps in our knowledge of reproductive plasticity, but no shortage of possibilities for future directions.
Gastropod Feeding Systems: Evolution of Neural Circuits that Generate Diverse Behaviors
Paul Benjamin and Michael Crossley
It is conceptually reasonable to explore how the evolution of behavior involves changes in neural circuitry. Progress in determining this evolutionary relationship has been limited in neuroscience because of difficulties in identifying individual neurons that contribute to the evolutionary development of behaviors across species. However, the results from the feeding systems of gastropod mollusks provide evidence for this concept of co-evolution because the evolution of different types of feeding behaviors in this diverse group of mollusks is mirrored by species-specific changes in neural circuitry. The evolution of feeding behaviors involves changes in the motor actions that allow diverse food items to be acquired and ingested. The evolution in neural control accompanies this variation in food and the associated changes in flexibility of feeding behaviors. This is present in components of the feeding network that are involved in decision making, rhythm generation, and behavioral switching but is absent in background mechanisms that are conserved across species, such as those controlling arousal state. These findings show how evolutionary changes, even at the single neuron level, closely reflect the details of behavioral evolution.