Philosophical transactions of the Royal Society of London. Series B, Biological sciences
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Philos. Trans. R. Soc. Lond., B, Biol. Sci. · Jan 2003
Genomic reduction and evolution of novel genetic membranes and protein-targeting machinery in eukaryote-eukaryote chimaeras (meta-algae).
Chloroplasts originated just once, from cyanobacteria enslaved by a biciliate protozoan to form the plant kingdom (green plants, red and glaucophyte algae), but subsequently, were laterally transferred to other lineages to form eukaryote-eukaryote chimaeras or meta-algae. This process of secondary symbiogenesis (permanent merger of two phylogenetically distinct eukaryote cells) has left remarkable traces of its evolutionary role in the more complex topology of the membranes surrounding all non-plant (meta-algal) chloroplasts. It took place twice, soon after green and red algae diverged over 550 Myr ago to form two independent major branches of the eukaryotic tree (chromalveolates and cabozoa), comprising both meta-algae and numerous secondarily non-photosynthetic lineages. ⋯ I discuss evidence from genome sequencing and other sources on the contrasting modes of protein targeting, cellular integration, and evolution of these two major lineages of eukaryote "cells within cells". They also provide powerful evidence for natural selection's effectiveness in eliminating most functionless DNA and therefore of a universally useful non-genic function for nuclear non-coding DNA, i.e. most DNA in the biosphere, and dramatic examples of genomic reduction. I briefly argue that chloroplast replacement in dinoflagellates, which happened at least twice, may have been evolutionarily easier than secondary symbiogenesis because parts of the chromalveolate protein-targeting machinery could have helped enslave the foreign plastids.
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Philos. Trans. R. Soc. Lond., B, Biol. Sci. · Sep 2001
ReviewEpisodic memory and common sense: how far apart?
Research has revealed facts about human memory in general and episodic memory in particular that deviate from both common sense and previously accepted ideas. This paper discusses some of these deviations in light of the proceedings of The Royal Society's Discussion Meeting on episodic memory. ⋯ The serial-dependent-independent (SPI) model of the relations among episodic, semantic and perceptual memory systems accounts for a number of puzzling phenomena, such as some amnesic patients' preserved recognition memory and their ability to learn new semantic facts, and holds that episodic remembering of perceptual information can occur only by virtue of its mediation through semantic memory. Although common sense endows many animals with the ability to remember their past experiences, as yet there is no evidence that humanlike episodic memory-defined in terms of subjective time, self, and autonoetic awareness-is present in any other species.
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Philos. Trans. R. Soc. Lond., B, Biol. Sci. · Sep 2001
ReviewMemory for events and their spatial context: models and experiments.
The computational role of the hippocampus in memory has been characterized as: (i) an index to disparate neocortical storage sites; (ii) a time-limited store supporting neocortical long-term memory; and (iii) a content-addressable associative memory. These ideas are reviewed and related to several general aspects of episodic memory, including the differences between episodic, recognition and semantic memory, and whether hippocampal lesions differentially affect recent or remote memories. Some outstanding questions remain, such as: what characterizes episodic retrieval as opposed to other forms of read-out from memory; what triggers the storage of an event memory; and what are the neural mechanisms involved? To address these questions a neural-level model of the medial temporal and parietal roles in retrieval of the spatial context of an event is presented. ⋯ Two experiments relating to the model are briefly described. A functional neuroimaging study of memory for the spatial context of life-like events in virtual reality provides support for the model's functional localization. A neuropsychological experiment suggests that the hippocampus does store an allocentric representation of spatial locations.
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Philos. Trans. R. Soc. Lond., B, Biol. Sci. · Sep 2001
ReviewEpisodic memory and autonoetic consciousness: a first-person approach.
Episodic memory is identified with autonoetic consciousness, which gives rise to remembering in the sense of self-recollection in the mental re-enactment of previous events at which one was present. Autonoetic consciousness is distinguished from noetic consciousness, which gives rise to awareness of the past that is limited to feelings of familiarity or knowing. Noetic consciousness is identified not with episodic but with semantic memory, which involves general knowledge. ⋯ There is evidence that remembering is selectively impaired in various populations, including not only amnesic patients and older adults but also adults with Asperger's syndrome. This first-person approach to episodic memory represents one way in which that most elusive aspect of consciousness, its subjectivity, can be investigated scientifically. The two kinds of conscious experiences can be manipulated experimentally in ways that are systematic, replicable and intelligible theoretically.
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Philos. Trans. R. Soc. Lond., B, Biol. Sci. · Sep 2001
ReviewEpisodic-like memory in animals: psychological criteria, neural mechanisms and the value of episodic-like tasks to investigate animal models of neurodegenerative disease.
The question of whether any non-human species displays episodic memory is controversial. Associative accounts of animal learning recognize that behaviour can change in response to single events but this does not imply that animals need or are later able to recall representations of unique events at a different time and place. The lack of language is also relevant, being the usual medium for communicating about the world, but whether it is critical for the capacity to represent and recall events is a separate matter. ⋯ A further value of studying episodic-like memory in animals is the opportunity it affords to model certain kinds of neurodegenerative disease that, in humans, affect episodic memory. An example is recent work on a transgenic mouse that over-expresses a mutation of human amyloid precursor protein (APP) that occurs in familial Alzheimer's disease, under the control of platelet derived (PD) growth factor promoter (the PDAPP mouse). A striking age- and amyloid plaque-related deficit is seen using a task in which the mice have to keep changing their memory representation of the world rather than learn a single fact.