Evolutionary Plasticity of Habenular Asymmetry with a Conserved Efferent Connectivity Pattern

The vertebrate habenulae (Hb) is an evolutionary conserved dorsal diencephalic nuclear complex that relays information from limbic and striatal forebrain regions to the ventral midbrain. One key feature of this bilateral nucleus is the presence of left-right differences in size, cytoarchitecture, connectivity, neurochemistry and/or gene expression. In teleosts, habenular asymmetry has been associated with preferential innervation of left-right habenular efferents into dorso-ventral domains of the midbrain interpeduncular nucleus (IPN). However, the degree of conservation of this trait and its relation to the structural asymmetries of the Hb are currently unknown. To address these questions, we performed the first systematic comparative analysis of structural and connectional asymmetries of the Hb in teleosts. We found striking inter-species variability in the overall shape and cytoarchitecture of the Hb, and in the frequency, strength and to a lesser degree, laterality of habenular volume at the population level. Directional asymmetry of the Hb was either to the left in D. rerio, E. bicolor, O. latipes, P. reticulata, B. splendens, or to the right in F. gardneri females. In contrast, asymmetry was absent in P. scalare and F. gardneri males at the population level, although in these species the Hb displayed volumetric asymmetries at the individual level. Inter-species variability was more pronounced across orders than within a single order, and coexisted with an overall conserved laterotopic representation of left-right habenular efferents into dorso-ventral domains of the IPN. These results suggest that the circuit design involving the Hb of teleosts promotes structural flexibility depending on developmental, cognitive and/or behavioural pressures, without affecting the main midbrain connectivity output, thus unveiling a key conserved role of this connectivity trait in the function of the circuit. We propose that ontogenic plasticity in habenular morphogenesis underlies the observed inter-species variations in habenular asymmetric morphology

Hypothesis on the Dual Origin of the Mammalian Subplate

The development of the mammalian neocortex relies heavily on subplate. The proportion of this cell population varies considerably in different mammalian species. Subplate is almost undetectable in marsupials, forms a thin, but distinct layer in mouse and rat, a larger layer in carnivores and big-brained mammals as pig, and a highly developed embryonic structure in human and non-human primates. The evolutionary origin of subplate neurons is the subject of current debate. Some hypothesize that subplate represents the ancestral cortex of sauropsids, while others consider it to be an increasingly complex phylogenetic novelty of the mammalian neocortex. Here we review recent work on expression of several genes that were originally identified in rodent as highly and differentially expressed in subplate. We relate these observations to cellular morphology, birthdating, and hodology in the dorsal cortex/dorsal pallium of several amniote species. Based on this reviewed evidence we argue for a third hypothesis according to which subplate contains both ancestral and newly derived cell populations. We propose that the mammalian subplate originally derived from a phylogenetically ancient structure in the dorsal pallium of stem amniotes, but subsequently expanded with additional cell populations in the synapsid lineage to support an increasingly complex cortical plate development. Further understanding of the detailed molecular taxonomy, somatodendritic morphology, and connectivity of subplate in a comparative context should contribute to the identification of the ancestral and newly evolved populations of subplate neurons

An Acoustic & Articulatory Analysis of Consonant Sequences across Word Boundaries in Tripolitanian Libyan Arabic

The main goal of this thesis is to provide a description of the articulatory and temporal interaction between stops spanning the word boundary in the four sequence types VC#CV, VC#CCV, VCC#CV, and VCC#CCV in Tripolitanian Libyan Arabic. A general aim of the study is to contribute to the Phonetic description of Libyan Arabic and to provide a better understanding of speech production and the temporal organisation of articulatory gestures. One of the principal objectives of this study is to investigate what effect an increase in the number of stops in a sequence will have on the timing of stop gestures. Furthermore, the study aims to identify the different patterns of gestural coordination and the types of inter-consonantal intervals occurring between stops in the four sequence types. Another aim of the study is to investigate the nature of the resulting inter-consonantal intervals occurring between these stops in order to understand the patterns of epenthesis. Factors affecting gestural coordination such as the order of place of articulation of stops and speech rate are also an objective of this study. Voice assimilation across the word boundary is also investigated in addition to the influence of inter-consonantal intervals on the process. The study adopts Articulatory Phonology as a theoretical framework to carry out the investigations. The data was collected through recordings of participants’ speech and was subjected to EPG and acoustic analysis. Ten native speakers of Tripolitanian Libyan Arabic took part in the acoustic part of the study. Two of the speakers also took part in the EPG part of the study. Results show that the effect of the number of stops in a sequence on gestural timing is not limited to within syllable-initial and final clusters but also spreads across the word boundary. The timing of syllable-final and syllable-initial stops decreases as a result of the increase in the number of stops across the word boundary. The results also show that the timing of syllable-final clusters is more variable than syllable-initial clusters in across word boundary sequences. Different sequences types exhibit different degrees of gestural coordination and epenthesis patterns between adjacent stops. Inter-consonantal intervals occurring as a result of lag durations between adjacent stop gestures fall into two types. The first type are typical of transitional excrescent vowels with a mean duration ranging from 14ms-20ms and their voice values exhibit more variation as a result of the voice context in which they occur. Inter-consonantal intervals of the second type are typical of epenthetic vowels with a mean duration ranging from 43ms-51ms and are usually specified as voiced. The patterns of epenthesis also show that Tripolitanian Libyan Arabic belongs to the VC type of languages where sequences of three stops CCC are broken up by epenthesis occurring between C1 and C2 of the sequence. Statistical tests show a significant effect for order place of articulation on gestural coordination across the word boundary in TLA in the C#C sequence where gestures are more closely coordinated in the coronal-dorsal order. Regressive voice assimilation is more frequent and the voice context of the stops involved plays a major role in determining the direction of voice assimilation spreading. Progressive voice assimilation is limited to the –V+V voice context and whereas regressive assimilation of voicelessness occurs in both. Furthermore, excrescent vowels are found to be transparent to voice assimilation and are dependent on the voicing of the trigger segment. On the other hand, epenthetic vowels block voice assimilation and are more dependent usually specified as voiced

Asymmetry and laterality index of the habenulae in teleosts.

<p>Abbreviations: L (Left), Max (maximum value), Min (minimum value), R (Right).</p

Relationship between habenular volume and laterality Index.

<p>(A–D) Plots showing the relation between habenular volume (rHb+lHb, in mm³×10⁻³) and asymmetry (rHb - lHb, in mm³×10⁻⁴) in different species of teleosts. Data corresponding to different species have been presented in two rows, each representing sex (female = top; male = bottom), and grouped into two columns according to the size of individuals (left = smaller fish; right = larger fish). Groups of dots sharing the same colour correspond to individuals of a single species, and the line of equivalent colour depicts the linear regression of that group. The abbreviation for each species is given on either left or right sides of the regression line, according to the code given in E. (E) Pearson's correlation coefficient (r) and p values (in parenthesis) for each species and sex. The asterisk indicates the presence of statistically significant correlation between habenular volume and asymmetry (p<0.05).</p

Comparative distribution of left and right habenular efferents in the interpeduncular nucleus of teleosts.

<p>(A) Drawings of adult male individuals belonging to different teleost species, placed in the context of a cladogram of the teleost lineage according to Nelson <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035329#pone.0035329-Nelson1" target="_blank">[28]</a>. (B) Schematic representation of a teleost brain (e.g. D rerio), showing the procedure of differential dye labelling in left (DiD, red) and right (DiO, green) Hb, and the location and orientation of the histological sections shown in B′, and C–I. (B′) Schematic drawing of a coronal section at the level of the IPN in a teleost brain (e.g. D rerio), showing dorsal and ventral aspects of the IPN in red and green, respectively. (C–I) Confocal microscopy images of 100 µm-thick vibratome sections taken according to B′, with dorsal to the top. The boundaries of dorsal (dIPN) and ventral (vIPN) IPN domains have been depicted with dashed lines. Panels C to I correspond to efferents labelled after placing crystals of DiD in the left Hb. Panels C′ to I′ correspond to efferents labelled after placing crystals of DiO in the right Hb. Abbreviations: A (anterior), D (dorsal), P (posterior), TeO (Optic Tectum), V (ventral). Scale bars: 100 µm.</p

Cytoarchitectonic organisation of the habenulae in teleosts.

<p>(A) Drawings of adult male individuals belonging to different teleost species, placed in the context of a cladogram of the teleost lineage according to Nelson <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035329#pone.0035329-Nelson1" target="_blank">[28]</a>. (B) Schematic representation of a teleost brain (e.g. D rerio), showing the location and orientation of histological sections shown in C–I. (C–I) Photomicrographs of cresyl-violet stained 10 µm-thick coronal sections taken at a midpoint between rostral and caudal ends of the Hb as shown in B. Each panel corresponds to a single species, as indicated in the letter code of the left diagram. C′ is a magnification of the square region depicted in C. Dorsal is to the top, and left is to the left. Arrowheads point to the subhabenular sulcus. Asterisks indicate the position of the dorsal-most neuropil region of the dorsal habenulae that is surrounded by a shell of cell bodies in some species. Abbreviations: A (anterior), D (dorsal), dHb (dorsal Hb), L (left), P (posterior), R (right), TeO (Optic Tectum), V (ventral), vHb (ventral Hb). Scale bars: 50 µm.</p

Relationship between body weight and habenular laterality Index.

<p>(A–D) Plots showing the relation between the weight of individual fish (in grams, g) and the score of habenular laterality index (LI) in different species of teleosts. Positive and negative scores of LI indicate right- and left- sided direction of habenular asymmetry, respectively. Data corresponding to different species have been presented in two rows, each representing a sex (female = top; male = bottom), and grouped into two columns according to the size of individuals (left = smaller fish; right = larger fish). Groups of dots sharing the same colour correspond to individuals of a single species, and the line of equivalent colour depicts the linear regression of that group. The abbreviation for each species is given on either left or right sides of the regression line, according to the code given in E. (E) Pearson's correlation coefficient (r) and p values (in parenthesis) for each species and sex. The asterisk indicates the presence of statistically significant correlation between body weight and habenular LI (p<0.05).</p

Sex-specific differences in volume, asymmetry and laterality index of the habenulae in teleosts.

<p>Abbreviations: male>female [m], female>male [f].</p