328 research outputs found
Egg shape changes at the theropod–bird transition, and a morphometric study of amniote eggs
The eggs of amniotes exhibit a remarkable variety of shapes,
from spherical to elongate and from symmetrical to asymmetrical.
We examine eggshell geometry in a diverse sample of fossil
and living amniotes using geometric morphometrics and linear
measurements. Our goal is to quantify patterns of morphospace
occupation and shape variation in the eggs of recent through to
Mesozoic birds (neornithe plus non-neornithe avialans), as well
as in eggs attributed to non-avialan theropods. In most amniotes,
eggs show significant deviation from sphericity, but departure
from symmetry around the equatorial axis is mostly confined
to theropods and birds. Mesozoic bird eggs differ significantly
from extant bird eggs, but extinct Cenozoic bird eggs do not. This
suggests that the range of egg shapes in extant birds had already
been attained in the Cenozoic. We conclude with a discussion
of possible biological factors imparting variation to egg shapes
during their formation in the oviduct
Boolean delay equations on networks: An application to economic damage propagation
We introduce economic models based on Boolean Delay Equations: this formalism
makes easier to take into account the complexity of the interactions between
firms and is particularly appropriate for studying the propagation of an
initial damage due to a catastrophe. Here we concentrate on simple cases, which
allow to understand the effects of multiple concurrent production paths as well
as the presence of stochasticity in the path time lengths or in the network
structure.
In absence of flexibility, the shortening of production of a single firm in
an isolated network with multiple connections usually ends up by attaining a
finite fraction of the firms or the whole economy, whereas the interactions
with the outside allow a partial recovering of the activity, giving rise to
periodic solutions with waves of damage which propagate across the structure.
The damage propagation speed is strongly dependent upon the topology. The
existence of multiple concurrent production paths does not necessarily imply a
slowing down of the propagation, which can be as fast as the shortest path.Comment: Latex, 52 pages with 22 eps figure
Effect of egg turning and incubation time on carbonic anhydrase gene expression in the blastoderm of the Japanese quail (Coturnix c. japonica)
(1) The gene expression of carbonic anhydrase, a key enzyme for the production sub-embryonic fluid (SEF), was assessed in turned and unturned eggs of the Japanese quail. The plasma membrane-associated isoforms CA IV, CAIX, CA XII, CA XIV, and the cytoplasmic isoform CA II, were
investigated in the extra-embryonic tissue of the blastoderm and in embryonic blood.
(2) Eggs were incubated at 37.6C, c. 60% R.H., and turned hourly (90 ) or left unturned. From 48 to 96 hours of incubation mRNA was extracted from blastoderm tissue, reverse-transcribed to cDNA and quantified by real-time qPCR using gene-specific primers. Blood collected at 96h was processed identically.
(3) Blastoderm CAIV gene expression increased with the period of incubation only in turned eggs, with maxima at 84 and 96h of incubation. Only very low levels were found in blood.
(4) Blastoderm CA II gene expression was greatest at 48 and 54h of incubation, subsequently declining to much lower levels and una ected by turning. Blood CA II gene expression was about 25-fold greater than that in the blastoderm.
(5) The expression of CA IX in the blastoderm was the highest of all isoforms, yet unaffected by turning.
CA XII did not amplify and CA XIV was present at unquantifiable low levels.
(6) It is concluded that solely gene expression for CA IV is sensitive to egg turning, and that increased CA IV gene expression could account for the additional SEF mass found at 84-96h of incubation.
in embryos of turned eggs
Egg shape in the Common Guillemot Uria aalge and Brunnich’s Guillemot U. lomvia: not a rolling matter?
The adaptive significance of avian egg shape is
poorly understood, and has been studied only in those
species producing pyriform (pear-shaped, or pointed) eggs:
waders and guillemots (murres) Uria spp., albeit to a
limited extent. In the latter, it is widely believed that the
pyriform shape has evolved to minimise their likelihood of
rolling off a cliff ledge: the idea being that the more
pointed the egg, the narrower the arc in which it rolls, and
the less likely it is it will fall from a cliff ledge. Previous
research also claimed that the rolling trajectory—the
diameter of the arc they describe—of Common Guillemot
U. aalge eggs is influenced not only by its shape but also
by its mass, with heavier (i.e. larger) eggs describing a
wider arc than lighter eggs. The finding that both shape and
mass determined the rolling trajectory of Common
Guillemot eggs (the shape–mass hypothesis) was used to
explain the apparent anomaly that Bru¨nnich’s Guillemot U.
lomvia produce eggs that are less pointed, yet breed on
narrower ledges than Common Guillemots. They are able
to do this, it was suggested, because Bru¨nnich’s Guillemot
eggs are smaller and lighter in mass than those of Common
Guillemots. However, since some populations of
Bru¨nnich’s Guillemots produce eggs that are as large or
larger than those of some Common Guillemot populations,
the shape–mass hypothesis predicts that that (1) larger (i.e.
heavier) eggs of both guillemot species will be more
pyriform (pointed) in shape, and (2) that eggs of the two
species of same mass should be similarly pointed. We
tested these predictions and found: (1) only a weak, positive
association between egg volume and pointedness in
both guillemot species (\3% of the variation in egg shape
explained by egg volume), and (2) no evidence that eggs of
the two species of similar mass were more similar in shape:
regardless of their mass, Brunnich’s Guillemot eggs were
less pointed than Common Guillemot eggs. Overall, our
results call into question the long-held belief that protection
from rolling is the main selective factor driving guillemot
egg shape
The Current State of Performance Appraisal Research and Practice: Concerns, Directions, and Implications
On the surface, it is not readily apparent how some performance appraisal research issues inform performance appraisal practice. Because performance appraisal is an applied topic, it is useful to periodically consider the current state of performance research and its relation to performance appraisal practice. This review examines the performance appraisal literature published in both academic and practitioner outlets between 1985 and 1990, briefly discusses the current state of performance appraisal practice, highlights the juxtaposition of research and practice, and suggests directions for further research
Molecular and cellular mechanisms underlying the evolution of form and function in the amniote jaw.
The amniote jaw complex is a remarkable amalgamation of derivatives from distinct embryonic cell lineages. During development, the cells in these lineages experience concerted movements, migrations, and signaling interactions that take them from their initial origins to their final destinations and imbue their derivatives with aspects of form including their axial orientation, anatomical identity, size, and shape. Perturbations along the way can produce defects and disease, but also generate the variation necessary for jaw evolution and adaptation. We focus on molecular and cellular mechanisms that regulate form in the amniote jaw complex, and that enable structural and functional integration. Special emphasis is placed on the role of cranial neural crest mesenchyme (NCM) during the species-specific patterning of bone, cartilage, tendon, muscle, and other jaw tissues. We also address the effects of biomechanical forces during jaw development and discuss ways in which certain molecular and cellular responses add adaptive and evolutionary plasticity to jaw morphology. Overall, we highlight how variation in molecular and cellular programs can promote the phenomenal diversity and functional morphology achieved during amniote jaw evolution or lead to the range of jaw defects and disease that affect the human condition
Meanings of existential uncertainty and certainty for people diagnosed with cancer and receiving palliative treatment: a life-world phenomenological study
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