The differential adhesion hypothesis: a direct evaluation

The differential adhesion hypothesis (DAH), advanced in the 1960s, proposed that the liquid-like tissue-spreading and cell segregation phenomena of development arise from tissue surface tensions that in turn arise from differences in intercellular adhesiveness. Our earlier measurements of liquid-like cell aggregate surface tensions have shown that, without exception, a cell aggregate of lower surface tension tends to envelop one of higher surface tension to which it adheres. We here measure the surface tensions of L cell aggregates transfected to express N-, P- or E-cadherin in varied, measured amounts. We report that in these aggregates, in which cadherins are essentially the only cell-cell adhesion molecules, the aggregate surface tensions are a direct, linear function of cadherin expression level. Taken together with our earlier results, the conclusion follows that the liquid-like morphogenetic cell and tissue rearrangements of cell sorting, tissue spreading and segregation represent self-assembly processes guided by the diminution of adhesive-free energy as cells tend to maximize their mutual binding. This conclusion relates to the physics governing these morphogenetic phenomena and applies independently of issues such as the specificities of intercellular adhesives.     

Emergence patterns in male butterflies: A hypothesis and a test

A game theoretical model is advanced to explain the emergence time schedule of male butterflies under temporal “apostatic” selection, so that males emerging on different days enjoy equal fitness in evolutionary equilibrium. The model predicts not only the position of the peak date but also the shape of the male emergence curve for any given female emergence schedule. Where the female emergence curve is smooth with one peak, a flight season can be divided into an earlier phase, when some males emerge every day, and a later phase in which no male emerges. The male emergence curve is truncated at the boundary of the phases. The position of the truncation point is determined by the difference between pre- and postemergence mortality. Preemergence mortality also determines the rate coefficient of the decrease in sex ratio through the season. The model is applied to a well-studied population of the butterfly Euphydryas editha. The male presence curve fits well, but no clear truncation exists in male emergence, and some males emerge earlier than predicted. Reasons for deviations are discussed.     

Evolving mechanisms of morphogenesis: on the interplay between differential adhesion and cell differentiation

Differential cell adhesion, mediated by e.g. integrin and cadherins/catenines, plays an important role in morphogenesis and it has been shown that there is intimate cross-talk between their expression and modification, and inter-cellular signalling, cell differentiation, cell growth and apoptosis. In this paper, we introduce and use a formal model to explore the morphogenetic potential of the interplay between these processes. We demonstrate the formation of interesting morphologies. Initiated by cell differentiation, differential cell adhesion leads to a long transient of cell migrations, e.g. engulfing and intercalation of cells and cell layers. This transient can be sustained dynamically by further cell differentiation, and by cell growth/division and cell death which are triggered by the (also long range) forces (stretching and squeezing) generated by the cell adhesion. We study the interrelation between modes of cell differentiation and modes of morphogenesis. We use an evolutionary process to zoom in on gene-regulation networks which lead to cell differentiation. Morphogenesis is not selected for but appears as a side-effect. The evolutionary dynamics shows the hallmarks of evolution on a rugged landscape, including long neutral paths. We show that a combinatorially large set of morphologies occurs in the vicinity of a neutral path which sustains cell differentiation. Thus, an almost linear molecular phylogeny gives rise to mosaic evolution on the morphological level.     

Brood desertion by female shorebirds: a test of the differential parental capacity hypothesis on Kentish plovers.

The aim of this study was to examine whether the energetic costs of reproduction explain offspring desertion by female shorebirds, as is suggested by the differential parental capacity hypothesis. A prediction of the hypothesis is that, in species with biparental incubation in which females desert from brood care after hatching, the body condition of females should decline after laying to a point at which their body reserves are too low for continuing parental care. We tested this prediction on Kentish plovers (Charadrius alexandrinus) in which both sexes incubate but the females desert from brood care before the chicks fledge. We found no changes in either the body masses or body compositions of both individual male and female plovers from early incubation and throughout early chick rearing. Furthermore, the timing of brood desertion by females was not affected by their body condition. Neither did we find gender differences in the energetic costs of incubation. There were no differences in the timing of brood desertion between experimental and control females in an experiment in which we lengthened or shortened the duration of incubation by one week. These results indicate that energetic costs do not explain offspring desertion by female Kentish plovers and that the needs of chicks for parental care rather than cumulative investment by females is what determines the timing of brood desertion.     

Patchy recruitment patterns in marine invertebrates: a spatial test of the density-dependent hypothesis in the bivalve Spisula ovalis

Density-dependent and density-independent processes have been shown to influence the population dynamics of marine invertebrates, especially recruitment. However, their relative importance has not been evaluated in natural populations. High adult densities have been suggested to inhibit recruitment, especially in suspension-feeders which may ingest incoming larvae. Age structure and juvenile abundance were investigated in the bivalve Spisula ovalis in order to evaluate the importance of density dependence in generating spatial patterns. Age structure is readily established in this species owing to annual shell lines. An extensive sample (from about 100 sites a few hundred meters apart over 4 consecutive years) was analyzed in the statistical framework of spatial analyses, avoiding spurious correlations due to non-independence between neighboring sites. The area studied supports about ten annual cohorts, though only a few occur at each site. The overall picture is a mosaic of kilometer-scale patches of contrasted age structures, as revealed by highly significant spatial autocorrelations. To our knowledge, such large-scale spatial patterns in age structure have not previously been described in benthic invertebrates. Strong patterns are detected even for juveniles, and are independent of the adult biomass present before settlement. Therefore, patchy patterns of age structure mainly reflect density-independent effects, such as spatial variations in larval supply, passive transport of juveniles, or predation on recruits. In the absence of detailed spatial analyses, such patterns have been misinterpreted previously as negative effects of adult density on settlement success. Received: 21 November 1996 / Accepted: 20 February 1997     

Homing and adhesion patterns determine the cellular composition of the bone marrow plasma cell niche

According to commonly held concepts, plasma cell (PC) longevity in bone marrow (BM) depends upon their access to survival niches. These are thought to exist in nursery cell types, which support PCs by secreting PC survival factors. To better define PC survival niches and their functioning, we adoptively transferred traceable Blimp-1-(GFP) PCs into recipient mice lacking a proliferation-inducing ligand (APRIL), IL-6, or macrophage migration inhibitory factor. Transferred BMPCs were preferentially associated with Ly-6C(high) monocytes (normalized colocalization index: 9.84), eosinophils (4.29), and megakaryocytes (2.12). Although APRIL was essential for BMPC survival, PC recruitment into the proximity of nursery cells was unimpaired in APRIL-deficient mice, questioning the concept that the same factors account for attraction/retention of PCs as for their local survival. Rather, the order of colocalization with BMPCs (monocytes > eosinophils > megakaryocytes) reflected these cells' relative expression of CXCR4, VLA-4, and LFA-1, the homing and adhesion molecules that direct/retain PCs in the BM. This suggests a scenario wherein the cellular composition of the BMPC niche is defined by a common pattern of attraction/retention on CXCL12-abundant reticular docking cells. Thereby, PCs are directed to associate in a functional BM niche with hematopoietic CXCR4(+)VLA-4(+)LFA-1(+) nursery cells, which provide PC survival factors.     

The neural mechanisms of mate choice: a hypothesis

Scientists have described many physical and behavioral traits in avian and mammalian species that evolved to attract mates. But the brain mechanisms by which conspecifics become attracted to these traits is unknown. This paper maintains that two aspects of mate choice evolved in tandem: 1) traits that evolved in the "display producer" to attract mates and, 2) corresponding neural mechanisms in the "display chooser" that enable them to become attracted to these display traits. Then it discusses our (in-progress) fMRI brain scanning project on human romantic attraction, what we believe is a developed form of "courtship attraction" common to avian and mammalian species as well as the primary neural mechanism underlying avian and mammalian mate choice. The paper hypothesizes that courtship attraction is associated with elevated levels of central dopamine and norepinephrine and decreased levels of central serotonin in reward pathways of the brain. It also proposes that courtship attraction is part of a triune brain system for mating, reproduction and parenting. 1)The sex drive evolved to motivate birds and mammals to court any conspecifics. 2) The attraction system evolved to enable individuals to discriminate among potential mating partners and focus courtship activities on particular individuals, thereby conserving mating time and energy. 3) The neural circuitry for attachment evolved to enable individuals to complete species-specific parental duties.     

Spatial patterns of herbivory by gall-forming insects: a test of the soil fertility hypothesis in a Mexican tropical dry forest

It has been proposed that fertile soils reduce the incidence of gall-forming insect (GFI) species in plant communities. This is known as the soil fertility hypothesis. The main objective of this study was to analyze the spatial distribution of GFI species under different habitats in a tropical dry forest at the Chamela-Cuixmala Biosphere Reserve, Mexico. Eight habitats which differ in soil type, topography, nutrient availability and vegetation were chosen. We found that 38 GFI species specialize on their host plant species. GFI species richness was negatively correlated with phosphorous and nitrogen availability. Using phosphorous as an indicator of soil fertility, we found low frequency and density of GFI on fertile soils. Our study indicates that soil fertility is one of the factors that negatively affects the patterns of spatial distribution of species richness, incidence and abundance of GFI at the community level in two different ways: i) indirectly affecting GFI species richness in plants adapted to infertile soils and ii) directly affecting GFI responses to plant traits of hosts found in a fertility gradient.     

Resource polyphenism increases species richness: a test of the hypothesis

A major goal of evolutionary biology is to identify the causes of diversification and to ascertain why some evolutionary lineages are especially diverse. Evolutionary biologists have long speculated that polyphenism—where a single genome produces alternative phenotypes in response to different environmental stimuli—facilitates speciation, especially when these alternative phenotypes differ in resource or habitat use, i.e. resource polyphenism. Here, we present a series of replicated sister-group comparisons showing that fishes and amphibian clades in which resource polyphenism has evolved are more species rich, and have broader geographical ranges, than closely related clades lacking resource polyphenism. Resource polyphenism may promote diversification by facilitating each of the different stages of the speciation process (isolation, divergence, reproductive isolation) and/or by reducing a lineage''s risk of extinction. Generally, resource polyphenism may play a key role in fostering diversity, and species in which resource polyphenism has evolved may be predisposed to diversify.     

Distinct evolutionary patterns between chemoreceptors of 2 vertebrate olfactory systems and the differential tuning hypothesis

Most tetrapod vertebrates have 2 olfactory systems, the main olfactory system (MOS) and the vomeronasal system (VNS). According to the dual olfactory hypothesis, the MOS detects environmental odorants, whereas the VNS recognizes intraspecific pheromonal cues. However, this strict functional distinction has been blurred by recent reports that both systems can perceive both types of signals. Studies of a limited number of receptors suggest that MOS receptors are broadly tuned generalists, whereas VNS receptors are narrowly tuned specialists. However, whether this distinction applies to all MOS and VNS receptors remains unknown. The differential tuning hypothesis predicts that generalist MOS receptors detect an overlapping set of ligands and thus are more likely to be conserved over evolutionary time than specialist VNS receptors, which would evolve in a more lineage-specific manner. Here we test this prediction for all olfactory chemoreceptors by examining the evolutionary patterns of MOS-expressed odorant receptors (ORs) and trace amine-associated receptors (TAARs) and VNS-expressed vomeronasal type 1 receptors (V1Rs) and vomeronasal type 2 receptors (V2Rs) in 7 tetrapods (mouse, rat, dog, opossum, platypus, chicken, and frog). The phylogenies of V1Rs and V2Rs show abundant lineage-specific gene gains/losses and virtually no one-to-one orthologs between species. Opposite patterns are found for ORs and TAARs. Analysis of functional data and ligand-binding sites of ORs confirms that paralogous chemoreceptors are more likely than orthologs to have different ligands and that functional divergence between paralogous chemoreceptors is established relatively quickly following gene duplication. Together, these results strongly suggest that the functional profile of the VNS chemoreceptor repertoire evolves much faster than that of the MOS chemoreceptor repertoire and that the differential tuning hypothesis applies to the majority, if not all, of MOS and VNS receptors.     

The size-grain hypothesis: a phylogenetic and field test

Abstract.  1. The size-grain hypothesis predicts that environmental rugosity results in positive allometric scaling of leg length on body length because of changes in locomotion costs.
2. The scaling of leg length and body length in ants was re-examined using phylogenetic independent contrast methods, and the allometric relationship found by Kaspari and Weiser ( Functional Ecology , 13 , 530–538, 1999) was supported.
3. The size-grain hypothesis was tested further by comparing the body sizes of ants from areas of contrasting habitat complexity in two different savanna habitats. No support for the size-grain hypothesis was found. Small body size classes were no more speciose in the rugose than in the more planar environment, and small ants were more abundant in the planar environment.     

Population regulation of Serengeti Wildebeeest: a test of the food hypothesis

Summary The food hypothesis proposes that density dependent mortality regulates populations through food shortage. For Serengeti wildebeest, we found an empirical relationship between dry season adult mortality rate, density and food supply. This relationship predicted that: (1) the population would stabilize between 1.0 and 1.5 million animals, (2) dry season mortality would be density dependent and sufficient to account for the levelling off of this large ungulate population. Recent observations have tested and confirmed these predictions.     

Gene-culture coevolution: a test of the steady-state hypothesis for gene-culture translation

Although there has been considerable discussion of the gene-culture theory (GCT) approach to biocultural systems, relatively little critical attention has focused on the specific approximations used in GCT, or on their improvement. Here we consider the steady-state hypothesis used in GCT models to connect activity on the level of individual choice behavior to the level of societies taken as wholes. The hypothesis is tested by using Monte Carlo techniques to estimate solutions to the dynamical equation for social order. The steady-state hypothesis is found generally to be invalid for all the categories of choice function originally considered by Lumsden and Wilson, but acceptable for the highly restricted parameter ranges used in their treatments of the gene-frequency change. The implications for the development of improved gene-culture modeling techniques are discussed.     

Risk taking during parental care: a test of the harm-to-offspring hypothesis

Amount of risk taking during parental care is often explainedin relation to the reproductive value of the offspring. The"harm-to-offspring hypothesis" focuses on the relative harma period of no parental care can do to the offspring. Accordingto this hypothesis, parents should take greater risks for offspringin poor condition than for offspring in good condition. We manipulatedoffspring condition in the pied flycatcher (Ficedula hypoleuca)and tested the harm-to-offspring hypothesis by exposing parentsto a predator model (a sparrowhawk, Accipiter nisus). Time elapseduntil a parent first entered the nest-box was used as a risk-takingmeasure. Parents spent significantly shorter time until first nestvisit for offspring in poor condition than for offspring ingood condition. Hence, the harm-to-offspring hypothesis wassupported.     

Habitat suitability in muskrats: a test of the food limitation hypothesis

Data on habitat selection, overwinter survival, and litter survival rate of muskrats indicate that fitness differs among habitats in marsh environments. We measured nine nutrient-dependent phenotypic variables of muskrats and one independent assessment of forage quality during the ice-free period (May-October) from 1992 to 1994. Data were used to test the hypothesis that differences in food resources limit the demographic performance of muskrats in marginal versus prime habitats (food limitation hypothesis, FLH). None of the null predictions relating habitat quality with reproductive parameters could be rejected statistically. Adult body mass and length were greater in prime habitats, but fat content did not differ, which suggested that the difference was associated with larger structural size of dominant animals in prime habitats. Growth rate of weaned juveniles was not different among habitats, which represented the strongest evidence for rejecting the FLH. Faecal crude protein content, which was used as an index of quality of food ingested, was marginally different between the two most suitable habitats, but the direction of the difference did not support our prediction. Overall, the temporal variation in faecal crude protein content was similar among habitats. We conclude that food is not a key factor limiting the demographic performance of muskrats among habitats during the ice-free season.