Natural Synchrony Research Articles

Chimera states among synchronous fireflies.

Systems of oscillators often converge to a state of synchrony when sufficiently interconnected. Twenty years ago, the mathematical analysis of models of coupled oscillators revealed the possibility for complex phases that exhibit a coexistence of synchronous and asynchronous clusters, known as "chimera states." Beyond their recurrence in theoretical models, chimeras have been observed under specifically designed experimental conditions, yet their emergence in nature has remained elusive. Here, we report evidence for the occurrence of chimeras in a celebrated realization of natural synchrony: fireflies. In video recordings of Photuris frontalis fireflies, we observe, within a single swarm, the spontaneous emergence of different groups flashing with the same periodicity but with a constant delay between them. From the three-dimensional reconstruction of the swarm, we demonstrate that these states are stable over time and spatially intertwined. We discuss the implications of these findings on the synergy between mathematical models and collective behavior.

MeioCapture: an efficient method for staging and isolation of meiocytes in the prophase I sub-stages of meiosis in wheat

BackgroundMolecular analysis of meiosis has been hindered by difficulties in isolating high purity subpopulations of sporogenous cells representing the succeeding stages of meiosis. Isolation of purified male meiocytes from defined meiotic stages is crucial in discovering meiosis specific genes and associated regulatory networks.ResultsWe describe an optimized method termed MeioCapture for simultaneous isolation of uncontaminated male meiocytes from wheat (Triticum spp.), specifically from the pre-meiotic G2 and the five sub-stages of meiotic prophase I. The MeioCapture protocol builds on the traditional anther squash technique and the capillary collection method, and involves extrusion of intact sporogenous archesporial columns (SACs) containing meiocytes. This improved method exploits the natural meiotic synchrony between anthers of the same floret, the correlation between the length of anthers and meiotic stage, and the occurrence of meiocytes in intact SACs largely free of somatic cells. The main advantage of MeioCapture, compared to previous methods, is that it allows simultaneous collection of meiocytes from different sub-stages of prophase I at a very high level of purity, through correlation of stages with anther sizes. A detailed description is provided for all steps, including the collection of tissue, isolation and size sorting of anthers, extrusion of intact SACs, and staging of meiocytes. Precautions for individual steps throughout the procedure are also provided to facilitate efficient isolation of pure meiocytes. The proof-of-concept was successfully established in wheat, and a light microscopic atlas of meiosis, encompassing all stages from pre-meiosis to telophase II, was developed.ConclusionThe MeioCapture method provides an essential technique to study the molecular basis of chromosome pairing and exchange of genetic information in wheat, leading to strategies for manipulating meiotic recombination frequencies. The method also provides a foundation for similar studies in other crop species.

Synchrony affects Taylor’s law in theory and data

Taylor's law (TL) is a widely observed empirical pattern that relates the variances to the means of groups of nonnegative measurements via an approximate power law: variance g ≈ a [Formula: see text] mean gb , where g indexes the group of measurements. When each group of measurements is distributed in space, the exponent b of this power law is conjectured to reflect aggregation in the spatial distribution. TL has had practical application in many areas since its initial demonstrations for the population density of spatially distributed species in population ecology. Another widely observed aspect of populations is spatial synchrony, which is the tendency for time series of population densities measured in different locations to be correlated through time. Recent studies showed that patterns of population synchrony are changing, possibly as a consequence of climate change. We use mathematical, numerical, and empirical approaches to show that synchrony affects the validity and parameters of TL. Greater synchrony typically decreases the exponent b of TL. Synchrony influenced TL in essentially all of our analytic, numerical, randomization-based, and empirical examples. Given the near ubiquity of synchrony in nature, it seems likely that synchrony influences the exponent of TL widely in ecologically and economically important systems.

Dissecting geographic variation in population synchrony using the common vole in central Europe as a test bed.

Spatial synchrony of population fluctuations is ubiquitous in nature. Theoretical models suggest that correlated environmental stochasticity, dispersal, and trophic interactions are important promoters of synchrony in nature to leave characteristic signatures of distance-dependent decays in synchrony. Recent refinements of this theory have clarified how distance-decay curves may steepen if local dynamics are governed by different density-dependent feedbacks and how synchrony should vary regionally if the importance and correlation of environmental stochasticity is location-specific. We analysed spatiotemporal data for the common vole, Microtus arvalis from 49 districts in the Czech Republic to examine the pattern of population synchrony between 2000 and 2014. By extending the nonparametric covariation function, we develop a quantitative method that allows a dissection of the effects of distance and additional variables such as altitude on synchrony. To examine the pattern of local synchrony, we apply the noncentered local-indicators of spatial association (ncLISA) which highlights areas with different degrees of synchrony than expected by the region-wide average. Additionally, in order to understand the obtained pattern of local spatial correlations, we have regressed LISA results against the proportion of forest in each district. The common vole abundances fluctuated strongly and exhibited synchronous dynamics with the typical tendency for a decline of synchrony with increasing distance but, not with altitude. The correlation between the neighbor districts decreases as the proportion of forest increases. Forested areas are suboptimum habitats and are strongly avoided by common voles. The investigation of spatiotemporal dynamics in animal populations is a key issue in ecology. Although the majority of studies are focused on testing hypotheses about which mechanisms are involved in shaping this dynamics it is crucial to understand the sources of variation involved in order to understand the underlying processes.

Survey on synchronization mechanisms in machine-to-machine systems

People have always tried to understand natural phenomena. In computer science natural phenomena are mostly used as a source of inspiration for solving various problems in distributed systems such as optimization, clustering, and data processing. In this paper we will give an overview of research in field of computer science where fireflies in nature are used as role models for time synchronization. We will compare two models of oscillators that explain firefly synchronization along with other phenomena of synchrony in nature (e.g., synchronization of pacemaker cells of the heart and synchronization of neuron networks of the circadian pacemaker). Afterwards, we will present Mirollo and Strogatz׳s pulse coupled oscillator model together with its limitations. As discussed by the authors of the model, this model lacks of explanation what happens when oscillators are nonidentical. It also does not support mobile and faulty oscillators. Finally, it does not take into consideration that in communication among oscillators there are communication delays. Since these limitations prevent Mirollo and Strogatz׳s model to be used in real-world environments (such as Machine-to-Machine systems), we will sum up related work in which scholars investigated how to modify the model in order for it to be applicable in distributed systems. However, one has to bear in mind that there are usually large differences between mathematical models in theory and their implementation in practice. Therefore, we give an overview of both mathematical models and mechanisms in distributed systems that were designed after them.

Transient expression of RAD51 in the late G2-phase is required for cell cycle progression in synchronous Physarum cells.

The homologous recombination factor RAD51 is highly conserved. This criterion enabled us to identify a RAD51 ortholog in Physarum polycephalum. We found that the Physarum protein presents a high homology to the human protein and cross-reacted with antibodies directed against the human RAD51. Taking advantage of the natural synchrony of millions of nuclei within a single cell of Physarum, we investigated the fluctuation of the amount of the PpRAD51 throughout the cell cycle. Our results showed that in the late G2-phase, RAD51 was transiently expressed in a large quantity. Furthermore, knocking-down RAD51 in the G2-phase abolished this transient expression before mitosis and affected cell cycle progression. These results support the idea that RAD51 plays a role in the progression of the cell cycle in the late G2-phase.

A dynamical study of pulse-coupled oscillators in the brain

In 1967, Winfree [1] proposed a novel mathematical approach to describe phenomena of collective synchrony in nature (i.e. flashing of fireflies, clapping in a theatre, alpha rhythms, etc.) using a large coupled network of phase oscillators with a diversity of natural frequencies. By analyzing this large heterogeneous network from a “mean field” approach, the spontaneous synchrony can be understood as a critical phase transition similar to most statistical mechanical systems. In this work, we employ this approach to model the phase transitions and bifurcation structures of a large network of pulse-coupled theta neurons [2] by appropriate choice of Winfree's response and functions, the latter of which is parameterized by a parameter n [1]. As this parameter increases, the influence function approximates the behavior of a pulse-coupled synapse. Assuming a Lorentzian distribution of natural frequencies of width Δ and mean value ω0, taking the thermodynamic limit, and employing the Ott-Antonsen reduction method [3], the collective dynamics of the pulse-coupled network can be analytically reduced to a single low-dimensional dynamical equation for the mean field parameter z(t). We analyze the bifurcation diagrams for different values of the sharpness parameter n. We find that more complex behavior is apparent with increasing sharpness of the influence function, and that equilibria outside the physically relevant region (within the unit circle) affect the transient dynamics of z(t) inside the circle. Further, we find that some level of coherence always exists in the network for non-zero coupling, in contrast to other mean field coupled phase oscillator networks [1,4]. Most interestingly, heterogeneity is observed to suppress complexity in the collective/macroscopic behavior. As the network becomes more homogeneous (∆→0), more complex dynamic including aperiodic and multistability emerge from the macroscopic mean field. Figure 1 A sample bifurcation diagram showing the complex structure of fixed points at various locations in parameter space, for a sharpness parameter of (n = 7). Several representative phase portraits from several distinct region of parameter space are included. ...

Phase locking, the Moran effect and distance decay of synchrony: experimental tests in a model system

Spatially separated populations of many species fluctuate synchronously. Synchrony typically decays with increasing interpopulation distance. Spatial synchrony, and its distance decay, might reflect distance decay of environmental synchrony (the Moran effect), and/or short-distance dispersal. However, short-distance dispersal can synchronize entire metapopulations if within-patch dynamics are cyclic, a phenomenon known as phase locking. We manipulated the presence/absence of short-distance dispersal and spatially decaying environmental synchrony and examined their separate and interactive effects on the synchrony of the protist prey species Tetrahymena pyriformis growing in spatial arrays of patches (laboratory microcosms). The protist predator Euplotes patella consumed Tetrahymena and generated predator-prey cycles. Dispersal increased prey synchrony uniformly over both short and long distances, and did so by entraining the phases of the predator-prey cycles. The Moran effect also increased prey synchrony, but only over short distances where environmental synchrony was strongest, and did so by increasing the synchrony of stochastic fluctuations superimposed on the predator-prey cycle. Our results provide the first experimental demonstration of distance decay of synchrony due to distance decay of the Moran effect. Distance decay of the Moran effect likely explains distance decay of synchrony in many natural systems. Our results also provide an experimental demonstration of long-distance phase locking, and explain why cyclic populations provide many of the most dramatic examples of long-distance spatial synchrony in nature.

Phase-locking and environmental fluctuations generate synchrony in a predator–prey community

Spatially synchronized fluctuations in system state are common in physical and biological systems ranging from individual atoms to species as diverse as viruses, insects and mammals. Although the causal factors are well known for many synchronized phenomena, several processes concurrently have an impact on spatial synchrony of species, making their separate effects and interactions difficult to quantify. Here we develop a general stochastic model of predator-prey spatial dynamics to predict the outcome of a laboratory microcosm experiment testing for interactions among all known synchronizing factors: (1) dispersal of individuals between populations; (2) spatially synchronous fluctuations in exogenous environmental factors (the Moran effect); and (3) interactions with other species (for example, predators) that are themselves spatially synchronized. The Moran effect synchronized populations of the ciliate protist Tetrahymena pyriformis; however, dispersal only synchronized prey populations in the presence of the predator Euplotes patella. Both model and data indicate that synchrony depends on cyclic dynamics generated by the predator. Dispersal, but not the Moran effect, 'phase-locks' cycles, which otherwise become 'decoherent' and drift out of phase. In the absence of cycles, phase-locking is not possible and the synchronizing effect of dispersal is negligible. Interspecific interactions determine population synchrony, not by providing an additional source of synchronized fluctuations, but by altering population dynamics and thereby enhancing the action of dispersal. Our results are robust to wide variation in model parameters representative of many natural predator-prey or host-pathogen systems. This explains why cyclic systems provide many of the most dramatic examples of spatial synchrony in nature.

Effects of altered gravity on the cell cycle, actin cytoskeleton and proteome in Physarum polycephalum

Some researchers suggest that the changes of cell cycle under the effect of microgravity may be associated with many serious adverse physiological changes. In the search for underlying mechanisms and possible new countermeasures, we used the slime mold Physarum polycephalum in which all the nuclei traverse the cell cycle in natural synchrony to study the effects of altered gravity on the cell cycle, actin cytoskeleton and proteome. In parallel, the cell cycle was analyzed in Physarum incubated (1) in altered gravity for 20 h, (2) in altered gravity for 40 h, (3) in altered gravity for 80 h, and (4) in ground controls. The cell cycle, the actin cytoskeleton, and proteome in the altered gravity and ground controls were examined. The results indicated that the duration of the G2 phase was lengthened 20 min in high aspect ratio vessel (HARV) for 20 h, and prolonged 2 h in altered gravity either for 40 h or for 80 h, whereas the duration of other phases in the cell cycle was unchanged with respect to the control. The microfilaments in G2 phase had a reduced number of fibers and a unique abnormal morphology in altered gravity for 40 h, whereas the microfilaments in other phases of cell cycle were unchanged when compared to controls. Employing classical two-dimensional electrophoresis (2-DE), we examined the effect of the altered gravity on P. polycephalum proteins. The increase in the duration of G2 phase in altered gravity for 40 h was accompanied by changes in the 2-DE protein profiles, over controls. Out of a total of 200 protein spots investigated in G2 phase, which were reproducible in repeated experiments, 72 protein spots were visually identified as specially expressed, and 11 proteins were up-regulated by 2-fold and 28 proteins were down-regulated by 2-fold over controls. Out of a total of three low-expressed proteins in G2 phase in altered gravity for 40 h, two proteins were unknown proteins, and one protein was spherulin 3b by MALDI-TOF mass spectrometry (MS). Our results suggest that a low level of spherulin 3b in G2 phase, which may lead to a reduction of Poly(b-L-malate) (PMLA), may contribute to the lengthened duration of G2 phase in altered gravity for 40 h. Present results indicate that altered gravity results in the prolongation of G2 phase with significantly altered actin cytoskeleton and proteome in P. polycephalum.

Analysis of chromatin assembled in vivo using exogenous histones in Physarum polycephalum

Histones are involved in the regulation of almost all events within the eukaryotic cell nucleus that utilize DNA as a substrate. We have developed a novel approach for examining the function of histone proteins and specific domains of these proteins in these various nuclear processes, and in particular assembly of chromatin throughout the cell cycle. This approach exploits several unique characteristics of the slime mold Physarum polycephalum, including the natural synchrony of all (∼10 8) nuclei throughout the cell cycle and the ability of this organism to take up exogenous proteins. Here, culture techniques and biochemical procedures for the incorporation of exogenous core histones into Physarum chromatin in vivo are described. The procedures for subsequent verification of the assembly of exogenous proteins into bona fide nucleosomes are also described.

Centriole duplication, centrosome maturation and spindle assembly in lysates of Spisula solidissima oocytes.

AbstractMarine oocytes and embryos have long been favored systems for the study of cell division and the cytoskeletal dynamics that accompany karyokinesis and cytokinesis. Several characteristics make marine egg systems ideal for the study of cell replication including: 1) the clarity of the eggs, which allows excellent real-time visualization of cytoskeletal components; 2) the natural synchrony of the cells, because the oocytes are typically arrested at a specific phase of the cell cycle until fertilized or artificially activated; 3) the rapid and synchronous cell divisions that follow fertilization or activation; and perhaps most importantly, 4) the large quantities of oocytes that can be obtained. Here we describe an in vitro system that utilizes cytoplasmic extracts prepared from surf clam, Spisula solidissima, oocytes for the analysis of the microtubule cytoskeleton during meiosis I spindle assembly.KeywordsGerminal VesicleHigh Voltage Electron MicroscopyGerminal Vesicle BreakdownSwing Bucket RotorSurf ClamThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Chromatin remodeling by cell cycle stage-specific extracts from Physarum polycephalum

Remodeling of chromatin is an essential process allowing the establishment of specific genetic programs. The slime mold Physarum polycephalum presents the attractive advantage of natural synchrony of the cell cycle in several million nuclei. Whole-cell extracts prepared at precise stages during the cell cycle were tested for the ability to induce remodeling in erythrocyte nuclei as monitored by microscopy, protamine competition assays, micrococcal nuclease digestions, and release of histone H5. Extracts derived from two specific cell cycle stages caused opposite types of changes in erythrocyte nuclei. An increase in chromatin compaction was imparted by extracts prepared during S-phase while extracts harvested at the end of G2-phase caused increases in nuclear volume, DNA accessibility, and release of linker histone. We also found that late G2 extracts had the ability to alter the DNase I digestion profile of mononucleosomes reconstituted in vitro in a classical nucleosomes remodeling assay. The relevance of these finding to the Physarum cell cycle is discussed.

Periodic events and cell cycle regulation in the plasmodia ofPhysarum polycephalum

The multinucleated plasmodia ofPhysarum polycephalum, a myxomycete, have been extensively used in cell cycle studies. The natural synchrony of mitosis and DNA synthesis, easy culture methods, the ready fusions obtainable between plasmodia, and the amenability to phase specific studies, employing physical and chemical perturbers, are some of the attractive features of this organism. Because of the absence of a Gl phase in the plasmodia, there is a crowding of cell cycle specific marker events at the G2/M boundary, which reflect features of both the G2/M and the Gl/S boundaries of a typical eukaryotic cell. Prominent among these are the synthesis and overall activity of thymidine kinase, the co-triggering of tubulin and histone genes, translation of their mRNA, the organization and duplication of the microtubular organizing centres of the mitotic spindle and the triggering of cdc 2 kinase activity. These above events have not only served as good markers to monitor the progress of the plasmodial cell cycle, but have also been fairly thoroughly analysed by means of specific perturbers such as DNA synthesis inhibitors, antimicrotubular drugs, UV-irradiation, heat-shock etc. Along with fusion studies, these perturbation studies have been helpful in the formulation of various models on regulation of mitosis. These above aspects as well as prospects for future studies employing this organism are discussed

Heterogeneity in the mouse epidermal cell cycle analysed by computer simulations.

Different sets of cell kinetic data obtained over many years from hairless mouse epidermis have been simulated by a mathematical model including circadian variations. Simulating several independent sets of data with the same mathematical model strengthens the validity of the results obtained. The data simulated in this investigation were all obtained with the experimental system in a state of natural synchrony. The data include cell cycle phase distributions measured by DNA flow cytometry of isolated epidermal basal cells, fractions of tritiated thymidine ([3H]TdR) labelled cells within the cell cycle phases measured by cell sorting at intervals after [3H]TdR pulse labelling, bivariate bromodeoxyuridine (BrdUrd)/DNA data from epidermal basal cells isolated at intervals after pulse labelling with BrdUrd, mitotic rate and per cent labelled mitosis (PLM) data from histologic sections. The following main new findings were made from the simulations: the second PLM peak observed at about 35 h after pulse labelling is hardly influenced by circadian variations; the peak is mainly determined by persisting synchrony of a rapidly cycling population with a G1-duration (TG1) of 20 h to 30 h; and there is a highly significant population of slowly cycling G1-cells (G1 sigma). However, no significant circadian variations were found in the number of these cells.

Mapping of a Physarum chromosomal origin of replication tightly linked to a developmentally-regulated profilin gene.

We compared the pattern of replication of two cell-type specific profilin genes in one developmental stage of the slime mold Physarum polycephalum. Taking advantage of the natural synchrony of S-phase within the plasmodium, we established that the actively transcribed profilin P gene is tightly linked to a chromosomal replication origin and is replicated at the onset of S-phase. In contrast, the inactive profilin A gene is not associated with a replication origin and it is duplicated in mid S-phase. Mapping by two-dimensional gel electrophoresis defines a short DNA fragment in the proximal upstream region of the profilin P gene from which bidirectional replication is initiated. We further provide an estimate of the kinetics of elongation of the replicon and demonstrate that the 2 alleles of the profilin P gene are coordinately replicated. All these results were obtained on total DNA preparations extracted from untreated cells. They provide a strong evidence for site specific initiation of DNA replication in Physarum.

Antibodies directed against a meiosis-specific, chromatin-associated protein identify conserved meiotic epitopes

The molecular mechanisms by which meiotic events are regulated are at present unknown. To approach this problem, we have exploited the natural synchrony of Lilium meiocytes to compare the nuclear protein profiles of a variety of stages of meiosis. This approach has facilitated the identification of a number of nuclear proteins that appear and disappear in a stage-specific fashion. Here we report the presence of an abundant nuclear protein that first appears during premeiotic interphase, a period during which the irreversible commitment to meiosis occurs. Antibodies directed against this protein demonstrate its meiosis specificity as well as conservation of the epitope(s) in both mono- and dicotyledonous plant species. Chromatin fractionation studies indicate that this protein, which we have termed meiotin-1, is associated with strings of nucleosomes. Implications for meiotic chromatin packaging and chromosome structure are discussed.

Physical relationship between a gene and its origin of replication in Physarum polycephalum

Taking advantage of the natural synchrony of the S-phase within the plasmodium of Physarum polycephalum, we extracted highly synchronous DNA samples at precise time points in early S-phase. We then separated, by electrophoresis under denaturating conditions, the newly synthesized DNA strands of the nascent chromosomal replicons from the parental DNA template. Using the cDNA clone of the early-replicating LAV1-2 gene as a probe, we could establish by filter hybridization that the elongation rate of the replicon which encompasses this gene is constant, at a rate of 1 kb/min during the first 30 min of S-phase. The smallest replication intermediate (RI) that we have detected by probing with the LAV1-2 cDNA was 5 kb long, suggesting that the LAV1-2 gene and its origin of replication are closely associated within the chromosome. This procedure should facilitate the mapping of replication origins within the genome of Physarum.

Amphibian oocyte maturation induced by extracts of Physarum polycephalum in mitosis.

The orderly progression of eukaryotic cells from interphase to mitosis requires the close coordination of various nuclear and cytoplasmic events. Studies from our laboratory and others on animal cells indicate that two activities, one present mainly in mitotic cells and the other exclusively in G1-phase cells, play a pivotal role in the regulation of initiation and completion of mitosis, respectively. The purpose of this study was to investigate whether these activities are expressed in the slime mold Physarum polycephalum in which all the nuclei traverse the cell cycle in natural synchrony. Extracts were prepared from plasmodia in various phases of the cell cycle and tested for their ability to induce germinal vesicle breakdown and chromosome condensation after microinjection into Xenopus laevis oocytes. We found that extract of cells at 10-20 min before metaphase consistently induced germinal vesicle breakdown in oocytes. Preliminary characterization, including purification on a DNA-cellulose affinity column, indicated that the mitotic factors from Physarum were functionally very similar to HeLa mitotic factors. We also identified a number of mitosis-specific antigens in extracts from Physarum plasmodia, similar to those of HeLa cells, using the mitosis-specific monoclonal antibodies MPM-2 and MPM-7. Interestingly, we also observed an activity in Physarum at 45 min after metaphase (i.e., in early S phase since it has no G1) that is usually present in HeLa cells only during the G1 phase of the cell cycle. These are the first studies to show that maturation-promoting factor activity is present in Physarum during mitosis and is replaced by the G1 factor (or anti-maturation-promoting factor) activity in a postmitotic stage. A comparative study of these factors in this slime mold and in mammalian cells would be extremely valuable in further understanding their function in the regulation of eukaryotic cell cycle and their evolutionary relationship to one another.

Effect of donor-embryo-recipient interactions on pregnancy rate in a large-scale bovine embryo transfer program

The effects on pregnancy rate of 23 factors relating to time, embryos, donors, and recipients in a commercial bovine embryo transfer program were analyzed retrospectively. Over 6 1 2 years, embryos were recovered on 1625 occasions from 825 different cows and heifers. Unfrozen embryos were transferred surgically to virgin Holstein heifers of a relatively uniform size and age, maintained under a single management system in the state of Pennsylvania in the USA. Transfer of 7652 embryos resulted in an average pregnancy rate of 71.3% with small, but significant differences among years and months. There were no differences among years, however, in the mean quality, stage, or age of the embryos transferred, or in the mean age of donors, number of embryos recovered, or the mean number of times that a recipient was used. The pregnancy rate was not affected by breed, fertility or lactational status of the donor, or day of the estrous cycle on which superovluatory treatment began; however, embryos from cows over 15 years of age resulted in lower pregnancy rates. While there was no influence of total number of ova recovered or percent fertilization, the number of fertilized ova had a small effect. Moreover, there were differences due to the morphological quality and stage of development of the embryos, and due to the interval from the donor's estrus to recovery. Pregnancy rates were lower with embryos recovered 9 or more days after estrus. Although duration of culture had no effect, Ham's F-10 was superior to modified PBS. Although the interval from treatment to estrus as well as the number of previous estrus synchronization treatments had no influence, recipients induced with prostaglandin to be in estrous-cycle synchrony with the donor had a distinctly higher pregnancy rate than those in natural synchrony. Estrous synchrony between the donor and recipient and the interval from the recipient's estrus to transfer affected the pregnancy rate, but the side and quality of the corpus luteum of the recipient and the number of times that a heifer had been used as a recipient did not. Interactions between the folowing factors influenced the pregnancy rate: 1) day of flush and embryo quality, 2) day of flush and stage of development of the embryo, 3) embryo quality and estrous synchrony, 4) stage of embryonic development and interval from the recipient's estrus to transfer, and 5) stage of embryonic development and estrous synchrony.