Investigations on vaccination against theileriosis in Morocco

Tropical Animal Health and Production -     

Microfilaments in cellular and developmental processes

In our opinion, all of the phenomena that are inhibited by cytochalasin can be thought of as resulting from contractile activity of cellular organelles. Smooth muscle contraction, clot retraction, beat of heart cells, and shortening of the tadpole tail are all cases in which no argument of substance for alternative causes can be offered. The morphogenetic processes in epithelia, contractile ring function during cytokinesis, migration of cells on a substratum, and streaming in plant cells can be explained most simply on the basis of contractility being the causal event in each process. The many similarities between the latter cases and the former ones in which contraction is certain argue for that conclusion. For instance, platelets probably contract, possess a microfilament network, and behave like undulating membrane organelles. Migrating cells possess undulating membranes and contain a similar network. It is very likely, therefore, that their network is also contractile. In all of the cases that have been examined so far, microfilaments of some type are observed in the cells; furthermore, those filaments are at points where contractility could cause the respective phenomenon. The correlations from the cytochalasin experiments greatly strengthen the case; microfilaments are present in control and "recovered" cells and respective biological phenomena take place in such cells; microfilaments are absent or altered in treated cells and the phenomena do not occur. The evidence seems overwhelming that microfilaments are the contractile machinery of nonmuscle cells. The argument is further strengthened if we reconsider the list of processes insensitive to cytochalasin (Table 2). Microtubules and their sidearms, plasma membrane, or synthetic machinery of cells are presumed to be responsible for such processes, and colchicine, membrane-active drugs, or inhibitors of protein synthesis are effective at inhibiting the respective phenomena. These chemical agents would not necessarily be expected to affect contractile apparatuses over short periods of time, they either do not or only secondarily interfere with the processes sensitive to cytochalasin (Table 1). It is particularly noteworthy in this context that microtubules are classed as being insensitive to cytochalasin and so are not considered as members of the "contractile microfilament" family. The overall conclusion is that a broad spectrum of cellular and developmental processes are caused by contractile apparatuses that have at least the common feature of being sensitive to cytochalasin. Schroeder's important insight (3) has, then, led to the use of cytochalasin as a diagnostic tool for such contracile activity: the prediction is that sensitivity to the drug implies presence of some type of contractile microfilament system. Only further work will define the limits of confidence to be placed upon such diagnoses. The basis of contraction in microfilament systems is still hypothetical. Contraction of glycerol-extracted cells in response to adenosine triphosphate (53), extraction of actin-like or actomyosin-like proteins from cells other than muscle cells (54), and identification of activity resembling that of the actomyosin-adenosine triphosphatase system in a variety of nonmuscle tissues (40, 54) are consistent with the idea that portions of the complex, striated muscle contractile system may be present in more primitive contractile machinery. In the case of the egg cortex, calcium-activated contractions can be inhibited by cytochalasin. If, as seems likely, microfilaments are the agents activated by calcium, then it will be clear that they have the same calcium requirement as muscle. Biochemical analyses of primitive contractile systems are difficult to interpret. Ishikawa's important observation (31), that heavy meromyosin complexes with fine filaments oriented parallel to the surface of chondrocytes and perpendicular to the surface of intestinal epithelial cells, implies that both types of filaments are "actin-like" in this one respect. Yet, it is very likely that these actin-like filaments correspond respectively to the cytochalasin-insensitive sheath of glial and heart fibroblasts and the core filaments of oviduct microvilli. No evidence from our studies links contractility directly to these meromyosin-binding filaments. Apart from this problem, activity resembling that of the myosin-adenosine triphosphatase has been associated with the microtubule systems of sperm tails and cilia (55), but those organelles are insensitive to cytochalasin in structure and function. Clearly, a means must be found to distinguish between enzymatic activities associated with microfilament networks, microfilament bundles, microtubules, and the sheath filaments of migratory cells. Until such distinctions are possible, little of substance can be said about the molecular bases of primitive contractile systems. Three variables are important for the control of cellular processes dependent upon microfilaments: (i) which cells of a population shall manufacture and assemble the filaments; (ii) where filaments shall be assembled in cells; and (iii) when contractility shall occur. With respect to distribution among cells, the networks involved in cell locomotion are presumed to be present in all cells that have the potential to move in cell culture. In this respect, the networks can be regarded as a common cellular organelle in the sense that cytoplasmic microtubules are so regarded. In some developing systems, all cells of an epithelium possess microfilament bundles (7, 13), whereas, in others, only discrete subpopulations possess the bundles (5, 6). In these cases the filaments can be regarded as being differentiation products associated only with certain cell types. These considerations may be related to the fact that microfilament networks are associated with behavior of individual cells (such as migration, wound healing, and cytokinesis), whereas the bundles are present in cells that participate in coordinated changes in shape of cell populations. With respect to placement in cells, two alternatives are apparent, namely, localized or ubiquitous association with the plasma membrane. Microfilament bundles of epithelial cells are only found extending across the luminal and basal ends of cells. In this respect they contrast with desmosomal tonofilaments and with microtubules, each of which can curve in a variety of directions through the cell. The strict localization of microfilament bundles probably rests upon their association with special junctional complex insertion regions that are only located near the ends of cells. In the case of mitotically active cells, the orientation of the spindle apparatus may determine the site at which the contractile ring of microfilaments will form (4, 56); this raises the question of what sorts of cytoplasmic factors can influence the process of association between filament systems and plasma membranes. In contrast to such cases of localized distribution, contractile networks responsible for cell locomotion are probably found beneath all of the plasma membrane, just as the network of thrombosthenin may extend to all portions of the periphery of a blood platelet. This ubiquitous distribution probably accounts for the ability of a fibroblast or glial cell to establish an undulating membrane at any point on its edge, or of an axon to form lateral microspikes along its length. The third crucial aspect of control of these contractile apparatuses involves the choice of when contraction shall occur (and as a corollary the degree or strength of contraction that will occur). In the simplest situation, contraction would follow automatically upon assembly of the microfilament bundles or networks. In cleavage furrows of marine embryos (4), for instance, microfilaments are seen beneath the central cleavage furrow and at its ends, but not beyond, under the portion of plasma membrane that will subsequently become part of the furrow. This implies that the furrow forms very soon after the contractile filaments are assembled in the egg cortex. In other cases, microfilaments are apparently assembled but not in a state of (maximal?) contraction. Thus, networks are seen along the sides of migratory cells, although such regions are not then active as undulating membrane organelles. Similarly, microfilament bundles occur in all epithelial cells of the salivary gland (13), or pancreatic anlage (7), although only the ones at discrete points are thought to generate morphogenetic tissue movements. Likewise, bundles begin to appear as early as 12 hours after estrogen administration to oviduct, although visible tubular gland formation does not start until 24 to 30 hours. Finally, streaming in plant cells can wax and wane, depending upon external factors such as auxin (57). All of these cases imply a control mechanism other than mere assembly of the microfilament systems and even raise the possibility that within one cell some filaments may be contracting while others are not. In discussing this problem, it must be emphasized that different degrees of contraction or relaxation cannot as yet be recognized with the electron microscope. In fact, every one of the cases cited above could be explained by contraction following immediately upon some subtle sort of "assembly." Inclusive in the latter term are relations between individual filaments, relations of the filaments and their insertion points on plasma membrane, and quantitative alterations in filament systems. Furthermore, the critical role of calcium and high-energy compounds in muscle contraction suggest that equivalent factors may be part of primitive, cytochalasinsensitive systems. The finding that calcium-induced contraction in the cortex of eggs is sensitive to cytochalasin strengthens that supposition and emphasizes the importance of compartmentalization of cofactors as a means of controlling microfilaments in cells.     

Potato genetic resources: Sources of resistance and systematics

The major potato of commerce,Solanum tuberosum L., is the fourth most important food crop in the world after rice, wheat and corn. Fortunately, the potato has many primitive cultivars and wild species relatives useful to reduce our reliance on chemical controls. These include resistances against diseases, pests, and traits for useful agronomic characters such as yield, specific gravity, chipping qualities, and suppression of enzymatic browning. This paper summarizes some of these qualities, and provides an overview of germplasm availability and taxonomy of the wild species. The major potato of commerce,Solanum tuberosum L., is the fourth most important food crop in the world after rice, wheat and maize. It is grown in more countries than any other crop but maize, and forms the staple crop of many societies. Over 280 million metric tons were grown worldwide in 1989, with Eastern Europe growing 46%, Asia 22%, Western Europe 17%, North America 7%, Latin America 5%, and Africa 3% (2). It is the leading vegetable crop in acreage and farm value in the United States, with 1.2 million acres planted in 1991, with a value of sales almost two and one-half billion dollars (53).Solanum tuberosum is one species of a group of seven cultivated and 216 additional tuber-bearing, and nine non-tuber-bearing wild relatives, all classified by Hawkes (41) in the genusSolanum, sectionPetota Dumort The purposes of this paper are threefold: 1) to provide examples of the proven and potential utility of wild and cultivated landrace members of sect.Petota for reducing our reliance on chemical controls for many pests and diseases that affect commercial cultivars, 2) to provide an overview of the status of germplasm availability of these species, and 3) to highlight the benefits for continuing germplasm collections and systematic studies of the group.     

Effects of landscape composition and connectivity on the distribution of an endangered parrot in agricultural landscapes

The extent and connectivity of individual habitat types strongly affects the distribution and abundance of organisms. However, little is known of how the level of connectivity and the interactions between different habitat types influences the distribution of species. Here, we used the geographically restricted and endangered regent parrot Polytelis anthopeplus monarchoides as a case study to examine the importance of composition and connectivity between different elements in 39 complex landscape mosaics (each 10 km radius). We compiled a database of 674 regent parrot nesting records, regional vegetation maps and measures of multipath connectivity between core vegetation types under different scenarios of resistance to movement provided by landscape elements. The occurrence of regent parrot nests was strongly affected by landscape composition, being positively related to the extent Eucalyptus camaldulensis riverine forest, but negatively related to the extent of semi-arid woodlands dominated by Eucalyptus largiflorens. Connectivity between E. camaldulensis forest (principal nesting habitat) and mallee (preferred feeding habitat) was a strong predictor of nest locations. Our study shows that the suitability of fragmented agricultural landscapes for supporting species can be greatly affected by connectivity and interactions between preferred and non-preferred habitats. For species that require complementary habitats such as the regent parrot, conservation management activities may be ineffective if they simply focus on a single core habitat type or the impacts of human land uses without regard to the interrelationships among landscape elements. While increasing the amount of primary preferred habitat should remain a cornerstone goal, increasing the extent and improving connectivity with alternative landscape elements also should be priority management objectives.     

Schemas and memory consolidation

Memory encoding occurs rapidly, but the consolidation of memory in the neocortex has long been held to be a more gradual process. We now report, however, that systems consolidation can occur extremely quickly if an associative "schema" into which new information is incorporated has previously been created. In experiments using a hippocampal-dependent paired-associate task for rats, the memory of flavor-place associations became persistent over time as a putative neocortical schema gradually developed. New traces, trained for only one trial, then became assimilated and rapidly hippocampal-independent. Schemas also played a causal role in the creation of lasting associative memory representations during one-trial learning. The concept of neocortical schemas may unite psychological accounts of knowledge structures with neurobiological theories of systems memory consolidation.     

A decade of collecting and research on wild potatoes of the Southwest USA

Potato is an important world crop with an abundant diversity of wild relatives for research and breeding. About 200 tuber-bearingSolarium relatives of the cultivated potato are distributed from southern Chile to the southwest USA. Only five of these have been reported in the USA, and only two exist with certainty (S. fendleri andS. jamesii). This paper reviews the procedures and outcome of 12 expeditions by the authors to the Southwest USA from 1992 to 2001 that resulted in 132 new germplasm accessions. Previously published information allowed successful collection from many documented sites, and many new sites were discovered and sampled. Incomplete or inaccurate records were improved and refined, making it possible for others to easily find these sites. When assessed for genetic diversity, re-collections from the same site were found to be nearly as genetically different as samples from different sites, and genetic differences between sites could not be linked with any ecogeographic parameter, even physical distance of separation. In conclusion, wild potato germplasm from the USA and associated knowledge was greatly expanded, but reaching the goal of obtaining and keeping the most complete sample possible of the genetic diversity will involve additional collecting and continued research on the reproductive behavior of these plants.     

Mexico, 1988 potato germplasm collecting expedition and utility of the Mexican potato species

A joint Danish, Mexican, United States wild potato (Solanum sect.Petota) germplasm collecting expedition was conducted in Mexico between Aug. 21-Oct. 20, 1988. The trip resulted in 93 seed and 25 tuber collections of 18 species and one putative natural hybrid,S. xmichoacanum. Rare species collected include:Solanum darum,S. hintonii,S. lesteri, andS. xmichoacanum. First germplasm collections were made of the disjunct populations ofS. fendleri from Baja California. The potential and realized breeding value of these species is discussed.     

Solarium SectionPetota in Costa Rica: Taxonomy and genetic resources

Prior to 1996, worldwide holdings of germplasm of wild potatoes from Costa Rica amounted to just two collections; this country therefore formed a priority for collecting. We mapped all localities of wild potatoes from herbarium specimen data from Costa Rica and collected throughout the country. We made 13 collections, 10 of these with botanical seeds. These collections considerably extend the numbers of accessions and geographic range of the germplasm available from Costa Rica. The taxonomic identity of the species of wild potatoes (Solanum sect. Petota) in Costa Rica was previously unresolved. Our fieldwork supports the concept that Costa Rican wild potatoes belong to a single species,S. longiconicum.