Lymphocyte locomotion and attachment on two-dimensional surfaces and in three-dimensional matrices

The adhesion and locomotion of mouse peripheral lymph node lymphocytes on 2-D protein- coated substrata and in 3-D matrices were compared. Lymphocytes did not adhere to, or migrate on, 2-D substrata suck as serum- or fibronectin-coated glass. They did attach to and migrate in hydrated 3-D collagen lattices. When the collagen was dehydrated to form a 2-D surface, lymphocyte attachment to it was reduced. We propose that lymphocytes, which are poorly adhesive, are able to attach to and migrate in 3-D matrices by a nonadhesive mechanism such as the extension and expansion of pseudopodia through gaps in the matrix, which could provide purchase for movement in the absence of discrete intermolecular adhesions. This was supported by studies using serum-coated micropore filters, since lymphocytes attached to and migrated into filters with pore sizes large enough (3 or 8 mum) to allow pseudopod penetration but did not attach to filters made of an identical material (cellulose esters) but of narrow pore size (0.22 or 0.45 mum). Cinematographic studies of lymphocyte locomotion in collagen gels were also consistent with the above hypothesis, since lymphocytes showed a more variable morphology than is typically seen on plane surfaces, with formation of many small pseudopodia expanded to give a marked constriction between the cell and the pseudopod. These extensions often remained fixed with respect to the environment as the lymphocyte moved away from or past them. This suggests that the pseudopodia were inserted into gaps in the gel matrix and acted as anchorage points for locomotion.     

Size homoplasy and mutational processes of interrupted microsatellites in two bee species, Apis mellifera and Bombus terrestris (Apidae)

Similar microsatellite electromorphs (PCR products of the same size) can arise from independent mutational events. Such alleles are not identical by descent. This phenomenon, termed size homoplasy, was studied by sequencing electromorphs of two microsatellite loci in which the stretch of basic repeats is interrupted by different short (1-2 bp) DNA motifs. The number and position of these interruptions were established for electromorphs from closely and distantly related populations of honeybees and bumblebees. No sequence difference was found when electromorphs came from the same subspecies or from closely related subspecies, suggesting that they were probably identical by descent. In contrast, sequence differences were often detected in distantly related subspecies, showing that size homoplasy frequently occurs at this level of population differentiation. Size homoplasy is increased by limits to free length variation of alleles, a phenomenon that seems to act on interrupted microsatellites when comparing distantly related taxa, that is, honeybee subspecies from different evolutionary lineages. Electromorph sequences suggest that, within the scope of these limits, large mutation events have occurred frequently at both interrupted loci studied. In good agreement with the molecular data, computations based on the observed heterozygosity and number of electromorphs and simulation studies showed that neither locus fits the one-step stepwise mutant model (SMM). We speculate that interrupted microsatellites in general could be characterized by a higher variance in repeat number and consequently a lower homoplasy rate than pure ones. Hence, interrupted microsatellites should be most appropriate for investigating population differentiation and evolutionary relationship between relatively distant populations.      

Isolation and characterization of bacteriophage T4 mutant preheads.

To determine the function of individual gene products in the assembly and maturation of the T4 prehead, we have isolated and characterized aberrant preheads produced by mutations in three of the T4 head genes. Mutants in gene 21, which codes for the T4 maturation proteases, produce rather stable preheads whose morphology and protein composition are consistent with a wild-type prehead blocked in the maturation cleavages. Mutants in gene 24 produce similar structures which are unstable because they have gaps at all of their icosahedral vertices except the membrane attachment site. In addition, greatly elongated "giant preheads" are produced, suggesting that in the absence of P24 at the vertices, the distal cap of the prehead is unstable, allowing abnormal elongation of broth the prehead core and its shell. Vertex completion by P24 is required to allow the maturation cleavages to occur, and 24- preheads can be matured to capsids in vitro by the addition of P24. Preheads produced by a temperature-sensitive mutant in gene 23 are deficient in core proteins. We show that the shell of these preheads has the expanded lattice characteristic of the mature capsid as well as the binding sites for the proteins hoc and soc, even though none of the maturation cleavage takes place. We also show that 21- preheads composed of wild-type P23 can be expanded in vitro without cleavage.