A genetic map of Dictyostelium discoideum based on mitotic recombination

A genetic map of the cellular slime mold Dictyostelium discoideum is presented in which 42 loci are ordered on five of the seven linkage groups. Although most of the loci were ordered using standing mitotic crossing-over techniques in which recessive selective markers were employed, use was also made of unselected recombined haploid strains. Consistent with cytological studies in which the chromosomes appear to be acrocentric, only a single arm has been found for each of the five linkage groups studied. The mating-type locus, matA, has been located in the tsgE-sprA interval on linkage group I on the basis of studies on diploids formed between strains of opposite mating type that have escaped from vegetative incompatibility.

Chromosome rearrangements in Dictyostelium discoideum

A tandem duplication (D350(III,III] of the whiB to radB interval of linkage group III has been characterized. The gene order on the duplication-bearing chromosome is: centromere, whiB500, radB+, whiB+, radB24, bsgA5, acrC4. Slow-growing, duplication-bearing strains (yellow-spored, radiation-resistant) produced four classes of faster growing sectors involving the whiB and radB loci: white-spored, radiation-sensitive (whiB500, radB24); white-spored, radiation-resistant (whiB500, radB+); yellow-spored, radiation-sensitive (whiB+, radB24); and yellow-spored, radiation-resistant. The first three classes can be explained as the products of single recombination events in which one copy of the whiB to radB interval was lost. The yellow-spored, radiation-resistant sectors probably arose by mutation elsewhere in the genome, but alternatively may represent multiple recombination events or deletion of part of one copy of the duplicated region. Loss of the duplicated segment was enhanced by irradiation with ultraviolet light (254 nm). Heterozygosity for a DNA repair mutation at the radB locus may have been involved in the formation of the duplication. It is proposed that translocations are a major cause of nonrandom segregation patterns such as the cosegregation of unlinked markers in Dictyostelium discoideum. Translocations involving all known linkage groups are tabulated and DNA damage by N-methyl-N'-nitro-N-nitrosoguanidine is implicated in the formation of translocations in D. discoideum.

Antigenic differences detected between prespore cells of Dictyostelium discoideum and Dictyostelium mucoroides using monoclonal antibodies

A method was developed to obtain frozen sections of paraformaldehyde-fixed Dictyostelium slugs. These sections were ideal for studies using monoclonal antibodies. Two different monoclonal antibodies were used to recognize specific antigens on the surface of D. discoideum prespore cells by fluorescence microscopy. Neither of the two monoclonal antibodies bound to D. discoideum prestalk cells. D. mucoroides, a closely related species to D. discoideum, failed to bind either monoclonal antibody to its prespore cells. These results contrast with those obtained with either D. mucoroides [1] or D. discoideum [2] polyspecific antisera.

Investigation of cell patterning in the asexual fruiting body of Dictyostelium discoideum using haploid and isogenic diploid strains

The cell patterning (proportion of spores, stalk cells, and basal disk cells) of individual asexual fruiting bodies of haploid and isogenic diploid strains of D. discoideum was examined to test the hypothesis that the patterning mechanism is based on the 'sensing' of only a single parameter, e.g., cell volume, in dividing the aggregate into the three cell types. If cell patterning is based on sensing only a single parameter, there is no reason to predict a change in cell patterning with ploidy change, and thus haploid and isogenic diploid strains should not differ in their cell patterning. The cell patterning of each of the three pairs of haploid and isogenic diploid strains examined was different. Therefore we conclude that the cell patterning mechanism must involve at least two components not changing in the same way with change in ploidy. The cell patterning of both the haploid and the diploid strains was qualitatively similar, i.e., relationships between the three cell types were described by equations of the same form in the haploid and diploid strains. However a quantitative change in cell patterning led to an increased percentage of stalk and basal disk cells in each diploid compared to its parent haploid. The ratio of basal disk to stalk cell was also greater in the diploids than in their parent haploids. We conclude that these are general ploidy-related changes because the cell patterning of each of the three parent haploid strains was different; the average percentage of stalk cells was 11.6% for X22 (12.4% for its diploid DU162), 20.5% for NP73 (27.2% for its diploid DP62), and 24.5% for HU127 (29.1% for its diploid DU310). One possible patterning mechanism could involve a diffusible signal (s), which shows gene dosage, interacting with cell-surface molecules which we predict occupy a limited number of sites per unit area of the cell membrane. The observed change in cell patterning leading to an increased percentage of stalk cells in diploid strains is predicted from such a model involving diffusible signal interacting with cell-surface molecules.

Genetic and cytological characterisation of fusion chromosomes of Dictyostelium discoideum

Abnormally large chromosomes which appear to result from the fusion of 2 chromosomes of the normal karyotype have been found in diploids of Dictyostelium discoideum formed by parasexual fusion of haploid strains HU483 (N = 7) and HU245 (n = 7). These fusion chromosomes appear to be the products of the tandem translocation of most, if not all, of one acrocentric chromosome to the telomere of a second acrocentric. Thus the chromosome number of the diploids is reduced from the normal 2n = 14 to 2n = 13 with the formation of an abnormally large acrocentric fusion chromosome. Experimental haploidisation of such diploids result in two types of products, those with a normal 7 chromosome karyotype and those with an abnormal 6 chromosome karyotype which contains the fusion chromosome. Genetic analysis of haploid segregants indicates that linkage groups II and VII are involved in this fusion. Phenotypes of recombinant diploids obtained following mitotic crossing-over establishes that linkage group II is proximal to linkage group VII. Cytological examination of the karyotypes of haploid strains bearing the fusion chromosome suggest that chromosome 2 may correspond to linkage group II and chromosome 3 to linkage group VII. Haploid strains bearing the fusion chromosome grow and develop normally so little or no genetic information can have been lost in the fusion event. While the nature of this event is unknown it may have involved aberrant recombinational DNA repair since the parental haploid strain HU483 bears the radB13 DNA repair mutation.

An extracellular chemical signal controlling phototactic behavior by D. discoideum slugs

Developing cells of the cellular slime mold Dictyostelium discoideum release a low molecular weight metabolite (Slug Turning Factor, STF) which, at high uniform concentrations, interferes with phototaxis and thermotaxis by D. discoideum slugs. D. discoideum slugs migrating in darkness are repelled by (exhibit negative chemotaxis to) crude STF exudates. Dose-response curves relating the accuracies of phototaxis and negative chemotaxis to STF concentration indicate that, in both phototaxis and chemotaxis, slugs "measure" the ratios of STF concentrations on their opposite sides. Net STF release is enhanced by light. We propose that light, focused onto the slug's distal side by its convex surface, generates a lateral STF gradient in response to which the slug turns toward the light source.

The age-dependent loss of cells from the rear of Dictyostelium discoideum slug is not tip controlled

Young slugs of the cellular slime mould Dictyostelium discoideum drop small numbers of individual amoebae (approximately 10/mm) in the slime trail. With increased time of migration, slugs develop trailing tails and leave clumps of cells in their slime trails. Using reciprocal transplants between tips of young and old slugs and between a wild-type strain and an 'aged" mutant it was shown that this age-dependent cell loss is due to changes in the bulk of cells comprising the slug, rather than to changes in the effectiveness of the tip (organizer region). Another property of the slug, the decision to continue migrating or form a fruiting body which is controlled by the tip, was less affected by age. This raises the possibility that cell autonomous properties of the slug are more subject to ageing than is the tip.

The assignment of four new loci, including the coumarin sensitivity locus couA, to linkage group VII of Dictyostelium discoideum

The assignment to linkage group VII of the coumarin sensitivity mutation couA351, which leads to a loss of colony-forming ability on agar containing coumarin, was established using the linkage group VII markers cobA1 and tsgK21. Complementation of the couA351 and bsgA5 mutations was an effective method of selecting heterozygous diploids at 21 +/- 1 degree C, i.e. without requiring a temperature sensitivity mutation. A morphological mutation, frtB353, which affects the distribution of fruiting bodies was also assigned to linkage group VII. Both the couA351 and frtB353 mutations were discovered in the tsgK21 strain NP187. Eight independently isolated, recessive mutations leading to resistance of 300 micrograms CoCl2 ml-1 were allelic with the cobA1 mutation. The partially dominant cob-353 mutation was shown to map in linkage group VII on the basis of its segregation relative to the couA351 and tsgK21 mutations and is almost certainly an allele of the cobA locus. These results are consistent with there being only a single locus at which mutations can lead to resistance to high concentrations of CoCl2. Two temperature sensitivity mutations tsgM357 and tsgG4 were also assigned to linkage group VII.

CHROMOSOME FRAGMENTS IN DICTYOSTELIUM DISCOIDEUM OBTAINED FROM PARASEXUAL CROSSES BETWEEN STRAINS OF DIFFERENT GENETIC BACKGROUND

The first aneuploid strains of Dictyostelium discoideum have been unambiguously characterized, using cytological and genetic analysis. Three independently isolated, but genetically similar, fragment chromosomes have been observed in segregants from diploids formed between haploid strains derived from the NC4 and VI2 isolates of D. discoideum. Once generated, the fragment chromosomes, all of which have Vl2-derived centromeres, can be maintained in a NC4 genetic background. Genetic evidence is consistent with the view that all three fragment chromosomes studied encompass the region from the centromere to the whiA locus of linkage group II and terminate in the interval between whiA and acrA. From cytological studies, one of the fragment chromosomes consists of approximately half of linkage group II.——We observed no deleterious effect on viability or asexual fruiting-body formation in either haploid or diploid strains carrying an additional incomplete chromosome and hence are disomic or trisomic, respectively, for part of linkage group II. The incomplete chromosome is lost at a frequency of 2 to 3% from disomic and trisomic strains, but surprisingly this loss is not increased in the presence of the haploidizing agent, benlate. A new locus (clyA), whose phenotype is altered colony morphology, is assigned to the region of linkage group II encompassed by the fragment chromosome.

Evidence for tip control of the 'slug/fruit' switch in slugs of Dictyostelium discoideum

Reciprocal transplants were performed between slugs of two strains of Dictyostelium discoideum. Slugs of one strain (NP84 'slugger') showed prolonged migration, while slugs of the other strain (AX3 'fruiter') migrated for a short period only. The transplant experiments showed that the 'slug/fruit' characteristic is tip dependent, since an NP84 tip induced an AX3 rear to migrate for a prolonged period, while an AX3 tip induced the rear of a NP84 slug to fruit without migration. These findings are not consistent with the hypothesis that tips of all stages release only one signal which is interpreted differently by the rest of the cells in the aggregate at different stages. Rather, we propose that the tip of a D. discoideum aggregate may release a number of signals, one of which is stage dependent, and triggers the 'slug/fruit' switch.

VEGETATIVE INCOMPATIBILITY AND THE MATING-TYPE LOCUS IN THE CELLULAR SLIME MOLD DICTYOSTELIUM DISCOIDEUM

The genetic basis of vegetative incompatibility in the cellular slime mold, Dictyostelium discoideum, is elucidated. Vegetatively compatible haploid strains form parasexual diploids at a frequency of between 10-6 and 10-5, whereas "escaped" diploids are formed between vegetatively incompatible strains at a frequency of ~10-8. There is probably only a single vegetative incompatibility site, which appears to be located at, or closely linked to, the mating-type locus. The nature of the vegetative incompatibility is deduced from parasexual diploid formation between wild isolates and tester strains of each mating type, examination of the frequency of formation of "escaped" diploids formed between vegetatively incompatible strains, and examination of the mating type and vegetative incompatibility of haploid segregants obtained from "escaped" diploids.

CHARACTERIZATION OF DOMINANT RESISTANCE TO COBALT CHLORIDE IN DICTYOSTELIUM DISCOIDEUM AND ITS USE IN PARASEXUAL GENETIC ANALYSIS

Strains of Dictyostelium discoideum resistant to cobaltous chloride have been isolated at a frequency of approximately 10-6. The resistant strains have one of three phenotypes, recessive to wild type, dominant to wild type and dominant to wild type but requiring the presence of cobaltous chloride to maintain resistance. Strains carrying a dominant cobaltous chloride resistance mutation and a recessive growth temperature-sensitive mutation can be mixed with wild-type haploid lines and then subjected to selection so that only diploid lines survive. Differential sensitivity to cycloheximide has also been observed. Hypersensitivity to cycloheximide in combination with dominant cobaltous chloride resistance provides a means of selecting diploids without the use of temperature-sensitive mutations.

The amino acid sequence of cytochrome c from the blowfly Lucilia cuprina

The amino acid sequence of cytochrome c isolated from the sheep blowfly Lucilia cuprina has been determined by comparison of the compositions of the tryptic peptides to those predicted from the published sequences of cytochromes c from other insects. Cytochrome c from L. cuprina differs at a single residue when compared to cytochrome c from the screw worm fly Haematobia irritans, a species belonging to the same order as the blowfly. This substitution, proline for alanine, has been located at position 44 in the protein chain.

Electron spin resonance studies of the membranes of the cellular slime mold Dictyostelium discoideum

Doxylstearic acid spin labels are used to study the fluidity of the membranes of the cellular slime mold, Dictyostelium discoideum. The tau omicron value of the wild-type cell membrane is close to that of egg lecithin indicating a rather fluid membrane. No detectable change in the fluidity of the bulk lipids at the 16-carbon depth occurs during differentiation of the myxamoebae into stalk and spore cells despite reported changes in the individual lipid components. The results of studies on temperature-sensitive and aggregationless mutants are also presented.

Isolation of strains of the cellular slime mold Dictyostelium discoideum capable of growing after a single passage in axenic medium

Wild-type strains of the cellular slime mold Dictyostelium discoideum grow on bacteria. Axenic strains of D. discoideum capable of growing in a simple salts-yeast extract-proteose peptone-glucose medium have been isolated from wild-type strains fo both mating types after a single passage in this medium. This result is consistent with the theory that the axenic phenotype has a simple genetic basis.

A genetic study of aggregation in the cellular slime mould Dictyostelium discoideum using complementation analysis

A series of aggregation-deficient (aggregateless) mutants were isolated in genetically marked haploid strains of the cellular slime mould Dictyostelium discoideum. Diploids were produced from pairs of such haploid mutants by a fusion system based on this organism's parasexual cycle. The diploids were isolated from the haploids by using complementation of non-allelic growth-temperature-sensitive mutations and selection at the restrictive temperature. Complementation between the aggregateless mutations uas then assessed in 419 diploids so formed. The non-complementing aggregateless mutations fell into five complementation groups (ago A, B, C, D, and E) and a dominant aggregation class that allowed little or no aggregation when present in a diploid with any of the other mutations tested or the parental wild type. Complicating factors, including partial dominance, multiple mutations, and possible interallelic conplementation, are discussed. Data on the linkage of the aggregateless mutations was obtained by using recessive drug resistance mutations on three linkage groups to segregate haploids from the diploids. Calculations from our results suggest a genetic complexity of about 50 genes that are specific and essential for aggregation.