Morphology,biology, and systematics ofCryptochlora perforans (Chlorarachniophyta), a phagotrophic marine alga

The life cycle ofCryptochlora perforans is described under culture conditions, including the following stages: Naked and walled amoebae, cysts, coccoid, palmelloid and flagellate stages. Phagotrophy of living diatoms, movement and reproduction are documented. Amoebae ofCryptochlora are chemotactically attracted by damaged algal filaments, perforate and penetrate such filaments, and subsequently engulf part of their contents. A comparison is made with a culture ofChlorarachnion reptans Geitler, according to earlier and own observations. Chloroplast structure as well as zoospore morphology confirm close affinities betweenCryptochlora andChlorarachnion. Generic characters as well as possible phylogenetic affinities with amoeboidXanthophyta andProtozoa are discussed.     

THE LIFE HISTORY OF COLEOCHAETE SCUTATA (CHLOROPHYCEAE) STUDIED BY A FEULGEN MICROSPECTROPHOTOMETRIC ANALYSIS OF THE DNA CYCLE1,2

The life history of Coleochaete scutata Bréb. was analyzed by Feulgen microspectrophotometry, a technique measuring DNA content in individual nuclei. By correlating nuclear DNA content with morphological structures or stages in the life history, changes in ploidy level are revealed. The microspectrophotometric study confirmed the earlier reports of a haploid vegetative thallus with mitotic division restricted primarily to the margin of the thallus. In the mitotic cycle the G₁ (pre-synthesis) phase is longer in duration than the synthesis find G₂ (post-synthesis) phases. Oogamous sexual reproduction results in resistant oospores which attain DNA levels of 2C 8C (1C being the DNA level of gamete nuclei).     

Fine structure of plasmodial nuclei in the slime moldPhysarum polycephalum I. Comparison of diploid and haploid vegetative mitosis

Summary The same basic ultrastructural features of interphase and mitotic nuclei were found for both the haploid Colonia and the diploid wild type strains of the myxomycete,Physarum polycephalum. Differences in nuclear size and chromocenter numbers were observed, but the nucleolar cycle and the intranuclear and acentriolar type of mitosis characteristic of the plasmodial stage of the diploid is present in haploid plasmodia, ruling out any relation between ploidy level and type of mitotic figure.     

STUDIES ON DIPLOID STRAINS OF DICTYOSTELIUM DISCOIDEUM

Ross, Ian K. (Yale U., New Haven, Conn.) Studies on diploid strains of Dictyostelium discoideum. Amer. Jour. Bot. 47 (1) : 54—59. Illus. 1960.–Three strains of Dictyostelium discoideum having the diploid number of chromosomes (14) at all stages of their life cycle were examined. No evidence of sexuality as shown by syngamy or meiosis was found in the diploid strains. Two of the diploid strains were unstable and reverted to the haploid type with 7 chromosomes. These haploid strains had a sexual phase in which syngamy and meiosis were observed. The nuclear behavior of both the diploid and haploid strains differed from that reported in previous papers.     

PLOIDY ANALYSIS OF THE TWO MOTILE FORMS OF CHRYSOCHROMULINA POLYLEPIS (PRYMNESIOPHYCEAE)1

In some cultures of the flagellate Chrysochromulina polylepis Manton et Parke, established from cells isolated from the massive bloom in Skagerrak and Kattegat in 1988, we observed, two motile cell types. They were termed authentic and alternate cells and differed with respect to scale morphology. To investigate whether or not the two cell forms were joined in a sexual life cycle, the relative DNA content per cell and relative size of cells of several clonal cultures of C. polylepis were determined by flow cytometry. Percentages of authentic and alternate cells in the cultures were estimated by transmission electron microscopy. Pure authentic cultures (α) contained cells with the lowest level of DNA and were termed haploid. Two pure alternate cultures (β) contained cells with double the DNA content of authentic cells and were termed diploid. Other pure alternate cultures contained haploid cells only, or both haploid and diploid cells. Three cell types were observed, each capable of vegetative propagation: authentic haploid, alternate haploid, and alternate diploid cells. Both the haploid and diploid alternate cells were larger than the haploid authentic cells. Cultures containing diploid cells appeared unstable: cell type ratio and ploidy ratio changed during the experiment where this cell type was present, particularly when grown in continuous light. In contrast, cultures with only haploid cells remained unchanged at all growth conditions tested. Light condition may influence cell type ratio and ploidy ratio. Our attempt to induce syngamy by mixing different authentic haploid clones did not result in mating. Assuming that the authentic and alternate cell types are of the same species, the life cycle of C. polylepis includes three flagellated scale-covered cell forms. Two of the cell types are haploid and may function as gametes, and the third is diploid, possibly being the result of syngamy.     

Nuclear behavior and roles indicate that Codiolum phase is a sporophyte in Monostroma angicava (Ulotrichales,Ulvophyceae)

The life‐cycle system of Ulotrichales, a major order of Ulvophyceae, remains controversial because it is unclear whether the Codiolum phase, a characteristic unicellular diploid generation in ulotrichalean algae, is a zygote or a sporophyte. This controversy inhibits the understanding of the diversified life cycles in Ulvophyceae. To distinguish between zygotes and sporophytes, we have to examine not only whether diploid generations function as sporophytes, but also whether mitosis occurs before meiosis in diploid generations. However, the nuclear behavior in the Codiolum phases is largely unknown, probably because no suitable methods are available. Using fluorescent microscopy with ethidium bromide and transmission electron microscopy of cell‐wall‐dissected specimens, we report the nuclear behavior in the Codiolum phases of an ulotrichalean alga with a representative life cycle, Monostroma angicava. Each vegetative Codiolum phase had a single polyploid nucleus due to endoreduplication, a type of mitosis without nuclear division. During zoosporogenesis, the nucleus had a structure that would be a meiosis‐specific complex. We quantitatively showed that Codiolum phases grew extremely large and produced numerous zoospores. Our results suggest that an event comparable to mitosis occurs before meiosis in the Codiolum phase of M. angicava. This nuclear behavior and the functions (growth and zoospore production abilities) correspond to those of sporophytes. Therefore, the life‐cycle system of M. angicava is a heteromorphic haplo‐diplontic cycle. This system appears to be widely adopted among other ulotrichalean algae.     

Markov Model of Haploid Random Mating with Given Distribution of Population Size

An exact Markov chain model is formulated and computed for random mating in a haploid gamete pool. There are two versions of the gamete, and there is a finite number of diploid monoecious organisms. The founder population is given, and the subsequent generations allow a prescribed statistical distribution over different population sizes. The non-homogeneous Markov chain works on the haploid gamete level provided the probability of self-fertilization is 1/n, where n is the number of diploid individuals. Standard deviations of gamete frequencies and fixation probabilities are calculated. Effective population sizes for different population size distributions are estimated, including periodic bottlenecks.     

Maintenance of Complex Life Cycles Via Cryptic Differences In The Ecophysiology Of Haploid And Diploid Spores Of An Isomorphic Red Alga1

Recognition of the wide diversity of organisms that maintain complex haploid–diploid life cycles has generated interest in understanding the evolution and persistence of such life cycles. We empirically tested the model where complex haploid–diploid life cycles may be maintained by subtle/cryptic differences in the vital rates of isomorphic haploid–diploids, by examining the ecophysiology of haploid tetraspores and diploid carpospores of the isomorphic red alga Chondrus verrucosus. While tetraspores and carpospores of this species did not differ in size or autofluorescence, concentrations of phycobiliproteins of carpospores were greater than that of tetraspores. However, tetraspores were more photosynthetically competent than carpospores over a broader range of photosynthetic photon flux densities (PPFDs) and at PPFDs found at both the depth that C. verrucosus is found at high tide and in surface waters in which planktonic propagules might disperse. These results suggest potential differences in dispersal potential and reproductive success of haploid and diploid spores. Moreover, these cryptic differences in ecological niche partitioning of haploid and diploid spores contribute to our understanding of some of the differences between these ploidy stages that may ultimately lead to the maintenance of the complex haploid–diploid life cycle in this isomorphic red alga.     

Nuclear conditions in haploid Pelargonium in vivo and in vitro

In the shoot apices of the haploid Pelargonium cultivar Kleine Liebling, all mitoses are haploid (n = 9); however, ca. 20% of the interphase nuclei have DNA contents greater than 2C (up to 4C), indicating a tendency to chromosome endoreduplication in this material. — In internodes in vivo, the few mitoses present are haploid (quite probably, cambium cells); in addition to haploid interphases (1C to 2C DNA contents), endoreduplicated (endopolyploid) nuclei and nuclei in the course of endoreduplication occur with a frequency of ca. 40–50% (DNA contents up to 8C). — When internodes are cultured in vitro, differentiated cells are stimulated to divide, thus forming a population of diploid and tetraploid mitoses in addition to the preexistent meristem (haploid) cell population. In the process of time, diploid and tetraploid mitoses continue to be present in the callus, whilst haploid mitoses may decrease in number and eventually disappear. All mitoses analyzed had euploid chromosome numbers (9, 18 and 36) and their DNA contents were correspondingly 2C, 4C and 8C. Since no extensive chromosome counts were made, aneuploidy in the cultured material cannot be excluded; but, if occurring, it should be rather rare. — Under the experimental conditions used, prolonged culture in vitro leads to the production of nuclei with DNA contents (16C and 32C) greater than those occurring in vivo (8C), due to one and two additional DNA replications respectively beyond the limits attained in vivo. Even in these cultures, however, a population of the meristematic haploid cell line (DNA values 1C to 2C) is still present. — The present results are discussed in their relations with previous works on nuclear conditions in vivo and in vitro and on regeneration processes in cultured tissues in plants.     

The ultrastructure of mitosis in myxamoebae and plasmodia of Physarum flavicomum

Electron microscopy of glutaraldehyde-osmium-fixed samples of haploid myxamoebae and diploid plasmodia of the myxomycete Physarum flavicomum Berk. reveal dissimilar spindle apparatus during mitosis in the two cell types. Myxamoebae exhibit an astral type of mitosis with centrioles at the poles and nuclear envelope breakdown during prophase. Plasmodial nuclei lack centrioles at mitosis and have an intranuclear spindle, with nuclear envelope persisting during the entire division. Coated vesicles are noted during prophase and telophase in myxamoebae and their role in spindle formation and dispersion is suggested.     

Unequal sister chromatid exchange in the rDNA array of Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae the nucleolar organiser region (NOR) is located on chromosome XII. It contains 100-200 copies of rDNA--a minimum of 20 rDNA genes in tandem--and is termed the RDN locus. Yeast cells may exist in either haploid or diploid form. There are two forms of life cycle: haploid and diploid cells double by mitosis, and diploid cells are reduced to the haploid state by meiosis. Diploid cells have two homologous chromosomes for each of the 16 chromosomes. They are usually of the same size. However, in this study it is shown that homologous chromosomes XII can become different in size due to unequal sister chromatid exchange during mitosis in 'old' cells.     

Hybridization between two cryptic filamentous brown seaweeds along the shore: analysing pre‐ and postzygotic barriers in populations of individuals with varying ploidy levels

We aimed to study the importance of hybridization between two cryptic species of the genus Ectocarpus, a group of filamentous algae with haploid–diploid life cycles that include the principal genetic model organism for the brown algae. In haploid–diploid species, the genetic structure of the two phases of the life cycle can be analysed separately in natural populations. Such life cycles provide a unique opportunity to estimate the frequency of hybrid genotypes in diploid sporophytes and meiotic recombinant genotypes in haploid gametophytes allowing the effects of reproductive barriers preventing fertilization or preventing meiosis to be untangle. The level of hybridization between E. siliculosus and E. crouaniorum was quantified along the European coast. Clonal cultures (568 diploid, 336 haploid) isolated from field samples were genotyped using cytoplasmic and nuclear markers to estimate the frequency of hybrid genotypes in diploids and recombinant haploids. We identified admixed individuals using microsatellite loci, classical assignment methods and a newly developed Bayesian method (XPloidAssignment), which allows the analysis of populations that exhibit variations in ploidy level. Over all populations, the level of hybridization was estimated at 8.7%. Hybrids were exclusively observed in sympatric populations. More than 98% of hybrids were diploids (40% of which showed signs of aneuploidy) with a high frequency of rare alleles. The near absence of haploid recombinant hybrids demonstrates that the reproductive barriers are mostly postzygotic and suggests that abnormal chromosome segregation during meiosis following hybridization of species with different genome sizes could be a major cause of interspecific incompatibility in this system.     

Studies on the cytotypes of Atrichum undulatum (Hedw.) P. Beauv. II. Chromosome length and relative DNA content

Abstract

Evidence is presented showing that the chromosomes of diploid and triploid races of Atrichum undulatum are significantly shorter than those of haploid plants. Relative DNA contents of the three cytotypes have been estimated and they differ significantly from an expected 1:2:3 ratio in haploid, diploid and triploid races.     

Quantitation of Absolute Pneumocystis carinii Nuclear DNA Content. Trophic and Cystic Forms Isolated from Infected Rat Lungs are Haploid Organisms

The Pneumocystis carinii carinii DNA content in nuclei of trophic forms and cysts (spore cases) containing 2, 4, or 8 intracystic bodies, were compared using quantitative fluorescence image analysis. The nuclear DNA content was found to be lower than the theoretical limits of Feulgen cytophotometry. Several fluorescent DNA dyes provide brighter staining, but these techniques suffer from nonspecific binding to other cellular components, such as RNA. It was demonstrated that the thick glycocalyx surfaces of trophic forms and the cyst walls of P. carinii organisms, as well as the cell wall of S. cerevisiae, bound all fluorescent dyes tested to varying degrees. Hence in this study, measurements were performed on cells in which the outer surfaces of organisms were first removed with lyticase. Two stains that appeared most specific for DNA, DB181 and 4′,6-diamidino-2-phenylindole (DAPI), were used for quantitations; lower deviations of fluorescence intensities were observed with DB181. Haploid wild type Saccharomyces cerevisiae and cdc-28 temperature-sensitive mutant cells, accumulated at the restrictive temperature (37° C), were used as quantitative internal standards for estimating the absolute nuclear DNA content of P. carinii. Haploid wild type and mutant nuclei stained with DAPI had the same relative fluorescence intensities. The P. carinii nuclear DNA content of trophic forms and individual intracystic bodies (spores), regardless of life cycle stage, were not different. The mean values obtained were 6.9 and 6.7 fg DNA/nucleus with DB181 and DAPI, respectively (approximately 9.26 and 8.99 Mbp nucleotides, respectively). Since these would include 2C (G-2 phase) and S-phase nuclei, a 1C population of nuclei was selected by histogram distributions of DB181-stained nuclei. Almost all nuclei analyzed in all life cycle stages fell within this population. The 1C mean of 6.55 fg DNA/nucleus (median, 6.62 fg DNA/nucleus) was estimated as representing 8.79 Mbp nucleotides, assuming only A-T binding of the dye and taking into account the G+C content of S. cerevisiae and P. carinii. A 4C (G-2-phase diploid nuclei) population was not detected in histograms of DB181- or DAPI-stained nuclei. The P. carinii nuclear DNA content values obtained in this study were similar to those independently obtained by calculating the total DNA in the organism's chromosomes resolved by electrophoretic techniques. Together, the data on total chromosome numbers and the estimated DNA content of those chromosomes, with our quantitation of nuclear DNA content of different life-cycle stages demonstrate that P. carinii carinii isolated from infected rat lungs are haploid organisms.     

Ploidy determination in Sphagnum samples from Svalbard,Arctic Norway,by DNA image cytometry

Abstract

We measured DNA content of cell nuclei, stained with the Feulgen method, using branch tips of 11 species of Sphagnum from Svalbard, Arctic Norway, as an alternative to chromosome counting. Nine species were haploid and two were diploid, with no intraspecific variation in ploidy level. The results conformed to known chromosome numbers and/or to expectations from isozyme studies. Ploidy levels were determined for the first time in S. tundrae and S. fimbriatum ssp. concinnum (haploid) and S. arcticum and S. olafii (diploid). No mitotic divisions were observed, but unreplicated interphase nuclei still allowed precise ploidy determinations. Basic DNA contents of all Sphagnum species were very similar, and measurement of a few nuclei proved sufficient to ascertain ploidy level despite very low nuclear DNA content. Advantages of the DNA image cytometry method are: mitotic or meiotic cells are not required to be found, and only a small amount of material is required.     

What haplodiploids can teach us about hybridization and speciation

Most evolutionary theory focuses on species that reproduce through sexual reproduction where both sexes have a diploid chromosome count. Yet a substantial proportion of multicellular species display complex life cycles, with both haploid and diploid life stages. A classic example is haplodiploidy, where females develop from fertilized eggs and are diploid, while males develop from unfertilized eggs and are haploid. Although haplodiploids make up about 15% of all animals (de la Filia et al. 2015 ), this type of reproduction is rarely considered in evolutionary theory. In this issue of Molecular Ecology, Patten et al. ( 2015 ) develop a theoretical model to compare the rate of nuclear and mitochondrial introgression in haplodiploid and diploid species. They show that when two haplodiploid species hybridize, nuclear genes are much less likely to cross the species barrier than if both species were to be diploids. The reason for this is that only half of the offspring resulting from matings between haplodiploid species are true hybrids: sons from such mating only inherit their mother genes and therefore only contain genes of the maternal species. Truly, hybrid males can only occur through backcrossing of a hybrid female to a male of one of the parental species. While this twist of haplodiploid transmission genetics limits nuclear introgression, mitochondrial genes, which are maternally inherited, are unaffected by the scarcity of hybrid males. In other words, the rate of mitochondrial introgression is the same for haplodiploid and diploid species. As a result, haplodiploid species on average show a bias of mitochondrial compared to nuclear introgression.     

NUCLEAR GENE DOSAGE EFFECTS ON MITOCHONDRIAL MASS AND DNA

In order to assess the effect of nuclear gene dosage on the regulation of mitochondria we have studied serial sections of a set of isogenic haploid and diploid cells of Saccharomyces cerevisiae, growing exponentially in the absence of catabolite repression, and determined the amount of mitochondrial DNA per cell. Mitochondria accounted for 14% of the cytoplasmic and 12% of the total cellular volume in all cells examined regardless of their ploidy or their apparent stage in the cell cycle. The mean number of mitochondria per cell was 22 in the diploid and 10 in the haploids. The volume distribution appeared unimodal and identical in haploids and diploids. The mitochondrial DNA accounted for 12.6 ± 1.2% and 13.5 ± 1.3% of the total cellular DNA in the diploid and haploid populations, respectively. These values correspond to 3.6 x 10^-15 g, 2.2 x 10⁹ daltons, or 44 genomes (50 x 10⁶ daltons each) per haploid and twice that per diploid cell. On this basis, the average mitochondrion in these cells contains four mitochondrial genomes in both the haploid and the diploid.     

Repair of DNA double-strand breaks requires two homologous DNA duplexes

DNA repair and cell survival in haploid and its diploid derivative strains ofSaccharomyces cerevisiae were studied after 100 krad X-ray irradiation. The cells were in theG ₁ stage of the cell cycle, where haploid cells had only one copy of genetic material per genome and diploid had two copies. It was found that diploid could repair double-strand breaks in its DNA after 48 hr of liquid holding which was accompanied by a four-fold rise in survival. In contrast a haploid strain failed to repair its DNA and showed no increase in survival after liquid holding. It is concluded that (1) repair of DNA double-strand breaks requires the availability of two homologous DNA duplexes, (2) restoration of cell viability during liquid holding is connected with repair of DNA double-strand breaks and (3) this repair is a slow process possibly associated with slow finding and conjugation of homologous chromosomes.