Sexual Generation Research Articles

Amplified fragment length polymorphism (AFLP) markers reveal that population structure of triploid dandelions (Taraxacum officinale) exhibits both clonality and recombination.

Highly variable amplified fragment length polymorphism (AFLP) fingerprints of triploid apomictic dandelions obtained from three localities in an area where diploids are lacking were analysed to infer the predominant modes of reproduction. The distribution of markers was analysed using character compatibility to infer whether many genotypes agree with a tree-like structure in the data set. The presence of incompatible character state combinations (matrix incompatibility; MI) was used as a measure of genetic exchange. The detection of overrepresented genotypes, of which some were widespread, confirmed asexual reproduction. Not all genotypes were overrepresented; approximately half of the genotypes in the three localities were found only once. Because, in terms of genotype frequencies, only a part of the genetic variation is described, more important aspects of the molecular data such as relationships between markers or genotypes have been studied. The analysis of character compatibility indicated a disagreement of the data with a clonal structure. Nearly all genotypes contributed to MI and this contribution varied considerably among genotypes in each sampled locality. A gradual decrease of matrix incompatibility upon successive deletion of genotypes showing the highest contribution to MI indicated that marker distribution of virtually all genotypes disagreed with a tree-like structure in the data. This result suggested that many genotypes were separated by one or more sexual generations. Consistent with this conclusion was the fact that markers that show a low probability of contributing to MI are different in every sampled locality, which is most easily explained as the result of recombination. Apparently, asexual reproduction has resulted in overrepresented, widespread genotypes but sexual recombination has also substantially contributed to genetic variation in the sites studied.

Evolution of Cyclic Sexual Reproduction Under Host–Parasite Interactions

Alternate changes of sexual and asexual generations was studied theoretically by numerical analyses, from the viewpoint of host-parasite infective interactions. The host species was considered a diploid organism, characterized by two loci determining parasite resistance type and sexual strategy, between which a linkage exists to a certain degree. The sexual reproductive strategy was assumed to be determined by three alleles: asexual, complete sexual and cyclic sexual alleles. The effect of the parasites was represented by decreasing fitness functions of each host-type frequency. Numerical analyses of various linkages between genes and frequency dependence of host fitness revealed that the dynamics varied depending on whether the cyclic sexual allele was dominant or recessive against the complete sexual allele. When the cyclic sexual allele was dominant, the cyclic sexual strain persisted under intermediate frequency dependence of host fitness. On the other hand, when the cyclic sexual allele was recessive, it always tended to spread and to be maintained in the population. In such situations, both complete and cyclic sexual strains coexist, but the asexual strain is lost.

The Ovary of Eve: Egg and Sperm and Preformation (review)

Reviewed by: The Ovary of Eve: Egg and Sperm and Preformation Virginia P. Dawson Clara Pinto-Correia. The Ovary of Eve: Egg and Sperm and Preformation. Chicago: University of Chicago Press, 1997. xxiii + 396 pp. Ill. $29.95. In this book, biologist and novelist Clara Pinto-Correia focuses on the history of preformation, an early modern concept of animal reproduction. Preformation presupposes that at the union of egg and sperm the orderly unfolding of a previously existing, tiny, preformed being begins. To a believer, it was inconceivable that new life could spring spontaneously from matter. Preformation supported the biblical view that God, not chance, governed all creation. Like her mentor, Stephen Jay Gould, Pinto-Correia approaches her subject with gusto, irreverence, and a sense of discovery. She hopes the reader will appreciate preformation as “an entertaining interlude in the study of reproduction” (p. 8). Further, presumably addressing other biologists, she says that her object in writing this book is to show that preformation was not a bizarre, aberrant theory, but one of the great mental constructions that take their places briefly in the history of scientific thought “when enough knowledge has been amassed to elicit them, but the limitations of further knowledge have not yet undermined their potential as explanatory models” (p. 10). Pinto-Correia criticizes earlier biologists-turned-historians such as Frederick J. Cole, who published Early Theories of Sexual Generation in 1930. Cole saw early modern biology as a progressive march toward greater objectivity, with epigenesis eventually prevailing over the “incorrect” idea of preformation. Pinto-Correia thinks too much has been made of the battle between epigenesis and preformation in the history of biology, but she does not cite Jacques Roger’s monumental and balanced account, Les sciences de la vie dans la pensée française du XVIIIe siècle (1963), recently translated by Robert Ellrich as The Life Sciences in Eighteenth-Century French Thought (1997). To Pinto-Correia, the history of early modern thought about reproduction was an internecine fight between two interpretations of preformation, one based on the egg (the ovist position), the other on the sperm. First the ovists take the field, thanks to Jan Swammerdam’s precise observations of insects and the religious justification he proposes for the idea of encasement of germs. The spermists gain the upper hand after Leeuwenhoek discovers “little animalcules” in semen. But ovism stages a comeback with the discovery of parthenogenesis by Charles Bonnet, followed on its heels by the discovery of regeneration by Abraham Trembley. Regeneration temporarily throws the ovists off balance, but spermism, [End Page 497] by then on the defensive, loses the final battles, thanks to the work of Lazzaro Spallanzani. Pinto-Correia claims that it is preformation’s compatibility with religion and with a mechanical view of the world that explains the theory’s hold over early modern thought. In her view, the biological discoveries themselves were “less important side effects” (p. 21). She suggests that part of the attraction of ovism was that human beings are naturally drawn to explanations based on the ancient belief that the circle is the image of perfection. More problematic was the question of why God had placed perfection within woman, an inferior creation. Regrettably, Pinto-Correia’s speculations on the universality of some aspects of human thought seem to negate her argument that preformation must be understood within its historical context. Because of her focus on ideas, she makes little use of her background in biology. She fails to examine critically what magnificent observers like Swammerdam, Leeuwenhoek, Bonnet, Haller, Trembley, and Spallanzani thought they saw and why. Nevertheless, The Ovary of Eve is an intriguing, if idiosyncratic, account of the history of an idea. Through diligent reading of primary sources and other historians of biology, the author has brought freshness and humor to what is generally regarded as an abstruse subject. Virginia P. Dawson Cleveland, Ohio Copyright © 1999 The Johns Hopkins University Press

The distribution of rates of spontaneous mutation over viruses, prokaryotes, and eukaryotes.

Although mutation has chaotic aspects, spontaneous mutation rates assume certain characteristic values when expressed per genome per genome duplication. The rate among lytic RNA viruses is roughly 1, while the rate among retroelements is roughly 0.2. The rate among viral and cellular microbes with DNA chromosomes is close to 0.0034. Mutation rates among higher eukaryotes, estimated from specific-locus studies, vary greatly. Most of this variation can be suppressed if the rates are expressed per cell division instead of per sexual generation, and if the genome size is taken to be only a little larger than the sum of the protein-encoding sequences; then, the mutation rate is roughly 0.01. The reasons for different characteristic mutation rates among different organism groups remain mysterious and pose a substantial challenge to students of evolution.

Complex Life Cycles in Andricus kollari (Hymenoptera, Cynipidae) and Their Impact on Associated Parasitoid and Inquiline Species

The cynipid gall wasp Andricus kollari was introduced to southwestern Britain in the middle of the nineteenth century and has since spread throughout the British Isles. A. kollari produces a sexual generation in spring on Turkey oak (Quercus cerris; an introduced tree in Britain) and a parthenogenetic generation in autumn on the native oak species Q. robur and Q. petraea. The galls of the two generations differ considerably in size ( 50% in the south to less than 5% in the north. Similarly, parasitoid abundance in the autumn galls decreased from an average of 6 individuals per gall in the south to I per gall in the north (in contrast to the spring galls, the autumn galls are sufficiently large to contain inquilines and can yield more than one individual per gall). Parasitism rates in the spring galls showed no geographical trend in 1994, but a clear decrease from south to north in 1995. This year, 1995, was a high-density year in the north, which suggests a satiation effect whereby the parasitoid species cannot follow the sudden increase in the host population.

Aristotle Against the Hippocratics on Sexual Generation: A Reply to Coles

An examination of the similarities and differences between Aristotle and the Hippocratic physicians is illuminating on a variety of points, as the work of G. E. R. Lloyd and other scholars has shown. Here, I consider and criticize A. Coles's recent contention («Biomedical models of reproduction in the fifth century B.C. and Aristotle's Generations of animals», in «Phronesis», 40, 1995, pp. 48-88) that, regarding the question of how sexual generation takes place, Aristotle was influenced so significantly by Cnidian medical ideas that Aristotle's and the Hippocratic theory are virtually the same. My position, on the contrary, is that Aristotle's insistence on the inclusion of form and telos in a causal account of generation makes the difference between their theories paramount. Recognizing this difference in its biological and philosophical significance is crucial for a proper assessment of their separate, and indeed unique, contributions. Against coles's idea that Aristotle, like the Hippocratic writer, has a pansomatic theory of generation I argue that: 1) the thesis is prima facie implausible given Aristotle's sustained critical discussion of pansomatism in GA I, 17; 2) the thesis cannot be sustained with the textual evidence Coles adduces; and 3) that the error in the thesis can be traced to Coles's attributing to Aristotle a reproductive physiology. This attribution is anachronistic in that Aristotle identifies no such a science. More importantly, however, it presumes in advance what needs to be demonstrated, namely that the material and mechanical parts of Aristotle's account of generation can be isolated and examined strictly on their own terms. But as I maintain, Aristotle's theory of generation cannot be separated from his commitment to form and final cause. At stake in this debate is the reducibility or the nonreducibility of the metaphysical dimension of Aristotle's theory

Chapter 27 Biolistic Transformation of Macro- and Micronuclei

This chapter presents methods for the biolistic transformation of both macro- and micronuclei of Tetrahymena thermophila. DNA-coated gold particles are fired into starved vegetative cells and, during late phases of conjugation, into mating cells to introduce gene sequences into the macronucleus. Early stages of mating are bombarded to add genes to the micronucleus. Methods for cell preparation and a detailed protocol for shooting and recovering transformed cells are also presented. Development of successful transformation techniques in any ciliate is complicated by the separation of somatic and germinal functions into two distinctly different nuclei: the polyploid macronucleus, which solely directs the cell's phenotype, and the transcriptionally inert diploid micronucleus, which contributes the genotype to subsequent sexual generations. Each of these nuclei is a unique target for DNA transformation, requires different strategies for the introduction of genes, and responds in different ways following successful transformation.

Homology-dependent maternal inhibition of developmental excision of internal eliminated sequences in Paramecium tetraurelia.

Thousands of single-copy internal eliminated sequences (IESs) are excised from the germ line genome of ciliates during development of the polygenomic somatic macronucleus, following sexual events. Paramecium IESs are short, noncoding elements that frequently interrupt coding sequences. No absolutely conserved sequence element, other than flanking 5'-TA-3' direct repeats, has been identified among sequenced IESs; the mechanisms of their specific recognition and precise elimination are unknown. Previous work has revealed the existence of an epigenetic control of excision. It was shown that the presence of one IES in the vegetative macronucleus results in a specific inhibition of the excision of the same element during the development of a new macronucleus, in the following sexual generation. We have assessed the generality and sequence specificity of this transnuclear maternal control by studying the effects of macronuclear transformation with 13 different IESs. We show that at least five of them can be maintained in the new macronuclear genome; sequence specificity is complete both between genes and between different IESs in the same gene. In all cases, the degree of excision inhibition correlates with the copy number of the maternal IES, but each IES shows a characteristic inhibition efficiency. Short internal IES-like segments were found to be excised from two of the IESs when excision between normal boundaries was inhibited. Available data suggest that the sequence specificity of these maternal effects is mediated by pairing interactions between homologous nucleic acids.

Organization and expression of the mitochondrial genome in the Nicotiana sylvestris CMSII mutant.

Previous analyses suggested that the Nicotiana sylvestris CMSII mutant carried a large deletion in its mitochondrial genome. Here, we show by cosmid mapping that the deletion is 60 kb in length and contains several mitochondrial genes or ORFs, including the complex I nad7 gene. However, due to the presence of large duplications in the progenitor mitochondrial genome, the only unique gene that appears to be deleted is nad7. RNA gel blot data confirm the absence of nad7 expression, strongly suggesting that the molecular basis for the CMSII abnormal phenotype, poor growth and male sterility, is the altered complex I structure. The CMSII mitochondrial genome appears to consist essentially of one of two subgenomes resulting from recombination between direct short repeats. In the progenitor mitochondrial genome both recombination products are detected by PCR and, reciprocally, the parental fragments are detected at the substoichiometric level in the mutant. The CMSII mtDNA organization has been maintained through six sexual generations.

Invaders on the move: parasitism in the sexual galls of four alien gall wasps in Britain (Hymenoptera: Cynipidae)

Four alien, host-alternating cynipid gall wasps are established in the British Isles: Andricus kollari, A. lignicola, A. quercuscalicis and A. corruptrix. Their current distributions divide Britain into four zones; all four invaders are present in the south and south-east of Britain, whereas only A. kollari is present in the far north of Scotland. The rank order, according to how far north the distributions of the four invading cynipid species reach, is: A. corruptrix→ A. quercuscalicis→ A. lignicola→ A. kollari. The life cycles of all four cynipid species involve a sexual generation in spring on Quercus cerris, and an agamic generation in autumn on Q. robur. Here we studied the parasitoid attack by four pteromalid species on the sexual generations of the invaders. We collected and reared the galls of all four species at eight sites from the south to the north of the country (two sites per zone). The geographical locations of the sites reflect the invasion history and thus the residence time of the alien species in the four zones. At each site we measured the densities of all host galls on Q. cerris and of both obligatory host-tree species. We also took a series of measures, such as host-tree density and mean host-tree size, to further characterize the tree stands. These measures are referred to as local parameters. Host densities varied between sites and between years. In A. kollari, galling rates were highest in the middle of the country (zone 2) in 1994, whereas in 1995 they increased from the south to the north. In A. lignicola, galling rates in both years were lowest at the sites in zone 3 (closest to its distribution boundary). In A. quercuscalicis, galling rates were found to be lowest at the site most to the north-west in both years, again the one furthest away from the area where this species was first recorded. Mortality caused by parasitoid attack differed from less than 10% to as high as 70% and varied between host species, sites and years. In four out of six cases the historical/regional variables (north/south and east/west) correlated significantly with parasitoid attack rates that were characteristically lowest at sites close to the distribution boundaries. Of the local factors, we found parasitoid attack rates correlated negatively in one case with host density, whereas they correlated positively in four cases with the density of alternative hosts of the parasitoids. In one of the models the local density of Q. cerris trees correlated negatively with parasitoid attack on A. quercuscalicis. For all three host species the terms retained in the minimal adequate models obtained for 1994 and 1995 differed, which might indicate that these communities of native parasitoids and invading host have not yet settled in any definite structure.

High Pressure Freezing and Freeze Substitution of Primitive Agglutinated Foraminifera

Abstract Freeze substitution techniques can provide exceptional fixation of many types of small biological materials. Ultrastructural studies on the Foraminiferida (marine rhizopods) traditionally have used standard chemical fixation protocols. The relatively large size (-25 μm - several cm) and the presence of a mineralized shell in most taxa precludes the application of many cryo-techniques. High pressure freezing, however, provides a method for freezing organisms as large as many of the smaller foraminifera often without extensive ice damage. Gamonts (sexual generation) from three representatives of the suborder Astrorhizina ﹛Myxotheca sp., Cribrothalammina alba, and Hyperammina sp.) were selected because they are among the most primitive foraminifera and have only very finely agglutinated shells with non-mineralized, organic cements. Myxotheca sp. and C. alba were originally collected from salt marsh environments on Sapelo Island, Georgia, and Hyperammina sp. was isolated from fine-grained carbonate sediments of the Florida Keys.

Polyploidy and transposons

In his interesting commentary on transposable elements (TEs), gene silencing and macroevolution, John McDonald[1xMcDonald, J. Trends Ecol. Evol. 1998; 13: 94–95Abstract | Full Text PDF | PubMed | Scopus (52)See all References[1]omitted a major player that has acted in concert with TEs to drive vertebrate and plant evolution, namely, polyploidy. The quantum increases in gene number that are correlated with landmark evolutionary events have often resulted from whole-genome duplications. The transition from invertebrates to jawed vertebrates involved two rounds of polyploidization[2xSpring, J. FEBS Lett. 1997; 400: 2–8Abstract | Full Text | Full Text PDF | PubMed | Scopus (208)See all References[2]. An estimated 70% of flowering plant species have polyploidy in their history[3xMasterson, J. Science. 1994; 264: 421–424Crossref | PubMedSee all References[3].Polyploidy permits extensive genome modification by TEs because, unlike diploid genomes (which are susceptible to insertional mutagenesis), polyploid genomes contain duplicates of all genes and, consequently, are relatively well buffered from the deleterious consequences of transposition[4xMatzke, M.A. and Matzke, A.J.M. Novartis Found. Symp. 1998; 214: 168–186PubMedSee all References[4]. Transposable elements will therefore tend to multiply and be maintained in polyploid genomes, because extra copies of genes compensate for losses or alterations in expression incurred through TE insertions. With respect to gene silencing, if it is assumed that TEs are the primary targets of epigenetic modifications, such as methylation[5xYoder, J.A., Walsh, C.P., and Bestor, T.H. Trends Genet. 1997; 13: 335–339Abstract | Full Text PDF | PubMed | Scopus (1221)See all References[5], and that polyploids tolerate transposition, then it would follow that polyploid genomes contain more TEs and are more highly methylated than diploid genomes[4xMatzke, M.A. and Matzke, A.J.M. Novartis Found. Symp. 1998; 214: 168–186PubMedSee all References[4]. Indeed, a rough correlation exists: global methylation is found in vertebrate and polyploid plant genomes, which contain a high proportion of TEs, whereas fractional methylation is observed in some invertebrates and the diploid plant Arabidopsis thaliana (which has not accumulated significant numbers of TEs)[4xMatzke, M.A. and Matzke, A.J.M. Novartis Found. Symp. 1998; 214: 168–186PubMedSee all References, 6xTweedie, S. et al. Mol. Biol. Cell. 1997; 17: 1469–1475See all References].Of course, factors in addition to polyploidy, such as the insertion-site preferences of different TEs, can also influence the distribution and abundance of TEs in genomes. For example, an element that preferentially integrates copies of itself, such as Tdd-1 in the slime mould Dictyostelium discoideum[7xSandmeyer, S., Hansen, L., and Chalker, D. Annu. Rev. Genet. 1990; 24: 491–518Crossref | PubMedSee all References[7], could potentially attain a high copy number in a diploid genome. Instead of epigenetic silencing systems having originated to accommodate increases in gene number[8xBird, A.P. Trends Genet. 1995; 11: 94–100Abstract | Full Text PDF | PubMed | Scopus (321)See all References[8], it is more likely that pre-existing defensive epigenetic modifications, targeted against TEs or viruses, were elaborated and became pervasive in polyploid genomes as TE populations expanded. The consequences would be the same as they are for vertebrates and many plants: an increased genomic methylation and apparent recruitment of a primordial genome defense (i.e. methylation) for regulating differential gene expression. These observations suggest that genes whose repression is correlated with methylation contain TEs or TE remnants in their promoters.Another point is that epigenetic silencing induced by transposable elements can have implications for evolution if meiotically heritable silencing occurs[9xSee all References[9]. We are only just beginning to understand the numerous ways in which TEs (and other sequences perceived as `foreign' by the genome defense system) can interact in cis- and in trans-associations, via DNA sequence identity, to induce gene silencing and epigenetic modifications[10xMatzke, M.A. and Matzke, A.J.M. Cell Mol. Life Sci. 1998; 54: 94–103Crossref | PubMed | Scopus (67)See all References, 11xMartienssen, R. Curr. Biol. 1996; 6: 810–813Abstract | Full Text | Full Text PDF | PubMed | Scopus (59)See all References]. In plants and, perhaps, other organisms, non-Mendelian inheritance can result from these interactions, because interacting alleles or loci exhibit altered expression after segregating in progeny. In some cases, epigenetic silencing induced by foreign sequences can persist and influence phenotypes for many sexual generations[10xMatzke, M.A. and Matzke, A.J.M. Cell Mol. Life Sci. 1998; 54: 94–103Crossref | PubMed | Scopus (67)See all References[10], and thus be indistinguishable from genetic mutations. In summary, the genetic and epigenetic consequences of transposition in polyploids are an important aspect of the creative power attributed to redundancy[12xSee all References[12].

Rates of spontaneous mutation.

Rates of spontaneous mutation per genome as measured in the laboratory are remarkably similar within broad groups of organisms but differ strikingly among groups. Mutation rates in RNA viruses, whose genomes contain ca. 10(4) bases, are roughly 1 per genome per replication for lytic viruses and roughly 0.1 per genome per replication for retroviruses and a retrotransposon. Mutation rates in microbes with DNA-based chromosomes are close to 1/300 per genome per replication; in this group, therefore, rates per base pair vary inversely and hugely as genome sizes vary from 6 x 10(3) to 4 x 10(7) bases or base pairs. Mutation rates in higher eukaryotes are roughly 0.1-100 per genome per sexual generation but are currently indistinguishable from 1/300 per cell division per effective genome (which excludes the fraction of the genome in which most mutations are neutral). It is now possible to specify some of the evolutionary forces that shape these diverse mutation rates.

Factors Affecting Parasitism in the Oak-Galler Neuroterus quercusbaccarum (Hymenoptera: Cynipidae)

We investigate the influence of seven explanatory variables based on individual characteristics of galls on parasitism in the oak-galler, Neuroterus quercushasbaccarum (sexual generation). The community consists of three species of parasitoid and one species of inquiline (which is lethal to the galler). Our analysis shows that there is considerable spatial heterogeneity in parasitism from site to site and from tree to tree within sites. With regard to the placement of galls on tree organs, galls on catkins are less parasitised than those on leaves. Gall size does not explain this difference because the external diameter of catkin galls is not significantly different from those on leaves. We hypothesise that the precocious abscission of catkin galls prevents their exploitation by parasitoid species with long developmental times. Moreover. there is a distinct sequence of parasitism, reflected by a partitioning in the sizes of galls attacked by each parasite species. However. the growth dynamics of the galls themselves show that just external diameter is not the only size parameter affecting the niches of the different parasites. Even though Synergus spp. is one of the earliest acting parasites, it attacks galls with thicker walls relative to external diameter than occurring in unparasitised galls from which the galler emerges. A delay between gall growth and feeding activity of the gallmaker would induce non-linear growth of gallwall thickness, with major consequences for the accessibility of the host larva to the parasitoid.

REVIEW Evolution of phytophagy in trombidiform mites

This paper reviews the evolutionary aspects of obligate phytophagy (excluding mycophagy and phycophagy) in the mite suborder Trombidiformes. Phytophagy in the other acariform suborder, Sarcoptiformes, is limited to just a few species, amidst otherwise saprophagous or fungivorous taxa, that attack the living tissues of higher plants. The phylogenetic relationships of lineages that contain taxa of plant-feeding mites are reviewed briefly, to facilitate hypotheses about the number of times that phytophagy has arisen within the Trombidiformes. The relationship between the two most important plant-feeding taxa, the Tetranychoidea and Eriophyoidea, is so distant that their obligate phytophagy represents independent events. Outgroup comparisons allow an estimate of the relative ages when phytophagy arose. This background facilitates analyses of the evolutionary patterns of attributes relevant to phytophagy as a way of life. Styliform modifications of chelate chelicerae for predation or fungivory were fundamental pre-adaptations for effective phytophagy. Dispersal among the major lineages of phytophagous mites seems generally passive, with little evidence of phoretic behaviour. Continued individual mobility seems to be needed during ontogeny and adulthood, such that no scale-like or sac-like instars have arisen. Trends towards physogastric reproduction and ovoviviparity are not evident. Arrhenotokous sex determination predominates among lineages of phytophagous mites. The primary sex ratios are not usually highly female biased. Direct sperm transfer does not seem to have been advantageous or disadvantageous to adaptive radiations of plant-feeding lineages. Adaptive trends towards thelytoky are scattered and do not seem to have played major roles in speciation, diversification or trends towards increasing host specificity in lineages. Alternate asexual and sexual generations and life cycles on different species of hosts, as occur among families of aphid and scale insects, are not known. Among unrelated lineages of trombidiform mites, there appears to have been convergent evolution of attributes, such as those noted above, in response to similar selective pressures for a phytophagous way of life. The patterns of attributes discussed need experimental analysis and detailed documentation to test their accuracy and generality and to understand the selective pressures that have formed them.

A mutation in Paramecium tetraurelia reveals functional and structural features of developmentally excised DNA elements.

The excision of internal eliminated sequences (IESs) from the germline micronuclear DNA occurs during the differentiation of a new macronuclear genome in ciliated protozoa. In Paramecium, IESs are generally short (28-882 bp), AT rich DNA elements that show few conserved sequence features with the exception of an inverted-terminal-repeat consensus sequence that has similarity to the ends of mariner/Tcl transposons (KLOBUTCHER and HERRICK 1995). We have isolated and analyzed a mutant cell line that cannot excise a 370-bp IESs (IES2591) from the coding region of the 51A variable surface protein gene. A single micronuclear C to T transition within the consensus sequence prevents excision. The inability to excise IES259 I has revealed a 28-bp IES inside the larger IES, suggesting that reiterative integration of these elements can occur. Together, the consensus sequence mutation and the evidence for reiterative integration support the theory that Paramecium IESs evolved from transposable elements. Unlike a previously studied Paramecium IES, the presence of this IES in the macronucleus does not completely inhibit excision of its Mild-type micronuclear copy through multiple sexual generations.

INBREEDING DEPRESSION UNDER JOINT SELFING, OUTCROSSING, AND ASEXUALITY.

Partial asexual reproduction was introduced into a model of inbreeding depression due to nearly recessive lethal mutations in a partially selfing population. The frequencies of asexuality, selfing, and outcrossing were either constant or occurred in cycles of a single sexual generation followed by one or more asexual generations. We found that increasing the degree of asexuality generally increases the inbreeding depression maintained in an equilibrium population with a given selfing rate. This is due to the increase in the number of mutations relative to sexual generations during which selfing-induced purging of mutations may take place. For very high genomic mutation rates, sufficient to produce a threshold rate of self-fertilization for purging recessive lethal mutations, asexuality can have the opposite effect, decreasing equilibrium inbreeding depression, because of an increase in the efficiency of selection against mutations in heterozygotes with asexuality.

A virtual mesocosm with artificial salps for exploring the conditions of swarm development in the pelagic tunicate Salpa fusiformis

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 154:1-16 (1997) - doi:10.3354/meps154001 A virtual mesocosm with artificial salps for exploring the conditions of swarm development in the pelagic tunicate Salpa fusiformis Laval P A virtual mesocosm is a software framework simulating a controlled oceanic environment. Artificial salps and their food are introduced into the mesocosm where their development can be watched on the computer screen. The artificial salps are software entities mimicking closely the life-history and behavior of their real counterparts (the tunicate Salpa fusiformis): they swim in the artificial space, where each individual feeds on the local resources; they metabolize, accumulate reserves, and grow; their reproduction alternates between asexual and sexual generations. Plausible values are given to 21 parameters characterizing their life-cycle and metabolism. A virtual mesocosm is in fact a model, focused upon individuals and their spatial interactions. The relationships present in the conceptual model are more completely and more precisely stated in a software design than in a mathematical model. Many computer experiments are possible with the artificial salps, without the difficult problems associated with the sampling and laboratory maintenance of the real, fragile, planktonic organisms. An example is given showing the effect of different spacing between food patches. Computer simulation · Artificial life · Conceptual model · Object-based design Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 154. Publication date: July 31, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Do respiratory metabolic rates of the scyphomedusa Aurelia aurita scale isometrically throughout ontogeny in a sexual generation

We measured the respiration rate of Aurelia aurita payingspecial attention to the relationship between its metabolic ratesand body mass throughout ontogeny of the jellyfish in a sexualgeneration. Two different regression lines between respirationrates and body dry weight were obtained in ephyra to young medusa(bell diameter 4.2–19 mm, 0.07–14 mg dry weight) and medusa(BD17-120 mm, 12-2100 mg DW), at respective temperatures of 10 and15 °C. The cut off point of the metabolic rates was foundat the developmental stage just being metamorphosed into medusashape (BD 12–20 mm). The slope value of medusa respiration ratewas close to isometric scaling (0.9), whereas that of ephyra toyoung medusa was lower (0.6). Ecological implications of allometricscaling in the early developmental stages of ephyrae and metephyraeare discussed.

Epigenetic regulation of programmed genomic rearrangements in Paramecium aurelia.

In ciliates, development of the polyploid somatic macronucleus after sexual events involves extensive and reproducible rearrangements of the germ-line genome, including chromosome fragmentation and precise excision of numerous internal sequence elements. In Paramecium aurelia, alternative macronuclear versions of the same germ-line genome can be maternally inherited across sexual generations, showing that rearrangement patterns are not strictly determined by the germ-line sequence. Homology-dependent maternal effects can be evidenced by transformation of the vegetative macronucleus with cloned macronuclear sequences: new fragmentation patterns or internal deletions are specifically induced during differentiation of a new macronucleus, in sexual progeny of transformed clones. Furthermore, transformation of the maternal macronucleus with germ-line sequences containing internal eliminated sequences (short single-copy elements) can result in a specific inhibition of the excision of the same elements in the zygotic macronucleus. These experiments show that the processing of many germ-line sequences in the developing macronucleus is sensitive to the structure and copy number of homologous sequences in the maternal macronucleus. The generality and sequence specificity of this trans-nuclear, epigenetic regulation of rearrangements suggest that it is mediated by pairing interactions between germ-line sequences and sequences imported from the maternal macronucleus.