Rules for assortative mating in relation to selection for linear merit functions

Summary This study examined how assortative mating (without selection) based on linear combinations of two traits could be used to change genetic parameters so as to increase efficiency of selection. The efficiency of the Smith-Hazel index for improvement of multiple traits is a function of phenotypic and genetic variances and covariances, and of the relative economic values of the traits involved. Assortative mating is known to change genetic variances and covariances. Recursive formulae were derived to obtain these variances and covariances after t generations of assortative mating on linear combinations (mating rules) of phenotypic values for two traits, with a given correlation between mates. Selection efficiency after t generations of assortative mating without selection was expressed as a function of random mating genetic parameters, economic values, the mating rule, and the correlation between mates. Selection efficiency was maximized with respect to the coefficients in the mating rule. Because the objective function was nonlinear, a computer routine was used for maximizing it. Two cases were considered. When random mating heritabilities for the two traits were h _X ² =0.25 and h _Y ² =0.50, the genetic correlation r_XY=-0.60, and the economic values were a_X=3 and a_Y=1, continued assortative mating based on the optimal mating rule for 31 generations (with a correlation of 0.80 between mates) increased selection efficiency by 29%. Heritabilities changed to 0.38 and 0.66, respectively, and the genetic correlation became – 0.79. When h _X ² =0.60, h _Y ² =0.60, r_XY=– 0.20, a₁=1 and a₂=1, 36 generations of continued assortative mating with the optimal mating rule increased the efficiency of selection by 17%, heritabilities became h _X ² = h _Y ² =0.71, and the genetic correlation changed to 0.25. Only three generations of assortative mating were required to change the sign of the genetic correlation.     

Cytogenetic,cross-mating and molecular evidence of four cytological races of Anopheles crawfordi (Diptera: Culicidae) in Thailand and Cambodia

Twenty-nine isolines of Anopheles crawfordi were established from wild-caught females collected from cow-baited traps in Thailand and Cambodia. Three types of X (X₁, X₂, X₃) and four types of Y (Y₁, Y₂, Y₃, and Y₄) chromosomes were identified, according to differing amounts of extra heterochromatin. These sex chromosomes represent four metaphase karyotypes, i.e., Forms A (X₁, X₂, X₃, Y₁), B (X₁, X₂, X₃, Y₂), C (X₂, Y₃) and D (X₂, Y₄). Forms C and D are novel metaphase karyotypes confined to Thailand, whereas forms A and B appear to be common in both Thailand and Cambodia. Cross-mating experiments between the four karyotypic forms indicated genetic compatibility in yielding viable progenies and synaptic salivary gland polytene chromosomes. The results suggest that the forms are conspecific and A. crawfordi comprises four cytological races, which is further supported by very low intraspecific variation (mean genetic distance = 0.000–0.018) of the nucleotide sequences in ribosomal DNA (ITS2) and mitochondrial DNA sequences (COI, COII).     

The variant landscape and function of DDX3X in cancer and neurodevelopmental disorders

RNA molecules rely on proteins across their life cycle. DDX3X encodes an X-linked DEAD-box RNA helicase with a Y-linked paralog, DDX3Y. DDX3X is central to the RNA life cycle and is implicated in many conditions, including cancer and the neurodevelopmental disorder DDX3X syndrome. DDX3X-linked conditions often exhibit sex differences, possibly due to differences between expression or function of the X- and Y-linked paralogs DDX3X and DDX3Y. DDX3X-related diseases have different mutational landscapes, indicating different roles of DDX3X. Understanding the role of DDX3X in normal and disease states will inform the understanding of DDX3X in disease. We review the function of DDX3X and DDX3Y, discuss how mutation type and sex bias contribute to human diseases involving DDX3X, and review possible DDX3X-targeting treatments.     

X-Y Exchange and the Coevolution of the X and Y Rdna Arrays in Drosophila melanogaster

The nucleolus organizers on the X and Y chromosomes of Drosophila melanogaster are the sites of 200-250 tandemly repeated genes for ribosomal RNA. As there is no meiotic crossing over in male Drosophila, the X and Y chromosomal rDNA arrays should be evolutionarily independent, and therefore divergent. The rRNAs produced by X and Y are, however, very similar, if not identical. Molecular, genetic and cytological analyses of a series of X chromosome rDNA deletions (bb alleles) showed that they arose by unequal exchange through the nucleolus organizers of the X and Y chromosomes. Three separate exchange events generated compound X·Y ^L chromosomes carrying mainly Y-specific rDNA. This led to the hypothesis that X-Y exchange is responsible for the coevolution of X and Y chromosomal rDNA. We have tested and confirmed several of the predictions of this hypothesis: First, X· Y^L chromosomes must be found in wild populations. We have found such a chromosome. Second, the X·Y^L chromosome must lose the Y^L arm, and/or be at a selective disadvantage to normal X⁺ chromosomes, to retain the normal morphology of the X chromosome. Six of seventeen sublines founded from homozygous X·Y^Lbb stocks have become fixed for chromosomes with spontaneous loss of part or all of the appended Y^L. Third, rDNA variants on the X chromosome are expected to be clustered within the X⁺ nucleolus organizer, recently donated (" Y") forms being proximal, and X-specific forms distal. We present evidence for clustering of rRNA genes containing Type 1 insertions. Consequently, X-Y exchange is probably responsible for the coevolution of X and Y rDNA arrays.     

Same-Sex Twin Pair Phenotypic Correlations are Consistent with Human <Emphasis Type="Italic">Y</Emphasis> Chromosome Promoting Phenotypic Heterogeneity

Junk DNA has been long appreciated as an evolutionary facilitator because it can participate in the causation of genetic variation such as chromosome rearrangements and can be exapted into coding or regulatory elements. Recently, it has been proposed that junk DNA variation within natural populations indirectly causes a phenotypic heterogeneity that subsequently promotes genetic capacitance, i.e., the random fluctuation of genetic variation. Junk DNA role as capacitor might drive population traits such as sexual dimorphism, spatiotemporal dynamics, or genetic diversification leading into speciation. Whether the human species also showed junk DNA-based capacitance manifested as a junk DNA-dependent phenotypic heterogeneity that contributed to the etiology and expression of diseases or the evolutionary history of human populations is intriguing. Because the human Y chromosome is highly enriched in junk DNA, humans are sexually dimorphic for the genomic content in junk DNA. Thus, it would be expected that junk DNA-based capacitance in humans were manifested as a sexual dimorphism for phenotypic heterogeneity. Here, I gather supporting evidence for the existence of a sexual dimorphism for putative junk DNA-based phenotypic heterogeneity by analyzing same-sex twin pairs phenotypic concordance.     

Depicting the phenotypic space of the annual plant Diplotaxis acris in hyperarid deserts

The phenotypic space encompasses the assemblage of trait combinations yielding well‐suited integrated phenotypes. At the population level, understanding the phenotypic space structure requires the quantification of among‐ and within‐population variations in traits and the correlation pattern among them. Here, we studied the phenotypic space of the annual plant Diplotaxis acris occurring in hyperarid deserts. Given the advance of warming and aridity in vast regions occupied by drylands, D. acris can indicate the successful evolutionary trajectory that many other annual plant species may follow in expanding drylands. To this end, we conducted a greenhouse experiment with 176 D. acris individuals from five Saudi populations to quantify the genetic component of variation in architectural and life history traits. We found low among‐population divergence but high among‐individual variation in all traits. In addition, all traits showed a high degree of genetic determination in our study experimental conditions. We did not find significant effects of recruitment and fecundity on fitness. Finally, all architectural traits exhibited a strong correlation pattern among them, whereas for life history traits, only higher seed germination implied earlier flowering. Seed weight appeared to be an important trait in D. acris as individuals with heavier seeds tended to advance flowering and have a more vigorous branching pattern, which led to higher fecundity. Population divergence in D. acris might be constrained by the severity of the hyperarid environment, but populations maintain high among‐individual genetic variation in all traits. Furthermore, D. acris showed phenotypic integration for architectural traits and, to a lesser extent, for life history traits. Overall, we hypothesize that D. acris may be fine‐tuned to its demanding extreme environments. Evolutionary speaking, annual plants facing increasing warming, aridity, and environmental seasonality might modify their phenotypic spaces toward new phenotypic configurations strongly dominated by correlated architectural traits enhancing fecundity and seed‐related traits advancing flowering time.     

Reproductive Flexibility: Genetic Variation,Genetic Costs and Long-Term Evolution in a Collembola

In a variable yet predictable world, organisms may use environmental cues to make adaptive adjustments to their phenotype. Such phenotypic flexibility is expected commonly to evolve in life history traits, which are closely tied to Darwinian fitness. Yet adaptive life history flexibility remains poorly documented. Here we introduce the collembolan Folsomia candida, a soil-dweller, parthenogenetic (all-female) microarthropod, as a model organism to study the phenotypic expression, genetic variation, fitness consequences and long-term evolution of life history flexibility. We demonstrate that collembola have a remarkable adaptive ability for adjusting their reproductive phenotype: when transferred from harsh to good conditions (in terms of food ration and crowding), a mother can fine-tune the number and the size of her eggs from one clutch to the next. The comparative analysis of eleven clonal populations of worldwide origins reveals (i) genetic variation in mean egg size under both good and bad conditions; (ii) no genetic variation in egg size flexibility, consistent with convergent evolution to a common physiological limit; (iii) genetic variation of both mean reproductive investment and reproductive investment flexibility, associated with a reversal of the genetic correlation between egg size and clutch size between environmental conditions ; (iv) a negative genetic correlation between reproductive investment flexibility and adult lifespan. Phylogenetic reconstruction shows that two life history strategies, called HIFLEX and LOFLEX, evolved early in evolutionary history. HIFLEX includes six of our 11 clones, and is characterized by large mean egg size and reproductive investment, high reproductive investment flexibility, and low adult survival. LOFLEX (the other five clones) has small mean egg size and low reproductive investment, low reproductive investment flexibility, and high adult survival. The divergence of HIFLEX and LOFLEX could represent different adaptations to environments differing in mean quality and variability, or indicate that a genetic polymorphism of reproductive investment reaction norms has evolved under a physiological tradeoff between reproductive investment flexibility and adult lifespan.     

Genetic Identification of Nascent Peptides That Induce Ribosome Stalling

Several nascent peptides stall ribosomes during their own translation in both prokaryotes and eukaryotes. Leader peptides that induce stalling can regulate downstream gene expression. Interestingly, stalling peptides show little sequence similarity and interact with the ribosome through distinct mechanisms. To explore the scope of regulation by stalling peptides and to better understand the mechanism of stalling, we identified and characterized new examples from random libraries. We created a genetic selection that ties the life of Escherichia coli cells to stalling at a specific site. This selection relies on the natural bacterial system that rescues arrested ribosomes. We altered transfer-messenger RNA, a key component of this rescue system, to direct the completion of a necessary protein if and only if stalling occurs. We identified three classes of stalling peptides: C-terminal Pro residues, SecM-like peptides, and the novel stalling sequence FXXYXIWPP. Like the leader peptides SecM and TnaC, the FXXYXIWPP peptide induces stalling efficiently by inhibiting peptidyl transfer. The nascent peptide exit tunnel and peptidyltransferase center are implicated in this stalling event, although mutations in the ribosome affect stalling on SecM and FXXYXIWPP differently. We conclude that ribosome stalling can be caused by numerous sequences and is more common than previously believed.     

Quantitative Genetics of DROSOPHILA MELANOGASTER. I. Sexual Dimorphism in Genetic Parameters for Wing Traits

Sexual dimorphism in genetic parameters is examined for wing dimensions of Drosophila melanogaster. Data are fit to a quantitative genetic model where phenotypic variance is a linear function of additive genetic autosomal variance (common to both sexes), additive genetic X-linked variances distinct for each sex, variance due to common rearing environment of families, residual environmental variance, random error variance due to replication, and variance due to measurement error and developmental asymmetry (left vs. right sides). Polygenic dosage compensation and its effect on genetic variances and covariances between sexes is discussed. Variance estimates for wing length and other wing dimensions highly correlated with length support the hypothesis that the Drosophila system of dosage compensation will cause male X-linked genetic variance to be substantially larger than female X-linked variance. Results for various wing dimensions differ, suggesting that the level of dosage compensation may differ for different traits. Genetic correlations between sexes for the same trait are presented. Total additive genetic correlations are near unity for most wing traits; this indicates that selection in the same direction in both sexes would have a minor effect on changing the magnitude of difference between sexes. Additive X-linked correlations suggest some genotype x sex interactions for X-linked effects.     

Mitotic Recombination in the Heterochromatin of the Sex Chromosomes of DROSOPHILA MELANOGASTER

The frequency of spontaneous and X-ray-induced mitotic recombination involving the Y chromosome has been studied in individuals with a marked Y chromosome arm and different XY compound chromosomes. The genotypes used include X chromosomes with different amounts of X heterochromatin and either or both arms of the Y chromosome attached to either side of the centromere. Individuals with two Y chromosomes have also been studied. The results show that the bulk of mitotic recombination takes place between homologous regions.     

The Genetic and Cytological Organization of the Y Chromosome of DROSOPHILA MELANOGASTER

Cytological and genetic analyses of 121 translocations between the Y chromosome and the centric heterochromatin of the X chromosome have been used to define and localize six regions on the Y chromosome of Drosophila melanogaster necessary for male fertility. These regions are associated with nonfluorescent blocks of the Y chromosome, as revealed using Hoechst 33258 or quinacrine staining. Each region appears to contain but one functional unit, as defined by failure of complementation among translocations with breakpoints within the same block. The distribution of translocation breakpoints examined appears to be nonrandom, in that breaks occur preferentially in the nonfluorescent blocks and not in the large fluorescent blocks.     

Assessing hidden species diversity in the coral Pocillopora damicornis from Eastern Australia

The incredible range of morphological plasticity present in scleractinian corals has confused the taxonomy of the group, prompting the introduction of “ecomorphs” to explain the observed correlation between local environmental conditions and phenotypic variation. Pocillopora damicornis (Linnaeus, 1758) represents one of the best known examples of eco-phenotypic variation in scleractinian corals with a variety of forms and reproductive strategies reported across its global distribution range. Here, we reconstruct genealogical relationships of P. damicornis colonies collected from thirteen locations along the East Australian coast to examine the relationship between genetic and phenotypic diversity in this species. Haplotype networks computed from two mitochondrial DNA regions (CR, ORF) indicate that the range of morphotypes observed within this taxon fall into at least five genetically distinct mitochondrial lineages. Nuclear (HSP70, ITS2) haplowebs on the other hand recover sharp genetic discontinuities among three of the morphological groups. We conclude that P. damicornis from Eastern Australia constitutes a cryptic species complex. The misinterpretation of taxonomical units within P. damicornis may well explain its perceived variation in the ecology, biology and life history across its range.     

NATURE OF THE ACTION CURRENT IN NITELLA : VI. SIMPLE AND COMPLEX ACTION PATTERNS

The experiments indicate that the protoplasm of Nitella consists of an aqueous layer W with an outer non-aqueous surface layer X and an inner non-aqueous surface layer Y. The potential at Y is measured by the magnitude of the action curve and the potential at X by the distance from the top of the action curve to the zero line. These potentials appear to be due chiefly to diffusion potentials caused by the activity gradients of KCl across the non-aqueous layers X and Y. The relative mobilities of K⁺ and Cl^- in X and in Y can be computed and an estimate of the activity of KCl in W can be made. In the complete resting state the mobilities of K⁺ and Cl^- in X are not very different from those in Y. The action curve is due to changes in Y which suddenly becomes very permeable, allowing potassium to move from the sap across Y into W, and thus losing its potential. A gradual loss may be due to changes in ionic mobility in Y. When recovery is incomplete and Y has not yet regained its normal potential a stimulus may cause a loss of the potential at Y giving an action curve of small magnitude. The magnitude may vary in successive action curves giving what is called a complex pattern in contrast to the simple pattern observed when recovery is complete and all the action curves are alike. Complex patterns occur chiefly in cells treated with reagents. Untreated cells usually give simple patterns. A variety of complex action patterns is discussed. It is evident that the cells of Nitella show much more variation than such highly specialized cells as muscle and nerve which give stereotyped responses. In some cases it may be doubtful whether the all-or-none law holds.     

Variations in the Number of the Y Chromosomal Rrna Genes in DROSOPHILA HYDEI

The number of rRNA cistrons is measured by filter saturation hybridization in different stocks of D. hydei, where the wild-type X chromosome has one nucleolus organizer (NO) and the wild-type Y has two separated NO's. (see PDF) females having no X chromosomal NO show an rDNA content exceeding that of a Y chromosome. An even greater increase in the rRNA cistron number is measured in two translocation stocks where the (see PDF) is combined with one half of a Y and, therefore, each stock contains only one of the two Y chromosomal NO's. But when the same Y fragments are brought together with a wild-type X chromosome they lose about one-half of their rRNA cistrons within one generation. Males with two complementary Y fragments but having no X chromosomal NO show a considerably higher rDNA content than the (see PDF) females, although both are equal in respect of their NO number. Consideration is given to related phenomena in Drosophila melanogaster.     

A New Resource for Characterizing X-Linked Genes in Drosophila melanogaster: Systematic Coverage and Subdivision of the X Chromosome With Nested,Y-Linked Duplications

Interchromosomal duplications are especially important for the study of X-linked genes. Males inheriting a mutation in a vital X-linked gene cannot survive unless there is a wild-type copy of the gene duplicated elsewhere in the genome. Rescuing the lethality of an X-linked mutation with a duplication allows the mutation to be used experimentally in complementation tests and other genetic crosses and it maps the mutated gene to a defined chromosomal region. Duplications can also be used to screen for dosage-dependent enhancers and suppressors of mutant phenotypes as a way to identify genes involved in the same biological process. We describe an ongoing project in Drosophila melanogaster to generate comprehensive coverage and extensive breakpoint subdivision of the X chromosome with megabase-scale X segments borne on Y chromosomes. The in vivo method involves the creation of X inversions on attached-XY chromosomes by FLP-FRT site-specific recombination technology followed by irradiation to induce large internal X deletions. The resulting chromosomes consist of the X tip, a medial X segment placed near the tip by an inversion, and a full Y. A nested set of medial duplicated segments is derived from each inversion precursor. We have constructed a set of inversions on attached-XY chromosomes that enable us to isolate nested duplicated segments from all X regions. To date, our screens have provided a minimum of 78% X coverage with duplication breakpoints spaced a median of nine genes apart. These duplication chromosomes will be valuable resources for rescuing and mapping X-linked mutations and identifying dosage-dependent modifiers of mutant phenotypes.MANY eukaryotes of biomedical and agricultural importance—including humans, other mammals, birds, and Drosophila—are heterogametic. Their sex chromosomes differ drastically in size and genetic composition. In species with X and Y chromosomes, males carry only one copy of each X-linked gene. This poses a serious challenge for experimental geneticists, because males inheriting a mutation in a vital X-linked gene die before they can be used in genetic crosses. In fact, the hemizygosity of X-linked genes in males has been a significant barrier to the functional analysis of many X-linked genes and is largely responsible for the poor genetic characterization of X chromosomes relative to autosomes in most organisms.The lethality of X-linked mutations can be rescued by providing a wild-type copy of the mutated gene elsewhere in the genome. This can be accomplished with a transgenic construct if the molecular identity of the mutated gene is known. In many cases, however, the mutated gene has not been identified and it is necessary to provide wild-type function with a multigene interchromosomal duplication, i.e., a segment of the X inserted in another chromosome. If the proximal and distal extents of the duplicated segment are known, phenotypic rescue maps the mutated gene to the defined X chromosome region.Multigene deletions can also be used to map X-linked mutations by complementation, but crosses between individuals carrying deletions and X-linked lethal mutations are impossible without rescuing the lethality of either the deletion or the lethal mutation in males. Projects at the Bloomington Drosophila Stock Center and elsewhere (Parks et al. 2004; Ryder et al. 2007) have generated large collections of deletions with molecularly defined breakpoints in Drosophila melanogaster, but the utility of the X deletions is limited without duplications of the corresponding chromosomal regions.Duplications are potentially important for gene discovery. Identifying sets of genes involved in the same cellular process is a major focus of functional genomics research and this can be accomplished genetically by identifying dosage-sensitive modifiers of mutant phenotypes. Often, increasing or decreasing the copy number of a gene will enhance or suppress the phenotype associated with mutating another gene involved in the same process. Screening collections of deletions is a popular way to identify interacting genes in Drosophila (for examples, see Seher et al. 2007; Zhao et al. 2008; Aerts et al. 2009; Salzer et al. 2010) and was a major impetus for the assembly of the Bloomington Stock Center “Deficiency Kit,” which provides maximal coverage of the genome with the fewest deletions. Though dosage-sensitive modifiers could also be identified using increased gene dosage, the use of duplications in enhancer and suppressor screens remains largely unexplored. Assembling sets of duplications providing efficient genomic coverage would likely popularize this experimental approach.The size of duplicated segments determines how duplication chromosomes are used experimentally. Small duplicated segments allow high resolution gene mapping, but they are not suitable for other purposes. Only large duplicated segments are capable of rescuing the lethality of sizable multigene X deletions. Likewise, large duplicated segments provide efficiency in initially localizing mutations and identifying dosage-dependent modifiers. Despite their usefulness, interchromosomal duplications of large segments are among the hardest chromosomal rearrangements to isolate. In Drosophila, many existing duplications were recovered fortuitously as three-breakpoint aberrations following irradiation, but such rearrangements are rare and difficult to identify in screens. Other duplications were methodically constructed from preexisting rearranged chromosomes. This approach works well when it is possible, but it can be used only when progenitor aberrations with appropriate breakpoints are available. Because of these difficulties, the selection of duplication strains generated by Drosophila workers over the past several decades is not satisfactory for many purposes. The duplications are often difficult to use experimentally, their breakpoints are sparsely distributed along the X chromosome and only roughly mapped, and substantial gaps in coverage exist. Obviously, improved duplication resources are needed.Here we present the methodology and progress of a project at the Bloomington Drosophila Stock Center to construct interchromosomal duplications of large, megabase-scale X segments. Our approach builds on the long history of manipulating Drosophila chromosomes in vivo (Novitski and Childress 1976; Ashburner et al. 2005), but we have eliminated the need for preexisting aberrations by generating progenitor chromosomes using the FLP-FRT system. Indeed, this site-specific recombination system has had an enormous impact on the ability of fly geneticists to engineer many kinds of novel chromosomes (Golic and Golic 1996; Parks et al. 2004; Ryder et al. 2007). We will demonstrate how we have combined FLP-mediated recombination and other chromosome manipulation techniques to produce Y-linked duplications of large X segments. As we will show, appending X segments to Y chromosomes rather than autosomes has advantages both for the synthesis and experimental use of X duplications.To date, we have generated a minimum of 78% X coverage with duplication breakpoints spaced a median of nine genes apart. We anticipate completion of the project within the coming year. Using these duplications, mutations and genetic modifiers can be mapped first to large X intervals using a tiling set of the largest duplicated segments and then to small chromosome intervals using subsets of the duplications. These duplications will also facilitate deletion mapping. The creation of a set of stocks providing complete duplication coverage and extensive breakpoint subdivision of the X chromosome in a consistent genetic background will remove an impediment to investigating the functions of X-linked genes that has frustrated generations of Drosophila geneticists.     

Inviability of hybrids between D. melanogaster and D. simulans results from the absence of simulans X not the presence of simulans Y chromosome

Interspecific crosses between D. melanogaster and D. simulans or its sibling species result in unisexual inviability of the hybrids. Mostly, crosses of D. melanogaster females X D. simulans males produce hybrid females. On the other hand, only hybrid males are viable in the reciprocal crosses. A classical question is the cause of the unisexual hybrid inviability on the chromosomal level. Is it due to the absence of a D. simulans X chromosome or is it due to the presence of a D. simulans Y chromosome? A lack of adequate chromosomal rearrangements available in D. simulans has made it difficult to answer this question. However, it has been assumed that the lethality results from the absence of the D. simulans X rather than the presence of the D. simulans Y. Recently I synthesized the first D. simulans compound-XY chromosome that consists of almost the entire X and Y chromosomes. Males carrying the compound-XY and no free Y chromosome are fertile. By utilizing the compound-XY chromosome, the viability of hybrids with various constitutions of cytoplasm and sex chromosomes has been examined. The results consistently demonstrate that the absence of a D. simulans X chromosome in hybrid genome, and not the presence of the Y chromosome, is a determinant of the hybrid inviability.     

Die Cytogenetik zweier norddeutscher Populationen von Nosopsyllus fasciatus Bosc (Aphaniptera)

Specimens of the populations Hamburg and Wilhelmshaven of the ratflea N. fasciatus exhibit variation of the chromosome number in the range of 2n=20–23 and 2n=20–27 respectively, resulting from individual differences in the number of supernumerary chromosomes beyond the basic chromosome complement of 2n=20. The supernumerary chromosomes are mostly euchromatic and partly or completely homologous to each other and to the 10. pair of the basic complement. The numerical variation in the population Wilhelmshaven is produced by recurrent mitotic non-disjunction of the supernumerary chromosomes in anaphase II of spermatogenesis. Constant mitotic non-disjunction and preferential segregation of the supernumerary chromosomes towards the pronucleus leads to their accumulation in the population.—A multiple sex-chromosome mechanism of the type X₁ X₂ Y₁ Y₂ (male): X₁ X₁ X₂ X₂ (female) has been demonstrated for the population Wilhelmshaven of N. fasciatus. The X₁ X₂ Y₁ Y₂-chain of four is restricted to the male meiosis, in oogenesis two sex bivalents (X₁ X₁ and X₂ X₂) are formed. — The cytogenetic data presented do not support the concept of a closer phylogenetic relationship between the Aphaniptera and Nematocera, but do not preclude the possibility of a kinship of Aphaniptera and Neomecoptera.     

Molecular evidence that the genes for dioecism and monoecism in Spinacia oleracea L. are located at different loci in a chromosomal region

Spinach (Spinacia oleracea L.) is widely known to be dioecious. However, monoecious plants can also occur in this species. Sex expression in dioecious spinach plants is controlled by a single gene pair termed X and Y. Our previous study showed that a single, incompletely dominant gene, which controls the monoecious condition in spinach line 03–336, should be allelic or linked to X/Y. Here, we developed 19 AFLP markers closely linked to the monoecious gene. The AFLP markers were mapped to a 38.2-cM chromosomal region that included the monoecious gene, which is bracketed between flanking markers with a distance of 7.1 cM. The four AFLP markers developed in our studies were converted into sequence-characterized amplified region (SCAR) markers, which are linked to both the monoecious gene and Y and are common to both populations segregating for the genes. Linkage analysis using the SCAR markers suggested that the monoecious gene (M) and Y are located in different intervals, between different marker pairs. Analysis of populations segregating for both M and Y also directly demonstrates linkage of the genes at a distance of ∼12 cM. The data presented in this study may be useful for breeding dioecious and highly male monoecious lines utilized as the pollen parents for hybrid seed production, as well as for studies of the evolutionary history of sexual systems in this species, and can provide a molecular basis for positional cloning of the sex-determining genes.     

Effects of shell morphological traits on the weight trait of the orange strain of the Manila clam

A new strain of Manila clam with orange shell color was produced after selection within a full-sib family for two generations. In the present study, the shell length, height, and width, and the live body weight of the orange strain were measured, and their correlation coefficients were calculated. The shell morphological traits were used as independent variables, and the live body weight was used as the dependent variable for calculating the path coefficients, correlation index, and determination coefficients. The results showed that the correlation coefficients between each shell morphological trait and the live body weight were all highly significant (P < 0.01). The correlation indices (R²) of morphological traits against the live body weight of clams were larger than 0.85, indicating that the morphology traits were the main factors affecting the body weight. Multiple regression equations were obtained to estimate shell length X₁ (cm), shell height X₂ (cm), and shell width X₃ (cm) against live body weight Y (g): Y = ? 2.62 + 0.34 X₁ + 0.145 X₂, (X₁ < 0.05, X₂ < 0.05). The results suggest that the shell length could be used as the main trait for selective breeding and could indirectly make a large improvement in the weight trait.     

Comparison of Energy and Growth Yields for Desulfitobacterium dehalogenans during Utilization of Chlorophenol and Various Traditional Electron Acceptors

Desulfitobacterium dehalogenans grew with formate as the electron donor and 3-chloro-4-hydroxyphenylacetate (3-Cl-4-OHPA) as the electron acceptor, yielding Y_X/formate, Y_X/2e⁻, and Y_X/ATP ranging from 3.2 to 11.3 g of biomass (dry weight)/mol, thus indicating that energy was conserved through reductive dechlorination. Pyruvate was utilized as the electron donor and acceptor, yielding stoichiometric amounts of acetate and lactate, respectively, and a Y_{X/reduced acceptor} of 13.0 g of biomass (dry weight)/mol. The supplementation of pyruvate-containing medium with additional electron acceptors, such as 3-Cl-4-OHPA, nitrate, fumarate, or sulfite, caused pyruvate to be replaced as the electron acceptor and nearly doubled the Y_X/ATP (Y_{X/acetate formed}). A comparison of the yields for 3-Cl-4-OHPA with those for other traditional electron acceptors indicates that the dehalogenation reaction led to the formation of similar amounts of energy equivalents. The various electron acceptors were used concomitantly with 3-Cl-4-OHPA in nonacclimated cultures, but the utilization rates and amounts utilized differed.