Expression pattern of anti-Müllerian hormone (amh) in the hybrid fish complex of Squalius alburnoides

In fish of the Squalius alburnoides complex, hybridisation and polyploidy have affected sex ratios, resulting in strong correlations between sex and genotype. The preponderance of females among triploids and the occurrence of an all male lineage among diploids seem to imply that sex ratio deviations should have a strong genetic basis. Until now, no information has been gathered regarding the molecular basis of sex determination in this intricate hybrid system. Thus, putative regulatory elements of the cascade that potentially are involved in sex determination in S. alburnoides have to be investigated. Being reported to have an important role in teleost sex determination, and more particularly in male gonad development, the anti-Müllerian hormone, amh was a good initial candidate. Here we report the isolation, cloning and characterization of the amh ortholog in S. alburnoides and the ancestral species S. pyrenaicus. In adult S. alburnoides and S. pyrenaicus of both sexes, amh shows a gonad specific expression pattern, restricted to the Sertoli cell lineage in testis and to granulosa cells in ovaries. During development, it plays an early role in male gonad differentiation in S. alburnoides. Overall the observed patterns are similar to what has been reported in other teleost species. This suggests a conserved role of amh and implies that its expression dynamics cannot be directly responsible for the sex ratio deviations reported in S. alburnoides. It is possible that a conjunction of other factors could be contributing for sex ratio imbalance. The present results constitute the starting point in the characterization of the S. alburnoides sex determination cascade, a process that we expect to shed some light on the molecular basis of sex distribution, within the context of hybrid system evolution.     

Programmed cell death in plant reproduction

Reproductive development is a rich arena to showcase programmed cell death in plants. After floral induction, the first act of reproductive development in some plants is the selective killing of cells destined to differentiate into an unwanted sexual organ. Production of functional pollen grains relies significantly on deterioration and death of the anther tapetum, a tissue whose main function appears to nurture and decorate the pollen grains with critical surface molecules. Degeneration and death in a number of anther tissues result ultimately in anther rupture and dispersal of pollen grains. Female sporogenesis frequently begins with the death of all but one of the meiotic derivatives, with surrounding nucellar cells degenerating in concert with embryo sac expansion. Female tissues that interact with pollen undergo dramatic degeneration, including death, to ensure the encounter of compatible male and female gametes. Pollen and pistil interact to kill invading pollen from an incompatible source. Most observations on cell death in reproductive tissues have been on the histological and cytological levels. We discuss various cell death phenomena in reproductive development with a view towards understanding the biochemical and molecular mechanisms that underlie these processes.     

The Non-Random Pattern of Insertion of IS2 into the hemB Gene of Escherichia coli

The hemB gene of Escherichia coli has been identified as a hot spot for the insertion of the transposable element IS2. The insertional specificity of IS2 is still unclear. This study reports on the attempt to sequence a statistically significant number of insertions in hemB, in order to determine whether there might be a basis for future studies to determine a molecular basis of IS2 insertional specificity. The results indicate that IS2 inserts in a non-random manner into a 240 bp segment at the 5′ end of the gene (region I). Twenty-one of 24 insertions occurred in region I. Three insertions have been identified in the two middle 250 bp segments of the 975 bp gene, and none in the 3′ terminal segment. A seventeen bp sequence showing 88.2% identity with a segment of IS2, 221 bp from the 3′ terminus has been identified in region I. Four instances of repeated insertion between the same pair of nucleotides have been observed at four different sites.     

Male production by apterous viviparae of the aphid Myzus persicae temporarily exposed to different scotoperiods

Male production by apterous viviparae of a holocyclic biotype of the green peach aphid, Myzus persicae, was induced by pre-natal and/or post-natal exposures to a long-night regime of 15 h dark per diem. When the apterae were exposed to three or more long nights immediately after birth and they subsequently developed under a short-night regime of 8 h dark per diem, they produced females (mostly alate viviparae) during the first 8–10 days of reproduction and a high proportion of males thereafter. When the apterae were exposed to two long nights immediately before their birth and to short nights thereafter, they produced relatively few females (mostly alate viviparae), and males were deposited already after 4–6 days of reproduction, i.e. 16–18 days after the first exposure to the long nights. The proportion of males among the progeny of these apterae was highest when the two prenatal exposures comprised scotophases of 11–15 h; under such long-night regimes many aphids switched to producing males exclusively. To achieve this effect, the two long scotophases had to be separated by a photophase of more than 1–2 h. Fewer males were produced and most of the apterae reverted to the production of females (apterous viviparae) when the duration of the two prenatal scotophases was 9 h 45 min-10 h 30 min, or 18 h and longer.One long night of 15–39 h could also induce temporary male production if the aphids were exposed to it late in the 4th larval instar or as teneral adults.     

Plant MITEs: useful tools for plant genetics and genomics

MTTEs (Miniature inverted-repeat transposabie elements) are reminiscence ot non-autonomous DNA (class Ⅱ) elements, which are distinguished from other transposable elements by their small size, short terminal inverted repeats (TIRs), high copy numbers, genie preference, and DNA sequence identity among family members. Although MITEs were first discovered in plants and still actively reshaping genomes, they have been isolated from a wide range of eukaryotic organisms. MITEs can be divided into Tourist-like, Stowaway-like, and pogo-like groups, according to similarities of their TIRs and TSDs (target site duplications). In despite of several models to explain the origin and amplification of MITEs, their mechanisms of transposition and accumulation in eukaryotic genomes remain poorly understood owing to insufficient experimental data. The unique properties of MITEs have been exploited as useful genetic tools for plant genome analysis. Utilization of MITEs as effective and informative genomic markers and pot     

Fitness effects of the timing of hatching may drive the evolution of temperature-dependent sex determination in short-lived lizards

In several species of short-lived Australian agamid lizards, an individual’s sex is determined by the nest temperatures encountered during incubation. The adaptive significance of such systems remains unclear. Here, we explore the hypothesis that (1) the optimal timing of hatching differs between the sexes, and thus (2) temperature-dependent sex determination (TSD) enhances maternal and offspring fitness by generating seasonal shifts in offspring sex ratios. Our model predicts that TSD can indeed enhance maternal fitness returns in short-lived lizards if (1) male–male competition is intense, thus reducing mating success of newly-matured males (but not females), and (2) the nesting season is prolonged, such that seasonal effects become significant. Available data on the distribution of TSD in Australian agamid lizards broadly support these predictions. Because both the level of male–male competition and the length of nesting season can vary at small spatial and temporal scales, selective forces on sex-determining mechanisms also should vary. Hence, our model predicts extensive small-scale (intraspecific) variation in sex-determining systems within agamid lizards, as well as among species.     

Abnormal and variable sex ratios in population samples of ladybirds

While most ladybird species are believed to show a conventional 1:1 sex ratio, population samples from five different species of ladybird have been found to show significant excesses of females. The species involved are Anatis ocellata, Exochomus quadripustulatus, Chilocorus renipustulatus, C. bipustulatus and C. nigritus. All possess neo-XY sex chromosome systems. It is possible that the excesses of females reflect the recombination of segments present at the ends of the neo-XY sex bivalent. If the products of recombination are more lethal in males than females, differences in sex ratio will result. An alternative hypothesis involves an interaction between Y-linked factors and maternally inherited factors, possibly of a transposable element type. The maintenance of such excesses of females in several species, in different taxonomic groups within the Coccinellidae, and the presence of differences in different populations of one of the species, must reflect a selective advantage for these excesses in natural populations of these species.     

Transposable element-medicated evolution of sex: A population genetic model

For a population made up of individuals capable of sexual as well as asexual modes of reproduction, conditions for the spread of a transposable element are explored using a one-locus, two-haplotype model. The analysis is then extended to include the possibility that the transposable element can modulate the probability of sexual reproduction, thus casting Hickey’s (1982,Genetics 101: 519–531) suggestion in a population genetics framework. The model explicitly includes the cost of sexual reproduction, fitness disadvantage to the transposable element, probability of transposition, and the predisposition for sexual reproduction in the presence and absence of the transposable element. The model predicts several kinds of outcome, including initial frequency dependence and stable polymorphism. More importantly, it is seen that for a wide range of parameter values, the transposable element can go to fixation. Therefore it is able to convert the population from a predominantly asexual to a predominantly sexual mode of reproduction. Viewed in conjunction with recent results implicating short stretches of apparently non-coding DNA in sex determination (McCoubreyet al. 1988,Science 242: 1146–1151), the model hints at the important role this mechanism could have played in the evolution of sexuality.     

Effects of ploidy and sex-locus genotype on gene expression patterns in the fire ant Solenopsis invicta

Males in many animal species differ greatly from females in morphology, physiology and behaviour. Ants, bees and wasps have a haplodiploid mechanism of sex determination whereby unfertilized eggs become males while fertilized eggs become females. However, many species also have a low frequency of diploid males, which are thought to develop from diploid eggs when individuals are homozygous at one or more sex determination loci. Diploid males are morphologically similar to haploids, though often larger and typically sterile. To determine how ploidy level and sex-locus genotype affect gene expression during development, we compared expression patterns between diploid males, haploid males and females (queens) at three developmental timepoints in Solenopsis invicta. In pupae, gene expression profiles of diploid males were very different from those of haploid males but nearly identical to those of queens. An unexpected shift in expression patterns emerged soon after adult eclosion, with diploid male patterns diverging from those of queens to resemble those of haploid males, a pattern retained in older adults. The finding that ploidy level effects on early gene expression override sex effects (including genes implicated in sperm production and pheromone production/perception) may explain diploid male sterility and lack of worker discrimination against them during development.     

果蝇P转座因子的研究进展

果蝇P因子是DNA转座子,在近几十年里受到很大关注。可用于确认有关基因,克隆基因以及安置基因回到基因组。P因子的高易动性及其保持和对内部序列强烈的修饰作用也是P因子的本质特征。P因子的另一重要用途是用于产生转基因果蝇。目的基因置于质粒内P因子中可在转座酶的作用下插入前胚盘胚。携带目的基因的P因子可从质粒转座到任意染色体上。据报道,在典型实验中,插入可育果蝇的10％～20％可产生转化体后裔。但是以这种可动DNA片段作为载体尚存在转移基因的不稳定性及与内源跳跃基因的相互影响。本文介绍了果蝇P转座因子的一些研究进展。这些因子的遗传可动性也使它们适用于建造载体产生转基因生物。若如此,载体导入外源基因组的遗传稳定性问题将是一个重要课题。Abstract：P elements in D.melanogaster are DNA transposons and received greater attention within the last few decades.P elements are used for identifying genes of interest,for cloning them,and for placing them back into the genome.The high mobility of P elements and their retention of this mobility and drastic modiffications to their internal sequences are also essential features.Another most important use of P elements is that of making transgenic flies.Desired gene is placed between P-element ends,usually within a plasmid,and injected into preblastoderm embryos in the presence of transposase.This P element then transposes from the plasmid to a random chromosomal site.Reported in a typical experiment,10％～20％ of the fertile injected flies produce transformant progeny.But the instability of the transferred gene carried on a piece of mobile DNA as a vector and its interaction with endogenous jumping genes.This paper introduced the studies advances of P transposable element in Drosophila.The genetic mobility of these elements can also make them suitable for the construction of vectors to create transgenic organisms.If so,the genetic stability of the vectors introduced to a foreign genome should be a important subject.     

Complementary sex determination in the genus Diadegma (Hymenoptera: Ichneumonidae)

In the evolution of sexual reproduction we would expect to see a close association between mating systems and sex determination mechanisms. Such associations are especially evident in the insect order Hymenoptera which shows great diversity with respect to both of these characteristics. The ancestral sex determination mechanism in this order is thought to be single‐locus complementary sex determination (sl‐CSD), which is inbreeding sensitive, and where inbreeding results in the production of sterile diploid males rather than daughters. Presently, however, there is insufficient data to give strong support to the hypothesis that sl‐CSD is truly the ancestral condition in the Hymenoptera, principally because of the difficulty of reliably determining the degree of male ploidy. Here we show that six ichneumonid parasitoids from the polyphyletic genus Diadegma are subject to sl‐CSD, using neuronal cell DNA flow cytometry to distinguish ploidy levels. The presence of sl‐CSD in these six species, together with earlier evidence from the authors for D. chrysostictos, provides considerable support for the notion that sl‐CSD was ancestral in the Aculeata/Ichneumonoidea clade, which contains all eusocial Hymenoptera. Moreover, because flow cytometry discriminates reliably between haploid and diploid males, and is independent of the maternal sex allocation or the need for genetic markers, it has considerable potential for the determination of ploidy more generally.     

Why are triploid zebrafish all male?

Adult triploid zebrafish Danio rerio has previously been reported to be all male. This phenomenon has only been reported in one other gonochoristic fish species, the rosy bitterling Rhodeus ocellatus, despite the fact that triploidy is induced in numerous species. To investigate the mechanism responsible, we first produced triploid zebrafish and observed gonad development. Histological sections of juvenile triploid gonads showed that primary growth oocytes were able to develop in the juvenile ovary, but no cortical alveolus or more advanced oocytes were found. All adult triploids examined were male (n = 160). Male triploids were able to induce oviposition by diploid females during natural spawning trials, but fertilization rates were low (1.0 ± 3.1%) compared with diploid male siblings (67.4 ± 16.6%). The embryos produced by triploid sires were aneuploid with a mean ploidy of 2.4 ± 0.1n, demonstrating that triploid males produce aneuploid spermatozoa. After confirming that adult triploids are all male, we produced an additional batch of triploid zebrafish and exposed them (and a group of diploid siblings) to 100 ng/L estradiol (E2) from 5 to 28 dpf. The E2 treated triploids and nontreated triploids were all male. The nontreated diploids were also all male, but the E2 treated diploids were 89% female. This demonstrates that triploidy acts downstream of estrogen synthesis in the sex differentiation pathway to induce male development. Based on this and the observations of juvenile gonad development in triploids, we suggest that triploidy inhibits development of oocytes past the primary growth stage, and this causes female to male sex reversal.