Are homologies in vertebrate sex determination due to shared ancestry or to limited options?

The same candidate genes and the same autosomes are repeatedly used as sex chromosomes in vertebrates. Are these systems identical by descent, or are some genes or chromosomes intrinsically better at triggering the first steps of sex determination?     

Is it possible to improve homologous recombination in Chlamydomonas reinhardtii?

Targeted modification of the genome has long been an aim of many geneticists and biotechnologists. Gene targeting is a main molecular tool to examine biological effects of genes in a controlled environment. Effective gene targeting depends on the frequency of homologous recombination that is indispensable for the insertion of foreign DNA into a specific sequence of the genome. The main problem associated with the development of an optimal procedure for gene targeting in a particular organism is the variability of homologous recombination (HR) in different species. Chlamydomonas reinhardtii is an attractive model system for the study of many cellular processes and is also an interesting object for the biotechnology industry. In spite of many advantages of this model system, C. reinhardtii does not readily express heterologous genes and does not allow targeted integration of foreign DNA into its genome easily. This paper compares data obtained from several different experiments designed for improving gene targeting in different organisms and reviews the suitability of particular techniques in C. reinhardtii cells. Presented at the International Symposium Biology and Taxonomy of Green Algae V, Smolenice, June 26–29, 2007, Slovakia.     

Larval RNAi in Drosophila?

RNA interference (RNAi) has become a common method of gene knockdown in many model systems. To trigger an RNAi response, double-stranded RNA (dsRNA) must enter the cell. In some organisms such as Caenorhabditis elegans, cells can take up dsRNA from the extracellular environment via a cellular uptake mechanism termed systemic RNAi. However, in the fruit fly Drosophila melanogaster, it is widely believed that cells are unable to take up dsRNA, although there is little published data to support this claim. In this study, we set out to determine whether this perception has a factual basis. We took advantage of traditional Gal4/upstream activation sequence (UAS) transgenic flies as well as the mosaic analysis with a repressible cell marker (MARCM) system to show that extracellular injection of dsRNA into Drosophila larvae cannot trigger RNAi in most Drosophila tissues (with the exception of hemocytes). Our results show that this is not due to a lack of RNAi machinery in these tissues as overexpression of dsRNA inside the cells using hairpin RNAs efficiently induces an RNAi response in the same tissues. These results suggest that, while most Drosophila tissues indeed lack the ability to uptake dsRNA from the surrounding environment, hemocytes can initiate RNAi in response to extracellular dsRNA. We also examined another insect, the red flour beetle Tribolium castaneum, which has been shown to exhibit a robust systemic RNAi response. We show that virtually all Tribolium tissues can respond to extracellular dsRNA, which is strikingly different from the situation in Drosophila. Our data provide specific information about the tissues amenable to RNAi in two different insects, which may help us understand the molecular basis of systemic RNAi.     

Are Drosophila preferences for yeasts stable or contextual?

Whether there are general mechanisms, driving interspecific chemical communication is uncertain. Saccharomycetaceae yeast and Drosophila fruit flies, both extensively studied research models, share the same fruit habitat, and it has been suggested their interaction comprises a facultative mutualism that is instigated and maintained by yeast volatiles. Using choice tests, experimental evolution, and volatile analyses, we investigate the maintenance of this relationship and reveal little consistency between behavioral responses of two isolates of sympatric Drosophila species. While D. melanogaster was attracted to a range of different Saccharomycetaceae yeasts and this was independent of fruit type, D. simulans preference appeared specific to a particular S. cerevisiae genotype isolated from a vineyard fly population. This response, however, was not consistent across fruit types and is therefore context‐dependent. In addition, D. simulans attraction to an individual S. cerevisiae isolate was pliable over ecological timescales. Volatile candidates were analyzed to identify a common signal for yeast attraction, and while D. melanogaster generally responded to fermentation profiles, D. simulans preference was more discerning and likely threshold‐dependent. Overall, there is no strong evidence to support the idea of bespoke interactions with specific yeasts for either of these Drosophila genotypes. Rather the data support the idea Drosophila are generally adapted to sense and locate fruits infested by a range of fungal microbes and/or that yeast–Drosophila interactions may evolve rapidly.     

An Adaptive Immune Response in Drosophila?

The ability to mount an adaptive immune response is thought to be an attribute restricted to vertebrates. A new study conducted in Drosophila demonstrates that invertebrate immunity can adapt to an immune challenge and mount a specific immune response.     

Is hobo permissivity related to I reactivity and sensitive to chromatin compaction in Drosophila melanogaster?

In Drosophila melanogaster, the hobo transposable element is responsible for a hybrid dysgenesis syndrome. It appears in the germline of progenies from crosses between females devoid of hobo elements (E) and males bearing active hobo elements (H). In the HE system, permissivity is the ability of females to permit hobo activity in their progeny when they have been crossed with H males. Permissivity displays both intra- and inter-strain variability and decreases with the age of the females. Such characteristics are reminiscent of those for the reactivity in the IR system. The reactivity is the ability of R females (devoid of I factors) to permit activity of the I LINE retrotransposon in the F1 females resulting from crosses with I males (bearing I factors). Here we investigated permissivity properties in the HE system related to reactivity in the IR system. Previously it had been shown that reactivity increases with the number of Su(var)3-9 genes, which increases chromatin compaction near heterochromatin. Using the same lines, we show that permissivity increases with the number of Su(var)3-9 genes. To investigate the impact of chromatin compaction on permissivity we have tested the polymorphism of position-effect variegation (PEV) on the white(mottled4) locus in RE strains. Our results suggest a model of regulation in which permissivity could depend on the chromatin state and on the hobo vestigial sequences.     

Convergence and extension in vertebrate gastrulae: cell movements according to or in search of identity?

During vertebrate gastrulation, convergence and extension cell movements both narrow and lengthen the forming embryonic axis. Concurrently, positional information established principally by the ventral-to-dorsal gradient of bone morphogenetic protein activity specifies cell fates within the gastrula. New data, primarily from zebrafish, have identified domains of distinct convergence and extension movements, and have established a role for the noncanonical Wnt signaling pathway in promoting the mediolateral cell polarization that underlies this morphogenetic process. Other observations suggest the intriguing possibility that positional information regulates convergence and extension movements in parallel with cell-fate specification.     

Is malarial anaemia homologous to neocytolysis after altitude acclimatisation?

Malaria patients frequently develop severe anaemia that can persist after Plasmodium infection has been cleared from the circulation. This puzzling phenomenon involves massive death of young uninfected erythrocytes at a time when parasitic infection is very low. We have observed striking similarities in erythrocyte homoeostasis during altitude acclimatisation and Plasmodium infection, both of which initially induce an increase in circulating erythropoietin (Epo). Decreasing levels of Epo after return to low altitudes induce the death of young erythrocytes, a phenomenon called neocytolysis. In a similar way, we propose that Epo, which peaks during acute malaria and decreases after parasite clearance, could be contributing to anaemia causing neocytolysis during recovery from Plasmodium infection.     

Are there morphologically abnormal early recombination nodules in the Drosophila melanogaster meiotic mutant mei-218?

Early recombination nodules have been suggested to perform a role in meiotic gene conversion recombination events. The meiotic recombination-defective mutant mei-218 greatly reduces the frequency of meiotic crossover (reciprocal) recombination events and reduces the number of late recombination nodules to the same extent. However, it does not reduce the frequency of simple gene conversion events, although they are abnormal in having shorter coconversion tracts than controls. The original cytological study yielded somewhat fewer early nodules in mei-218 than in controls, although very abnormal ones might have been missed. The present study failed to identify a mei-218 specific abnormal category. However, because recombination nodules are at present recognizable only by their morphology, a definitive answer to this question must await a specific probe for recombination nodules. Moreover, the possibility remains that early nodules in mei-218 are more ephemeral than are early nodules in wild type.     

Are annulate lamellae in the Drosophila embryo the result of overproduction of nuclear pore components?

Annulate lamellae are cytoplasmic organelles composed of stacked sheets of membrane containing pores that are structurally indistinguishable from nuclear pores. The functions of annulate lamellae are not well understood. Although they may be found in virtually any eucaryotic cell, they occur most commonly in transformed and embryonic tissues. In Drosophila, annulate lamellae are found in the syncytial blastoderm embryo as it is cleaved to form the cellular blastoderm. The cytological events of the cellularization process are well documented, and may be used as temporal landmarks when studying changes in annulate lamellae. By using morphometric techniques to analyze electron micrographs of embryos, we are able to calculate the number of pores per nucleus in nuclear envelopes and annulate lamellae during progressive stages of cellularization. We find that annulate lamellae pores remain at a low level while nuclear envelopes are expanding and acquiring pores in early interphase. Once nuclear envelopes are saturated with pores, however, the number of annulate lamellae pores increases more than 10-fold in 9 min. Over the next 30 min it gradually declines to the initial low level. On the basis of these results, we propose (a) that pore synthesis and assembly continues after nuclear envelopes have been saturated with pores; (b) that these supernumerary pores accumulate transiently in cytoplasmic annulate lamellae; and (c) that because these pores are not needed by the embryo they are subsequently degraded.     

Planar cell polarization: do the same mechanisms regulate Drosophila tissue polarity and vertebrate gastrulation?

Many types of cell show different aspects of polarization. Epithelial cells display a ubiquitous apical-basolateral polarity but often are also polarized in the plane of the epithelium - a feature referred to as 'planar cell polarity' (PCP). In Drosophila all adult epithelial cuticular structures are polarized within the plane, whereas in vertebrates examples of PCP include aspects of skin development, features of the inner ear epithelium, and the morphology and behavior of mesenchymal cells undergoing the morphogenetic movement called 'convergent extension'. Recent advances in the study of PCP establishment are beginning to unravel the molecular mechanisms that underlie this aspect of cell and tissue differentiation. Here I discuss new developments in our molecular understanding of PCP in Drosophila and compare them towhat is known about the regulation of convergent extension in vertebrates.     

Pervasive Natural Selection in the Drosophila Genome?

Over the past four decades, the predominant view of molecular evolution saw little connection between natural selection and genome evolution, assuming that the functionally constrained fraction of the genome is relatively small and that adaptation is sufficiently infrequent to play little role in shaping patterns of variation within and even between species. Recent evidence from Drosophila, reviewed here, suggests that this view may be invalid. Analyses of genetic variation within and between species reveal that much of the Drosophila genome is under purifying selection, and thus of functional importance, and that a large fraction of coding and noncoding differences between species are adaptive. The findings further indicate that, in Drosophila, adaptations may be both common and strong enough that the fate of neutral mutations depends on their chance linkage to adaptive mutations as much as on the vagaries of genetic drift. The emerging evidence has implications for a wide variety of fields, from conservation genetics to bioinformatics, and presents challenges to modelers and experimentalists alike.