Centrioles in flies: The exception to the rule?

Centrioles and basal bodies are MT based structures that present a highly conserved ninefold symmetry. Centrioles can be found at the core of the centrosome where they participate in PCM recruitment and organization, contributing to cytoplasmic MT nucleation. Basal bodies are normally located closely to the plasma membrane where they are responsible for axoneme assembly to form structures such as cilia or flagella. While it is well accepted that these organelles have important roles in cell and tissue organization, their contribution to certain phases of animal development is still not entirely established. Here we review the role of centrosomes and cilia in Drosophila melanogaster and briefly discuss the implications of these findings to other model organisms.     

Extra-Mediterranean refugia: The rule and not the exception?

ABSTRACT: Some decades ago, biogeographers distinguished three major faunal types of high importance for Europe: (i) Mediterranean elements with exclusive glacial survival in the Mediterranean refugia, (ii) Siberian elements with glacial refugia in the eastern Palearctic and only postglacial expansion to Europe and (iii) arctic and/or alpine elements with large zonal distributions in the periglacial areas and postglacial retreat to the North and/or into the high mountain systems. Genetic analyses have unravelled numerous additional refugia both of continental and Mediterranean species, thus strongly modifying the biogeographical view of Europe. This modified notion is particularly true for the so-called Siberian species, which in many cases have not immigrated into Europe during the postglacial period, but most likely have survived the last, or even several glacial phases, in extra-Mediterranean refugia in some climatically favourable but geographically limited areas of southern Central and Eastern Europe. Recently, genetic analyses revealed that typical Mediterranean species have also survived the Last Glacial Maximum in cryptic northern refugia (e.g. in the Carpathians or even north of the Alps) in addition to their Mediterranean refuge areas.     

Signal-induced repression: the exception or the rule in developmental signaling?

Cell-cell communication plays a key role in organ formation and patterning in multicellular animals and is carried out by a few evolutionarily conserved signaling pathways. The modes of action of these pathways share a number of general properties, or habits, that allow them to strongly activate target genes in a ligand-dependent manner in the proper cellular contexts. Recent studies have revealed that some developmental signaling pathways can also strongly repress genes in a ligand-dependent manner. These new findings raise the interesting possibility that this repressive mode of action is shared by many or most developmental signaling pathways.     

The role of Quaternary environmental change in plant macroevolution: the exception or the rule?

The Quaternary has been described as an important time for genetic diversification and speciation. This is based on the premise that Quaternary climatic conditions fostered the isolation of populations and, in some instances, allopatric speciation. However, the 'Quaternary Ice-Age speciation model' rests on two key assumptions: (i) that biotic responses to climate change during the Quaternary were significantly different from those of other periods in Earth's history; and (ii) that the mechanisms of isolation during the Quaternary were sufficient in time and space for genetic diversification to foster speciation. These assumptions are addressed by examining the plant fossil record for the Quaternary (in detail) and for the past 410 Myr, which encompasses previous intervals of icehouse Earth. Our examination of the Quaternary record indicates that floristic responses to climate changes during the past 1.8 Myr were complex and that a distinction has to be made between those plants that were able to withstand the extremes of glacial conditions and those that could not. Generation times are also important as are different growth forms (e.g. herbaceous annuals and arborescent perennials), resulting in different responses in terms of genetic divergence rates during isolation. Because of these variations in the duration of isolation of populations and genomic diversification rates, no canonical statement about the predominant floristic response to climatic changes during the Quaternary (i.e. elevated rates of speciation or extinction, or stasis) is currently possible. This is especially true because of a sampling bias in terms of the fossil record of tree species over that of species with non-arborescent growth forms. Nevertheless, based on the available information, it appears that the dominant response of arborescent species during the Quaternary was extinction rather than speciation or stasis. By contrast, our examination of the fossil record of vascular plants for the past 410 Myr indicates that speciation rates often increased during long intervals of icehouse Earth (spanning up to 50 Myr). Therefore, longer periods of icehouse Earth than those occurring during the Quaternary may have isolated plant populations for sufficiently long periods of time to foster genomic diversification and allopatric speciation. Our results highlight the need for more detailed study of the fossil record in terms of finer temporal and spatial resolution than is currently available to examine the significance of intervals of icehouse Earth. It is equally clear that additional and detailed molecular studies of extant populations of Quaternary species are required in order to determine the extent to which these 'relic' species have genomically diversified across their current populations.     

PCNA-coupled p21 degradation after DNA damage: The exception that confirms the rule?

While many are the examples of DNA damaging treatments that induce p21 accumulation, the conception of p21 upregulation as the universal response to genotoxic stress has come to an end. Compelling evidences have demonstrated the existence of converging signals that negatively regulate p21 bellow basal levels when replication forks are blocked. Moreover, conclusive reports identified the E3-ligase CRL4^CDT2 (CUL4–DDB1–CDT2) as the enzymatic complex that promotes p21 proteolysis when treatments such as UV irradiation trigger replication fork stress. A pre-requisite for CRL4^CDT2-driven proteolysis is the interaction of p21 with PCNA. Interestingly as well, CRL4^CDT2-dependent proteolysis is not limited to p21 and affects other PCNA partners, including the specialized DNA polymerase η (pol eta). These recent discoveries are particularly intriguing since the UV-induced degradation of p21 has been shown to be required for efficient pol η recruitment to DNA lesions. Herein we review the findings that lead to the identification of the molecular mechanism that triggers damage-induced PCNA-coupled protein proteolysis. We propose a novel model in which CRL4^CDT2-dependent protein degradation facilitates a sequential and dynamic exchange between PIP box bearing proteins at stall forks during Translesion DNA synthesis (TLS). Moreover, given the tight spatiotemporal control that CRL4^CDT2-driven proteolysis is able to confer to PCNA-regulated processes, we discuss the impact that this degradation mechanism might have in other molecular switches associated with the repair of damaged DNA.     

Inositol pyrophosphates and Akt/PKB: Is the pancreatic β-cell the exception to the rule?

The inositol pyrophosphate, diphosphoinositol pentakisphosphate (IP₇), is thought to negatively regulate the critical insulin signaling protein Akt/PKB. Knockdown of the IP₇-generating inositol hexakisphosphate kinase 1 (IP6K1) results in a concomitant increase in signaling through Akt/PKB in most cell types so far examined. Total in vivo knockout of IP6K1 is associated with a phenotype resistant to high-fat diet, due to enhanced Akt/PKB signaling in classic insulin regulated tissues, counteracting insulin resistance. In contrast, we have shown an important positive role for IP6K1 in insulin exocytosis in the pancreatic β-cell. These cells also possess functional insulin receptors and the feedback loop following insulin secretion is a key aspect of their normal function. Thus we examined the effect of silencing IP6K1 on the activation of Akt/PKB in β-cells. Silencing reduced the glucose-stimulated increase in Akt/PKB phosphorylation on T308 and S473. These effects were reproduced with the selective pan-IP6K inhibitor TNP. The likely explanation for IP₇ reduction decreasing rather than increasing Akt/PKB phosphorylation is that IP₇ is responsible for generating the insulin signal, which is the main source of Akt/PKB activation. In agreement, insulin receptor activation was compromised in TNP treated cells. To test whether the mechanism of IP₇ inhibition of Akt/PKB still exists in β-cells, we treated them at basal glucose with an insulin concentration equivalent to that reached during glucose stimulation. TNP potentiated the Akt/PKB phosphorylation of T308 induced by exogenous insulin. Thus, the IP₇ regulation of β-cell Akt/PKB is determined by two opposing forces, direct inhibition of Akt/PKB versus indirect stimulation via secreted insulin. The latter mechanism is dominant, masking the inhibitory effect. Consequently, pharmacological strategies to knock down IP6K activity might not have the same positive output in the β-cell as in other insulin regulated tissues.     

Physiological regulation of evaporative water loss in endotherms: is the little red kaluta (Dasykaluta rosamondae) an exception or the rule?

It is a central paradigm of comparative physiology that the effect of humidity on evaporative water loss (EWL) is determined for most mammals and birds, in and below thermoneutrality, essentially by physics and is not under physiological regulation. Fick''s law predicts that EWL should be inversely proportional to ambient relative humidity (RH) and linearly proportional to the water vapour pressure deficit (Δwvp) between animal and air. However, we show here for a small dasyurid marsupial, the little kaluta (Dasykaluta rosamondae), that EWL is essentially independent of RH (and Δwvp) at low RH (as are metabolic rate and thermal conductance). These results suggest regulation of a constant EWL independent of RH, a hitherto unappreciated capacity of endothermic vertebrates. Independence of EWL from RH conserves water and heat at low RH, and avoids physiological adjustments to changes in evaporative heat loss such as thermoregulation. Re-evaluation of previously published data for mammals and birds suggests that a lesser dependence of EWL on RH is observed more commonly than previously thought, suggesting that physiological independence of EWL of RH is not just an unusual capacity of a few species, such as the little kaluta, but a more general capability of many mammals and birds.     

Fine-scale spatial genetic structure in predominantly selfing plants with limited seed dispersal: a rule or exception?

Gene flow at a fine scale is still poorly understood despite its recognized importance for plant population demographic and genetic processes. We tested the hypothesis that intensity of gene flow will be lower and strength of spatial genetic structure (SGS) will be higher in more peripheral populations because of lower population density. The study was performed on the predominantly selfing Avena sterilis and included: (1) direct measurement of dispersal in a controlled environment; and (2) analyses of SGS in three natural populations, sampled in linear transects at fixed increasing inter plant distances. We found that in A.sterilis major seed dispersal is by gravity in close (less than 2m) vicinity of the mother plant, with a minor additional effect of wind. Analysis of SGS with six nuclear SSRs revealed a significant autocorrelation for the distance class of 1m only in the most peripheral desert population, while in the two core populations with Mediterranean conditions, no genetic structure was found. Our results support the hypothesis that intensity of SGS increases from the species core to periphery as a result of decreased within population gene flow related to low plant density. Our findings also show that predominant self pollination and highly localized seed dispersal lead to SGS at a very fine scale, but only if plant density is not too high.     

Fine-scale spatial genetic structure in predominantly selfing plants with limited seed dispersal:a rule or exception?

Gene flow at a fine scale is still poorly understood despite its recognized importance for plant population demographic and genetic processes.We tested the hypothesis that intensity of gene flow will be lower and strength of spatial genetic structure(SGS) will be higher in more peripheral populations because of lower population density.The study was performed on the predominantly selfing Avena sterilis and included:(1) direct measurement of dispersal in a controlled environment;and(2) analyses of SGS in three natural populations,sampled in linear transects at fixed increasing inter-plant distances.We found that in A.sterilis major seed dispersal is by gravity in close(less than 2 m) vicinity of the mother plant,with a minor additional effect of wind.Analysis of SGS with six nuclear SSRs revealed a significant autocorrelation for the distance class of 1 m only in the most peripheral desert population,while in the two core populations with Mediterranean conditions,no genetic structure was found.Our results support the hypothesis that intensity of SGS increases from the species core to periphery as a result of decreased within-population gene flow related to low plant density.Our findings also show that predominant self-pollination and highly localized seed dispersal lead to SGS at a very fine scale,but only if plant density is not too high.     

Comment: Low dental caries rate in Neandertals: The result of diet or the oral flora composition?

Dental caries is an infectious disease caused by oral acidophilic bacteria feeding on fermentable sugars, e.g. Streptococcus mutans. The frequency of dental caries in Neandertals was very low. This was usually explained as the result of a low-sugar diet. Recent research, however, revealed some regional differences between European and Near Eastern Neandertals, with the latter consuming considerable amounts of plants including highly cariogenic dates. This discovery, compared with the results of research on genetic diversity of S. mutans, may suggest that this species, and perhaps other most virulent species, were absent in the oral flora of Neandertals.     

Are <Emphasis Type="Italic">Drosophila</Emphasis> telomeres an exception or the rule?

At the ends of eukaryotic chromosomes are telomeres, specialized structures with unusual properties. The repetitive structure of telomere regions makes them difficult to deal with in general genome-sequencing projects. Specific efforts to compare sequences and properties of telomeres across species can reveal the generalities of telomere properties.     

The absence of crossovers on chromosome 4 in Drosophila melanogaster: Imperfection or interesting exception?

Drosophila melanogaster chromosome 4 is an anomaly because of its small size, chromatin structure, and most notably its lack of crossing over during meiosis. Earlier ideas about the absence of crossovers on 4 hypothesize that these unique characteristics function to prevent crossovers. Here, we explore hypotheses about the absence of crossovers on 4, how these have been addressed, and new insights into the mechanism behind this suppression. We review recently published results that indicate that global crossover patterning, in particular the centromere effect, make a major contribution to the prevention of crossovers on 4.