Testing the Cambrian explosion hypothesis by using a molecular dating technique

Molecular studies have the potential to shed light on the origin of the animal phyla by providing independent estimates of the divergence times, but have been criticized for failing to account adequately for variation in rate of evolution. A method of dating divergence times from molecular data addresses the criticisms of earlier studies and provides more realistic, but wider, confidence intervals. The data are not compatible with the Cambrian explosion hypothesis as an explanation for the origin of metazoan phyla, and provide additional support for an extended period of Precambrian metazoan diversification.

Omega images at the active zone may be endocytotic rather than exocytotic: Implications for the vesicle hypothesis of transmitter release

A Ca2+-dependent synaptic vesicle-recycling pathway emanating from the plasma membrane adjacent to the dense body at the active zone has been demonstrated by blocking pinch-off of recycling membrane by using the Drosophila mutant, shibire. Exposure of wild-type Drosophila synapses to low Ca2+/high Mg2+ saline is shown here to block this active zone recycling pathway at the stage in which invaginations of the plasma membrane develop adjacent to the dense body. These observations, in combination with our previous demonstration that exposure to high Ca2+ causes “docked” vesicles to accumulate in the identical location where active zone endocytosis occurs, suggest the possibility that a vesicle-recycling pathway emanating from the active zone may exist that is stimulated by exposure to elevated Ca2+, thereby causing an increase in vesicle recycling, and is suppressed by exposure to low Ca2+ saline, thereby blocking newly forming vesicles at the invagination stage. The presence of a Ca2+-dependent endocytotic pathway at the active zone opens up the following possibilities: (i) electron microscopic omega-shaped images (and their equivalent, freeze fracture dimples) observed at the active zone adjacent to the dense body could represent endocytotic images (newly forming vesicles) rather than exocytotic images; (ii) vesicles observed attached to the plasma membrane adjacent to the dense body could represent newly formed vesicles rather than vesicles “docked” for release of transmitter.

Dictyostelium myosin II null mutant can still cap Con A receptors

Cross-linked antigens on the surface of a motile cell cap at the trailing end of the cell. In Dictyostelium discoideum, myosin II null mutants have previously been reported to be unable to cap Con A receptors, although they are able to locomote. This finding implicated myosin II as an essential component of the capping mechanism, although not of the machinery for locomotion. Here we show that myosin II null mutants do cap Con A receptors, albeit less efficiently than does wild type. This shows that cap formation is not absolutely dependent on myosin II and that a close mechanistic relationship between capping, particle movement, and cell migration may still exist.

Wheat cytosolic acetyl-CoA carboxylase complements an ACC1 null mutation in yeast

Spores harboring an ACC1 deletion derived from a diploid Saccharomyces cerevisiae strain, in which one copy of the entire ACC1 gene is replaced with a LEU2 cassette, fail to grow. A chimeric gene consisting of the yeast GAL10 promoter, yeast ACC1 leader, wheat cytosolic acetyl-CoA carboxylase (ACCase) cDNA, and yeast ACC1 3′ tail was used to complement a yeast ACC1 mutation. The complementation demonstrates that active wheat ACCase can be produced in yeast. At low concentrations of galactose, the activity of the “wheat gene” driven by the GAL10 promoter is low and ACCase becomes limiting for growth, a condition expected to enhance transgenic yeast sensitivity to wheat ACCase-specific inhibitors. An aryloxyphenoxypropionate and two cyclohexanediones do not inhibit growth of haploid yeast strains containing the yeast ACC1 gene, but one cyclohexanedione inhibits growth of the gene-replacement strains at concentrations below 0.2 mM. In vitro, the activity of wheat cytosolic ACCase produced by the gene-replacement yeast strain is inhibited by haloxyfop and cethoxydim at concentrations above 0.02 mM. The activity of yeast ACCase is less affected. The wheat plastid ACCase in wheat germ extract is inhibited by all three herbicides at concentrations below 0.02 mM. Yeast gene-replacement strains will provide a convenient system for the study of plant ACCases.

A single amino acid substitution converts a carboxylesterase to an organophosphorus hydrolase and confers insecticide resistance on a blowfly

Resistance to organophosphorus (OP) insecticides is associated with decreased carboxylesterase activity in several insect species. It has been proposed that the resistance may be the result of a mutation in a carboxylesterase that simultaneously reduces its carboxylesterase activity and confers an OP hydrolase activity (the “mutant ali-esterase hypothesis”). In the sheep blowfly, Lucilia cuprina, the association is due to a change in a specific esterase isozyme, E3, which, in resistant flies, has a null phenotype on gels stained using standard carboxylesterase substrates. Here we show that an OP-resistant allele of the gene that encodes E3 differs at five amino acid replacement sites from a previously described OP-susceptible allele. Knowledge of the structure of a related enzyme (acetylcholinesterase) suggests that one of these substitutions (Gly137 → Asp) lies within the active site of the enzyme. The occurrence of this substitution is completely correlated with resistance across 15 isogenic strains. In vitro expression of two natural and two synthetic chimeric alleles shows that the Asp137 substitution alone is responsible for both the loss of E3’s carboxylesterase activity and the acquisition of a novel OP hydrolase activity. Modeling of Asp137 in the homologous position in acetylcholinesterase suggests that Asp137 may act as a base to orientate a water molecule in the appropriate position for hydrolysis of the phosphorylated enzyme intermediate.

Possible ecological risks of transgenic organism release when transgenes affect mating success: Sexual selection and the Trojan gene hypothesis

Widespread interest in producing transgenic organisms is balanced by concern over ecological hazards, such as species extinction if such organisms were to be released into nature. An ecological risk associated with the introduction of a transgenic organism is that the transgene, though rare, can spread in a natural population. An increase in transgene frequency is often assumed to be unlikely because transgenic organisms typically have some viability disadvantage. Reduced viability is assumed to be common because transgenic individuals are best viewed as macromutants that lack any history of selection that could reduce negative fitness effects. However, these arguments ignore the potential advantageous effects of transgenes on some aspect of fitness such as mating success. Here, we examine the risk to a natural population after release of a few transgenic individuals when the transgene trait simultaneously increases transgenic male mating success and lowers the viability of transgenic offspring. We obtained relevant life history data by using the small cyprinodont fish, Japanese medaka (Oryzias latipes) as a model. Our deterministic equations predict that a transgene introduced into a natural population by a small number of transgenic fish will spread as a result of enhanced mating advantage, but the reduced viability of offspring will cause eventual local extinction of both populations. Such risks should be evaluated with each new transgenic animal before release.

Further examination of the Xist promoter-switch hypothesis in X inactivation: Evidence against the existence and function of a P0 promoter

The onset of X inactivation coincides with accumulation of Xist RNA along the future inactive X chromosome. A recent hypothesis proposed that accumulation is initiated by a promoter switch within Xist. In this hypothesis, an upstream promoter (P0) produces an unstable transcript, while the known downstream promoter (P1) produces a stable RNA. To test this hypothesis, we examined expression and half-life of Xist RNA produced from an Xist transgene lacking P0 but retaining P1. We confirm the previous finding that P0 is dispensable for Xist expression in undifferentiated cells and that P1 can be used in both undifferentiated and differentiated cells. Herein, we show that Xist RNA initiated at P1 is unstable and does not accumulate. Further analysis indicates that the transcriptional boundary at P0 does not represent the 5′ end of a distinct Xist isoform. Instead, P0 is an artifact of cross-amplification caused by a pseudogene of the highly expressed ribosomal protein S12 gene Rps12. Using strand-specific techniques, we find that transcription upstream of P1 originates from the DNA strand opposite Xist and represents the 3′ end of the antisense Tsix RNA. Thus, these data do not support the existence of a P0 promoter and suggest that mechanisms other than switching of functionally distinct promoters control the up-regulation of Xist.

Ligand-dependent development of the endothelial and hemopoietic lineages from embryonic mesodermal cells expressing vascular endothelial growth factor receptor 2

The existence of a common precursor for endothelial and hemopoietic cells, termed the hemangioblast, has been postulated since the beginning of the century. Recently, deletion of the endothelial-specific vascular endothelial growth factor receptor 2 (VEGFR2) by gene targeting has shown that both endothelial and hemopoietic cells are absent in homozygous null mice. This observation suggested that VEGFR2 could be expressed by the hemangioblast and essential for its further differentiation along both lineages. However, it was not possible to exclude the hypothesis that hemopoietic failure was a secondary effect resulting from the absence of an endothelial cell microenvironment. To distinguish between these two hypotheses, we have produced a mAb directed against the extracellular domain of avian VEGFR2 and isolated VEGFR2+ cells from the mesoderm of chicken embryos at the gastrulation stage. We have found that in clonal cultures, a VEGFR2+ cell gives rise to either a hemopoietic or an endothelial cell colony. The developmental decision appears to be regulated by the binding of two different VEGFR2 ligands. Thus, endothelial differentiation requires VEGF, whereas hemopoietic differentiation occurs in the absence of VEGF and is significantly reduced by soluble VEGFR2, showing that this process could be mediated by a second, yet unidentified, VEGFR2 ligand. These observations thus suggest strongly that in the absence of the VEGFR2 gene product, the precursors of both hemopoietic and vascular endothelial lineages cannot survive. These cells therefore might be the initial targets of the VEGFR2 null mutation.

AP-2-null cells disrupt morphogenesis of the eye, face, and limbs in chimeric mice

The homozygous disruption of the mouse AP-2 gene yields a complex and lethal phenotype that results from defective development of the neural tube, head, and body wall. The severe and pleiotropic developmental abnormalities observed in the knockout mouse suggested that AP-2 may regulate several morphogenic pathways. To uncouple the individual developmental mechanisms that are dependent on AP-2, we have now analyzed chimeric mice composed of both wild-type and AP-2-null cells. The phenotypes obtained from these chimeras indicate that there is an independent requirement for AP-2 in the formation of the neural tube, body wall, and craniofacial skeleton. In addition, these studies reveal that AP-2 exerts a major influence on eye formation, which is a critical new role for AP-2 that was masked previously in the knockout mice. Furthermore, we also have uncovered an unexpected influence of AP-2 on limb pattern formation; this influence is typified by major limb duplications. The range of phenotypes observed in the chimeras displays a significant overlap with those caused by teratogenic levels of retinoic acid, strongly suggesting that AP-2 is an important component of the mechanism of action of this morphogen.

Epidermal Growth Factor Signaling and Mitogenesis in Plcg1 Null Mouse Embryonic Fibroblasts

Gene targeting techniques and early mouse embryos have been used to produce immortalized fibroblasts genetically deficient in phospholipase C (PLC)-γ1, a ubiquitous tyrosine kinase substrate. Plcg1−/− embryos die at embryonic day 9; however, cells derived from these embryos proliferate as well as cells from Plcg1+/+ embryos. The null cells do grow to a higher saturation density in serum-containing media, as their capacity to spread out is decreased compared with that of wild-type cells. In terms of epidermal growth factor receptor activation and internalization, or growth factor induction of mitogen-activated protein kinase, c-fos, or DNA synthesis in quiescent cells, PLcg1−/− cells respond equivalently to PLcg1+/+ cells. Also, null cells are able to migrate effectively in a wounded monolayer. Therefore, immortalized fibroblasts do not require PLC-γ1 for many responses to growth factors.

Primary Mesenchymal Cells Isolated from SPARC-null Mice Exhibit Altered Morphology and Rates of Proliferation

SPARC (secreted protein acidic and rich in cysteine)/BM 40/osteonectin is a matricellular protein shown to function as a counteradhesive factor that induces cell rounding and as an inhibitor of cell proliferation. These activities have been defined in cell culture, in which interpretation has been complicated by the presence of endogenous SPARC. We therefore sought to determine whether cell shape and proliferation would be affected by the absence of SPARC. Mesangial cells, fibroblasts, and aortic smooth muscle cells were isolated from SPARC-null and age-matched, wild-type mice. In contrast to wild-type cells, SPARC-null mesangial cells exhibited a flat morphology and an altered actin cytoskeleton. In addition, vinculin-containing focal adhesions were distributed over the center of SPARC-null cells, whereas in wild-type cells, the number of focal adhesions was reduced, and these structures were restricted largely to the cell periphery. Although the SPARC-null fibroblasts did not display overt differences in cell morphology, the cells responded to exogenous recombinant SPARC by rounding up in a manner similar to that of wild-type fibroblasts. Thus, the expression of endogenous SPARC is not required for the response of cells to SPARC. Additionally, SPARC-null mesangial cells, fibroblasts, and smooth muscle cells proliferated faster than their respective wild-type counterparts. Null cells also showed a greater sensitivity to the inhibition of cell cycle progression by the addition of recombinant SPARC. The increased proliferation rate of SPARC-null cells appeared to be mediated, at least in part, by an increase in the cell cycle regulatory protein cyclin A. We conclude that the expression of SPARC influences the cellular architecture of mesangial cells and that SPARC plays a role in the regulation of cell cycle in mesangial cells, fibroblasts, and smooth muscle cells.

Circadian rhythms in Neurospora crassa: Lipid deficiencies restore robust rhythmicity to null frequency and white-collar mutants

The conidiation rhythm in the fungus Neurospora crassa is a model system for investigating the genetics of circadian clocks. Null mutants at the frq (frequency) locus (frq9 and frq10) make no functional frq gene products and are arrhythmic under standard conditions. The white-collar strains (wc-1 and wc-2) are insensitive to most effects of light, and are also arrhythmic. All three genes are proposed to be central components of the circadian oscillator. We have been investigating two mutants, cel (chain-elongation) and chol-1 (choline-requirer), which are defective in lipid synthesis and affect the period and temperature compensation of the rhythm. We have constructed the double mutant strains chol-1 frq9, chol-1 frq10, chol-1 wc-1, chol-1 wc-2, cel frq9, cel frq10, and cel wc-2. We find that these double mutant strains are robustly rhythmic when assayed under lipid-deficient conditions, indicating that free-running rhythmicity does not require the frq, wc-1, or wc-2 gene products. The rhythms in the double mutant strains are similar to the cel and chol-1 parents, except that they are less sensitive to light. This suggests that the frq, wc-1, and wc-2 gene products may be components of a pathway that normally supplies input to a core oscillator to transduce light signals and sustain rhythmicity. This pathway can be bypassed when lipid metabolism is altered.

DGD1-independent biosynthesis of extraplastidic galactolipids after phosphate deprivation in Arabidopsis

The galactolipids, mono- and digalactosyldiacylglycerol (DGDG), are the most common nonphosphorous lipids in the biosphere and account for 80% of the membrane lipids found in green plant tissues. These lipids are major constituents of photosynthetic membranes (thylakoids), and a large body of evidence suggests that galactolipids are associated primarily with plastid membranes in seed plants. A null-mutant of Arabidopsis (dgd1), which lacks the DGDG synthase (DGD1) resulting in a 90% reduction in the amount of DGDG under normal growth conditions, accumulated DGDG after phosphate deprivation up to 60% of the amount present in the wild type. This observation suggests the existence of a DGD1-independent pathway of galactolipid biosynthesis. The fatty acid composition of the newly formed DGDG was distinct, showing an enrichment of 16-carbon fatty acids in the C-1 position of the glycerol backbone of DGDG. Roots with their rudimentary plastids accumulated large amounts of DGDG after phosphate deprivation, suggesting that this galactolipid may be located in extraplastidic membranes. Corroborating evidence for this hypothesis was obtained directly by fractionation of subcellular membranes from leaf tissue and indirectly by lipid analysis of the phosphate-deprived fad3 mutant primarily deficient in extraplastidic fatty acid desaturation. The discovery of extraplastidic DGDG biosynthesis induced by phosphate deprivation has revealed a biochemical mechanism for plants to conserve phosphate. Apparently, plants replace phospholipids with nonphosphorous galactolipids if environmental conditions such as phosphate deprivation require this for survival.