Environmentally induced development defects in Drosophila

A theoretical perspective on Waddington's genetic assimilation experiments

Genetic assimilation is the process by which a phenotype that is initially induced by an environmental stimulus becomes stably inherited in the absence of the stimulus after a few generations of selection. While the concept has attracted much debate after being introduced by C. H. Waddington 70 y ago, there have been few experiments to quantitatively characterize the phenomenon. Here, we revisit and organize the results of Waddington's original experiments and follow-up studies that attempted to replicate his results. We then present a theoretical model to illustrate the process of genetic assimilation and highlight several aspects that we think require further quantitative studies, including the gradual increase of penetrance, the statistics of delay in assimilation, and the frequency of unviability during selection. Our model captures Waddington's picture of developmental paths in a canalized landscape using a stochastic dynamical system with alternative trajectories that can be controlled by either external signals or internal variables. It also reconciles two descriptions of the phenomenon-Waddington's, expressed in terms of an individual organism's developmental paths, and that of Bateman in terms of the population distribution crossing a hypothetical threshold. Our results provide theoretical insight into the concepts of canalization, phenotypic plasticity, and genetic assimilation.

Pathway to a phenocopy: Heat stress effects in early embryogenesis

BACKGROUND

Heat shocks applied at the onset of gastrulation in early Drosophila embryos frequently lead to phenocopies of U-shaped mutants-having characteristic failures in the late morphogenetic processes of germband retraction and dorsal closure. The pathway from nonspecific heat stress to phenocopied abnormalities is unknown.

RESULTS

Drosophila embryos subjected to 30-min, 38 °C heat shocks at gastrulation appear to recover and restart morphogenesis. Post-heat-shock development appears normal, albeit slower, until a large fraction of embryos develop amnioserosa holes (diameters > 100 µm). These holes are positively correlated with terminal U-shaped phenocopies. They initiate between amnioserosa cells and open over tens of minutes by evading normal wound healing responses. They are not caused by tissue-wide increases in mechanical stress or decreases in cell-cell adhesion, but instead appear to initiate from isolated apoptosis of amnioserosa cells.

CONCLUSIONS

The pathway from heat shock to U-shaped phenocopies involves the opening of one or more large holes in the amnioserosa that compromise its structural integrity and lead to failures in morphogenetic processes that rely on amnioserosa-generated tensile forces. The proposed mechanism by which heat shock leads to hole initiation and expansion is heterochonicity, i.e., disruption of morphogenetic coordination between embryonic and extra-embryonic cell types.

Dropping like Flies: Environmentally Induced Impairment and Protection of Locomotor Performance in AdultDrosophila melanogaster

In Drosophila, heat shock (HS) during the pupal stage chronically hinders adult locomotor performance by disrupting wing development and cellular and/or tissue-level mechanisms that support walking and flight. Furthermore, heat pretreatment (PT) protects locomotor function against these disruptions. HS flies with abnormal wings were less able to alter trajectory in free fall relative to control, PT-only, and PT+HS wild-type flies. This deficit was less severe but still present in HS-only flies with wild-type wings. Transgenic increases in the copies of genes encoding the major inducible heat-shock protein of Drosophila melanogaster, Hsp70, also protected walking ability from disruption due to pupal HS. Walking velocity did not differ between excision (five natural hsp70 copies) and extra-copy (five natural and six transgenic hsp70 copies) flies in the control, PT, and PT+HS groups, nor did velocity vary among these thermal treatment groups. HS dramatically reduced walking velocity, however, but this effect occurred primarily in the excision flies. These results suggest that Hsp70 and other mechanisms protect against heat-induced locomotor impairment.

Effect of heat shock, pretreatment and hsp70 copy number on wing development in Drosophila melanogaster

Naturally occurring heat shock (HS) during pupation induces abnormal wing development in Drosophila; we examined factors affecting the severity of this induction. The proportion of HS-surviving adults with abnormal wings varied with HS duration and intensity, and with the pupal age or stage at HS administration. Pretreatment (PT), mild hyperthermia delivered before HS, usually protected development against HS. Gradual heating resembling natural thermal regimes also protected wing development against thermal disruption. Because of the roles of the wings in flight and courtship and in view of natural thermal regimes that Drosophila experience, both HS-induction of wing abnormalities and its abatement by PT may have marked effects on Drosophila fitness in nature. Because PT is associated with expression of heat-inducible molecular chaperones such as Hsp70 in Drosophila, we compared thermal disruption of wing development among hsp70 mutants as well as among strains naturally varying in Hsp70 levels. Contrary to expectations, lines or strains with increased Hsp70 levels were no more resistant to HS-disruption of wing development than counterparts with lower Hsp70 levels. In fact, wing development was more resistant to HS in hsp70 deletion strains than control strains. We suggest that, while high Hsp70 levels may aid cells in surviving hyperthermia, high levels may also overly stimulate or inhibit numerous signalling pathways involved in cell proliferation, maturation and programmed death, resulting in developmental failure.

Targeted gene expression without a tissue-specific promoter: creating mosaic embryos using laser-induced single-cell heat shock

We have developed a method to target gene expression in the Drosophila embryo to a specific cell without having a promoter that directs expression in that particular cell. Using a digitally enhanced imaging system to identify single cells within the living embryo, we apply a heat shock to each cell individually by using a laser microbeam. A 1- to 2-min laser treatment is sufficient to induce a heat-shock response but is not lethal to the heat-shocked cells. Induction of heat shock was measured in a variety of cell types, including neurons and somatic muscles, by the expression of beta-galactosidase from an hsp26-lacZ reporter construct or by expression of a UAS target gene after induction of hsGAL4. We discuss the applicability of this technique to ectopic gene expression studies, lineage tracing, gene inactivation studies, and studies of cells in vitro. Laser heat shock is a versatile technique that can be adapted for use in a variety of research organisms and is useful for any studies in which it is desirable to express a given gene in only a distinct cell or clone of cells, either transiently or constitutively, at a time point of choice.

Preferential deadenylation of Hsp70 mRNA plays a key role in regulating Hsp70 expression in Drosophila melanogaster

Following a standard heat shock, approximately 40% of Hsp70 transcripts in Drosophila melanogaster lack a poly(A) tail. Since heat shock disrupts other aspects of RNA processing, this observation suggested that heat might disrupt polyadenylation as well. We find, however, that as the temperature is increased a larger fraction of Hsp70 RNA is polyadenylated. Poly(A)-deficient Hsp70 RNAs arise not from a failure in polyadenylation but from the rapid and selective removal of poly(A) from previously adenylated transcripts. Poly(A) removal is highly regulated: poly(A) is (i) removed much more rapidly from Hsp70 RNAs than from Hsp23 RNAs, (ii) removed more rapidly after mild heat shocks than after severe heat shocks, and (iii) removed more rapidly after a severe heat shock if cells have first been conditioned by a mild heat treatment. Poly(A) seems to be removed by simple deadenylation rather than by endonucleolytic cleavage 5' of the adenylation site. During recovery from heat shock, deadenylation is rapidly followed by degradation. In cells maintained at high temperatures, however, the two processes are uncoupled and Hsp70 RNAs are deadenylated without being degraded. These deadenylated mRNAs are translated with low efficiency. Deadenylation therefore allows Hsp70 synthesis to be repressed even when degradation of the mRNA is blocked. Poly(A) tail shortening appears to play a key role in regulating Hsp70 expression.

Preferential deadenylation of Hsp70 mRNA plays a key role in regulating Hsp70 expression in Drosophila melanogaster

Following a standard heat shock, approximately 40% of Hsp70 transcripts in Drosophila melanogaster lack a poly(A) tail. Since heat shock disrupts other aspects of RNA processing, this observation suggested that heat might disrupt polyadenylation as well. We find, however, that as the temperature is increased a larger fraction of Hsp70 RNA is polyadenylated. Poly(A)-deficient Hsp70 RNAs arise not from a failure in polyadenylation but from the rapid and selective removal of poly(A) from previously adenylated transcripts. Poly(A) removal is highly regulated: poly(A) is (i) removed much more rapidly from Hsp70 RNAs than from Hsp23 RNAs, (ii) removed more rapidly after mild heat shocks than after severe heat shocks, and (iii) removed more rapidly after a severe heat shock if cells have first been conditioned by a mild heat treatment. Poly(A) seems to be removed by simple deadenylation rather than by endonucleolytic cleavage 5' of the adenylation site. During recovery from heat shock, deadenylation is rapidly followed by degradation. In cells maintained at high temperatures, however, the two processes are uncoupled and Hsp70 RNAs are deadenylated without being degraded. These deadenylated mRNAs are translated with low efficiency. Deadenylation therefore allows Hsp70 synthesis to be repressed even when degradation of the mRNA is blocked. Poly(A) tail shortening appears to play a key role in regulating Hsp70 expression.

Heat-shock gene expression and cell cycle changes during mammalian embryonic development

Synchronized regulation of cell division during gastrulation is essential for the regional proliferation of cells and pattern formation of the early CNS. The neural plate and neuroectoderm cells are a rapidly dividing and differentiating population of cells with a unique and rapid heat-shock response. Heat shock and the heat-shock genes were studied during neural plate development in a whole rat embryo culture system at 9.5-11.5 days. A lethal shock can cause cell death and severe developmental defects to the forebrain and eye during organogenesis. Heat shock can also result in acquired thermotolerance whereby cell progression is delayed at the G1/S and S/G2 boundaries of the cell cycle. This delay in cell cycle progression caused an overall lengthening of the cell cycle time of at least 2 hr. The heat shock genes may therefore function as cell cycle regulators in neuroectoderm induction and differentiation. The kinetics and expression of the hsp genes were examined in neuroectodermal cells by flow cytometry and Northern analysis. The levels of hsp mRNA 27, 71, 73, and 88 were identified following exposure at 42 degrees C (nonlethal), 43 degrees C (lethal) and 42 degrees/43 degrees C (thermotolerant) heat shock. Examination of hsp gene expression in the neural plate showed tight regulation in the cell cycle phases. Hsp 88 expression was enhanced at Go and hsp71 induction at G2 + M of the cell cycle. Cells exposed to a thermotolerant heat shock of 42 degrees C induced hsp71 mRNA expression in all phases of the cell cycle with the mRNA levels of hsp27, 73, and 88 increased but relatively constant. Following a lethal heat shock, dramatic changes in hsp expression were seen especially enhanced hsp71 induction in late S phase. The regulated expression of hsps during the cell cycle at various phases could play a unique and important role in the fate and recovery of neuroectoderm cells during early mammalian embryo development.