Homozygous clones in the brains of mutant third instar larvae show axon fasciculation defects.
sim2 mutants are characterized by the absence of all midline cells after the migration of only a few peripheral glial cells.
The longitudinal axon tracts are collapsed in mutant embryos. Peripheral glial cells show migration defects and stall in the central nervous system and exit area, resulting in fewer cells distal to the SNc than normal.
In sim2 mutant embryos the number of medial and lateral glia in each segment is normal until late stage 11, but is severely decreased at stage 13. Symmetric clusters of lateral glia take up positions closer to the ventral midline than in the wildtype. The numbers of lateral peripheral glia is reduced by 60%.
In sim2 mutant embryos, a single longitudinal glioblast is present in each hemisegment at stage 11, but a short time later only 32% of hemisegments show 2-3 divided longitudinal glia, instead of the 4 observed in wild-type. At stage 13, the longitudinal glia are disconnected in some segments and fused at the midline.
In sim2 mutant embryos, A and B glia are absent in 90% of hemisegments starting from stage 13-15.
In sim2 mutant embryos, initial formation of the eg-positive neuroglioblasts and neuroblasts occurs normally at stage 10. But at stage 11 and 12, only 2 medial cell body glia are generated instead of 3. At stage 13, eg-positive medial cell body glia and neurons are reduced in number by 20%.
In sim2 mutant embryos, exit glia are absent from many segments at stage 15. The remaining exit glia are not properly localised at the junction of the segmental and intersegmental nerves. Peripheral glia are absent from 38% of hemisegments, and some peripheral glia do not move properly along the motor and sensory axon pathways at stage 16.
There is a reduction in the number of mitotic lateral glia by 51% in sim2 mutant embryos compared to wild-type.
In sim2/simry75 mutant larval brains, more developing lamina neurons remain in the pre-assembling domain than normal and R axons that project to the smaller assembling domain contain much fewer than the normal number of lamina neurons. The mutant brains are comparable to control brains with regard to the number of differentiated ommatidia and the total number of lamina neurons.
The ventral nerve cord fails to fully condense in sim2 mutant embryos. This phenotype is fully penetrant.
Normal separation of primordial germ cells into lateral clusters on each side of the embryo is observed in homozygous and heterozygous sim2.
Primordial germ cells that remain over the midline are eliminated in sim2 mutants, as in wild-type.
Mutants exhibit a loss of midbrain.
ventral midline cells do not develop properly in mutant embryos.
All central nervous system axons collapse at the midline in mutant embryos. The invagination of the genital disc anlage starts normally in mutant embryos, but by stage 16, the presumptive disc cells are found in a large ectodermal fold at the posterior end of the embryo instead of being invaginated into the interior. simJ1-47/sim2 embryos raised at 17oC show a mild fused commissure phenotype in the ventral nerve cord, while at 29oC they show a complete collapse of the nervous system. Homozygous larvae have smaller anal pads than wild type and no anal slit can be detected. The anal slit is shortened in simJ1-47/sim2 larvae raised at the permissive temperature (17oC), while at the restrictive temperature (29oC) the anal slit is completely missing. At 29oC, simJ1-47/sim2 adults are not recovered. At 17oC, rare escapers (0.1% of the expected number of flies) are seen. These flies are sterile and frequently lack external genitalia and the anal plate. Of flies without external defects, simJ1-47/sim2 males do not perform normal courtship behaviour and simJ1-47/sim2 females ignore wild-type males. Most simJ1-47/sim2 adults only walk in circles and they show abnormal responses in Buridan's paradigm; they show a markedly reduced walking speed, the activity period is usually shorter and they show no measurable influence from visual landmarks on their orientation behaviour. Optomotor tests indicate that the flies are not entirely blind. Leg movement appears to be coordinated normally, although step length differs between the right and left body side. The flies show defects associated with the inner chiasm of the optic lobe and in the central complex. In the central complex, the protocerebral bridge is thinner and less compact than normal in 70% of brains and the fan-shaped body is divided posterior-sagittally in 15% of brains. Ectopic fibre bundles cut into the lobula either unilaterally or bilaterally in 40% of optic lobes.
Homozygous mutant exhibit a collapsed axon phenotype in the embryonic central nervous system.
The GMC-1 in mutants frequently (in about 9% of hemisegments) divide symmetrically to generate two RP2s instead of an RP2 and a sib.
Homozygous mutant embryos exhibit defective mitotic cell division in the ventral neurectoderm. CycE expressing neuroblasts at stage 11 are disorganised. throughout neurogenesis, mitotic cells are much reduced both in the midline and the ventral neurectoderm. Mitotic cells are absent in 56% of hemisegments at stage 9 and in 82% in stage 10. TUNEL analysis shows that there is a about a two fold increase in apoptotic cell death in mutant embryos, but this is not sufficient to account for the entire sim phenotype.
Muscle founder cell specification does occur in sim2 embryos, but the cells are present in 11 segmentally-repeated groups that are clustered along the midline.
Midline progenitor cells fail to divide and do not migrate in the CNS layer where they normally differentiate into neurons and glia. Removal of midline cells (using a capillary tube shortly after gastrulation) from wild type embryos phenocopies the sim mutant phenotype, indicating that the midline cells are required for the formation of the dorsal medial cells. Transplantation of wild type midline progenitors into sim mutant hosts leads to a local rescue of the phenotype by inducing the formation of dorsal medial cells.
Most midline cells die (mutants are lacking about 20 midline cells per neuromere) and correspondingly the CNS cortex is thin. Longitudinal connectives are collapsed at the midline and no commissures can be detected. The size of the central connective is reduced. Heat shock reduces the average number of cells per neuromere from about 490 cells (wild type level) to 411 cells: mutants lack about 15% of the normal complement of lateral ventral cord cells. Scer\GAL4sim.PS-mediated expression allows partial rescue of the sim2 mutant axon pattern phenotype. Transplantation of wild type midline cells also leads to partial rescue of the sim phenotype and commissures develop.
Mutant embryos do not show a tracheal phenotype.
In stage 11 embryos the dorsal median cells are completely absent.
Wild type midline cells heterotypically transplanted into a sim2 mutant background are unable to migrate to the midline, instead becoming part of the lateral CNS cortex.
Homozygous embryos exhibit a lack of migrating macrophages along the ventral midline.
Mutation has no effect on rhohs.sev rough eye phenotype.
Embryonic ventral muscles are shifted ventrally and some span the midline due to mislocalisation of ventral muscle precursor cells. Dorsolateral embryonic muscles appear normal.
Strong CNS phenotype. All or nearly all of the ventral epidermal cells are absent in mutant embryos.
Salivary placodes are fused at the ventral midline to yield one large placode.
Absence of commissures and the collapse of the longitudinal tracts into a single fused tract at the midline. The midline precursor cells divide at least once but die during germband retraction.
Midline cells fail to undergo a characteristic synchronised cell division, they do not migrate into the nerve cell precursor layer but remain along the ventral epidermis and they retain the nonpolarised, rounded shape of blastoderm neuroectodermal cells.
The presence of Ecol\lacZsim.7.8 construct fails to rescue lethality or the CNS phenotype. The construct shows that the midline cells are disorganised compared to wild type at stage 11 and at stage 13 are clustered near the ventral surface of the embryo leading to the collapse of the two lateral CNS hemiganglia and fusion of the longitudinal connectives.
CNS defect: specific alterations in the pattern of precursors that give rise to the 2 MP1 progeny, ventral unpaired median neurons and the specialized ectodermal cells, the midline ectodermal cells (MEC).