msi1 and msi1/msi2 mutant testes exhibit fewer brightly stained cells at the apex of the testis, indicative of a loss of early germ cells. msi1 mutant testis also exhibit a swelling of the apical region in ~90% of testes analyzed.
msi1 mutant third-instar larvae exhibit a hub in 97% of testes analysed and exhibit milder morphological defects than those observed in pharate adults.
In msi1 mutants, there are on average 3.9 germ stem cells per testis, compared to 8.4 per testis in wild-type.
Germline stem cell clones homozygous for msi1 are found at the same frequency as control clones 2 days after clone induction. Although wild-type clones are maintained at similar levels over time, msi1 stem cell clones are not maintained at the same frequency. At 8 days after induction, msi1 mutant clones are observed significantly less frequently than control wild-type germ line stem cells of the same age.
msi1 spermatogonia clones are found in 47% of testes 5 days after clone induction, this is a similar number to that found in control clones.
Acridine orange staining does not reveal the presence of any excess dying cells in msi1 mutant clones. In 15% of cases, msi1 spermatogonial clones are found 8 days after induction in the absence of msi1 germline stem cells, indicating that msi1 germline stem cells can differentiate into spermatogonial cells without regeneration of germline stem cells.
In msi1 mutants, early round spermatids commonly contain two or four nuclei and a large mitochondrial derivative, indicating that one or both meiotic cytokinetic divisions have failed. These mutants often display haploid nuclei of different sizes, resulting from errors in chromosome segregation during meiosis.
Mutants show a double bristle phenotype.
Homozygotes show a low penetrance phenotype of abnormal ommatidia with deformed rhabdomeres and abnormal ommatidial orientation. The number of photoreceptor cells is normal, in affected ommatidia.
Lethality occurs during late pupal and early adult stages. The total number of sensilla is similar to wild type but the number of outer support cells (sockets and shafts) is variable. Microchaetae on the notum have variable phenotypes, some having up to 4 sockets and no shaft. The presence of extra support cells is correlated to the absence of neurons. Macrochaetae also display variability, the most frequent phenotype is two shafts and one socket. Macrochaetae with additional support cells frequently had one or more neurons but did not typically contain glial cells. Anterior wing margin has extra support cells in the singly innervated bristles and multiple innervated bristles and extra outer support cells in the twin campaniform sensilla.