Spermatid elongation in park25
mutants follows the usual pattern observed in wild-type testis. The overall length of the fully elongated spermatid cysts varies from 1.65 to 2.03mm in park25
mutants and 1.85 to 2.13mm in control flies. In testes from park25
mutants, not all the spermatid nuclei are aligned in register; some of them are mispositioned along the cyst length. The investment cones (ICs) are located in position distant from the nuclear region, indicating that they have translted along the spermatid bundles. However, they are not aligned as in wild-tpye, but are scattered along the sperm tail, suggesting that they do not move in synchrony. The individual ICs are cone-shaped as in wild-type. Normally, their apex points toward the apical region of the cyst, where the nuclei are aligned, and their basis is toward the tips of the tails. Among the fully elongated spermatid cysts examined, 19% of them display 9-10% of misoriented ICs. 'Waste bags' in park25
testes are not distinct as in wild-type, suggesting that the process of individualisation is not properly executed in these mutants. This is confirmed by the finding of irregularly coiled distal ends of the spermatid bundles.
Mitochondria aggregate in a loose cluster around the nuclei in both wild-type and mutant spermatids at the end of meiosis. No significant differences in nebenkern morphology are observed in young wild-type or mutant spermatids. During earlier stages of elongation, the round wild-type spermatids still show a spherical nuclues, but the nebenkern starts to elongate and a distinct paried structure is readily observed. park25
mutant spermatids at the same stage show an elongated nebenkern that does not resolve into two parts. While mitochondria form concentric ring-like structures in wild-type nebenkern, their arrangement is more iiregular in mutant spermatocytes. The park25
nebenkern is, indeed, formed by a large portion of rather ring-like concentric mitochondria, and a small portion of loose peripheral mitochondria, that do not seem to integrate into the tightly packed main cluster. The concentric mitochondria, moreover, appear more electron-dense in mutant than in wild-type spermatids.
Cross-sections through park25
spermatocytes reveal only one large mitochondrion associated with the growing flagellum during axoneme elongation, while in wild-type there are two mitochondrial derivatives.
Sections through park25
mutant spermatid bundles that appear slightly elongated (as judged by the hooked peripheral arms growing by the B-subfiber), show both ring-like and spherical mitochondria. This suggests that park25
mitochondria undergo compaction as the spermatid elongates.
At a comparable developmental stage, the pre-individualization park25
spermatids maintain some cytoplasmic ground substance and are connected by intercellular bridges. The spermatids usually contain a single irregularly shaped and sized mitochondrion surrounded by disorganised sheaths of microtubules. These microtubules are scattered in the spermatid cytoplasm and occasionally localised near the axoneme. The organisation of the axonemal microtubules is normal, whereas the orientation of the mitochondrial derivatives varies within the same cyst. Mitochondrial condensation is highly defective: some mitochondria are filled with dense material in the region adjacent to the axoneme, whereas others do not show appreciable signs of condensation or the condensation is barely detectable. Closely paired axonemes are also found in these cysts, whereas wild-type cysts contain evenly spaced axonemes.
Towards the distal region of post-elongating wild-type spermatid bundles, the size of the mitochondrial derivatives decreases. In the same stage in park25
mutant speramtids, one mitochondrion alone, abnormally shaped and of varying size, is usually found. Mitochondrial condensation is also defective. Mitochondria are found filled with large dark masses, along with pale mitochondria with evident cristae remnants in the same cyst.
Sections of mutant spermatids that appear quite mature on the basis of axonemal decoration reveal that only a few spermatids are apparently cellularized. Most of the spermatids are immersed in a large amount of cytoplasmic ground substance, and their mitochondrial derivatives appear irregularly condensed and abnormally sized.