The ultrastructure of reproduction in the brown algaPylaiella littoralis

Summary

The ultrastructure of zoosporogenesis in unilocular sporangia ofPylaiella littoralis is described. Zoosporogenesis begins with the formation of spore mother cells from intercalary vegetative cells. A meiotic division then occurs in each spore mother cell, which is usually not synchronous in each member of a spore mother cell pair. Centrioles replicate at 1 pole of the nucleus during the meiotic division, and during centriole migration, Golgi bodies and microtubules are present near the centrioles. Synaptonemal complexes have been observed in spore mother cells. Following meiosis, four nuclei are present in each sporangium and a series of mitotic divisions takes place as the sporangium enlarges. At the beginning of the mitotic period nuclei and plastids migrate to the sporangium periphery and vacuoles and vesicles gather in the middle of the sporangium. Eventually nuclei and plastids are redistributed throughout the sporangium and the mitotic divisions continue. Centriole behavior during the early stages of mitosis is similar to that which occurs in meiosis. Microtubules end near the centrioles often in amorphous material. The formation of flagella and mastigonemes occurs after the end of the mitotic period. Flagella form within the cytoplasm in vesicles, the two flagella developing in a slightly different manner. Flagella show various degrees of enclosure within the cytoplasm, this probably being related to their development. Mastigoneme formation occurs in ER cisternae and Golgi cisternae but mastigonemes were not observed in the perinuclear continuum, although it is believed this stage does occur but was overlooked. Mastigonemes are often tightly packed together in the flagellar vesicle and occur over about half of the flagellar circumference at this time. During the time of flagellum and mastigoneme formation the mass of cytoplasm in the sporangium shrinks away from the cell wall and cleavage divides up the cytoplasm into individual zoospores. During later stages of zoosporogenesis, characteristic vesicles, previously rare in the cytoplasm, develop in large numbers. Also at this time mitochondrial cristae contain tubular structures 5–10 mμ in diameter. Zoospores, when mature, are released through an aperture in the cell wall. The inner half of the cell wall of a sporangium undergoing release is partially broken down. The remnants of the inner half of the wall surround the zoospores as they are released. Flagellum and mastigoneme formation inPylaiella are discussed in detail and compared to other organisms. Zoospore release is also discussed.