The sexual reproduction and auxospore formation of Centrales had been misinterpreted for a long time. It was believed until recently that auxospore formation had no connection with sexual reproduction. More recently, however, STOSCH
2) demonstrated oogamy clearly in
Melosira varians for the first time, and since then STOSCH
4-8) and GEITLER
9, 10) studied the sexual reproduction in many marine Centrales.
In this work, the author observed the sexual reproduction of
Skeletonema costatum in axenic culture, and the following results were obtained.
1. When the narrow filamentous colonies of
Skeletonema less than 5-6
μ in breadth were cultured at the temperature of 17-23°C, formation of spermatogonia and oogonia was observed.
2. The spermatogonia are produced after 1 or 2 cell-divisions of vegetative cells, and four spermatozoa are formed in each spermatogonium. The spermatozoa are globular in shape, 3-3.5
μ in diameter, poor in plastids, and they each have one flagellum being 7-10
μ in length.
3. The oogonia are produced from vegetative cells by extension of the cells, and one egg cell is formed in each oogonium.
4. After fertilization, a zygote develops into an auxospore without any resting stage. The fully grown auxospore is generally about three times as large in diameter as the mother cell.
5. The spermatogonia and oogonia are produced in the same clone, and accordingly this species is monoecious.
6. The auxospore formation is found more abundantly at the temperature of 20°C than 15 or 25°C. The sex determination in this species is influenced by light intensity. Clones of vegetative cells produce female cells under strong light, but they produce only male cells under weak light.
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