The role of egg flagella in the settlement and development of zygotes in two Saccharina species

ABSTRACT

The settlement of zygotes in the right place is the most critical step for the survival of young kelp sporophytes in the marine environment. The eggs of Saccharina species were connected to oogonia with two flagella projected from its basal part, and this connection lasted even after fertilisation and first cell division of the zygote. Transmission electron microscopy showed that the egg flagella have a unique structure and arrangement of basal bodies different from those of other flagella appearing throughout the life cycle. Fluorescein isothiocyanate (FITC)-lectin labelling showed that a significant modification of carbohydrate composition occurred at the surface of the zygote after fertilisation. When the eggs and zygotes were artificially detached from the oogonia at early developmental stage, most sporophytes failed to attach to a substrate and remained in two- to three-cell stage even in 15 days after fertilisation; whereas, fluorescein diacetate (FDA) staining showed that most zygotes (> 95%) detached from the oogonia were still alive and had intact cell membranes. On the contrary, zygotes connected to the oogonia developed into multicellular blades composed of hundreds of cells 15 days after fertilisation with elongated rhizoids extending from the basal portion of the blade. These results suggest that the egg flagella ensure close contact and orientation of the zygote to the oogonial wall, and later binding between zygote and oogonium occurs by means of glycoconjugates. These consecutive events appear to be essential for the survival and development of young sporophytes in Saccharina species.

KEYWORDS:

• Anchor flagella
• Egg flagella
• Kelp
• Lectin
• Oogonium
• Saccharina

Additional information

Funding

We are grateful to funding from Hokkaido University for supporting stay of T.A. Klochkova and G.H. Kim in Muroran Marine Station as visiting Professors. This research was also partially supported by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, and Forestry (IPET) through Golden Seed Project (213008-05-3-SB810), funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA), Ministry of Oceans and Fisheries (MOF), Rural Development Administration (RDA) and Korea Forest Services (KFS), and supported by the Marine Biotechnology Program of the Korea Institute of Marine Science and Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (MOF) (No. 20170431).