Isolation of Fucus serratus Gametes and Cultivation of the Zygotes.

Zygotes of the Fucale species are a powerful model system to study cell polarization and asymmetrical cell division (Bisgrove and Kropf, 2008). The Fucale species of brown algae grow in the intertidal zone where they reproduce by releasing large female eggs and mobile sperm in the surrounding seawater. The gamete release can be induced from sexually mature fronds in the laboratory and thousands of synchronously developing zygotes are easily obtained. In contrast to other eukaryotic models, such as land plants (Brownlee and Berger, 1995), the embryo is free of maternal tissues and therefore readily amenable to pharmacological approaches. The zygotes are relatively large (up to 100 µm in diameter), facilitating manipulations and imaging studies. During the first hours of zygote development, the alignment of the axis to external cues such as light is labile and can be reversed by light gradients from different directions. A few hours before rhizoid emergence, the alignment of the axis and the polarity are fixed and the cells germinate accordingly. At this stage the zygotes are naturally attached to the substratum through the secretion of cell wall adhesive materials ( Kropf et al., 1988 ; Hervé et al., 2016 ). The first cell division occurs about 24 h after fertilisation and the early embryo is composed of only two cell types that differ in size, shape and developmental fates (i.e., thallus cells and rhizoid cells) ( Bouget et al., 1998 ). The embryo can be successfully cultivated in the laboratory for a few more days (4 weeks maximum) and has an invariant division pattern during the early stages, which allows cell lineages to be traced histologically.