Shedding of the zygote wall by Durvillaea potatorum (Durvillaeales, Phaeophyta) embryos

Abstract

The zygote wall in developing Durvillaea potatorum embryos is secreted soon after fertilization. Mucilage covers the surface of the zygote wall ensuring rapid adhesion of the newly fertilized egg. The zygote wall is replaced by new cell walls secreted by growing, dividing embryonic cells. Separation of the zygote wall from the embryo is evident 4 days after fertilization and is visible at the edge of the cap flanking the side of the developing embryo and/or at the curved part of the cap where it lifts away from the top of the embryo. At this stage the embryo has undergone an average of 8 cell divisions, increased 3 times in length, and started to increase in width. The zygote wall is eventually shed as it cannot accommodate embryo expansion. The growing embryo flicks out of the rigid zygote wall, which usually remains connected to the developing rhizoid by a mucilaginous strand. By day 5 the zygote wall has detached from the embryo in 22% of individuals and this increases to nearly 70% of individuals by day 10. Removal of alginate-gelling cations from the artificial sea medium indicates that the zygote wall has different properties from the underlying embryonic cell walls. Shedding of the zygote wall allows continued embryo growth, frees the embryonic thallus from the substratum and cleans the embryo surface of phenolic bodies and mucilage that are secreted soon after fertilization; it also removes micro-organisms colonizing the mucilage.