Notes on the attachment of zygotes and germlings of Bifurcaria bifurcata Ross (Phaeophyceae, Fucales)

Abstract

In a previous paper (Hardy & Moss, 1978) we have described the development of zygotes and germlings of Halidrys siliquosa (L.) Lyngb. (Fucales, Cysto­ seiraceae) and showed that a zygote wall is produced which attaches the young plant to the substratum until such time as the rhizoids develop. A similar study was carried out using the closely-related species Bifurearia bi/ureata Ross and similar results were obtained. Fertile material of B. bijureata was collected from rock pools at Blackstone Rocks, Wembury, Devon, England, and Galway Bay, County Galway, Ireland, between November and March. The material from Ireland was collected for us by Dr C. Pybus and posted to Newcastle immediately after collection. The zygotes and germlings were cultured and prepared for examination with light microscopy, SEM and TEM in the same way as those of H. siliquosa (Hardy & Moss, 1978). Spherical oospheres are produced which, after fertilization, settle and attach on the substratum. Zygotes settling on smooth substrata, such as glass, become flattened so that a large surface area is in contact with this surface. The degree of moulding to the shape of the substratum seemed to be greater than that observed in Halidrys (Hardy & Moss, 1978). Settling on rough substrata the zygotes are more or less spherical and their base fits into the contours of the substratum. Strands of mucilage appear to help anchor them in position. A rigid zygote wall is produced soon after fertilization as in H. sili­ quosa. The zygote wall of Bijurearia had an average thickness of only 0·37 fLm 18 h after fertilization whereas that in Halidrys measured 1·33 fLm after only 12 h, and 2·55 fLm after 24 h (Hardy 1977). The first cell division occurs at approximately right angles to the substratum and this is followed by a second set of divisions at right angles to the first so that a four-celled germling is produced which does not have any obvious polarity of differentiation. The cell walls laid down around the zygote, and internally as a result of these divisions, stain predominantly blue with alcian dyes (Parker & Diboll, 1966) indi­ cating the presence of sulphated polysaccharides, most probably fucoidan. Occasionally traces of yellow staining are visible which might indicate that the walls have a framework of an acidic polysac­ charide such as alginate. The mucilage around the outside of these germlings stains blue with alcian dyes. These staining reactions obtained for Bijurearia by light microscopy with alcian dyes correspond to those obtained for Halidrys (Hardy & Moss, 1978). As in Halidrys four primary rhizoids differentiate from the multicellular germling. In living, unstained, material these rhizoids appear colourless compared with the brown thallus, indicating that they contain fewer plastids or less pigment. The zygote wall being rigid, acts as a physical barrier to the continuing development of the germling and, as they develop, the rhizoid cells break through leaving jagged edges around the ruptured zygote wall. Transverse sections of the thallus of germlings at this stage revealed a multicellular structure surrounded by the zygote wall and its associated mucilages. Once the rhizoids have been produced any function that the zygote wall may have had in attachment becomes obsolete as these structures will anchor the germling firmly to the substratum. Consequently the zygote wall is shed from the thallus in the form of a rigid cap in exactly the same way as that of Halidrys. The rhizoids grow across the substratum duplicat­ ing the surface morphology and penetrating into any pores or cavities in the substratum. The continued production of these rhizoids results in the develop­ ment of the attaching holdfast. In the adult plant this attaching organ is unusual among those of the Fucales in being a richly branched rhizomatous structure with small insignificant adhesive discs rather than a conical discoid structure. It is apparent that early development in B. bij ureata is very similar to that observed in H. siliquosa. The function of the elaborate zygote wall