Sexual Recognition and Fertilization in Brown Algae

Abstract

SUMMARY Fertilization in the brown marine algae known as fucoids, is oogamous. The naked egg cell (80 μm diam.) is fertilized by small biflagellate spermatozoids and both monoecious a d dioecious species are found. Fertilization is highly species-specific and this appears to be controlled during plasmogamy. Following fusion of egg and sperm, a rapid (<1 min) release of polyuronide cell wall material takes place from cytoplasmic vesicles within the egg. This is easily visualized using the fluorescent brightener Calcofluor, which therefore provides the basis of a quantitative fertilization bioassay. It has not proved possible to measure direct sperm binding to eggs. In experiments to investigate the molecular basis of egg–sperm recognition, the effect of exogenous agents on the initial rate of fertilization was examined. Predigestion of eggs with low concentrations of two glycosidases, α-fucosidase and α-mannosidase, caused inhibition of fertilization. The lectins concanavalin A and RCA120 bound strongly to egg surfaces, as detected using fluorescent labels, but not to sperm. The binding to eggs inhibited fertilization. On the other hand, Fucose Binding Protein bound only weakly to eggs but strongly to sperm, again causing inhibition of fertilization. It has not proved possible to quantitate lectin binding since high levels of lectin-non-specific binding were detected using iodinated lectins. These inhibition experiments suggest that specific sugar residues may be involved in egg–sperm recognition, but the effects of lectins must be treated with caution since a large amount of variability in the sensitivity of gametes was detected. Attempts to isolate receptor fractions from egg cells have been partially successful. Egg membrane preparations bind sperm and sodium dodecyl sulphate-solubilized fractions, purified by concanavalin A affinity chromatography, have yielded low levels of a soluble receptor-like fraction that has not yet been fully characterized. Antisera raised against surface antigens of Fucus serratus sperm flagella, cause inhibition of fertilization in a species-specific manner, possibly by binding directly to the sperm fertilization receptor. A number of flagellar antigens were detected and future attempts to pinpoint the sperm receptor will make use of monoclonal antibodies.