Quantitative data are available by placing droplets of an inert solvent with known concentrations of the test substance into close vicinity of the fertile gynogametophyte. For example, the mass release of male gametes of Laminaria digitata occurs within 8-12 sec at a threshold of ˜50 pmol of lamoxirene (8, 23, 24), and male gametes of Perithalia caudata are released within ˜10 sec, triggered by caudoxirene at concentrations down to 30 pmol (25).
Considering the large number of plant species and the limited number of only 11 different pheromones, it becomes obvious that these signals cannot be specific at the level of the species or even the genus. Moreover, ectocarpene, hormosirene, and dictyotene are typically present in most of the pheromone blends, and, hence, their presence may reflect nothing but a phylogenetic reminiscence of a biosynthetic pathway that converts a whole array of appropriate precursor molecules (see below) into olefinic hydrocarbons. The quantitative determination of the released volatiles from signaling females shows that females of E. siliculosus (26) release within 1 h ˜0.6 fmol of ectocarpene per individual; ˜75 fmol per h per egg of hormosirene was reported (27) as the initial rate of the secretory capacity of individual females of Hormosira banksii.
Much larger amounts of volatiles are present in thalli of several members of the genus Dictyopteris, but their production is obviously not linked to sexual events (ref. 28 and references cited therein). Interestingly, Dictyopteris divaricata from Japan (29) and Dictyopteris zonarioides from California (30, 31) produce sesquiterpenoids instead of C11 hydrocarbons. In the case of the Mediterranean D. membranaceae, the compounds are continuously released to the environment. The thalli of this alga (1 kg wet weight) release 10-50 mg of C11 hydrocarbons into seawater within 24 h (10). This exceeds by far the required threshold concentrations for chemotaxis, and one may suspect that the compounds can act as ''mating disruptants" in the control of the habitat. This idea, however, awaits experimental confirmation. According to recent analyses, the occurrence of large amounts of C11 hydrocarbons in thalli of brown algae is not limited to the genus Dictyopteris. Two Fucales from the Red Sea—namely, Sargassum asporofolium and Sargassum latifolium (S. Fatallah and W.B., unpublished data)—contain, besides sesquiterpenes, ectocarpene and dictyotene as the major volatiles.
Recent advances in gas chromatographic separations of enantiomers allow precise determination of the enantiomeric purity of the algal pheromones. The cis-disubstituted cyclopentenes, such as multifidene, viridiene, and caudoxirene, are of high optical purity [=95% enantiomeric excess (e.e.)] whenever they have been found (32, 33). The situation is different with the cyclopropanes and the cycloheptadienes, as shown in Table 2 and Figure 1. Hormosirene from female gametes or thalli of