SELECTIVE MORTALITY ON HAPLOID AND DIPLOID MICROSCOPIC STAGES OF LESSONIA-NIGRESCENS BORY (PHAEOPHYTA, LAMINARIALES)

In seaweeds, potential selective events on juveniles are particularly important because the highest mortality rates usually affect these mic roscopic stages rather than the macroscopic ones. This study evaluates the degree to which the mortality induced by herbivory and wave impac t are selective on the size (mean and whole distribution) of haploid, gametophytic, and of diploid, early sporophytic stages of the intertid al kelp Lessonia nigrescens. In theory, in species with a complex life history, the potential effect of selective mortality on haploid and d iploid individuals may range between two extremes. One of inhibition, where the same feature, conferring higher survival in one phase, may c ause the opposite effect in the following phase. The other extreme is synergism, where selection operates similarly among the two alternatin g Life history phases. Controlled experiments showed that herbivorous snails had a more significant negative impact on haploid gametophytes than on the slightly larger, diploid, sporophytes. Conversely, separat e experiments showed that wave impact eliminated more efficiently the larger diploid plantules than the smaller haploid ones. Size selectivi ty by both mortality agents occurred only on the microscopic diploid s tages, when plantule sizes were longer than 20 mu m. Directionality of selection indicated that larger individuals, with more developed adhe sive rhizoids, had higher survival rates. The directional selection on diploid plants is neither reinforced, or cancelled out, in the altern ate haploid stages. Thus, no inhibitory or synergetic effects were obs erved for the directional selection of size between these two Life his tory phases. These results suggest that, compared to organisms with di rect life cycles, those having complex life histories, with independen t phases, have at least one more restriction for the occurrence of evo lutionary change. Namely, that of equivalent expression and response o f involved traits on both alternate haploid and diploid phases to the same putative selective agents. (C) 1998 Elsevier Science B.V. All rig hts reserved.