Selective mortality on haploid and diploid microscopic stages of Lessonia nigrescens Bory (Phaeophyta, Laminariales)

Abstract

In seaweeds, potential selective events on juveniles are particularly important because the highest mortality rates usually affect these microscopic stages rather than the macroscopic ones. This study evaluates the degree to which the mortality induced by herbivory and wave impact are selective on the size (mean and whole distribution) of haploid, gametophytic, and of diploid, early sporophytic stages of the intertidal kelp Lessonia nigrescens. In theory, in species with a complex life history, the potential effect of selective mortality on haploid and diploid individuals may range between two extremes. One of inhibition, where the same feature, conferring higher survival in one phase, may cause the opposite effect in the following phase. The other extreme is synergism, where selection operates similarly among the two alternating 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, separate experiments showed that wave impact eliminated more efficiently the larger diploid plantules than the smaller haploid ones. Size selectivity by both mortality agents occurred only on the microscopic diploid stages, when plantule sizes were longer than 20 μm. Directionality of selection indicated that larger individuals, with more developed adhesive rhizoids, had higher survival rates. The directional selection on diploid plants is neither reinforced, or cancelled out, in the alternate haploid stages. Thus, no inhibitory or synergetic effects were observed for the directional selection of size between these two life history phases. These results suggest that, compared to organisms with direct life cycles, those having complex life histories, with independent phases, have at least one more restriction for the occurrence of evolutionary change. Namely, that of equivalent expression and response of involved traits on both alternate haploid and diploid phases to the same putative selective agents.