Effects of self-fertilization in the giant kelp, Macrocystis pyrifera

The costs of self-fertilization were evaluated for the giant kelp, Macrocystis pyrifera. Macrocystis, like other kelps, has a biphasic life history in which the dispersal stage is the IN spore generation. Once spores settle, they grow into sedentary and microscopic male or female gametophytes, which produce sperm and eggs. Fertilization ensues, and the resultant 2N sporophyte, the spore-producing form, grows rapidly to a height of LIP to 25 m. Based on previous empirical studies and modeling field Surveys, the potential for self-fertilization is expected to be high. largely due to the shape and extent of spore dispersal distributions. The costs of self-fertilization were assessed with laboratory and field experiments over the entire life history of the sporophyte stage. There was no evidence for self-incompatibility, and fitness was dramatically lower in selfed compared to outcrossed individuals. In addition, there was evidence that costs were more severe under intraspecific competition With outcrossed individuals. The demonstrated severity of the consequences of self-fertilization Suggests a species that typically outcrosses (at least under the assumption that costs result from rare recessive deleterious alleles). However, costs were more pronounced ill later life history phases, Suggesting routine self-fertilization. One implication of significant levels of self-fertilization in Macrocystis is a sort of inbreeding-mediated localized population senescence, leading to striking oscillations in Populations, a pattern seen in many Californian kelp beds.