Effects of Self-Fertilization in the Giant Kelp, Macrocystis Pyrifera

Abstract

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 1N 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 up 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 in 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.

Towards domestication of giant kelp (Macrocystis pyrifera) in Chile: selection of haploid parent genotypes, outbreeding, and heterosis

“…Our results with sexual crosses of clonal gametophytes of M. pyrifera from different localities along a 250-km stretch of South Chilean coastline correspond well with the observations in a natural kelp bed (Raimondi et al 2004): Fig. 2 Three cases of heterosis in outbreeding experiments with different cultivars of M. pyrifera.…”

“…Consequently, natural kelp populations show a high degree of self-fertilization, which results in inbreeding-depression, and is considered to be responsible for periodic kelp bed fluctuations. Raimondi et al (2004) showed for Californian M. pyrifera that outbreeding caused significant positive effects (heterosis) in offspring produced by geographically distant parents. Since these experiments, as well as traditional seeding of mariculture ropes with natural spore suspensions (Gutierrez et al 2006), make use of natural spore batches that contain many millions of genetically different meiospores, they have only limited value to reveal heterosis.…”

The mixed mating system of the sea palm kelpPostelsia palmaeformis: few costs to selfing

“…Some abundant brown algae species (such as the rockweed species of temperate shores) show high levels of selfing [17,18], while other species such as the giant kelp (Macrocystis pyrifera) self while also showing conspicuous inbreeding depression [22]. Despite estimates of spore dispersal for kelp in excess of 1 km [23,24] and spore viability for several days [25], many kelp spores settle near to the parental source, a feature that may lead to selfing in nature.…”

Macrocystis mariculture in Chile: growth performance of heterosis genotype constructs under field conditions

“…Graham (2003) and Raimondi et al (2004) studied natural Macrocystis beds in California. Meiospores have a limited swimming capacity and settle down near their origin to form gametophytes.…”

“…Survival of the population requires a high density of gametophytes, since the distance for successful interaction between egg and sperm does not exceed the range of millimetres. Raimondi et al (2004) estimated that the area within which a given sporophyte can reliably produce new progeny only reaches out a few metres from its basal holdfast. This spatial limitation strongly enhances inbreeding, i.e.…”