FERTILIZATION SUCCESS CAN DRIVE PATTERNS OF PHASE DOMINANCE IN COMPLEX LIFE HISTORIES¹

Abstract

Many algal life cycles alternate between two freeliving generations. Life histories in which the two generations look identical (isomorphic) are common, particularly in the Rhodophyta. Reports of natural populations dominated by one generation of the life history have sought explanation in terms of phase-specific differences in mortality and reproductive output, yet in many cases identification of these adaptations has been elusive or inconsistent with predictions. We hypothesized that the gametophyte-to-sporophyte ratio of ecologically equivalent isomorphs could result from variation in fertilization rate. We developed two models to test this hypothesis: one representing a generalized isomorphic life history and the other specific to red algae with a Polysiphonia-type life history. Fertilization rate affected the gametophyte-to-sporophyte ratio, especially at low fertilization rates. In the general model, gametophytes dominated the population regardless of fertilization rate unless egg production greatly exceeded meiospore production. In the red algal model, phase dominance depended on the combination of fertilization rate and the number of carpospores produced per fertilization. The generational composition of model multiphasic algal populations results from their inherent reproductive characteristics and the dynamic environment to which fertilization and mortality rates are tied.

Ecophysiological studies of the non-indigenous speciesGracilaria vermiculophylla(Rhodophyta) and its abundance patterns in Ria de Aveiro lagoon, Portugal

“…A revision by Fierst et al (2005) pointed to a dominance of the tetrasporophytes in the Gracilariales. For G. vermiculophylla, Rueness (2005) described a higher abundance of vegetative individuals in European waters.…”

Susceptibility of spores of different ploidy levels from AntarcticGigartina skottsbergii(Gigartinales, Rhodophyta) to ultraviolet radiation

“…Analysis of variance (ANOVA) is presented in Table 2. phase dominance in most Gigartinales species and that of G. skottsbergii population in South America (Piriz 1996;Avila et al 1999;Faugeron et al 2004). Phase dominance of specific macroalgal life stage may result from differences in spore production between generations (Scrosati & De Wreede 1999;Thornber & Gaines 2004) or fertilization rate (Fierst et al 2005). In the triphasic life history of red macroalgae, the carposporophyte phase represents an adaptation to minimize the effect of both low and variable fertilization rates, where the cloning of the zygote amplifies the production ratio of carpospores relative to tetraspores (Searles 1980).…”

Responses of the haploid-to-diploid ratio of isomorphic biphasic life cycles to time instability

“…These encompass the spore survival range observed for most seaweeds and were further expanded to the highest survival possible (10 0 = 1 = 100%). Testing a wide range of spore survival was fundamental because the H:D dynamics varies extremely with the amount of fertility output [7,33,34].…”