Population dynamics and stage structure in a haploid‐diploid red seaweed, Gracilaria gracilis

Abstract

Summary 1Many red seaweeds are characterized by a haploid-diploid life cycle in which populations consist of dioecious haploid (gametophyte) and diploid (tetrasporophyte) individuals as well as an additional diploid zygote-derived sporangium (carposporophyte) stage. A demographic analysis of Gracilaria gracilis populations was carried out to explore and evaluate the population dynamics and stage structure of a typical haploid-diploid red seaweed. 2 Four G. gracilis populations were studied at two sites on the French coast of the Strait of Dover. Survival, reproduction and recruitment rates were measured in each population for up to 4 years. Eight two-sex stage-based population projection matrices were built to describe their demography. 3 All four populations were characterized by high survival and low recruitment rates. Population growth rates ( λ ) were similar between populations and between years and ranged from 1.03 to 1.17. In addition, generation times were found to be as long as 42 years. 4 Sex and ploidy ratios were variable across populations and over time. Female frequencies ranged from 0.31 to 0.59 and tetrasporophyte frequencies from 0.44 to 0.63. However, in most cases, the observed population structures were not significantly different from the calculated stage distributions. 5 Eigenvalue elasticity analysis showed that λ was most sensitive to changes in matrix transitions that corresponded to survival of the gametophyte and tetrasporophyte stages. In contrast, the contribution of the fertility elements to λ was small. Eigenvector elasticity analysis also showed that survival elements had the greatest impact on sex and ploidy ratios.

Mating system and gene flow in the red seaweed Gracilaria gracilis: effect of haploid–diploid life history and intertidal rocky shore landscape on fine-scale genetic structure

“…In addition, although differential selection between haploids (males) and diploids (females) has been detected in haplodiploid insects (Pamilo, 1993;Parker and Hedrick, 2000), we did not observe any significant differences in haploid and diploid within-population allele frequencies, suggesting the absence of strong differential selection at our microsatellite, or closely linked, loci. Second, migrationdrift disequilibrium is the most plausible explanation in agreement with our results: since pairwise genetic differentiation between pools was not correlated between the two ploidy levels (Table 5), there may be a lag between migration and (local) breeding due to the long generation times in G. gracilis (420 years, Engel et al, 2001). Finally, in the case of migration-drift disequilibrium, differential migration rates of haploid and diploid spores could account for the higher haploid y y ST values.…”

“…However, a demographic study of Aud1, Aud2, GN2 and GN3 revealed similar population dynamics in all the pools (Engel et al, 2001). In the absence of differences in survival, fecundity or extinction probabilities and even habitat quality, biased spore dispersal may generate directional gene flow (cf.…”

“…A recent model showed that drift is a weak force in the population genetic structure of treestypically long lived with long juvenile phases -because new migrants arrive before (founding) individuals contribute to the gamete pool (Austerlitz et al, 2000). In light of this model, the high survival rates and long generation times typical of G. gracilis populations (Engel et al, 2001) probably help maintain high genetic diversity within populations (relative to among populations). Similarly, according to the 'storage effect', genetic drift is buffered in long-lived organisms due to the storage of genotypes in long-lived stages (Gaggiotti and Vetter, 1999;see also, Ellner and Hairston, 1994), much as a seed bank slows down coalescence rates (Kaj et al, 2001).…”

Seaweed reproductive biology: environmental and genetic controls

“…More opportunistic fucoids like the invasive Sargassum muticum, which displays a very large reproductive allocation, seem to rely on the same strategy, irrespective of the state of invasion (Engelen and Santos 2009). Also, the red seaweed Gracilaria gracilis was characterized by high survival and low recruitment rates, consistent with an estimated longevity of 42 years (Engel et al 2001). In contrast, short-lived ephemeral species are predicted to have a very high reproductive effort and associated costs as they do not have to balance costs with future vital rates.…”

Population Demographics in Species With Biphasic Life Cycles

“…However, few studies combine field surveys with demographic studies to determine the relative effects of mortality and fecundity on population structure (but see Engel et al 2001).…”