Convergent Biological Processes in Coalescing Rhodophyta

Abstract

Sporeling coalescence in Gracilaria chilensis Bird, McLachlan et Oliveira produces genetically polymorphic, chimeric individuals. If this is common in red algae, it may have significant biological consequences. In this study, we evaluate the hypotheses that coalescence is widespread among the Rhodophyta and that specific and convergent morphological and ecological responses characterize this as a unique growth style among marine algae. A literature survey on coalescence was undertaken to assess the distribution of this condition in the Florideophycidae. Sixty-two (54.9%) of 113 species considered germinated to form a disk. Subsequent development in 37 of these species showed crust formation and coalescence during development with other crusts in 31 species (84%). Coalescing red algae were members of the orders Ahnfeltiales, Corallinales, Gigartinales, Gracilariales, Halymeniales, Palmariales, and Rhodymeniales. Ultrastructural studies in species of Ahnfeltiopsis, Chondrus, Gracilaria, Mazzaella, and Sarcothalia suggested a common pattern of early development. Newly released, naked spores may fuse into a single cell, as they do in Chondrus canaliculatus, or they may develop individual cell walls that later are surrounded by a thickened common wall. Ultrastructural studies demonstrated two kinds of immediate development after the first mitotic division: direct development by symmetric divisions resulting in discoid sporelings or an indirect asymmetric arrangement of divisions before a diskoid sporeling was formed. Germination in coalescing species is a linear function of the initial spore density, whereas in noncoalescing species maximum germination occurs at intermediate densities. In the field, coalescing species may recruit either from solitary or aggregated spores. However, survival is significantly higher for plantlets grown from a larger number of coalescing spores. Total number of erect axes formed by the coalesced mass is a logarithmic function of the initial number of spores. Thus, germlings grown from a larger number of coalescing spores exhibited a larger photosynthetic canopy than do plantlets grown from a few spores. Juveniles and mature clumps grown from a coalescing mass may exhibit size inequalities among erect axes, with the larger axes located toward the center of the clump. These larger axes mature first or, in some cases, are the only to produce spores. The widespread occurrence of coalescence in 1 roughly half the number of orders of the Florideophycidae, the similarity of the coalescence process, and the finding of various adaptive traits associated with coalescence characterizes this as a unique growth style, splitting the diversity of species now included in the Florideophycidae into two major groups: coalescing and noncoalescing Rhodophyta.

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

“…DNA samples were amplified according to the protocol in Luo et al (1999), with slight modifications for Gv2CT . Although reports indicate that in many orders of red seaweeds, including Gracilariales, spores may coalesce and form a single, genetically chimeric plant (Santelices et al, 1999), no genotypes with supernumerary alleles were observed in this study.…”

Field testing of inter- and intraspecific coalescence among mid-intertidal red algae

“…Erect fronds may be up to 30 cm long and 5 cm wide. However, usually only 1 or 2 blades per plant reach a large size and there is a tendency for larger and thicker blades to be located closer to the center of the holdfast, generating size inequalities among the fronds arising from the same disc (Martínez & Santelices 1992, Santelices et al 1999. In this species, growth meristems are located along the border of the disc and at the base and borders of erect fronds.…”

“…To evaluate whether coalescence was occurring among inter-and intraspecific contacting partners, ultrastructural studies were undertaken. Characterization of the process of coalescence at the cellular level followed (with modifications) the protocol described in previous studies (Santelices et al 1996(Santelices et al , 1999. Contacting holdfasts were carefully removed from the rocks about 30 d after contact, and fixed in the field for ultrastructural analysis.…”

“…species in the genera Ahnfeltia, Ahnfeltiopsis, Callophyllis, Champia, Chondrus, Corallina, Endocladia, Fosliella, Gastroclonium, Gigartina, Gracilaria, Grateloupia, Gymnogongrus, Hildenbrandia, Hypnea, Iridaea, Lithophyllum, Lithothamnion, Masto-carpus, Mazzaella, Microcladia, Plocamium, Petrocelis, Phyllophora, Porolithon, Rhodymenia, Sarcothalia, Schizymenia) are important space users in many middle to low intertidal and shallow subtidal habitats at diverse latitudes (see descriptions in Stephenson & Stephenson 1972, Mathieson & Nienhuis 1991and Raffaelli & Hawkins 1996 and, as for colonial animals, it is possible that coalescence may affect key ecological processes such as recruitment, competitive ability and susceptibility to grazing. However, coalescence has only been described from laboratory cultures and in connection with spores and germlings (see Santelices et al 1999). A first step in evaluating the relative importance of coalescence is to determine whether the process occurs among well-established clumps in the field and whether it is restricted to conspecific partners or also includes interspecific fusions.…”

Evolution of Individuality: A Case Study in the Volvocine Green Algae

“…In many plants, somatic mutations accumulate between cell lineages (Whitham and Slobodchikoff 1981). Somatic mutations have also been reported in fungi (Peabody et al 2000;Stenlid 2000), red algae (Santelices and Varela 1993;Meneses et al 1999;Santelices et al 1999), invertebrates (Rinkevich and Weissman 1987;Barki et al 2002;Sommerfeldt et al 2003), and vertebrates (De 2011).…”