Algal Clades Research Articles

Geographic variation in algal partners of Cladonia subtenuis (Cladoniaceae) highlights the dynamic nature of a lichen symbiosis

Multiple interacting factors may explain variation present in symbiotic associations, including fungal specificity, algal availability, mode of transmission and fungal selectivity. To separate these factors, we sampled the lichenized Cladonia subtenuis and associated Asterochloris algae across a broad geographic range. We sampled 87 thalli across 11 sites using sequence data to test for fungal specificity (phylogenetic range of association) and selectivity (frequency of association), fungal reproductive mode, and geographic structure among populations. Permutation tests were used to examine symbiont transmission. Four associated algal clades were found. Analysis of molecular variation (amova) and partial Mantel tests suggested that the frequency of associated algal genotypes was significantly different among sites and habitats, but at random with respect to fungal genotype and clade. The apparent specificity for Clade II algae in the fungal species as a whole did not scale down to further within-species lineage-dependent specificity for particular algae. Fungal genotypes were not structured according to site and appeared to be recombining. We suggest that ecological specialization exists for a specific lichen partnership and a site, and that this selectivity is dynamic and environment-dependent. We present a working model combining algal availability, fungal specificity and selectivity, which maintains variation in symbiotic composition across landscapes.

Strong fungal specificity and selectivity for algal symbionts in Florida scrub Cladonia lichens.

Symbiosis is a major theme in the history of life and can be an important force driving evolution. However, across symbioses, it is difficult to tease apart the mechanisms that structure the interactions among potential partners. We used genetic similarity and frequency-based methods to qualitatively and quantitatively examine the patterns of association among several co-occurring Cladonia lichen fungi and their algal photobionts in six disjunct Florida scrub sites. The patterns of association were described by the degree of specificity, i.e. the phylogenetic range of associated partners, and of selectivity, i.e. the frequency of association among partners. Six fungal species associated with only one algal internal transcribed spacer clade, with the remaining two fungi being associated with two algal clades. In all cases, the fungi associated in unequal frequencies with the observed algal photobiont genotypes within those clades--suggesting that both specificity and selectivity were higher than expected. Fungal species can be grouped into three significantly different specificity classes: photobiont specialists, intermediates and generalists. In contrast to the pronounced specificity for photobionts among fungal species, the different Florida scrub sites do not harbour distinct photobiont pools, and differential photobiont availability cannot explain the patterning of lichen associations at this spatial scale. Therefore, we conclude that fungal specificity and selectivity for algal photobionts are major factors in determining the local composition of symbiotic partnerships.

Early ontogenetic expression of specificity in a cnidarian-algal symbiosis

Most cnidarian-microalgae symbioses exhibit some degree of specificity in host-sym- biont pairing. Among taxa with aposymbiotic larvae, specificity must be established each generation. Newly settled polyps of the gorgonians Plexaura kuna and Pseudoplexaura porosa rapidly acquired zooxanthellae, in both laboratory and field settings. Initial zooxanthella acquisition by polyps was non-selective and did not reflect adult host specificity. When placed in shallow backreef and forereef habitats, and on the deep forereef, newly settled polyps naturally acquired zooxanthellae belonging to Symbiodinium clades A, B and C (based on restriction fragment length analysis of small subunit ribosomal DNA). Over time the taxonomic diversity changed such that by 3 mo, the majority of polyps (77%) harbored only algae belonging to Symbiodinium clade B, the same algal clade found in the adult hosts. A survey of naturally occurring juvenile gorgonians (those 10 cm or less) showed that all contained only algae belonging to Symbiodinium clade B at this life history stage. These data are the first to characterize the early ontogenic change in zooxanthella diversity and confirm models of speci- ficity in cnidarians in which a broad group of dinoflagellates initially enter the host followed by a change in algal genotypes that leads to the specificity observed in the adult host.

Pre-Ediacaran multicellular life: harbinger of a Phanerozoic radiation

Multicellular organisms must have had a substantial pre-Ediacaran history, but the fossil evidence is sparse and often equivocal. The taxonomic resolution necessary to constrain phylogenetic hypotheses is limited largely to Lagerstätte-grade fossils that retain evidence of diagnostic cell division patterns. Uniseriate and multiseriate filaments in the 1267-723 Ma Hunting Formation, Somerset Island, derive from transverse and longitudinal intercalary cell division programs indistinguishable from those of the modern red alga Bangia, and establish a significantly pre-Ediacaran datum point for the Rhodophyta. Likewise, on the basis of a distinctive “segregative cell division”, three taxa of siphonocladalean green algae (Chlorophyta) are identified in the ca. 750 Ma Svanbergfjellet Formation, Spitsbergen. Process-bearing vesicles in the Svanbergfjellet sequence further compare with the germinating zoospores of the modern chromophyte alga Vaucheria, while convincing Vaucheria-like thalli are reported from ca. 900 Ma deposits in Siberia. The broad co-occurrence of multicellular Rhodophyta, Chlorophyta, and Chromophyta by at least 750 Ma ago accords well with molecular evidence that suggests the three principal algal clades diverged from a common ancestor during a brief but marked radiation relatively late in eukaryote evolution.Complex multicellularity featuring cellular and tissue(?) differentiation is also encountered in the pre-Ediacaran fossil record. A large ornate form in the Svanbergfjellet succession preserves at least six readily distinguishable cell types and, despite its taxonomic uncertainty, can be characterized as at least as complex as the most complex modern algae or fungi. No cellularity is preserved in the late Proterozoic macrofossil Tawuia; however, SEM and light microscopy of its isolated wall reveal a complex histology suggestive of a relatively advanced grade of multicellularity. Another Svanbergfjellet macrofossil has a distinct wall structure and bears a terminal pair of large reniform structures; if these prove to be truly bilaterally symmetrical, this fossil represents a grade of organization otherwise not recognized until the Ediacaran. Further analysis of these and other pre-Ediacaran ‘problematica’ may clarify their taxonomic relationships and promises to resolve at least some of the ‘Cambrian explosion’ into a meaningful sequence of evolutionary change.