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The systematics of the Prasiolales was investigated by phylogenetic inference based on analyses of the rbcL and 18S rRNA genes for representatives of all four genera currently attributed to this order ( Prasiococcus, Prasiola, Prasiolopsis, Rosenvingiella), including all type species. The rbcL gene had higher sequence divergence than the 18S rRNA gene and was more useful for phylogenetic inference at the ranks of genus and species. In the rbcL gene phylogeny, three main clades were observed, corresponding to Prasiola, Prasiolopsis, and Rosenvingiella. Prasiococcus was nested among species of Prasiola occurring in subaerial and supralittoral habitats. Trichophilus welckeri Weber Bosse, a subaerial alga occurring in the fur of sloths in Amazonia, was closely related to Prasiolopsis ramosa Vischer. The species of Prasiola were grouped into three well-supported clades comprising (i) marine species, (ii) freshwater and terrestrial species with linear blades, and (iii) terrestrial species with rounded or fan-shaped blades. Sequence divergence was unexpectedly low in the marine group, which included species with different morphologies. For the 18S rRNA gene, the phylogenetic analyses produced several clades observed for the rbcL gene sequence analysis, but, due to very little sequence variation, it showed considerably lower resolution for inference at the species and genus levels. Due to the low support of some internal branches, the results of the analyses did not allow an unambiguous clarification of the origin and the early evolution of the Prasiolales. [ABSTRACT FROM AUTHOR]

A UV-absorbing mycosporine-like amino acid (324 nm-MAA), so far only known from the green macroalgal genusPrasiola(Trebouxiophyceae), was also identified in other morphologically diverse green algae closely related toPrasiolaspp. in 18S rDNA phylogenies. Using HPLC, a second UV-absorbing compound was found only inMyrmecia incisaReisigal among all studied strains. This substance showed an absorption maximum at 322 nm and hence was designated as putative 322 nm-MAA. Preliminary UV-exposure experiments indicated that all species containing one or the other MAA showed a strong accumulation of the respective compound, thus supporting their function as putative UV sunscreen. Both UV-absorbing substances were only identified in the studied members of the Trebouxiophyceae but were absent in members of the Ulvophyceae and Chlorophyceae. When mapped on an 18S rDNA phylogeny, the distribution of 324 nm-MAA was found to be scattered within the Trebouxiophyceae but was consistent with a distribution that follows phylogenetic patterns rather than ecological adaptations. The 324 nm-MAA was also detected in two phylogenetically related species from freshwater as well as from subaerial habitats,Watanabea reniformisHanagata et al. and isolate UR7/5, which were phylogenetically independent ofPrasiolaand its closer allies. MAAs were absent in another Trebouxiophyceae clade comprising lichen photobionts (Coccomyxa pringsheimiiJaag) as well as freshwater picoplanktonic algae (Choricystis minor(Skuja) Fott). The data presented suggest a chemotaxonomic value of the 324 nm-MAA in green algal taxonomy. To address the paraphyly of the genusMyrmeciaPrintz as presently circumscribed, we propose the new combinationLobosphaera incisa. [ABSTRACT FROM AUTHOR]