Symmetry breaking of the cellular lobes closely relates to phylogenetic structure within green microalgae of the Micrasterias lineage (Zygnematophyceae).

Green microalgae of the Micrasterias lineage are unicellular microorganisms with modular morphology consisting of successively differentiated lobes. Due to their morphological diversity and peculiar morphogenesis, these species are important model systems for studies of cytomorphogenesis and cellular plasticity. Interestingly, the phylogenetic structure of the Micrasterias lineage and most other Desmidiales is poorly related to the traditional morphological characters used for delimitation of taxa. In this study, we focused on symmetry breaking between adjacent cellular lobes in relation to phylogeny of the studied species. While pronounced morphological asymmetry between the adjacent lobes is typical for some species, others have been characterized by the almost identical morphologies of these structures. We asked whether there is any detectable average shape asymmetry between the pairs of lobes and terminal lobules in 19 Micrasterias species representing all major clades of this desmidiacean lineage. Then, we evaluated whether the asymmetric patterns among species are phylogenetically structured. The analyses showed that the phylogeny was in fact strongly related to the patterns of morphological asymmetry between the adjacent cellular lobes. Thus, evolution of the asymmetric development between the adjacent lobes proved to be the key event differentiating cellular shape patterns of Micrasterias. Conversely, the phylogeny was only weakly related to asymmetry between the pairs of terminal lobules. The subsequent analyses of the phylogenetic morphological integration showed that individual hierarchical levels of cellular morphology were only weakly coordinated with regard to asymmetric variation among species. This finding indicates that evolutionary differentiation of morphogenetic processes leading to symmetry breaking may be relatively independent at different branching levels. Such modularity is probably the key to the evolvability of cellular shapes, leading to the extraordinary morphological diversity of these intriguing microalgae.

Asymmetry and integration of cellular morphology in Micrasterias compereana.

BACKGROUND: Unicellular green algae of the genus Micrasterias (Desmidiales) have complex cells with multiple lobes and indentations, and therefore, they are considered model organisms for research on plant cell morphogenesis and variation. Micrasterias cells have a typical biradial symmetric arrangement and multiple terminal lobules. They are composed of two semicells that can be further differentiated into three structural components: the polar lobe and two lateral lobes. Experimental studies suggested that these cellular parts have specific evolutionary patterns and develop independently. In this study, different geometric morphometric methods were used to address whether the semicells of Micrasterias compereana are truly not integrated with regard to the covariation of their shape data. In addition, morphological integration within the semicells was studied to ascertain whether individual lobes constitute distinct units that may be considered as separate modules. In parallel, I sought to determine whether the main components of morphological asymmetry could highlight underlying cytomorphogenetic processes that could indicate preferred directions of variation, canalizing evolutionary changes in cellular morphology. RESULTS: Differentiation between opposite semicells constituted the most prominent subset of cellular asymmetry. The second important asymmetric pattern, recovered by the Procrustes ANOVA models, described differentiation between the adjacent lobules within the quadrants. Other asymmetric components proved to be relatively unimportant. Opposite semicells were shown to be completely independent of each other on the basis of the partial least squares analysis analyses. In addition, polar lobes were weakly integrated with adjacent lateral lobes. Conversely, higher covariance levels between the two lateral lobes of the same semicell indicated mutual interconnection and significant integration between these parts. CONCLUSIONS: Micrasterias cells are composed of several successively disintegrated parts. These integration patterns concurred with presumed scenarios of morphological evolution within the lineage. In addition, asymmetric differentiation in the shape of the lobules involves two major patterns: asymmetry across the isthmus axis and among the adjacent lobules. Notably, asymmetry among the adjacent lobules may be related to evolutionary differentiation among species, but it may also point out developmental instability related to environmental factors.

Static allometry of unicellular green algae: scaling of cellular surface area and volume in the genus Micrasterias (Desmidiales).

The surface area-to-volume ratio of cells is one of the key factors affecting fundamental biological processes and, thus, fitness of unicellular organisms. One of the general models for allometric increase in surface-to-volume scaling involves fractal-like elaboration of cellular surfaces. However, specific data illustrating this pattern in natural populations of the unicellular organisms have not previously been available. This study shows that unicellular green algae of the genus Micrasterias (Desmidiales) have positive allometric surface-to-volume scaling caused by changes in morphology of individual species, especially in the degree of cell lobulation. This allometric pattern was also detected within most of the cultured and natural populations analysed. Values of the allometric S:V scaling within individual populations were closely correlated to the phylogenetic structure of the clade. In addition, they were related to species-specific cellular morphology. Individual populations differed in their allometric patterns, and their position in the allometric space was strongly correlated with the degree of allometric S:V scaling. This result illustrates that allometric shape patterns are an important correlate of the capacity of individual populations to compensate for increases in their cell volumes by increasing the surface area. However, variation in allometric patterns was not associated with phylogenetic structure. This indicates that the position of the populations in the allometric space was not evolutionarily conserved and might be influenced by environmental factors.

Polyphasic evaluation of Xanthidium antilopaeum and Xanthidium cristatum (Zygnematophyceae, Streptophyta) species complex.

We investigated twenty-six strains of Xanthidium antilopaeum Kütz. and seven strains of X. cristatum Ralfs isolated from various European localities or obtained from public culture collections. A combination of molecular, geometric morphometric, and morphological data were used to reveal the patterns of the phylogenetic and morphological differentiation of these taxonomically very compli-cated desmid taxa. The molecular data based on trnG(ucc) and ITS rDNA sequences illustrated the monophyly of both the complexes, which indicated that their traditional morphology-based discriminative criteria, such as the different number of spines, may generally continue to be considered relevant. The single exception was X. antilopaeum var. basiornatum B. Eichler et Raciborski, which was positioned outside the X. antilopaeum/cristatum clade. The independent status of this taxon was also confirmed on the basis of the geometric morphometric data, so that we concluded that it probably represents a separate species. Within X. cristatum complex, the traditional varieties X. cristatum var. cristatum Ralfs, X. cristatum var. uncinatum Ralfs, and X. cristatum var. scrobiculatum Scott et Grönblad turned out to be separate taxa. Conversely, X. cristatum var. bituberculatum Lowe lacked any taxonomical value. Our data on X. antilopaeum illustrated extensive phylogenetic as well as phenotypic variability within this species complex. However, our data did not result in any unambiguous pattern that would allow sound taxonomic classification. Finally, we also found out that the morphologically peculiar Staurastrum tumidum Ralfs belongs to the genus Xanthidium based on the combined rbcL + cox III data set. Consequently, this species was formally transferred to this genus.

Levantamento taxonômico de desmídias (Chlorophyta) do lago Novo (Amapá, Brasil): gêneros Staurastrum, Staurodesmus e Xanthidium / Desmids (Chlorophyta) from lake Novo (Amapá, Brazil): genera Staurastrum, Staurodesmus e Xanthidium

Neste estudo são descritas e ilustradas as espécies de três gêneros de desmídias (Staurastrum, Staurodesmus e Xanthidium) do lago Novo, um ambiente localizado na região dos lagos, na planície costeira do Estado do Amapá. Um total de 35 táxons foi registrado, sendo 23 do gênero Staurastrum, sete do gênero Staurodesmus e cinco do gênero Xanthidium. Todos os táxons apresentados neste trabalho constituem primeira citação para o estado do Amapá.(AU)

This study describes and sketches the species of three desmids genus (Staurastrum, Staurodesmus and Xanthidium) from Lake Novo, located in the coastal plain lake region of Amapá State. In total, 35 taxa were registered, being 23 of Staurastrum, seven of Staurodesmus and five of Xanthidium genus. These results are the first documented taxa registered in the Amapá State.

Annual and interannual fluctuation of desmids species in a black water Amazon floodplain lake (Lago Cutiuaú, Amazonas State, Brazil) - DOI: 10.4025/actascibiolsci.v31i3.1050 / Flutuação anual e interanual da riqueza de espécies de desmídias (Chlorophyta Conjugatophyceae) em um lago de inundação amazônico de águas pretas (Lago Cutiuaú, Estado do Amazonas, Brasil) - DOI: 10.4025/actascibiolsci.v31i3.1050

In this study, the seasonal and interannual fluctuations of desmids species richness was investigated for three years in an Amazon black water floodplain lake in the Negro River basin, Amazon State. The studied environment showed a great annual fluctuation of water column depth and high temperature values with little seasonal fluctuations. In general, pH and electrical conductivity values were low. One hundred five desmids species were identified distributed among families Closteriaceae, Gonatozygaceae and Desmidiaceae. Twenty three species are new references to Amazon State and are illustrated in this study. A marked seasonal fluctuation of desmids species was registered in association with the inundation pulse. The increased water level was the period of more species richness; on the other hand, the others periods showed low and similar species number. An interannual fluctuation of desmids species number was also registered, with the highest number in 2003, especially in the increased water level. Staurastrum Meyen and Closterium Nitzch were the genera with the highest species numbers. Based on this occurrence, the majority of species registered was rare or sporadic, registered in less than 10% or between 10 and 30% of total analyzed sample, respectively.

Nesta pesquisa foi avaliada a flutuação sazonal e interanual da riqueza específica de desmídias ao longo de diferentes períodos do ciclo hidrológico de 2002 a 2004, em um lago de inundação de águas pretas da bacia do rio Negro, no Estado do Amazonas. O lago estudado apresentou marcada flutuação anual na profundidade da coluna dágua e elevados valores de temperatura ao longo de todo o estudo, enquanto os valores de pH e condutividade elétrica foram baixos. Foram identificadas 105 espécies de desmídias distribuídas nas famílias Closteriaceae, Gonatozygaceae e Desmidiaceae. Destas, 23 constituem primeira citação para o Estado do Amazonas e se encontram ilustradas no presente estudo. O período de enchente foi o que apresentou maior riqueza de espécies, em oposição aos demais períodos, que apresentaram valores similares. Em escala interanual, o ano de 2003 foi caracterizado por apresentar maior número de táxons. Os gêneros Staurastrum Meyen e Closterium Nitzch foram os que apresentaram maior número de espécies durante o estudo. A maioria das espécies registradas esteve presente em menos de 10% ou entre 10 e 30% das amostras analisadas, sendo consideradas raras ou esporádicas, respectivamente.

PHYLOGENY OF THE CONJUGATING GREEN ALGAE (ZYGNEMOPHYCEAE) BASED ON rbc L SEQUENCES.

Sequences of the gene encoding the large subunit of RUBISCO (rbcL) for 30 genera in the six currently recognized families of conjugating green algae (Desmidiaceae, Gonatozygaceae, Mesotaeniaceae, Peniaceae, and Zygnemataceae) were analyzed using maximum parsimony and maximum likelihood; bootstrap replications were performed as a measure of support for clades. Other Charophyceae sensu Mattox and Stewart and representative land plants were used as outgroups. All analyses supported the monophyly of the conjugating green algae. The Desmidiales, or placoderm desmids, constitute a monophyletic group, with moderate to strong support for the four component families of this assemblage (Closteriaceae, Desmidiaceae, Gonatozygaceae, and Peniaceae). The analyses showed that the two families of Zygnematales (Mesotaeniaceae, Zygnemataceae), which have plesiomorphic, unornamented and unsegmented cell walls, are not monophyletic. However, combined taxa of these two traditional families may constitute a monophyletic group. Partitioning the data by codon position revealed no significant differences across all positions or between partitions of positions one and two versus position three. The trees resulting from parsimony analyses using first plus second positions versus third position differed only in topology of branches with poor bootstrap support. The tree derived from third positions only was more resolved than the tree derived from first and second positions. The rbcL-based phylogeny is largely congruent with published analyses of small subunit rDNA sequences for the Zygnematales. The molecular data do not support hypotheses of monophyly for groups of extant unicellular and filamentous or colonial desmid genera exhibiting a common cell shape. A trend is evident from simple omniradiate cell shapes to taxa with lobed cell and plastid shapes, which supports the hypothesis that chloroplast shape evolved generally from simple to complex. The data imply that multicellular placoderm desmids are monophyletic. Several anomalous placements of genera were found, including the saccoderm desmid Roya in the Gonatozygaceae and the zygnematacean Entransia in the Coleochaetales. The former is strongly supported, although the latter is not, and Entransia's phylogenetic position warrants further study.