Phylogenetic, ecological and intraindividual variability patterns in grass phytolith shape.

BACKGROUND AND AIMS: Grass silica short cell (GSSC) phytoliths appear to be the most reliable source of fossil evidence for tracking the evolutionary history and paleoecology of grasses. In recent years, modern techniques that quantitatively assess phytolith shape variation have widened opportunities for the classification of grass fossil phytoliths. However, phylogenetic, ecological and intraindividual variability patterns in phytolith shape remain largely unexplored. METHODS: The full range of intraindividual phytolith shape variation [3650 two-dimensional (2-D) outlines] from 73 extant grass species, 48 genera, 18 tribes and eight subfamilies (particularly Pooideae) was analysed using geometric morphometric analysis based on semi-landmarks spanning phytolith outlines. KEY RESULTS: The 2-D phytolith shape is mainly driven by deep-time diversification of grass subfamilies. There is distinct phytolith shape variation in early-diverging lineages of Pooideae (Meliceae, Stipeae). The amount of intraindividual variation in phytolith shape varies among species, resulting in a remarkable pattern across grass phylogeny. CONCLUSIONS: The phylogenetic pattern in phytolith shape was successfully revealed by applying geometric morphometrics to 2-D phytolith shape outlines, strengthening the potential of phytoliths to track the evolutionary history and paleoecology of grasses. Geometric morphometrics of 2-D phytolith shape is an excellent tool for analysis requiring large numbers of phytolith outlines, making it useful for quantitative palaeoecological reconstruction.

Inter- and intraspecific variation in grass phytolith shape and size: a geometric morphometrics perspective.

BACKGROUND AND AIMS: The relative contributions of inter- and intraspecific variation to phytolith shape and size have only been investigated in a limited number of studies. However, a detailed understanding of phytolith variation patterns among populations or even within a single plant specimen is of key importance for the correct taxonomic identification of grass taxa in fossil samples and for the reconstruction of vegetation and environmental conditions in the past. In this study, we used geometric morphometric analysis for the quantification of different sources of phytolith shape and size variation. METHODS: We used landmark-based geometric morphometric methods for the analysis of phytolith shapes in two extant grass species (Brachypodium pinnatum and B. sylvaticum). For each species, 1200 phytoliths were analysed from 12 leaves originating from six plants growing in three populations. Phytolith shape and size data were subjected to multivariate Procrustes analysis of variance (ANOVA), multivariate regression, principal component analysis and linear discriminant analysis. KEY RESULTS: Interspecific variation largely outweighed intraspecific variation with respect to phytolith shape. Individual phytolith shapes were classified with 83 % accuracy into their respective species. Conversely, variation in phytolith shapes within species but among populations, possibly related to environmental heterogeneity, was comparatively low. CONCLUSIONS: Our results imply that phytolith shape relatively closely corresponds to the taxonomic identity of closely related grass species. Moreover, our methodological approach, applied here in phytolith analysis for the first time, enabled the quantification and separation of variation that is not related to species discrimination. Our findings strengthen the role of grass phytoliths in the reconstruction of past vegetation dynamics.

Disentangling phylogenetic and functional components of shape variation among shoot apical meristems of a wide range of herbaceous angiosperms.

PREMISE: The shoot apical meristem (SAM) is the basic determinant of plant body organization, but interspecific variation in SAM shape and its relationship to stem and leaf morphological traits is not well known. Here we tested the hypothesis that different SAM shapes are associated with specific shoot traits of the plant body and examined the phylogenetic conservatism of these relationships. METHODS: We used geometric morphometrics of SAM outlines for a phylogenetically representative set of 110 herbaceous angiosperms and examined their relationship to a number of shoot traits. RESULTS: We found large variations in SAM shapes across angiosperm lineages, but covering only a subset of geometrically possible shapes. Part of this variation was allometric (due to SAM size), but the dominant shape variation (dome-shaped vs. flat surface) was size-independent and strongly phylogenetically conserved. SAM shapes were largely independent of their cell size and therefore of the number of cells involved. Different patterns in shape variation of outer and inner SAM boundaries were associated with stem thickness, leaf area, and leafiness of the stem. CONCLUSIONS: The findings show that geometric interdependence of meristem zones gives rise to correlations among organ numbers, sizes, and their proportions. Phylogenetic conservatism in these correlations indicates conservatism in regulatory processes that underlie the correlations, or the individual traits, that give rise to plant architecture.

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.

Shape differences between the faces of homosexual and heterosexual men.

Previous studies have shown that homosexual men differ from heterosexual men in several somatic traits and lay people accurately attribute sexual orientation based on facial images. Thus, we may predict that morphological differences between faces of homosexual and heterosexual individuals can cue to sexual orientation. The main aim of this study was to test for possible differences in facial shape between heterosexual and homosexual men. Further, we tested whether self-reported sexual orientation correlated with sexual orientation and masculinity-femininity attributed from facial images by independent raters. In Study 1, we used geometric morphometrics to test for differences in facial shape between homosexual and heterosexual men. The analysis revealed significant shape differences in faces of heterosexual and homosexual men. Homosexual men showed relatively wider and shorter faces, smaller and shorter noses, and rather massive and more rounded jaws, resulting in a mosaic of both feminine and masculine features. In Study 2, we tested the accuracy of sexual orientation judgment from standardized facial photos which were assessed by 80 independent raters. Binary logistic regression showed no effect of attributed sexual orientation on self-reported sexual orientation. However, homosexual men were rated as more masculine than heterosexual men, which may explain the misjudgment of sexual orientation. Thus, our results showed that differences in facial morphology of homosexual and heterosexual men do not simply mirror variation in femininity, and the stereotypic association of feminine looking men as homosexual may confound judgments of sexual orientation.

Leptochlorella corticola gen. et sp. nov. and Kalinella apyrenoidosa sp. nov.: two novel Chlorella-like green microalgae (Trebouxiophyceae, Chlorophyta) from subaerial habitats.

The diversity of green microalgae in subaerial habitats remains largely unexplored and a number of new genus- and species-level lineages have been discovered recently. The traditional green algal genus, Chlorella, which accommodated coccoid unicellular green algal species with globular to oval cells, reproducing entirely by autospores, has been found to be polyphyletic. In this study, we provide a detailed characterization of two strains of microalgae isolated from tree bark in the Mediterranean. These algae share the general Chlorella-like morphology and their 18S rRNA and rbcL gene sequences place them in the Trebouxiophyceae. Strain CAUP H8401 forms an independent trebouxiophycean lineage, together with three previously published 18S rRNA gene environmental sequences of undescribed microalgae, which were retrieved from profoundly different habitats. In contrast, strain CAUP H7902 is related to Kalinella bambusicola in the Watanabea clade of the Trebouxiophyceae on the basis of its 18S rRNA gene sequence. This relationship is also supported by the rbcL gene sequence, acquired from the type strain of K. bambusicola. The investigated strains are described as representatives of a novel species in a new genus, Leptochlorella corticola gen. et sp. nov., and a novel species, Kalinella apyrenoidosa sp. nov., according to the International Code of Nomenclature for Algae, Fungi and Plants.

Halimeda tuna (Bryopsidales, Ulvophyceae) calcification on the depth transect in the northern Adriatic Sea; carbonate production on the microscale of individual segments.

Halimeda tuna (J. Ellis & Solander) J.V. Lamouroux is the only Halimeda species found in the Mediterranean Sea, and it is an important habitat former. In the northern Adriatic, H. tuna is among the ten most abundant seaweeds in the upper-infralittoral belt in spring and autumn. The modular thalli consist of serially arranged calcified segments. Calcification is closely related to photosynthesis, which causes alkalinization of the inter-utricular space and triggers aragonite formation. Understanding of the complex patterns of segment shape plasticity in relation to CaCO3content at different depth levels is still incomplete. Geometric morphometrics was used to investigate H. tuna segment shape variation on the depth transect at Cape Madona Nature Monument in the northern Adriatic Sea. The position on the thallus and the CaCO3 content of each studied segment were recorded, allowing slight changes in mineral content to be detected at the microscale of the segments. Our results showed that shape, size, or asymmetry of H. tuna segments were not significantly affected by depth. On the other hand, plants that grew deeper were generally more calcified. The apical and subapical segments contributed to the increase in CaCO3 content at the deeper sites, whereas the basal segments did not. This indicates that reniform or oval segments positioned apically or subapically play a key role in calcification of H. tuna in Mediterranean ecosystems.

A multilocus phylogeny of the desmid genus Micrasterias (Streptophyta): evidence for the accelerated rate of morphological evolution in protists.

Micrasterias, the name of which is derived from the Greek for 'little star', comprises possibly the most spectacularly shaped desmids (Desmidiales, Streptophyta). Presently, the genus Micrasterias includes about 60 traditional species, the majority of which were described in the early 19th century. We used a comprehensive multigene dataset (including SSU rDNA, psaA, and coxIII loci) of 34 Micrasterias taxa to assess the relationships between individual morphological species. The resulting phylogeny was used to assess the patterns characterizing the morphological evolution of this genus. The phylogenetic analysis led to the recognition of eight well-resolved lineages that could be characterized by selected morphological features. Apart from the members of Micrasterias, three species belonged to different traditional desmid genera (Cosmarium, Staurodesmus, and Triploceras) and were inferred to be nested within the genus. Morphological comparisons of these species with their relatives revealed an accelerated rate of morphological evolution. Mapping morphological diversification of the genus on the phylogenetic tree revealed profound differences in the phylogenetic signal of selected phenotypic features. Whereas the branching pattern of the cells clearly correlated with the phylogeny, cell complexity possibly reflected rather their adaptive morphological responses to environmental conditions. Finally, ancestral reconstruction of distribution patterns indicated potential origin of the genus in North America, with additional speciation events occurring in the Indo-Malaysian region.

Male sterility significantly elevates shape variation and fluctuating asymmetry of zygomorphic corolla in gynodioecious Glechoma hederacea (Lamiaceae).

Female flowers of gynodioecious plants usually have smaller corollas than bisexual flowers. This difference is explained as a developmental consequence of stamen abortion and as a result of stronger selection for larger corolla size in hermaphroditic flowers that solely ensure male function within populations. This study evaluated whether the size difference of zygomorphic corollas in a widely distributed gynodioecious herb Glechoma hederacea is accompanied by variation in shape and bilateral fluctuating asymmetry of sexually differentiated flowers. Geometric morphometric analyses of bilateral symmetry in the shapes of corolla lower lips showed that male-sterile flowers were significantly more plastic and asymmetric, implying that they may be subjected to weaker stabilizing selection for corolla shape in comparison to hermaphrodites. These results illustrated that sexual differentiation may be an important factor contributing to bilateral fluctuating asymmetry in the shape of zygomorphic flowers.

Gynodioecy in the common spindle tree (Euonymus europaeus L.) involves differences in the asymmetry of corolla shapes between sexually differentiated flowers.

Gynodioecy is typically associated with a smaller perianth size in purely pistillate flowers than in hermaphrodite flowers. However, it is unclear whether this size differentiation is associated with any differences in flower shape between the two sexual groups. A geometric morphometric analysis of the symmetry of tetrameric corolla shapes was used in the study of Euonymus europaeus L., Darwin's classical system of floral sexual differentiation. I investigated whether there are any shape differences between the female and bisexual flowers, with respect to both purely symmetric variation involving coordinated shape changes of the four petals and asymmetry among petals within flowers. The corolla shapes of the female and bisexual flowers and the variability among flowers within each sexual group were very similar in the purely symmetric components of shape variation. However, the female flowers were significantly more asymmetric with respect to both the lateral and transversal asymmetry of their corolla shapes. This is the first study to apply geometric morphometrics in the analysis of morphological patterns in a sexually differentiated gynodioecious plant system. The results showed that subtle shape differences in corolla asymmetry differ between the sexual groups and indicate diverging developmental or selection signals between the sexes.

Morphological allometry constrains symmetric shape variation, but not asymmetry, of Halimeda tuna (Bryopsidales, Ulvophyceae) segments.

Green algae of the genus Halimeda have modular siphonous thalli composed of multiple repeated segments. Morphological variation among the segments has been related to various environmental factors, which often jointly affect their size and shape. The segments are bilaterally symmetric, which means that their shape variation can be decomposed into the symmetric and asymmetric components. Asymmetric variation might reflect both environmental heterogeneity and developmental instability of morphogenetic processes during the development of segments. In the present study, we examined if segment shape in H. tuna is related to their size and if an allometric relationship can also be found with respect to their asymmetry. Relative contributions of directional and fluctuating asymmetry to the segment shape variation within individual plants were investigated at two close localities in the northern Adriatic Sea. A series of equidistant semilandmarks were set along the outline of the segments, and analyzed by geometric morphometrics using two parallel methods to optimize their final position. Symmetric variation was strongly constrained by allometry, which also explained differences between populations. Smaller segments were significantly more asymmetric, but the difference in asymmetry between populations could not be explained solely by this allometric relationship. These differences between populations might have been caused by variation in local environmental factors. We conclude that members of the genus Halimeda represent an intriguing model system for studies of morphometric symmetry and asymmetry of sessile marine organisms, including effects of allometric relationships and infraspecific variation in relation to environmental factors of the benthic coastal habitats.

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.

Distribution of the genus Mallomonas (Synurophyceae) - ubiquitous dispersal in microorganisms evaluated.

The neutral dispersal model in protists was suggested as a general principle resulting in either cosmopolitan or ecologically restricted distribution of individual species. The high local diversity results in "flat" species-area curves of individual protist groups. We investigated the local and regional diversity of the genus Mallomonas in the alluvial plain of upper Luznice in the Czech Republic. About 86.5% of species previously reported from all types of freshwater biotopes within the country were found in our investigated localities. However, there was a considerable increase of species numbers in relation to the total area of available habitats on the continent and global scales. In three species found in our localities, the floristic data indicate a possible geographically restricted distributional pattern. Here, we discuss possible reasons for this phenomenon.

Microalgal biofilms on common yew needles in relation to anthropogenic air pollution in urban Prague, Czech Republic.

Excessive occurrence of microalgae on needles of gymnosperms was reported for the first time in the 1980s from the Scandinavian countries. Since then, it has been repeatedly encountered on needles from various European forest habitats. The abundance of these biofilms has been related to the climatic conditions, such as temperature and precipitation, as well as to the air pollution by nitrogen and sulfur oxides. Urban areas typically have relatively homogenous climates and profound variation in levels of air pollution. Therefore, variation in the occurrence of biofilms in localities within an urban area may be related to local anthropogenic air pollution. We investigated the abundance of biofilms occurring on needles of the common yew (Taxus baccata) in the city of Prague, Czech Republic. The biofilms were composed of algae, fungi and particulate matter. The cover area of the biofilms was marginally explained by a positive influence of short-term maximum atmospheric levels of nitrogen dioxide (NO2). The amounts of the microalgae were also positively influenced by short-term maximum NO2 levels. In addition, high atmospheric levels of particulate matter (PM10) were related to low abundance of algae. The microbial biofilms growing on widely cultivated conifers, such as the common yew, form one of the few commonly occurring natural communities in highly urbanized central areas of temperate European cities. Consequently, we propose that microscopic analysis of biofilms may be used as a rapid and cheap method to collect ecological data. Such data may be used in biomonitoring schemes illustrating the effects of anthropogenic air pollution on natural microcommunities in urban areas.

DNA content variation and its significance in the evolution of the genus Micrasterias (Desmidiales, Streptophyta).

It is now clear that whole genome duplications have occurred in all eukaryotic evolutionary lineages, and that the vast majority of flowering plants have experienced polyploidisation in their evolutionary history. However, study of genome size variation in microalgae lags behind that of higher plants and seaweeds. In this study, we have addressed the question whether microalgal phylogeny is associated with DNA content variation in order to evaluate the evolutionary significance of polyploidy in the model genus Micrasterias. We applied flow-cytometric techniques of DNA quantification to microalgae and mapped the estimated DNA content along the phylogenetic tree. Correlations between DNA content and cell morphometric parameters were also tested using geometric morphometrics. In total, DNA content was successfully determined for 34 strains of the genus Micrasterias. The estimated absolute 2C nuclear DNA amount ranged from 2.1 to 64.7 pg; intraspecific variation being 17.4-30.7 pg in M. truncata and 32.0-64.7 pg in M. rotata. There were significant differences between DNA contents of related species. We found strong correlation between the absolute nuclear DNA content and chromosome numbers and significant positive correlation between the DNA content and both cell size and number of terminal lobes. Moreover, the results showed the importance of cell/life cycle studies for interpretation of DNA content measurements in microalgae.

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.

JENUFA GEN. NOV.: A NEW GENUS OF COCCOID GREEN ALGAE (CHLOROPHYCEAE, INCERTAE SEDIS) PREVIOUSLY RECORDED BY ENVIRONMENTAL SEQUENCING(1).

The diversity of eukaryotic microorganisms is far from fully described, as indicated by the vast number of unassigned genotypes retrieved by environmental sequencing or metagenomics. We isolated several strains of unicellular green algae from algal biofilms growing on tree bark in a Southeast Asian tropical rainforest and determined them to be relatives of an unidentified lineage of environmental 18S rDNA sequences, thus uncovering its cellular identity. Light, confocal, and electron microscope observations and sequencing the 18S rRNA gene revealed that the strains represent two different species within an apparently new genus, described here as Jenufa gen. nov. Both species formed minute coccoid cells with an irregular globular outline, a smooth cell wall, and a single parietal chloroplast without a pyrenoid. The two species, described herein as J. perforata and J. minuta, differed in chloroplast morphology and cell wall structure. Phylogenetic analyses of 18S rRNA gene sequences showed a firm relationship between the two species and placed the Jenufa lineage in an unresolved position within the CS clade (Chlamydomonadales + Sphaeropleales) of the class Chlorophyceae, although possible affinities to the genus Golenkinia were suggested both by maximum-likelihood (ML) and Bayesian methods. Furthermore, two almost identical environmental 18S rDNA sequences from an endolithic microbial community occurring in dolomite rock in the central Alps turned out to be specifically related to, yet apparently distinct from, the sequence of J. minuta, indicating the existence of an undescribed Jenufa species occurring in the temperate zone.

Pseudocryptic diversity versus cosmopolitanism in diatoms: a case study on Navicula cryptocephala Kütz. (Bacillariophyceae) and morphologically similar taxa.

Despite the significance of diatoms in biomonitoring, many aspects of their biodiversity and geographical distribution are poorly understood. Recent evidence from molecular data has shown that traditional cosmopolitan and euryvalent morphospecies are often heterogeneous, containing cryptic or pseudo-cryptic species. It is important to establish whether these more finely differentiated species are also cosmopolitan or show restricted distributions. According to the standard freshwater diatom floras, Navicula cryptocephala and morphologically similar species (N. veneta, N. trivialis, N. gregaria and N. cryptotenella) are common, cosmopolitan freshwater pennate diatoms. Although allopatric and even sympatric populations of N. cryptocephala are extremely similar morphologically, they have previously been found to be highly polymorphic with respect to reproductive and nuclear characteristics; however, molecular data supporting the existence of cryptic diversity were lacking. Phylogenetic analyses (LSU rDNA, ITS of the rRNA operon) of 52 strains of N. cryptocephala-like diatoms confirmed the existence of genetically distinct lineages within N. cryptocephala, and revealed a close relationship between N. trivialis and N. cryptocephala. Cytological, reproductive and morphological variation, investigated by means of landmark-based geometric morphometrics, were in congruence with molecular data. Two pseudo-cryptic species within N. cryptocephala coexist sympatrically and are widely distributed, occurring in both European and Australian lakes.

Hylodesmus singaporensis gen. et sp. nov., a new autosporic subaerial green alga (Scenedesmaceae, Chlorophyta) from Singapore.

The algal flora of subaerial habitats in the tropics remains largely unexplored, despite the fact that it potentially encompasses a wealth of new evolutionary diversity. Here we present a detailed morphological and molecular characterization of an autosporic coccoid green alga isolated from decaying wood in a natural forest in Singapore. Depending on culture conditions, this alga formed globular to irregularly oval solitary cells. Autosporulation was the only mode of reproduction observed. The cell periphery was filled with numerous vacuoles, and a single parietal chloroplast contained a conspicuous pyrenoid surrounded by a bipartite starch envelope. The cell wall was composed of a thick inner layer and a thin trilaminar outer layer, and the cell surface was ornamented with a few delicate ribs. Phylogenetic analyses of 18S rRNA gene sequences placed our strain in the family Scenedesmaceae (Sphaeropleales, Chlorophyceae) as a strongly supported sister branch of the genus Desmodesmus. Analyses of an alternative phylogenetic marker widely used for the Scenedesmaceae, the ITS2 region, confirmed that the strain is distinct from any scenedesmacean alga sequenced to date, but is related to the genus Desmodesmus, despite lacking the defining phenotypic features of Desmodesmus (cell wall with four sporopolleninic layers ornamented with peculiar submicroscopic structures). Collectively, our results establish that we identified a novel, previously undocumented, evolutionary lineage of scenedesmacean algae necessitating its description as a new species in a new genus. We propose it be named Hylodesmus singaporensis gen. et sp. nov. A cryopreserved holotype specimen has been deposited into the Culture Collection of Algae of Charles University in Prague, Czech Republic (CAUP) as CAUP C-H8001.