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Temperature-Related Phenotypic Plasticity in the Green Microalga Micrasterias Rotata

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Temperature-Related Phenotypic Plasticity in the Green Microalga Micrasterias Rotata Vol. 51: 77–86, 2008 AQUATIC MICROBIAL ECOLOGY Published April 24 doi: 10.3354/ame01184 Aquat Microb Ecol OPENPEN ACCESSCCESS Temperature-related phenotypic plasticity in the green microalga Micrasterias rotata Jirˇí Neustupa*, Jan 2t’astn´y, Ladislav Hoda< Department of Botany, Charles University of Prague, Benatska 2, Prague 2, 128 01, Czech Republic ABSTRACT: Temperature-related morphological variation of Micrasterias rotata (Desmidiales) strain CAUP K604 was investigated using landmark-based geometric morphometrics. Cultured morpho- types were compared with natural populations of M. rotata collected in different seasons at a central European lowland peat bog. In addition, we compared temperature-related variation with morpho- metric differentiation among other species within the genus Micrasterias. As temperatures increased, the population size of cultured M. rotata generally decreased and shape of the individual tempera- ture groups differed significantly. The high temperature (30°C) population contained the vegetative diploid cells. The shape variation related to temperature was similar to the size-related change in shape. Natural populations of M. rotata were consistently similar to the low temperature cultured populations throughout the season, while the high temperature morphotypes seen in culture were not present in natural samples. KEY WORDS: Geometric morphometrics · Desmidiales · Phenotypic plasticity · Thin-plate spline Resale or republication not permitted without written consent of the publisher INTRODUCTION environmental stress, and high temperatures can affect morphogenetic processes, leading to more vari- A temperature-related decrease in the body size of able or even teratogenic shapes of the high-tempera- protists has been demonstrated by several authors ture populations (Kristensen et al. 2003). In the present (Atkinson et al. 2003, De Boer et al. 2005, Gächter & study temperature-related phenotypic plasticity of a Weisse 2006). Atkinson et al. (2003) proposed a model, freshwater unicellular green alga, Micrasterias rotata based on meta-analysis of literature data covering a (Desmidiales, Viridiplantae), was investigated. wide range of aquatic protist organisms, indicating an The complex morphology and relatively large size of approximately 2.5% reduction in cell size for each 1°C Micrasterias cells have made members of this genus increase in temperature within a range that is normally the favorite model organism for morphogenetic and encountered by an organism in nature. However, cell plasticity studies (Kallio 1953, Meindl 1993, Weiss et al. size usually increases in low sublethal temperatures 1999). As seen from the front, Micrasterias cells are and this partially hampers the population growth composed of 2 flat symmetrical semicells that are bilat- (Atkinson et al. 2003). erally symmetrical (Fig. 1A). Each semicell consists of a A temperature-related decrease in cell size was central polar lobe and 2 lateral lobes that are further explained by either a higher fitness in populations that divided into individual lobules (Prescott et al. 1977). divide earlier as population growth rate increases in a Meindl (1993) demonstrated that the bilateral symme- higher temperature environment, or as an adaptation try of semicells and standard pattern of lobes and lob- to facilitate the consumption of resources, whose avail- ules is completely lost in Micrasterias denticulata cells ability is reduced by increased temperature (Atkinson grown at temperatures elevated to about 35°C. Exten- et al. 2003). However, little data exist on temperature- sive morphological malformations also occurred in related shape variation of eukaryotic microorganisms. populations of the same species exposed to short-term Increased phenotypic plasticity has been correlated to approximately 40°C heat shocks (Weiss et al. 1999). *Email: [email protected] © Inter-Research 2008 · www.int-res.com 78 Aquat Microb Ecol 51: 77–86, 2008 ticity levels using morphological disparity measures of individual species or populations (Zelditch et al. 2004). In contrast to conventional morphometric methods, geometric morphometric data are able to represent variation in shape as a whole in an investigated data set. The higher statistical power of geometric morpho- metric methods compared with traditional measure- ments to discriminate biological objects has been demonstrated many times in different organisms (e.g. Rohlf 2000, Monteiro et al. 2002), including microalgae (Beszteri et al. 2005). These methods have been used increasingly in algal and protist research in both taxo- nomic (Beszteri et al. 2005, Neustupa & Neˇ mcová 2007) and ecological studies (Potapova & Hamilton 2007). In desmids, correct infrageneric discrimination of populations belonging to 14 central European spe- cies of the genus Micrasterias was demonstrated by Neustupa & 2t’astn´y (2006) on the basis of geometric morphometric data. In addition, patterns of plasticity spanned by principal component analysis (PCA) of geometric morphometric data in 5 Micrasterias species populations were closely correlated with molecular phylogenetic data of the genus (Neustupa & 2kaloud 2007). In the present study, species-specific variation in morphology relative to temperature level and the pos- sible increase in phenotypic plasticity as a result of eventual stress-related morphogenetic disturbances in higher temperatures were investigated. In addition, using the comparative generic data set of Neustupa & 2t’astn´y (2006), the Micrasterias rotata data were com- pared with other species of the genus to illustrate infra- specific plasticity in relation to infrageneric differenti- ation. We asked whether the different experimental temperature conditions can shift resulting morphology so that the morphometric analysis would then not be able to unambiguously separate the M. rotata popula- tions from other species of the genus. Finally, morpho- logical variation in the cultured M. rotata strain with seasonal variation of natural populations of the same species was compared. We asked which temperature levels produced morphotypes most similar to those Fig. 1. Micrasterias rotata. Morphology of vegetative cells and found in natural populations and whether the morphol- positions of landmarks. (A) The standard haploid cell with ogy of natural populations varied significantly during positions of 31 landmarks (indicated by numbers) on the the season. adult semicell. (B) The vegetative diploid cell from the 30°C population MATERIALS AND METHODS However, the effect of higher temperatures on pheno- typic plasticity at levels that would not completely dis- The strain of Micrasterias rotata used in this study rupt the morphogenetic processes was not investi- (CAUP K604) was originally obtained as a single cell gated. isolate from benthos of flooded quarry pools near Cep Landmark-based geometric morphometrics (Book- village, South Bohemia, Czech Republic, in October stein 1991) can be used to precisely characterize pat- 2005. Cultures were initiated with an inoculum of 15 terns of morphological variation and quantify the plas- cells and grown for 2 wk in 250 ml Erlenmeyer flasks Neustupa et al.: Phenotypic plasticity in Micrasterias rotata 79 containing liquid oligotrophic medium developed by TpsRelw, ver. 1.42, was used for GPA of our data set. the Culture Collection of Algae of Charles University Correlation between Procrustes and the Kendall tan- of Prague (CAUP) (http://botany.natur.cuni.cz/algo/ gent space distances was assessed using TpsSmall, caup.html). Cultures were maintained at temperatures ver. 1.20, to ensure that the variation in shape was of 10, 15, 20, 25 and 30°C and illuminated at 40 µmol small enough to allow subsequent statistical analyses m–2 s–1 from 18 W cool fluorescent tubes (Philips TLD (Zelditch et al. 2004). The correlation of Procrustes and 18W/33), at a light:dark regime of 12:12 h. Micro- the Kendall shape spaces was very high (r = 0.999 to graphs were taken on an Olympus BX51 light micro- 1.000 in all analysed data sets), so we proceeded with scope with Olympus Z5060 digital microphotographic further analyses. As Micrasterias semicells are bilater- equipment. As a comparative set of natural popula- ally symmetrical and anterior and posterior sides do tions, 6 samples of phytobenthos dominated by Micras- not differ, cells were symmetrized prior to analysis terias rotata were collected from the benthos of a following the standard formula of Klingenberg et al. single 10 to 15 cm shallow peat bog pool in Borkovická (2002). This involved reflecting the cells (by multiply- Blata Nature Reserve in South Bohemia, Czech Repub- ing the x-coordinates in all landmarks by –1), then re- lic, altitude 410 m above sea level (a.s.l.), pH 5.75 to labelling the paired landmarks in the reflected copy of 6.10. Samples were taken on 22 April, 19 May, 15 June, landmark configurations and averaging the original 26 July, 9 October and 7 December 2004. and mirrored configurations in the Procrustes super- For each temperature level and for each natural imposition (GPA). The averages of original and sample, 50 adult semicells were randomly chosen and reflected/relabelled cells are ideal symmetric shapes, photographed. On each semicell, 31 structurally corre- where each half, together with landmarks lying on sponding landmarks were depicted (Fig. 1A, Table 1). median axis, bears all the information on the shape of For most of the geometric morphometric analyses the a symmetric object averaged from its halves.
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