GENETIC DIVERSITY OF PARALIA SULCATA
GENETIC DIVERSITY OF PARALIA SULCATA
Genetic diversity of Paralia sulcata (Bacillariophyta) analysed by Inter Simple Sequence Repeats (ISSRs)
Christina Gebühr*, Gunnar Gerdts, Antje Wichels & Karen H. Wiltshire
Prepared for re-submission to: European Journal of Phyclogy
Biologische Anstalt Helgoland, Alfred Wegener Institute for Polar and Marine Research
POB 180
27489 Helgoland, Germany
*Corresponding Author:
e-mail: Christina.Gebuehr@awi.de Phone: +49(4725)819-3255
Fax: +49(4725)819-3283
Running head: Genetic diversity of Paralia sulcata
GENETIC DIVERSITY OF PARALIA SULCATA
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BSTRACTParalia sulcata is a centric, marine diatom and is abundant in the water column at Helgoland Roads (North Sea, Germany) mainly in the winter months. In recent years we observed a trend towards a less seasonal appearance of P. sulcata and not only restricted to low water temperatures. We hypothesised that this change in the ecological niche might be reflected in a genetically different population of P. sulcata.
Therefore, we evaluated the genetic diversity of P. sulcata at Helgoland Roads based on strains isolated over one year using an inter simple sequence repeat (ISSR) fingerprinting method. Surprisingly, the ISSR approach unveiled large intraspecific diversity. On the other hand analysis of similarity (ANOSIM) and non-metric multidimensional scaling (nMDS) based on Jaccard’s similarities of combined ISSR patterns revealed well separated P. sulcata populations. Strains isolated in January were clearly different to those isolated in July, September, October and December.
Significant correlations of the ISSR pattern with environmental parameters were detected. Specifically, the phosphate concentration, Secchi depth and temperature were shown to structure the P. sulcata population at Helgoland Roads. Furthermore, the high genetic diversity indicates that P. sulcata is well adapted to its frequently changing environment suggesting a more general ecological niche within its marine habitat.
Key words: 18S rDNA, ecological niche, Helgoland Roads, inter simple sequence repeat (ISSR), long-term data, marine diatoms, Paralia sulcata
GENETIC DIVERSITY OF PARALIA SULCATA
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NTRODUCTIONIntraspecific diversity in terrestrial plant communities has been studied for a long time.
It is supposed that the success of ecological communities is partly dependent on the genetic diversity of their populations which enables the adaptation of the species to a dynamic environment (Whitlock et al. 2007). Thus, the physiological response of species combined with environmental parameters is strongly dependent on the genetic diversity of the population and enables a high adaptation rate to new ecological species niches. In this context fitness can be considered as a product of genotype and environmental interactions (Aarssen 1989, Whitlock et al. 2007). To investigate the community structure of species and the genetic diversity within plant populations molecular markers are an important tool (Serra et al. 2007).
Phytoplankton, in particular the diatoms, are at the basis of the marine food web. The identification and monitoring of the phytoplankton species is currently mostly based on microscopic investigations of taxonomic characteristics. Due to optical limitations the differentiation of single species is sometimes difficult and also time intensive (Evans et al. 2007). Moreover, morphological features can often be very similar and thus, differentiation next to impossible. Genetic markers can provide a fast and specific means for identifying differences in the genetic composition and species dynamics of phytoplankton both on a spatial and a temporal scale (Medlin et al. 1988, Medlin 1990). Different molecular techniques, such as the analysis of ribosomal or functional genes and other molecular markers as well as random amplified polymorphic DNA (RAPD), simple sequence repeats (microsatellites, SSR) or amplified fragment length polymorphisms (AFLP), are increasingly important in investigations on intraspecific interactions in plant communities and also marine phytoplankton (Bornet & Branchard 2001, Serra et al. 2007). However, for the use of these molecular markers basic knowledge of specific genome sequences is necessary and the costs can be higher for developing initial primers (Zietkiewicz et al. 1994, Jarne & Lagoda 1996, McGregor et al. 2000, Serra et al. 2007). An advantage of microsatellite (SSR) markers is the fine-scale recognition of species on population levels allowing detection of intraspecies variations on both temporal and spatial scales (Evans et al. 2005, Alverson 2008).
Rynearson & Armbrust (2004) evaluated the high genetic diversity of Ditylum brightwellii populations using microsatellites. The authors identified three distinct D. brightwellii populations related to different sampling locations.
GENETIC DIVERSITY OF PARALIA SULCATA
Over the past decades the use of inter simple sequence repeats (ISSRs) as molecular markers is becoming more common. Within genomes ubiquitously distributed short tandem repeated sequence pattern exist (Zietkiewicz et al. 1994). Furthermore, each band amplified with ISSR-PCR corresponds to a DNA sequence with an individual fragment length and is enclosed by two inverted ISSR which leads to highly polymorphic band patterns (Bornet & Branchard 2001). In addition, ISSR markers require no genomic information, are very abundant and easy to use (Zietkiewicz et al.
1994, McGregor et al. 2000, Bornet & Branchard 2001, Bornet et al. 2004).
ISSR-PCR are well established in the identification of plant populations or plant cultivars via to differences in the mean number of fragments and polymorphic bands (Barth et al. 2002, Galvan et al. 2003, Serra et al. 2007) as well as for genetic characterisation and diversity studies on phytoplankton species (Bornet & Branchard 2001, Bornet et al. 2004). Bornet et al. (2004) showed that comparisons of ISSR pattern of all their tested species were discriminated between toxic and nontoxic species of Alexandrium and Pseudo-nitzschia as well on a local scale.
In this study we use ISSR fingerprints for investigation of intraspecific diversity of Paralia sulcata (Ehrenberg; Cleve, 1873). P. sulcata is a tychopelagic, centric marine diatom species (McQuoid & Nordberg 2003a) occurring in the water column at Helgoland Roads over the whole year (Wiltshire & Dürselen 2004, Gebühr et al.
2009). Figure 1 shows the abundance data of P. sulcata from January 2007 to January 2008 with maximal cell numbers in winter (over 2000 cells ml-1) and lower numbers in summer. This finding is in contrast to the literature where P. sulcata is mostly described as a winter algae (Hobson & McQuoid 1997). The multivariate statistical analysis on the long-term data set at Helgoland Roads exhibited that the changing environmental conditions lead to a shift in the ecological occurrence of P. sulcata from a winter diatom to a diatom occurring throughout the whole year (Gebühr et al. 2009) accompanied by niche shifts. Furthermore, the genetic diversity might either play a role in the general fitness and adaptation of the P. sulcata population or it could indicate species shifts. We assume that this observed shift in the ecological niche could be reflected in the genetic diversity of P. sulcata. To determine the genetic diversity a sampling campaign with the isolation of P. sulcata from the water column at different times of the year was started in 2007 at Helgoland Roads and P. sulcata cultures were analysed using 18S rDNA sequence and ISSR fingerprint method.
GENETIC DIVERSITY OF PARALIA SULCATA
Figure 1: Abundance data of Paralia sulcata (cells l-1) from January 2007 to January 2008 based on the long-term data set at Helgoland Roads, North Sea. The dots represent the abundance data, the running average are shown by the black line and the bars indicate the sampling dates of the water samples to isolate P. sulcata.