Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Vol

Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Vol. XXXVIII, No. 4 Institute of Oceanography (153-164) University of Gdańsk ISSN 1730-413X 2009 eISSN 1897-3191 Received: July 03, 2008 DOI 10.2478/v10009-009-0052-2 Review paper Accepted: April 22, 2009 The effect of the ploidy level and genetic background of Sphagnum denticulatum on its morphology and ecological requirements Iwona Melosik1 Department of Genetics, Adam Mickiewicz University ul. Umultowska 89, 61-614 Poznań, Poland Key words: Sphagnum denticulatum, polyploidy, morphological variability, genetic variability, ecological requirements, epigenetic remodeling Abstract This paper presents a current study on the morphology, genetic variability, and ecological requirements of the gametophytically diploid S. denticulatum (Bryophyta, Sphagnaceae). Its broad variations in morphology and physiology, coupled with its low genetic variability, may be explained by epigenetic remodeling in response to environmental heterogeneity. Phenotypes initiated via a plastic response can be canalized in the stable and predictable conditions on the bottom of Lobelia lakes. The problem of the different development of these isolated populations is a matter for further taxonomic studies and discussion. Taking into account the great physiological tolerance and massive development of S. denticulatum, predominantly in man-made and man-modified habitats, the question arises: how far should we go to protect this species? This is particularly important at sites where it threatens the survival of other protected plants. 1 [email protected] Copyright© by Institute of Oceanography, University of Gdańsk, Poland www.oandhs.org 154 I. Melosik INTRODUCTION Sphagnum denticulatum (Bryophyta, Sphagnaceae) is in Poland a native species protected by law. It is scattered throughout most parts of the country and is more frequent only in the lake lands in the northwest and in the heavily- polluted Silesian Upland in the southwest (Melosik 2000). It can be found both in natural ecosystems (e.g., in oligotrophic Lobelia lakes), and in man-made habitats that have been additionally acidified by air pollution, among other factors. This study considers the physiological and morphological features as well as ecological requirements of this species in the context of its genetic background. ORIGIN OF S. DENTICULATUM Species of the S. subsecundum complex, to which S. denticulatum belongs, are diploid or haploid. Sphagnum denticulatum and two other names, S. inundatum and S. lescurii that are both well-known in the literature, are considered to be gametophytically diploids (Newton 1993), with gametophytic DNA content of 0.672-0.790 pg DNA (2x, 1C) (Melosik et al. 2005). Allopolyploidy is considered to be a prevalent mechanism of speciation among bryophytes and other groups of plants (for a review see Såstad 2004). However, in spite of extensive molecular studies on the S. subsecundum complex (Shaw et al. 2005), the origin of S. denticulatum is still unknown and several scenarios may be considered, e.g., a “homoploid hybrid speciation” from a diploid, or different evolutionary lineages of S. inundatum in which reproductive isolation might arise, e.g., through ecological divergence (see also: Ungerer et al. 1998, Buerkle et al. 2000, Doyle et. al. 2002, Shaw et al. 2005, for review, see: Gross and Rieseberg 2005, Rosenthal et al. 2002). Irrespective of their origin, all polyploids have some common features: a high level of gene duplication; in some cases, functional divergence; and a higher level of heterozygosity compared with their progenitors (Soltis and Soltis 1999, 2000). ECOLOGICAL REQUIREMENTS The features of polyploids can alter ecological tolerance and might play some role in the establishment and diversification of their lineages (Otto and Whitton 2000, Osborn et al. 2003). Sphagnum denticulatum has a very wide ecological amplitude, growing both in terrestrial habitats and on semi-submerged or completely submerged sites, sometimes under extreme non-equilibrium conditions, disturbed either naturally or by human activity. Copyright© by Institute of Oceanography, University of Gdańsk, Poland Effect of ploidy level of Sphagnum denticulatum 155 On the one hand, aquatic morphotypes of this species occur in the well- oxygenated, transparent, cold waters of so-called Lobelia lakes or artificial ponds. They occur predominantly in the littoral zone, but in the Lobelia lakes can even reach depths of 9 m (Kraska, unpublished data). In the littoral zone of Lobelia lakes, it is usually found in communities neighboring Isoëto-Lobelietum (Koch 1926) R. Tx. 1937 em. Dierss. 1975 or rarely Isoëtetum echinosporae W. Koch 1926. (class Littorelletea uniflorae Br.-Bl. et R. Tx. 1943). Sphagnum denticulatum penetrates into patches of these associations when lake water becomes more acidic (see also, Brzeg et al. 2000). On the other hand, terrestrial morphotypes of this species occur in floristically poor associations (here it also has an optimum in the communities of the Littorelletea): usually on a narrow strip of sandy ground bordering lakes, which is alternately exposed or covered by water; on initial bogs among dunes on the sea shores, where the soil is not well stabilized; in coastal heaths; or in lower parts of mountains among acid, wet rocks, where vegetation is often destroyed by local flows. Besides, S. denticulatum is a common and dominant species in altered habitats more often in industrialized areas with all forms of acid deposition, such as in ditches, along footpaths, on disturbed bogs (around peaty pools, etc.), and rarely on wet meadows (Melosik 2000, Brzeg et al. 2000). Such niches are partially an effect of ecosystem fragmentation, hydrological changes, acid rain, or clearing of coniferous forest around lakes, and the subsequent acidification of lake waters (Roelofs 1983, see also: Kraska and Piotrowicz 1994 and Kraska et al. 1999, Melosik 2000). S. denticulatum can, therefore, be considered here as an auxochorous species (i.e., a native species that occurs predominantly in anthropogenic habitats). The high human pressure observed in places where S. denticulatum establishes itself may be related to its origin, presumably due to natural or anthropogenic habitat disturbance and the breakdown of reproductive barriers within its parental species/lineages (see also: Gross and Rieseberg 2005). In this kind of niche populations of various species are often founded from only a few individuals that are isolated from source populations; so inter- and intra-specific competition is reduced considerably. This presumably helps to increase fitness after a number of generations. MORPHOLOGICAL VARIABILITY In the observed extreme habitats colonized by Sphagnum denticulatum, the acquisition of extreme traits seems to have been necessary (see also Rosenthal et al. 2002, Gross and Rieseberg 2005). The larger plant size and larger stem and branch leaves in comparison with its closest polyploid relatives www.oandhs.org 156 I. Melosik (S. inundatum and S. lescurii), can be a plastic response to more favorable conditions with reduced competition, where resource availability is increased, but it also can be an effect of changes in gene expression, connected with its polyploid level (Melosik et al. 2005) or an effect of its parental composition (see also: Osborn et al. 2003), and indirectly a result of its predominantly clonal reproductive strategy (see also Thompson and Eckert 2004, Brown and Eckert 2005). Sphagnum denticulatum is considered as the most morphologically variable species in the genus Sphagnum, and clear connections between particular morphotypes and their habitats is observed (e.g. Daniels 1993, Wojtuń 2006, Melosik 2008). On the one hand, in various unpredictable terrestrial habitats, selection reduces the plasticity of fitness, but it favors plasticity of morphological traits (Baker 1965, Schlichting and Smith 2002). This could explain the great ability of this species to invade various terrestrial habitats and its plasticity. On the other hand, the permanently submerged aquatic populations of S. denticulatum have evolved in relatively constant and predictable conditions. This leads to fixation of adaptive traits including some key traits – pore traits in stem leaves), which continue to be expressed even if the environment changes and these traits become harmful (canalization of the trait) (Melosik 2008, see also Waddington 1942, Stearns 1994, Schlichting and Smith 2002, Alpert and Simms 2002, Proulx and Phillips 2005, Debat and Dawid 2001). These traits that are highly correlated with fitness exhibit low plasticity in this environment (Melosik 2008). The relatively high fitness of S. denticulatum may result from either the production of novel epistatic gene combinations, or through the combining of advantageous alleles across additive loci (see also: Burke and Arnold 2001). The great variability and relative instability of phenotypic characteristics, particularly in terrestrial conditions, may also be a hallmark of nascent allopolyploidy (see also Comai et al. 2000, Liu and Wendel 2002). BREEDING SYSTEM Sphagnum denticulatum is an organism with facultative sexual reproduction. Environmental stress is correlated with an increased tendency for sexual reproduction, e.g. individuals introduced into new terrestrial habitats usually tend to disperse more than individuals in well-adapted terrestrial populations (Melosik, unpublished observations; see also: Hadany and Beker 2003).

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