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Abstract

During laboratory and field experiments on Nacella concinna on the west coast of Admiralty Bay, King George Island (Antarctica) clear morphological and behavioural differences between two limpet forms (N. concinna polaris and N. concinna concinna) were found. They suggested presence of genetic divergence. AFLP (amplified fragment length polymorphism) profiling of N. concinna individuals representing the two forms revealed nearly 32% of polymorphic bands; only 2% of them differed between the forms. Our results suggest that the observed phenotypic variation seems to be a result of adaptation to environ− mental conditions and not of any genetic divergence.
Low genetic differentiation between two morphotypes
of the gastropod Nacella concinna from Admiralty Bay,
Antarctica
Katarzyna J. CHWEDORZEWSKA1, Małgorzata KORCZAK1, Piotr T. BEDNAREK2
and Marta MARKOWSKA−POTOCKA1
1Zakład Biologii Antarktyki PAN, Ustrzycka 10/12, 02−141 Warszawa, Poland
<kchwedorzewska@go2.pl>
2Instytut Hodowli i Aklimatyzacji Roślin, 05−870 Warszawa – Radzików, Poland
Abstract: During laboratory and field experiments on Nacella concinna on the west coast
of Admiralty Bay, King George Island (Antarctica) clear morphological and behavioural
differences between two limpet forms (N. concinna polaris and N. concinna concinna) were
found. They suggested presence of genetic divergence. AFLP (amplified fragment length
polymorphism) profiling of N. concinna individuals representing the two forms revealed
nearly 32% of polymorphic bands; only 2% of them differed between the forms. Our results
suggest that the observed phenotypic variation seems to be a result of adaptation to environ−
mental conditions and not of any genetic divergence.
Key wo rd s : Antarctica, Nacella concinna, genetic diversity, AFLP.
Introduction
Herbivorous gastropod Nacella concinna is a characteristic representative of
the Maritime Antarctic benthos. It occurs in two forms: N. concinna polaris and
N. concinna concinna (Berry and Rugge 1973; Branch and Branch 1981). Mor
phological and behavioral differences between these two forms were observed by
many authors (Nolan 1991; Markowska and Kidawa 2007; de Aranzamendi et al.
2008). N. concinna polaris occurs on stony bed rock in the littoral zone, and
N. concinna concinna is present in the sublittoral and in the intertidal zones below
4 m down to 110 m depth (Walker 1972; Berry and Rugge 1973; Castilla and
Rozbaczylo 1985; Nolan 1991) along the Antarctic Peninsula and adjacent islands
(Picken 1980). The intertidal morphotype has a taller and heavier shell as com
pared to the rock pool morphotype having a lighter and flatter shell (Powell 1951;
Nolan 1991). These two morphotypes also differ in important physiological traits
(Waller et al. 2006; Weihe and Abele 2008).
Pol. Polar Res. 31 (2): 195–200, 2010
vol. 31, no. 2, pp. 195–200, 2010 doi: 10.4202/ppres.2010.11
In previous studies limpets Nacella concinna were gathered from rock pools
during the low tide and by divers from the subtidal area at the depths of 5–8 m.
Limpets collected from rock pools were significantly smaller than the subtidal
individuals (ttest, P< 0.005), ranging from 19.8 to 43.3 mm shell length (mean
= 21.2, SE = 0.32 mm, n= 222), in comparison to 23.7–47.0 mm (mean = 37.4,
SE = 0.49, n= 191) for the subtidal ones. Visible difference in reaction to preda
tors was also noted between the two described limpet populations in laboratory
and field experiments (Markowska and Kidawa 2007; Markowska 2008).
The aim of this study was to explore the genetic relationships between the two
forms described above: the rock pool N. concinna polaris and the subtidal N.
concinna concinna form from the Admiralty Bay.
Material and methods
DNA extraction. — Samples were collected during the previous experiment
run by Markowska (2008). Forty eight individuals from both forms were taken for
the AFLP analysis. DNA was extracted from about 20 mg of tissue, stored in 70%
ethanol. Samples were dried on Watman paper, then followed overnight proteinase
K digestion. Next steps of extraction followed the manufacturer’s protocol recom−
mendation (NucleoSpin®Tissue, Macherzy−Nagel, AQUA LAB). Purity and quan−
tity of the samples were determined spectrophotometrically. DNA integrity and lack
of RNA impurities were tested by agarose gels electrophoresis (1 × TBE buffer with
0.5 μg/ml ethidium bromide at 20 V/cm).
AFLP analysis. — The AFLP technique was performed according to the previ−
ously described procedures with minor modifications (Chwedorzewska et al. 2006).
Briefly, 250 ng of genomic DNA was digested simultaneously with two restrictive
enzymes EcoRI and MseI. This was followed by ligation of the appropriate adaptors,
preselective and, finally, selective amplification steps (core sequence Table 1). For
196 Katarzyna J. Chwedorzewska et al.
Table 1
Adapter and primer sequences
Adapter/Primer code Sequence (5’−>3’)
Adapter EcoRI CTCGTAGACTGCGTACC
CATCTGACGCATGGTTAA
Adapter MseITACTCAGGACTCATA
GAGTCCTGAGTAGCAG
EcoRI preselective primer GACTGCGTACCAATTCA
MseI preselective primer GATGAGTCCTGAGTAAC
E−Azz − selective primer GACTGCGTACCAATTCAzz
M−Cyy − selective primer GATGAGTCCTGAGTAACyy
E and M are for any selective primer complementary to EcoRI and MseI adaptors respectively; −zz,
−yy − any combination of the nucleotides at the primers 3’ends.
the selective amplification seven primer pair combinations (EcoRI – AGG/MseI–
CAC, EcoRI – ACG/MseI – CAC, EcoRI – ACT/MseI – CGC, EcoRI – ACC/MseI
–CAC,EcoRI – AAA/MseI – CAG, EcoRI – AGT/MseI – CAA, EcoRI –
AAA/MseI – CGG) were used (Table 1). The EcoRI compatible primers were la
belled at their 5’−ends with gamma – 32P ATP. PCR products were separated on 5%
PAGE and X−ray films were exposed to the gels at −70°C overnight.
Data analysis and statistics. — Reproducible (experiment was run twice), in
dividual AFLP fragments were scored as either present (1) or absent (0). Their fre
quencies were calculated for all the markers. Xlstat v.7.5.2 excel add−in software
(www.xlstat.com, Addinsoft) was used to perform clustral analysis (Aggregation
criterion: unweighted pair−group average UPGMA, Jaccard coefficient of dissimi
larity) and construct the dendrogram. Principal Component Analysis (PCA) and
Analysis of Molecular Variance (AMOVA) was carried out using GeneAlex5.1
excel add−in software (Peakall and Smouse 2001).
Results and discussion
DNA profiling of the samples representing animals collected from rock pools
and from subtidal zone revealed 374 scoreable fragments amplified by seven
primer pair combinations. The number of the DNA fragments generated by an in−
dividual primer pair varied from 31 to 72 with an average of 53. In total all primer
pairs generated 121 (32% of all signals) polymorphic fragments with an average of
17 per primer combination (Table 1).
Clustral and PCA analyses using all the polymorphic bands failed to differentiate
among all samples collected from rock pools and from subtidal. Nevertheless, the first
main axis of the PCA explained 88.00% and the second 4.34% of the identified vari
ability (Fig. 1). Based on the mentioned methods samples were randomly distributed
and formed no separate groups. The similarity matrix was analysed using UPGMA.
Dendrogram was based on all DNA fragments obtained with seven selective primer
pairs for all individuals. Cluster analysis did not reveal any groups (the results of both,
cluster and PCA analysis were similar, thus only PCA diagram is presented).
Although molecular profiling of N. concinna individuals revealed nearly 32%
of the polymorphic bands, few (2%) were discriminated between the analyzed
populations as demonstrated by AMOVA (Table 2).
Genetic diversity of Nacella concinna 197
Table 2
Results of AMOVA of N. concinna samples from all analysed populations. Significance
tested by a permutation analysis against alternative random partitioning of individuals
(999) across populations.
Source of variation df SS Estimate variance Total variance (%) P−value
Among populations 1.0 5.312 0.066 1.7 <0.001
Within populations 46.0 172.001 3.739 98.3 <0.001
Genetic differentiation of the two forms was studied by de Aranzamendi et al.
(2008) with use of ISSR−PCR markers and indicated that the two analyzed forms
can be considered as genetically distinct populations maintaining low levels of
gene flow. Our results, obtained by a different genetic method focused on total
genomic DNA, showed much smaller differences between the analyzed forms (ac
cording to AMOVA less than 2%) and did not confirm results of de Aranzamendi
et al. (2008) study. These differences may be linked to too small sample size taken
for analysis by de Aranzamendi (only eleven individuals per population).
Thus, our results may point to specificity of this species reproduction physiol
ogy. Nacella concinna produces and disperses long−lived planktonic larvae, which
can survive for two months in the water column (Bowden et al. 2006). The pelagic
198 Katarzyna J. Chwedorzewska et al.
Fig. 1. Principal Component Analysis (PCA) of two population of Nacella concinna collected from
rockpools (A) and from subtidal zones (B), based on AFLP data.
long−lived larvae are able to colonise new sites rapidly, thus recruitment occurs
within a common gene pool and there is little opportunity for local genetic diver
gence. In N. concinna the reproduction takes place about 3 weeks after the water
temperature exceeds −1.4°C (Picken 1980, Picken and Allan 1983). Thus spawn
ing appears to be synchronized with the increasing temperature and probably the
availability of food (Brathes et al. 1994). However, it does not seem to be a suffi
cient barrier to avoid gene exchange in form of partial reproductive isolation.
Moreover, the intertidal morphotype (equivalent of rock pool in presented study)
is also migratory, moving seasonally between the intertidal and subtidal zones
(Walker 1972). Probably these phenotypic and behavioural variations (Mar
kowska 2008), specific to these populations, might be the result of phenotypic
plasticity expressed in particular environmental conditions, rather than it reflects
genetic differences. The heterogeneous environment that characterises intertidal
region (different wave exposures, temperature, ultraviolet irradiation and variable
chemical−physic parameters over a short vertical distance, different predators)
could be responsible for such a significant morphological variability.
Acknowledgments. — We would like to thank two anonymous reviewers for constructive
advice that has improved our paper.
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Received 21 October 2009
Accepted 28 April 2010
200 Katarzyna J. Chwedorzewska et al.
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