Evolution of the limpet Patella candei d'Orbigny (Mollusca, Patellidae) in Atlantic archipelagos

13 Pages·2002·1 MB·English

Blackwell Science, LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066The Linnean Society of London, 2002
77
Original Article
GENETIC STRUCTURE OF PATELLA CANDEI
L. I. WEBER and S. J. HAWKINS
Biological Journal of the Linnean Society, 2002, 77, 341–353. With 9 figures
Evolution of the limpet Patella candei d’Orbigny
(Mollusca, Patellidae) in Atlantic archipelagos:
human intervention and natural processes
LAURA I. WEBER1* and STEPHEN J. HAWKINS2†
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1Port Erin Marine Laboratory, University of Liverpool, Isle of Man ow
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2Biodiversity and Ecology Division, School of Biological Sciences, University of Southampton, lo
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Boldrewood Building, Southampton d
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Received 30 January 2002; accepted for publication 21 June 2002 m
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The genetic and morphological structure of Patella candei in the Macaronesian Islands were studied by allozyme a
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electrophoresis and multivariate analysis (canonical discriminant analysis), respectively. Sixteen enzymes, yielding e
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21 resolvable loci were analysed for all populations using the BIOSYS-1 program. A disjunct distribution of genet- ic
ically similar populations from the Azores and Selvagens Islands (P.c. gomesii–P.c. candei) was found, which showed .ou
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two diagnostic loci with P.c. ordinaria from Madeira. Patella c. crenata from the Canaries showed an intermediate .c
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composition of 92% P.c. ordinaria and 8% P.c. gomesii–P.c. candei. Hypotheses are presented to explain the evolution m
of the genetic and morphological structure of P.c. candei in the Macaronesian Islands, including the interaction of /b
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natural processes and human intervention. Patella c. candei is considered to represent the ancestral form. An extinc- lin
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tion or near extinction event may have happened in Madeira after which P.c. ordinaria arose, carrying only a small e
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part of the diversity of the original form, which evolved in isolation. The ancestral form, P.c. candei, was depleted by n
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human extraction from high levels of the shore in the Azores and Canaries. Patella c. gomesii, from the Azores rtic
evolved by phyletic evolution, colonizing low levels of the shore, leaving only a relict population in the Selvagens le
Islands, that remained almost free of human exploitation. Patella c. crenata is a hybrid with a high degree of intro- -ab
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gression to P.c. ordinaria, which is maintained by the forces of human extraction at high-tide levels and by intraspe- tra
cific competition at low-tide levels. © 2002 The Linnean Society of London, Biological Journal of the Linnean c
Society, 2002, 77, 341-353. t/77
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ADDITIONAL KEYWORDS: patellogastropoda – relict – human exploitation – selection. /34
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INTRODUCTION et al., 1996; Hodgson et al., 1996; Ridgway et al., 1998; b
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Koufopanou et al., 1999; Weber & Hawkins, unpub- g
Patella candei d’Orbigni 1840, is an endemic species of lished data). In this paper we particularly focus on ues
all north-west African archipelagos, the Macaronesian understanding the genetic relationships of P. candei at t o
Islands (Christiaens, 1973; Côrte-Real, Hawkins & the intraspecific level. n 0
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Thorpe, 1996), which comprise the Azores, Madeira, Patella candei has been highly exploited for human A
Sphelyvlaoggeennes,t icC apnoasritieiosn aonfd PCaateblola Vcearndde eiI swlainthdisn. Tthhee consumption in the Canaries, Madeira and the Azores. pril 2
In the Azores, Patella species form a substantial fish- 01
Patellidae has been extensively studied (Côrte-Real 9
ery and in 1986 they were reported as the sixth most
valuable commercially exploited species (Martins,
Santos & Hawkins, 1987; Hawkins et al., 1990b). The
stocks of Patella spp. in this archipelago have been
*Correspondence. Present address: LaboratoryBioquímica e
Biologia Molecular, Universidade do Vale do Itajaí-UNIVALI, declining steadily since the 1970s (Martins et al.,
Bloco 20, Sala 127, CEP 88302–202, Santa Catarina, SC, 1987; Hawkins et al., 2000). Because of difficulty of
Brazil. E-mail: [email protected] access, Patella have not been harvested in the Sel-
†Present address: The Marine Biological Association of the
vagens Islands and so this is considered a preserved
United Kingdom, The Laboratory, Citadel Hill, Plymouth
PL12PB, UK. area.
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353 341
342 L. I. WEBER and S. J. HAWKINS
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Figure 1. Patella candei subspecies, dorsal view. A, P. candei gomesii from the Azores. B, P.c. ordinaria from Madeira. C, de
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P.c. ordinaria from Deserta Grande. D, P.c. candei from Selvagens. E, P.c. candei, rare individuals from Fuerteventura, ic
Canaries. F, P.c. crenata from the Canaries. .o
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Figure 2. Patella candei subspecies. Internal view. A, P. candei gomesii from the Azores. B, P.c. ordinaria from Madeira. C, 07
P.c. ordinaria from Deserta Grande. D, P.c. candei from Selvagens. E, P.c. candei, rare individuals from Fuerteventura, Ap
Canaries. F, P.c. crenata from the Canaries. ril 2
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Christiaens (1973), in the last revision of the genus, Internally, the scar is bluish or grey, surrounded by
described four subspecies of P. candei (Figs 1,2): a white band, and with broad yellow-to-orange
P. candei candei, P. candei crenata, P. candei ordinaria, border. Its type locality is Santa Cruz from Tenerife
and P. candei gomesii. (Canaries), but nowadays it is most abundant in the
Patella candei candei d’Orbigny 1840 (Figs 1D,E; Selvagens Islands, reaching large sizes (Christiaens,
2D,E), is characterized by its highly conical and thick 1973; Hernández-Dorta, 1992). Although Christiaens
shell with smooth growth lines and ribs (Christiaens, (1973) suggested that this subspecies is now absent
1973), mostly whitish and very eroded on the outside. from the Canaries and Madeira, individuals of this
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
GENETIC STRUCTURE OF PATELLA CANDEI 343
form have been found by Hernández-Dorta (1992) between these presumptive subspecies. Nevertheless,
and by S.J. Hawkins during collections in 1992 in they found that P.c. gomesii from the Azores is distinct
Fuerteventura, Canaries (Figs 1E,2E). Drouët (1858) from P.c. ordinaria and P.c. crenata in its shell sculp-
cited this form for the Azores, but as rare in occurrence ture, shape and allozyme characters. Côrte-Real et al.
and as more conical and lighter in colour than (1996) suggested that P.c. gomesii might be regarded
P.c. gomesii. Hawkins et al. (1990b) described, from as a distinct species from those forms found in
São Miguel of the Azores, a form called ‘lapa mosca’ Madeira and the Canaries. However, they recognized
due to its small size, that was very rare in that locality the need for further studies on the other presumptive
(see Hawkins et al., 1990b: figs 23,24). ‘Lapa mosca’ subspecies, P. candei candei, a depleted form actually
lives on exposed surfaces above high-tide levels, sug- present only in Fuerteventura and the Selvagens.
gesting by its radular fraction (length of the radula/ Therefore, this study aimed to increase our knowl-
total length) to be a habitat form of P. candei (Hawkins edge of the genetic structure and morphological
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et al., 1990b). Patella c. candei also lives above high- patterns of P. candei populations to better understand o
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tide levels over smooth surfaces on the Selvagens and the striking pattern observed for this species by Côrte- nlo
Fuerteventura (Hernández-Dorta, 1992). Real et al. (1996) among the Macaronesian Islands. To ad
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Patella candei ordinaria Mabille 1888 (Fig. 1B,C; this end, new samples were taken and analysis was d
2B,C), has a suboval to stellate form. Internally, it has expanded to the subspecies P.c. candei and to interme- fro
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a white or yellow-to-orange scar, surrounded by a blu- diate islands, such as Santa Maria from the Azores, h
ish-purple iridescent colour with a thin light brown Deserta Grande from Madeira and the Selvagens. ttps
border (Christiaens, 1973). It has few, thick ribs (9–15 ://a
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approximately) running from the apex to the border of a
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the shell, where it shows extensions that give the shell MATERIAL AND METHODS m
a stellate outline. This subspecies is found in all of the SAMPLING SITES ic.ou
Madeira archipelago, including the Deserta Islands p
(Christiaens, 1973). Christiaens suggested that this Samples of Patella candei were taken at six localities .co
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form is also found in the Canaries, but rarely. How- along the north-west African archipelagos (Fig. 3): /b
ever, Hernández-Dorta (1992) searching a total of Faial (FAI), north-western Azores (N = 47, February iolin
56 localities around the Canaries, did not find any 1993, type P.c. gomesii); Santa Maria (SMA), south- ne
individuals of P.c. ordinaria. This subspecies lives on eastern Azores (N = 50, June 1993, type P.c. gomesii); an
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intertidal habitats, like most forms of P. candei, and Porto Moniz, Madeira (MAD), north-west of Madeira rtic
occasionally, in shallow subtidal waters. Archipelago (N = 50, July 1994, type P.c. ordinaria); le
Patella candei crenata d’Orbigny 1840 (Figs 1F,2F), Deserta Grande (DES), south-east of Madeira Archi- -ab
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is a round-outlined form with a more thickly crenel- pelago (N = 50, March 1995, type P.c. ordinaria); north tra
lated, regular and highly ribbed shell. Internally, it of the Selvagens Islands (SEL), situated at the north- ct/7
has a white scar, surrounded by a bluish iridescent west of the main islands of Canary Archipelago (N = 9, 7/3
colour. The border is thin, brown to yellowish, and February, 1995, type P.c. candei); and Fuerteventura /34
transparent, allowing the external ribs of the shell to (FUE), south-east of Canary Archipelago (N = 22, 1/2
be seen. This form is described for all the Canary June 1992, 20 types P.c. crenata and 2 types P.c. can- 63
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islands (Christiaens, 1973; Hernández-Dorta, 1992), dei). The samples from Madeira and Deserta were 64
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where it is mainly intertidal. Larger individuals analysed previously by Weber et al. (1998) for stock b
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can also be found in shallow subtidal habitats identification. g
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(Hernández-Dorta, 1992). All samples were sent alive to the Port Erin Marine es
Patella candei gomesii Drouët (1858) (Figs 1A,2A), is Laboratory, Isle of Man. Live animals were dissected t o
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a depressed and rounded form, very smoothly ribbed to remove the radula and shell for morphometric 0
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with an obtuse and blunted apex. Externally, it is analysis, and samples from the foot muscle were A
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pinkish, grey to brown. Internally, it is brownish or also taken and stored at -78∞C, until required for ril 2
pinkish with a slightly bluish iridescent colour. The electrophoresis. 01
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scar is slightly drawn with whitish to milky colours
(Christiaens, 1973). It is an intertidal and occasionally
shallow subtidal form from all islands of the MORPHOMETRIC ANALYSIS
Azores archipelago (Christiaens, 1973; Hawkins et al., Seven morphological parameters were used to study
1990a,b). the morphological variation of P. candei within
Work by Côrte-Real et al. (1996), involving morpho- and between archipelagos by multivariate analysis.
logical and genetic characterization of P.c. gomesii, Five of the seven parameters were taken from the
P.c. ordinaria and P.c. crenata, showed no differentia- shell (Fig. 4): total length (TOTL), maximum width
tion on the soft-body anatomy and radular characters (MAXWD), apical length (APICL), apical width
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
344 L. I. WEBER and S. J. HAWKINS
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Figure 3. Sampling sites at the Macaronesian Islands. Arrows indicate the location where samples of Patella candei were e
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collected. ic
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(APICWD), and apical height (APICHG). The other io
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morphological parameters were radular length n
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(RADL), and the ratio between RADL and TOTL, n
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termed ‘radular ratio’ (RADRT). Canonical discrimi- rtic
nant analysis (CDA) was used to determine which of le
the variables discriminate between groups (locations -ab
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and archipelagos) and to find out how well discrimi- tra
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nant functions allow classification of individuals to t/7
groups. Some of the assumptions of CDA were not 7/3
met. APICWD, APICHG, RADL and RADRT showed /3
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some deviations from normality and homoscedasticity 1/2
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(P < 0.01). CDA was carried out, however, since it is a 3
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powerful analysis, giving reliable results even with 6
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some deviation from the assumptions (Pimentel, b
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1979). Significant differences between group centroids g
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of the predefined groups were found for both classifi- e
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cations: between locations (Pillai-Barlette Trace = t o
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2.0763, P < 0.001) and between archipelagos (Pillai- 0
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Barlett Trace = 1.7566, P < 0.001). The following spec- A
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ifications were used for the analysis: Forward Step- ril 2
wise, Tolerance = 0.010, F to enter = 1.0, F to remove 01
= 0.00, and a priori probabilities were estimated to be 9
proportional to group sizes. For all morphometric
analyses the STATISTICA for Windows program,
version 4.3 (Statsoft et al., 1993) was used.
Figure 4. Morphometric variables taken from the shell
and used for multivariate analysis. A, Dorsal view:
(APICWD) width at the line of the apex (MAXWD) maxi-
GENETIC ANALYSIS
mum width. B, Lateral view: (TOTL) total length (APICL)
length of the shell until the line of apical height (APICHG) Standard 12.5% Starch-gel (Sigma, Co.) electro-
height of the shell at the apex. phoresis was carried out for all collected samples of
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
GENETIC STRUCTURE OF PATELLA CANDEI 345
P. candei. Homogenates were prepared by macerating intervals were obtained by Bootstrapping after 5000
foot muscle in Tris-HCl, pH 8.0 buffer. Supernatants resamplings.
obtained after 4 min in centrifugation at 10 000 r.p.m. Potential hybridization of subspecies was explored
were used to load the gels. Five buffer systems (BS) by a two-dimensional Principal Components plot and
were used for electrophoresis: (I) Tris-citrate, pH 8.0 the proportions of subspecies admixture were calcu-
(Siciliano & Shaw, 1976); (II) Tris-citrate-EDTA, lated in accordance with Campton (1988).
pH 7.0 (Ayala et al., 1972); (III) Tris-citrate-borate The relationship between genetic differentiation
discontinuous, pH 8.2–8.7 (Poulik, 1957); (IV) Tris- and geographical distance was assessed by regression
citrate-borate-LIOH, pH 8.26–8.31 (Redfield & Salini, analysis of pair-wise number of migrants (Nm) and
1980); and (V) Tris-borate-EDTA, pH 9.0 (Ayala et al., pair-wise geographical distances (DIS).
1974). The following 16 enzymes, which gave a total of
21 loci, were resolved for all populations: aspartate
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aminotransferase (AAT, EC: 2.6.1.1, BS: III), malic RESULTS ow
enzyme (NADP) (MEP, EC: 1.1.1.40, BS: I), esterases nlo
(EST, EC: 3.1.1.-, BS: IV), esterase-D (EST-D, EC: MORPHOMETRIC ANALYSIS ad
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3.1.1.-, BS: III), phosphoglucomutase (PGM, EC: The discriminant function analysis was highly signi- d
5.4.2.2, BS: II), phosphogluconate dehydrogenase ficant (Wilks’ lambda = 0.02716, P <0.0001, R = 0.904) from
(PGDH, EC: 1.1.1.44, BS: I), fructose-diphosphate for all sampled localities (six groups). The apical h
aldolase (FBALD, EC: 4.1.2.13, BS: V), glyceraldehyde- width (APICWD) was not important for discriminat- ttps
3-phosphate dehydrogenase (GAPDH, EC: 1.2.1.12, ing between populations and therefore was excluded ://a
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BS: V), glucose-6-phospahte isomerase (GPI, EC: from the analysis. Five roots were obtained, but only a
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5.3.1.9, BS: I), glutamate dehydrogenase (GLUDH, four were significant. The first and second functions m
EC: 1.4.1.-, BS: V), isocitrate dehydrogenase (NADP) provided the best overall discrimination between ic.o
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(IDHP, EC: 1.1.1.42, BS: I), L-lactate dehydrogenase groups (Table 1). The classification functions assigned p
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(LDH, EC: 1.1.1.27, BS: II), malate dehydrogenase individuals correctly to the predefined locations in o
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(MDH, EC: 1.1.1.37, BS: IV), mannose-6-phosphate 76% of the cases (Table 2). When the analysis was /b
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isomerase (MPI, EC: 5.3.1.8, BS: IV), purine- done by grouping individuals into archipelagos lin
nucleoside phosphorilase (PNP, EC: 2.4.2.1, BS: I), and (Azores, Madeira, Selvagens and Canaries), the dis- ne
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superoxide dismutase (SOD, EC: 1.15.1.1, BS: III). criminant function (without APICWD) was also highly n
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Enzyme-specific staining procedures followed the significant (Table 3). Figure 5 shows the individuals of rtic
same techniques used by Weber et al. (1998). each archipelago distributed over the Canonical Dis- le
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The BIOSYS-1 program (Swofford & Selander, criminant Space, formed by Root 1 and Root 2. In this b
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1981) was used to obtain allele frequencies and vari- case, correct assignment of individuals to archipelagos tra
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ability measures; Nei’s (1978) genetic identities and was 93%, with P. candei from Selvagens being t/7
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Prevosti Distances were used to construct a phenetic morphologically the most distinct form. Individuals /3
tree by Distance Wagner Procedure; and a c2 test was from the Selvagens were 100% correctly assigned by /34
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used for testing independence in allele frequencies the classification functions (Table 4). The Azores and /2
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between samples. The estimator of F-statistic F Madeira are partially separated in the Discriminant 3
ST 9
(Theta) (Weir & Cockerham, 1984) and the number of Space, showing some overlap in their morphology. 64
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migrants (Slatkin, 1993) between pairs of populations Individuals from the Canaries, appear as intermedi- b
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were obtained using the FSTAT program, version ate forms between the Selvagens and the other g
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1.2 (Goudet, 1995). Standard deviations for Theta groups, with Root 1 overlapping mainly with Azorian es
were calculated by jack-knifing and 95% confidence individuals (Fig. 5). Root 1 has major contributions t o
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Table 1. Patella candei: chi-square tests with successive canonical removed roots for the case of grouping individuals into 0
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Removed roots Eigenvalue Canonical R Wilk’s lambda Chi-square df P-level
0 4.492 0.904 0.027 796.949 30 <0.00001
1 2.508 0.846 0.149 420.536 20 <0.00001
2 0.636 0.624 0.523 143.195 12 <0.00001
3 0.160 0.371 0.856 34.374 6 0.00001
4 0.007 0.084 0.993 1.561 2 0.45810
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
346 L. I. WEBER and S. J. HAWKINS
Table 2. Patella candei: classification matrix for all populations. Percent of correct assignment of the predicted (columns)
versus the observed (rows) classifications. Code for Islands: (FAI) Faial and (SMA) Santa Maria from the Azores; (MAD)
Madeira and (DES) Deserta Grande from the Madeira Archipelago; (SEL) Selvagens; and (FUE) Fuerteventura from the
Canaries
Azores Madeira Selvagens Canaries
Percentage of FAI SMA MAD DES SEL FUE
Population correct assignment P = 0.21491 P = 0.21491 P = 0.21491 P = 0.21930 P = 0.03947 P = 0.09649
FAI 67.35 33 14 1 0 0 1
SMA 59.18 15 29 3 2 0 0
MAD 85.71 0 1 42 6 0 0 D
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DES 88.00 1 0 5 44 0 0 w
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SEL 100.00 0 0 0 0 9 0 lo
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FUE 72.73 1 3 0 2 0 16 de
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Total 75.88 50 47 51 54 89 17 fro
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Table 3. Patella candei: chi-square tests with successive removed roots for the case of grouping individuals into ://ac
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archipelagos d
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Removed roots Eigenvalue Canonical R Wilk’s lambda Chi-square df P-level .o
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0 4.426 0.903 0.044 695.192 18 <0.00001 o
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1 2.118 0.824 0.237 319.749 10 <0.00001 /b
2 0.354 0.511 0.738 67.297 4 <0.00001 iolin
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Figure 5. Patella candei individuals distributed in the Canonical Discriminant Space. 95% confidence intervals are shown 9
as ellipses.
from radula-related variables (RADRT, RADL, API- GENETIC ANALYSIS
CHG), while Root 2 has contributions from size-
related variables (TOTL, MAXWD, APICHG). The Allele frequencies for all localities, mean number of
third root explained only 5% of the discrimination alleles, polymorphism (99%), and mean observed
among groups. and expected heterozygosity are available from the
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
GENETIC STRUCTURE OF PATELLA CANDEI 347
Table 4. Patella candei: classification matrix for the case of grouping individuals into archipelagos. Observed (rows) and
predicted classifications (columns)
Percentage of Canaries Selvagens Azores Madeira
Archipelago correct assignment P = 0.09649 P = 0.03947 P = 0.42982 P = 0.43421
Canary 72.73 16 0 5 1
Selvagens 100.00 0 9 0 0
Azores 93.88 1 0 92 5
Madeira 94.95 0 0 5 94
Total 92.54 17 9 102 100
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Figure 6. Diagnostic loci observed between Patella candei subspecies at the Macaronesian Islands. 6
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authors. Contingency Chi-square tests showed that 1600), and P.c. candei (Pgm79). Two diagnostic loci (Pgdh g
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populations were independent in their allele frequen- and Gpi) were found between P.c. gomesii (from the es
cies (P < 0.05). Independence in allele frequencies was Azores) and P.c. ordinaria (from the Madeira archipel- t o
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also evident within archipelagos. For example, within ago): that is, they share no alleles at these loci. 0
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the Azores, Faial (FAI) was significantly different P.c. candei from Selvagens showed the same pattern as A
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from Santa Maria (SMA), after Bonferroni correction P.c. gomesii at the Pgdh locus, sharing no alleles with ril 2
for multiple tests, due to three loci (Mpi, Est-2 and P.c. ordinaria at this locus. At the Gpi locus, P.c. candei 01
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Pnp, P < 0.0002); and within the Madeira archipelago, from Selvagens also showed the pattern of P.c. gomesii
Madeira Island (MAD) was significantly different but with one individual sharing a rare allele with
from Deserta Grande (DES), after Bonferroni correc- P.c. ordinaria. P.c. crenata and P.c. candei found in the
tion, due to two loci (Mdh-2 and Aat-1, P < 0.001). The Canaries have no diagnostic loci with the other two
Selvagens sample (SEL) was also different from subspecies found in the Azores and Madeira archipel-
Fuerteventura, Canaries (FUE), where six loci were agos, because they showed alleles that are common to
responsible for the overall significance (P < 0.002). both subspecies (see Fig. 6). Other loci such as Mpi
Exclusive alleles were observed for P.c. ordinaria and Est-1 showed partial discrimination between
(Est-D145, Est-151, and Ldh-286,75), for P.c. gomesii (Mep- P.c. gomesii and P.c. ordinaria. P.c. crenata does not
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
348 L. I. WEBER and S. J. HAWKINS
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Figure 7. Distribution of Patella candei individuals on the two-dimensional space formed by the first and second Principal h
Components based on individual allele characterization at three diagnostic loci. ttps
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Table 5. Patella candei: mean unbiased genetic identity between and within archipelago. Range of identity values in m
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brackets .o
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Archipelago om
Number of /b
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Archipelago populations Azores Madeira Selvagens Canaries lin
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Azores 2 0.959 n
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Madeira 2 0.636 (0.634–0.638) 0.995 rtic
Selvagens 1 0.889 (0.861–0.917) 0.728 (0.723–0.733) – le
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Canaries 1 0.678 (0.674–0.682) 0.930 (0.923–0.938) 0.821 – b
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appear as a distinct genetic subspecies, and it seems to column. P.c. ordinaria is separated from the former /34
1
be a hybrid resulting from admixture proportions of subspecies and it is distributed mainly in the first col- /2
6
92% P.c. ordinaria and 8% P.c. gomesii–P.c.candei (cal- umn of quadrants. Individuals from Fuerteventura of 3
9
culated as the mean admixture proportions obtained the Canaries are mainly distributed in the same col- 64
8
from loci Pgdh, Gpi, and Mpi in accordance to Camp- umn as P.c.ordinaria, but a high proportion of them b
y
ton, 1988). Two individuals collected in Fuerteventura are spread over the second column, indicating hybrids g
u
and morphologically classified as P.c. candei are likely with a high level of introgression with P.c. ordinaria. es
to be hybrids between P.c. gomesii–P.c.candei and The individuals situated in the central columns rep- t o
n
P.c. ordinaria as shown by their heterozygote condi- resent first generation hybrids (heterozygotes for the 07
tions at Gpi and Pgdh loci. Figure 7 shows the diagnostic loci), which correspond to those individuals A
p
individuals from each locality distributed on the two- classified morphologically as P.c. candei, while all the ril 2
dimensional space formed by the first and second others belong to P.c. crenata. One individual from Sel- 01
9
principal components based on individual allele char- vagens was also classified as a hybrid (5th column),
acterization at the two diagnostic loci (Gpi, Pgdh) and but with a high level of introgression with P.c. candei,
the partial one (Mpi; see Principal Component Analy- and corresponds to the only one from Selvagens that
sis for detecting hybrids in Campton & Utter, 1985 shares a rare allele with P.c. ordinaria.
and Campton, 1988). It is clear that P.c. gomesii from Nei’s (1978) genetic identities for pair-wise compar-
the Azores and P.c. candei from Selvagens share the isons of subspecies situated in different archipelagos
same quadrants (6th column) of the two dimensional are shown in Table 5. The genetic relationships
space, indicating that they are the same subspecies. between the populations are visualized in the dendro-
Only one individual of Selvagens is situated in the 5th gram shown in Figure 8. All pair-wise F values are
ST
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
GENETIC STRUCTURE OF PATELLA CANDEI 349
Table 6. Patella candei: pair-wise number of migrants
(above the diagonal) and pair-wise F values (below the
ST
diagonal)
Populations FAI SMA MAD DES SEL FUE
FAI 1.19 0.11 0.11 0.60 0.12
SMA 0.174 0.12 0.12 0.40 0.13
MAD 0.702 0.684 6.91 0.13 0.38
DES 0.687 0.671 0.035 0.15 0.51
SEL 0.296 0.389 0.656 0.622 0.19
Figure 8. Dendrogram showing the genetic relationship
FUE 0.674 0.654 0.399 0.330 0.568
among all Patella candei populations. Wagner tree based on
D
Prevosti Distance. o
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6
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9
Figure 9. Cline at Idh-2 locus along the geographical distribution of Patella candei. 6
4
8
b
y
g
u
given in Table 6. The mean deviation from random DISCUSSION es
mating within populations measured as heterozygote t o
n
rdaenfidcioemnc ym, FatISi,n gw aisn 0.t0h8e7 ±to 0ta.0l 38p;o pthuela dtieovni,a tiFon, frwoams Tthhee ecxopmlopilteexdit yli mofp tehte Pp. ocpaunldaetii own agse nsehtoiwc nst rbuyc tCurôer toef- 07 A
IT p
0.631 ± 0.073; and the degree of genetic differentiation Real et al. (1996), with genetic identities as low as ril 2
between population, FST, was 0.596 ± 0.075 (95% 0.660 within the species, indicating allopatric specia- 019
confidence interval: 0.424–0.713). The number of tion between the different archipelagos. Similar val-
migrants estimated from F (Nm = 1/4F - 1/4) ues of genetic identities (Nei, 1978) were also found by
ST ST
between pairs of populations are also given in Table 6. us for the Macaronesian Islands, from 0.995 (within
A cline at the Idh-2 locus was found along the geo- the Madeira archipelago) to 0.636 (between the Azores
graphical distribution of P. candei (Fig. 9). The geo- and Madeira archipelagos). The high value of F
ST
graphical distances separating the Macaronesian obtained (0.596 ± 0.075) also showed the high level of
Islands explained only 38% of the genetic differentia- structuring within this species complex.
tion among them (log Nm = 1.695 - 0.806 Log DIS; Our results confirm the complexity of the morpho-
R2 = 0.383). logical and genetic structure of P. candei in the
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353
350 L. I. WEBER and S. J. HAWKINS
Macaronesian islands, suggesting that both human the surrounding islands (founder effect), or by a near-
intervention and natural processes have determined extinction event of the ancestral form of P.c. candei,
the present situation of P. candei subspecies. followed by renewal of the population out of a few indi-
The morphological dispersion of P. candei subspecies viduals. In either case, the new population would have
in the canonical space seems to agree well with their carried only a partial sample of the diversity of the
position in the tidal level. The most distinctive form original population. The new subspecies, P.c. ordi-
was P.c. candei from Selvagens, the only one still found naria, successfully established itself, most probably in
surviving at high levels on the shore, having big and isolation, developing new alleles (and/or perhaps
tall shells with long radula. But within species (e.g. retaining old, rare alleles) at different loci, afterwards
P. vulgata) high shore forms have coned shells and expanding its distribution to the south-east through
long radulae. the Canary Current, which is characteristic of the
P.c. gomesii from the Azores is not genetically dis- Recent period, allowing hybridization with the ances-
D
tinct from P.c. candei collected in the Selvagens. They tral P.c. candei subspecies of the Canary Archipelago. o
w
share all alleles in two loci that are diagnostic with Madeira must have been isolated for a long time to nlo
respect to P.c. ordinaria from Madeira and Deserta allow the evolution of P.c. ordinaria, independently ad
e
Grande. Therefore, genetically, P.c. gomesii and P.c. from the influence of the ancestral form still in the d
candei are the same subspecies (P.c. candei–P.c. Azores and Selvagens. It is easy to explain the isola- fro
m
gomesii). In contrast, P.c. crenata has no diagnostic loci tion of the Madeira archipelago over the last 3 Myr h
with the other subspecies, but is more similar to from the other north-west African Islands. There is a ttps
P.c. ordinaria, reflecting their occurrence on adjacent thermohaline front between the Azores and Madeira ://a
c
archipelagos. P.c. crenata seems to be the result of the (Fernández & Pingree, 1996) that could have acted as a
d
e
hybridization of the former subspecies with a high a barrier for larval dispersal, adding to the difficulty m
level of introgression to P.c. ordinaria. The calculated of traversing the great distance separating both archi- ic.o
u
relative composition was 92% of P.c. ordinaria and 8% pelagos. Furthermore, the Azores current flows along p
.c
of P.c.candei–P.c.gomesii. Furthermore, two individuals the front in a south-east direction passing further o
m
collected in Fuerteventura, identified morphologically north and east than Madeira, turning south and join- /b
io
as P.c. candei are clearly first generation hybrids of the ing the Canary current only close to the African coast lin
genetic subspecies P.c. candei–P.c. gomessi with P.c. (Fernández & Pingree, 1996). Some deviations have ne
a
ordinaria. P.c. candei–P.c. gomessi and P.c. ordinaria been observed and occasionally it turns in a direction n
/a
can be considered as subspecies because they still south-west from Madeira. The present circulation of rtic
interbreed successfully as shown by the genetic com- the North Atlantic was established after the eleva- le
-a
position of the population of P. candei in the Canaries tion of the Isthmus of Panama about 3.5 Myr ago b
s
including finding first generation hybrids with mor- (Berggren & Hollister, 1974), although latitudinal tra
c
phological features of P.c. candei. Thus, P.c. candei– gradients, sea level, and therefore circulation was t/7
7
P.c. gomessi is distributed in the Azores and the affected temporally during glaciation periods. Popula- /3
Selvagens with only rare cases of pre-hybridized indi- tions in the Madeira archipelago might have little /34
1
viduals in the Canaries due to the widespread estab- interchange with the Selvagens, because south-east /2
6
lishment of the introgressed hybrid.. water movements are stronger than northward flow 3
9
(Fernández & Pingree, 1996). These facts may explain 64
8
the lack of gene flow from the other archipelagos to b
EVOLUTION BY NATURAL PROCESSES: A HYPOTHESIS Madeira and Deserta in the Recent, but do not explain y g
u
Many questions arise from this pattern of distribution. the extinction of the ancestral P.c. candei in Madeira es
For example, how can the present disjunct distribu- and Deserta. If Madeira was isolated for a long time it t o
n
tion of the genetic subspecies P.c.candei–P.c.gomesii might have had the time to diverge by phyletic 07
be explained? How can the extinction of P.c. candei changes from the ancestral P.c. candei. Nevertheless, A
p
in Madeira and the subsequent emergence of P.c. ordi- it seems more likely that an extinction–recolonization ril 2
naria be explained? P.c. candei should be the ancestral event might have caused the resurgence of previously 01
9
form that colonized all the archipelagos in the past. rare alleles of the ancestral form with the establish-
The Azores must have been colonized last, probably in ment of some new ones. Changes in allele frequencies
the Upper Pliocene. The ancestral form might be much over a short temporal scale can be dramatic in inver-
older than the Azores and certainly first colonized the tebrate populations, as shown by many recent studies
Canaries and Madeira during the Miocene (23–5 mil- (Hedgecock, 1994a,b; Lessios, Weinberg & Starczak,
lion years BP) or even earlier. P.c. ordinaria could have 1994; Weber, 2000), especially, in cases of a sudden
arisen in the Madeira archipelago after an extinction reduction of the effective genetic population size.
process caused by environmental change followed by One explanation for the extinction of the ancestral
the recolonization by a few individuals coming from P.c. candei in Madeira might be related to the many
© 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 341–353