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Sea Anemone Anthothoe Albocincta MARINE ECOLOGY PROGRESS SERIES Vol. 156: 121-13( Published September 25 Mar Ecol Prog Ser Asexual reproduction and genetic determination of colour patterns within populations of the subtidal sea anemone Anthothoe albocincta Martin R. Billingham*, David J. Ayre Department of Biological Sciences and Australian Flora and Fauna Research Centre, University of Wollongong, Wollongong, New South Wales 2522, Australia ABSTRACT: Southeastern Australian populations of the dioecious, subtidal sea anemone Anthothoe albocincta display considerable spatial variation in the colour patterns of the constituent polyps, and the most visually striking feature of local populations are sets of dense aggregations each comprised of polyps with a single colour pattern. We used a combination of genetic data, observations and manipu- lative experiments to infer that A. albocincta generates these monomorphic aggregations via asexual reproduction and that asexual reproduction plays the major role in the maintenance of established pop- ulation~.Allozyme electrophoresis of 2 to 5 polyps from each of 20 mapped aggregations (in 2 popula- tion~)revealed that in 18 cases (9OU4~)all polyps were electrophoretically identical and always distinct from differently coloured polyps tdken from each of 26 adjacent aggregations. Furthermore, the genetlc structure of each of 13 populations separated by up to 930 km was consistent with the predicted effects of localised asexual recruitment. We detected 16 heterozygote excesses and 11 heterozygote def~cits(p < 0.05) in a total of 66 tests for departures from Hardy-Weinberg equilibria. Moreover, each population contained significantly less [p < 0.005) multi-locus genotypic diversity (G,) than was expected for a randomly mating population (G,) (mean Go:G, = 0.30) and contained relatively few unique 7-locus genotypes. Some genotypes were found within many aggregations, suggesting suc- cessful clones may have extensive and discontinuous distributions within populations. In addition, we observed longitudinal fission of adult polyps in both field and laboratory conditions, and found that over a 6 mo period 82% (i.e. 1469 of 1790) of new recruits into 30 experimentally cleared patches, within 3 populations. were morphologically identical to those within surrounding aggregations. These data, together with our earlier analysis of genetic variation among populations, and observations of gonad development, imply that this specles uses fission to maintain populations, consisting predomi- nantly of spatially restricted clones (each with an almost unique colour pattern), but uses sexual repro- duction to produce widely dispersed colonists. KEY WORDS: Cnidarian . Asexual - Sexual . Colour morph . Longitudinal fission INTRODUCTION 1986, Grosberg 1987, 1991, Ayre & Dufty 1994). In many of these cases the proximate cause of subdivision Despite the potential of marine organisms for disper- has been inferred to be the philopatric dispersal of sal over vast spatial scales, recent studies have found gametes and/or sexually or asexually produced that populations of many marine organisms are highly propagules. However, there is increasing evidence genetically subdivided (e.g. Ayre 1983, 1984, Johnson that subdivision may also be reinforced by, as yet & Black 1984a, 1991, Burton 1986, 1990, Hoffmann poorly studied, fine-scale genotype by environment interactions (e.g. Johnson & Black 1984b, Ayre 1985, 1995, Behrens Yamada 1989, Johannesson et al. 1990) 'Present address: Department of Plant Sciences. University of and barriers gene produced by the formation Of co-ada~tedgene complexes (e.g. Burton 1986, 1990, Oxford, South Parks Road. Oxford OX1 3RB. United Kina- .2 dom. E-mail: [email protected] Grosberg 1987, 1991) 0 Inter-Research 1997 Resale of full al-ticle not permitted 122 Mar Ecol Prog Se In sea anemones sexual reproduction may involve possibility that either: (1) colour is genetically deter- one of many tactics including self-fertilisation and mined and aggrcgations are produced by the external brooding of larvae (e.g Epiactis prolifera; philopatnc dispersal of sexually generated but closely Bucklin et al. 1984),philopatric dispersal of outcrossed related indivld.uals or (2) colour varies simply in larvae (e.g.Diadumene leucolena; Shick & Lamb 1977) response to underlying patterns of environmental vari- or outcrossed, broadcast spawning (e.g Oulactis mus- ation (phenotypic plasticity). Each alternative explana- cosa; Hunt & Ayre 1989) and so have the capacity to tion wouId be distinguished by a different pattern of either create or overwhelm genetic structuring within genotypic structuring within populations. populations. Asexual reproduction may occur via If each monomorphic aggregation conslsts of a single processes ranging from fission to parthenogenetic pro- clone then individual polyps within each aggregation duction of larvae (reviewed by Shick 1991),and, in the should share the same multilocus genotype and be intertidal, asexual reproduction is usually associated genetically different to dissimilarly coloured neigh- with spatially restricted clonal proliferation leading to bouring anemones. Single-locus genotype frequencies highly structured 'clonal' populations (e.g.Shick et al. within such 'clonal' populations should differ from 1979, Ayre 1983, Shaw 1991, McFadden et al. 1997). expectations for Hardy-Weinberg equilibria, with both The philopatric dispersal of either sexually or asexu- excesses and deficits of heterozygotes present (e.g. ally generated propagules provides the proximate Ayre 1983). Alternatively, if the limited dispersal of cause of fine-scale (within populat~on)and meso-scale sexually derived progeny gives rise to aggregations (within region) genetic subdivision within at least sev- then individuals within aggregations would be eral sea anemone species (reviewed by Shick 1991). expected to be genetically similar (though not identi- Nevertheless, our understanding of the ecological or cal) and fine-scale population subdivision should result evolutionary significance of fine-scale genetic struc- in lower than expected numbers of heterozygotes (i.e. ture within local populations of most species remains Wahlund effect; Hedrick 1985). Finally, if colour has no uncertain and experimental studies are seriously simple genetic basis but reflects a phenotypically plas- impeded by the need to identify individual genotypes. tic response to fine-scale environmental heterogeneity Such identification typically requires the time consum- then any genotypic and phenotypic variation should ing assessment of variation in protein or nucleic acid vary independently. markers. In this study we used allozyme electrophoresis Local populations of the dioecious, subtidal sea together with field and laboratory observations, and anemone Anthothoe albocincta (Stuckey) (Tribe experimental manipulations to show that: (1) within 2 Mesomyaria, Family Sagartiidae) are characterised by populations single colour aggregations of Anthothoe striking spatial variation in the colours of the oral albocincta result from asexual reproduction via longi- discs and tentacles of their constituent polyps (Carl- tudinal fission; (2) successful clones may have exten- gren 1950, 1954, Thomas & Shepherd 1982, Billing- sive and discontinuous distributions within popula- ham & Ayre 1996).A. albocincta is common along the tion~;and (3) asexually derived recruitment plays the shores of southeastern Australia, forming large popu- major role in maintaming each of 13 populations of A. lation~ (some with 1000s of polyps). Populations albocincta over 930 km of coastline extend across a wide vertical range (0 to >24 m) and occur on many substrates (most commonly rock, but also sponges, algae, gastropod shells and ascidians; METHODS Billingham pers. obs.). Numerous different colour morphs exist within most populations, although most Collection and electrophoresis. We collected polyps are found within dense aggregations of identi- anemones irom between 4 and 10 m depth by prising cally coloured individuals, which are themselves in them from the substrata using a small knife. We then close proximity to other aggregations (Billingham & transported them live to the laboratory in individual Ayre pers, obs.). plastic bags '4 filled with cool sea water and stored Asexual replication of identically colou.red polyps them at -80°C pending electrophoresis. We used hori- would provide the simplest explanation of the colour zontal starch gel electrophoresis (12.5% w/v starch) as variation seen within populations of Anthothoe described in Billingham & Ayre (1996) to determine a albocincta. Qualitatively similar interclonal variation multilocus genotype for each sampled polyp of Antho- has been reported for the fissiparous, North American thoe albocincta. We assessed the variation at each of intertidal anemone Anthopleura elegantissima (Fran- 7 variable enzyme-encoding loci, using the elec- cis 1973, Ayre & Grosberg 1995).Alternative explana- trophoretlc buffers and assay conditions described tions for the presence of monomorphic aggregat~onsof by Selander et al. (1971)-on buffer number 5: gluta- polyps within populations of A. albocincta include the mate dehydrogenase (GDH; E.C. 1.4.1.3),6-phospho- Billingham & Ayre: Asexual reproduction in a subtidal sea anemone 123 gluconate dehydrogenase (6PGD; E.C.1.1.1.44) and xanthine dehydro- genase (XDH); on buffer number 6: leucine-alanine peptidase (PEP- C; E.C. 3.4.11);and on buffer number 9: glu- cosephosphate isomerase
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