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Temporal variation in malate dehydrogenase loci from Nerita albicilla (L.) (Gastropoda: Neritidae) at the Cape d'Aguilar Marine Reserve, Hong Kong
- Hong Kong University Press, HKU
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The Marine Flora and Fauna ofHong Kong and Southern China IV (ed. B. Morton). Proceedings of the Eighth International Marine Biological Workshop: The Marine Flora and Fauna of Hong Kong and Southern China, Hong Kong, 2-20 April 1995. Hong Kong: Hong Kong University Press, 1997. TEMPORAL VARIATION IN MALATE DEHYDROGENASE LOCI FROM NERITA ALBICILLA (L.) (GASTROPODA: NERITIDAE) AT THE CAPE D'AGUILAR MARINE RESERVE, HONG KONG Q. Huang The Swire Institute of Marine Science, The University of Hong Kong, Cape d'Aguilar, Shek 0, Hong Kong ABSTRACT Four samples of Nerita albicilla were collected between 1993 and 1996 from Lobster Bay, Cape d'Aguilar, Hong Kong Island and investigated for allelic variations at gene loci sMDH-l * and sMDH-2 * which have been assumed to encode supernatant malate dehydrogenase (E.C. No. 1.1.1.37) in the Neritidae. The isozymes were separated using isoelectric focusing electrophoresis. Three alleles were detected at the locus sMDH-l * and four at the locus sMDH-2 *. The two gene loci were shown to be linked. Five electrophoretic patterns were detected, involving three genotypes of the locus sMDH1 *and four of the locus sMDH-2*. No significant differences in allele frequencies and heterozygosities were observed between the different years and seasons of sampling. No significant deviations from Hardy-Weinberg equilibrium were detected. INTRODUCTION Nerita albicilla Linnaeus, 1758 is common on Hong Kong's rocky shores (Hill 1980; Huang 1995) and is especially associated with intertidal rocks and boulders. Individuals of this species often occur in high densities, either grouped inside rock crevices or attached to the undersurfaces of boulders below mid-tide level. In Lobster Bay, Cape d' Aguilar, Hong Kong Island, individuals of N. albicilla are distributed from extreme low-tide to the mid-tide zone. In a previous gel electrophoretic investigation (Huang in press), twenty-three enzymes were scored for Hong Kong populations of Nerita albicilla. Of the eighteen gene loci resolved for this species, only two, i.e., sMDH-l * and sMDH-2*, which have been assumed to encode the supernatant NAD+ dependent malate dehydrogenases (E.C. No. 1.1.1.37) in the Neritidae, were shown to be polymorphic. No variation has yet been detected at these two loci in other local species of Nerita (Huang in press). 316 Q. HUANG Selective responses of malate dehydrogenase (MDH) allozymes to temperature variation have been reported for both animals and plants (Liu et al. 1978; Richmond and Zimmerman 1978; Snyder and Ayala 1979; Smith et al. 1983). If the sMDH loci polymorphism in Hong Kong's populations of Nerita albicilla has been maintained in response to temperature variation, it might be expected that allele frequencies at these loci fluctuate both seasonally and between tidal heights. The first of these hypotheses can be tested by sampling between seasons. Two methods can be applied to the examination of temporal genetic phenomena, i.e., sequential long-term sampling and an integration of genetic data with respect to population age structures (Lewis and Thorpe 1994). In the present work, four samples of Nerita and Thorpe albicilla were collected from Lobster Bay between 1993 and 1996 and scored electrophoretically for sMDH variation. Two of the four samples were obtained in spring when air temperature varied between 18 and 22°C and the others were collected in summer when the mean air temperature was 28.5°C (data from the Royal Observatory, Hong Kong). In addition to the study of environmental genetic correlations (DiMichele and Powers 1982; Burton and Feldman 1983; Nevo 1983; Nevo et al. 1987), allozyme data have been used widely to investigate population structure and phylogenetic relationships between taxa (Avise 1974; Nei 1987; Leary and Booke 1990). While information on geography, biodiversity and ecology at Cape d' Aguilar, which has recently been designated as Hong Kong's first marine reserve, has accumulated (Morton and Harper 1995), little is known about the population genetics of animals and plants within the area. In addition to the temporal genetic hypothesis tested, i.e., the null hypothesis of no seasonal variation, this study also provides the first baseline population genetic data for the Cape d' Aguilar marine reserve. Such data are an essential prerequisite for conservation genetic management. MATERIALS AND METHODS Samples The first record of sMDH polymorphism in Nerita albicilla from Lobster Bay, Cape d'Aguilar, was obtained in August 1993. Seventeen individuals were collected. The three other samples were obtained in March 1994, April 1995 and July 1996, respectively, with sample sizes being 26 (1994), 50...