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Genetic structure of ®ssiparous populations of Holothuria (Halodeima) atra on the Great Barrier Reef
| Content Provider | Semantic Scholar |
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| Abstract | Allozyme variation at ®ve polymorphic loci was surveyed in a total of 311 individuals of the sea cucumber Holothuria (Halodeima) atra (JaÈ ger, 1833) collected from two nearshore and two midshelf populations in the Great Barrier Reef in November 1996. Strong deviations in genotype frequencies from those expected under Hardy±Weinberg equilibrium, particularly a large number of heterozygote excesses, con®rmed the occurrence of asexual reproduction. Females and males diered signi®cantly in genotype frequencies as a result of dierences in either the amount of ®ssion in each sex or in the mortality of asexually produced recruits. The estimated maximum sexual input (number of sexually produced individuals: sample size N*:Ni) to the two nearshore reefs (38 to 67%) was low relative to that to the two midshelf reefs (74 to 87%). The three ratios and Go:Ge, Ngo:Ni, N*:Ni, (where Go observed genotypic diversity, Ge expected genotypic diversity, Ngo number of genotypes) considered to be indicators of the extent of asexual reproduction, showed a consistent trend in the degree of asexual reproduction similar to that derived from the number of regenerating individuals observed in the populations for which data were available. F-statistic analyses of clonal gene frequencies demonstrated that all populations received sexual recruits from the same gene pool. There was evidence of restricted sexual recruitment to the Fantome population, suggesting that asexual reproduction was dominant only in areas where sexual recruitment was limited by other factors. Introduction Cloning by transverse ®ssion is an important means of population-size maintenance in several echinoderm species (Emson and Wilkie 1980; Ottesen and Lucas 1982) including sediment-feeding sea cucumbers of the order Aspidochirotida (Chao et al. 1993; Conand 1996; Uthicke 1997, 1998). Asexual reproduction is known to occur in eight aspidochirotide species (summarised in Uthicke 1997), and may be arti®cially induced in several others (Reichenbach et al. 1996). Fission is a seasonally occurring event in Holothuria atra (Conand and De Ridder 1990; Chao et al. 1993; Uthicke 1997),H. parvula (Emson and Mladenov 1987), H. edulis and Stichopus chloronotus (Uthicke 1997). All ®ssiparous species also reproduce sexually by broadcast-spawning of gametes in distinct spawning seasons (e.g. Conand 1993). Males and females occur in similar numbers in most populations (Franklin 1980; Harriott 1982; Conand 1993), although exceptions from this rule have been observed (Harriott 1982). Holothuria (Halodeima) atra, is amongst the most widespread holothurians in the Indo-Paci®c region (Clark and Rowe 1971; Conand 1989). On the Great Barrier Reef (GBR), it is the most abundant species in shallow-water reef̄at areas (Harriott 1980; Hammond et al. 1985; Uthicke 1994). Fission rates are very low during the main sexual spawning period in the GBR in December (Harriott 1980), and there is a seasonal peak in asexual reproduction in the austral winter (Uthicke 1997). Several authors have inferred dierences in the rate of asexual reproduction on dierent reefs or at dierent locations on the same reef (Chao et al. 1993; Conand 1996; Uthicke1997) on the basis of observations of differences in the percentage of recently divided or regenerating individuals in the total population. Similar data has indicated that asexual reproduction is higher on nearshore reefs that are subjected to enhanced nutrient loads (Conand 1996; Uthicke 1997). Although several Marine Biology (1998) 132: 141±151 Ó Springer-Verlag 1998 Communicated by G.F. Humphrey, Sydney S. Uthicke (&) á J.A.H. Benzie á E. Ballment Australian Institute of Marine Sciences, PMB No. 3, Townsville, Queensland 4810, Australia Fax: +61-74772-5852 e-mail: S.Uthicke@aims.gov.au S. Uthicke Institut fuÈ r Hydrobiologie und Fishereiwissenschaft, Zeiseweg 9, D-22765 Hamburg, Germany factors, such as emersion or high water temperatures during low tide (Bonham and Held 1963; Conand and De Ridder 1990; Chao et al. 1993), have been discussed in the literature, the triggers for asexual reproduction and the reasons for between-reef dierences in ®ssiparity remain unknown. This is at least partially due to the lack of an easily applicable methodology to estimate interreef dierences in ®ssion rates. The only methods available to date are long-term observations of populations over periods of at least one year. Allozyme electrophoresis provides a means of determining the genetic structure of populations and of inferring the level of asexual as opposed to sexual reproduction in those populations. The method has been successfully applied to sessile marine animals such as sea anemones (e.g. Black and Johnson 1979; Ayre 1984; Homann 1986), zoanthids (Burnett et al. 1995) and corals (Stoddart 1984; Ayre and Dufty 1994; McFadden 1997) and amongst mobile megafauna in asteroids (Johnson and Threlfall 1987; Kwast et al. 1990) and ophiuroids (Garret et al. 1997), but has not yet been used for holothurians. In this paper, the genetic structures ofHolothuria atra populations that were known to dier in their degree of ®ssiparity are described, using allozyme electrophoresis, in an attempt to quantify the extent of asexual reproduction and to estimate the importance of sexual recruitment in this ®ssiparous holothurian. Materials and methods |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://page-one.springer.com/pdf/preview/10.1007/s002270050380 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
