Genetic structure of fissiparous populations of Holothuria atra on the Great Barrier Reef

Population genetic structure was studied in two nearshore (Fantome and Great Palm Islands) and two midshelf (Rib and 18-026) reef populations of Holothuria atra on the Great Barrier Reef. Two stations approximately 500 m apart were sampled on each reef (only 150 m apart at Fantome Island due to the limited area in which the species occurs). At each station, 30-40 adult individuals (generally > 50 g) were sampled within a radius of 30m from a marker buoy, in early November 1996. The sampling time in November was chosen to increase the likelihood of finding mature gonads in the sampled animals. Female gonads were dark red, male gonads were orange; several small gonads were examined microscopically to determine whether they were ovaries or testes. Several inidivduals had gonads which were either immature or not detectable, these were exluded from the male/female analyses but included in other analyses.Genetic variation at 5 polymorphic loci was examined using allozyme electrophoresis. The polymorphic enzymes examined were: GPI, HK, LP, PGD and PGM. Measures describing the genetic variables for females, males and unsexed individuals were: sample size per locus; number of alleles per locus; percentage of loci that were polymorphic; heterozygosity (observed=Ho, expected=He); genotypic diversity (observed=Go, expected=Ge, and the ratio of Go to Ge); sample size (Ni); number of genotypes ((observed=Ngo, expected=Nge).Calculation were made on: the number of sexually produced individuals (N*); and the ratios of Ngo to Nge; Ni to Ngo; N* to Ni.Average weight (g), for females, males and unsexed individuals were recorded; and sex ratios for all sexed individuals and estimated annual fission rates calculated.F-statistics were used to partition genetic variation into that occurring within populations (FIS) and that occurring between populations (FST) for males, females and total individuals for both original and clonal genotype frequencies.A dendrogram was created to show genetic relationships between the populations of the 8 sites using UPGMA cluster algorithm and Nei's unbiased genetic distance. To describe the genetic structures of Holothuria atra using allozyme electrophoresis to quantify the extent of asexual reproduction and to extimate the importance of sexual recruitment in this fissiparous holothurian.

本研究针对大堡礁（Great Barrier Reef）上的黑海参（Holothuria atra）4个礁群种群开展遗传结构分析，其中2个为近岸礁群（范托姆岛礁与大棕榈岛礁），另外2个为陆架中部礁群（里布礁与18-026礁）。每个礁群设置2个采样站位，间距约500米；因范托姆岛该物种栖息区域有限，其采样站位间距仅为150米。1996年11月上旬，在每个站位距标记浮标30米半径范围内，采集30~40头成体个体，其体重通常大于50克。选择11月作为采样时段，旨在提高采集到性腺成熟个体的概率。该物种雌性个体性腺呈深红色，雄性为橙色；对部分小型性腺进行镜检，以确认其为卵巢还是精巢。部分个体的性腺未成熟或无法辨别性别，这类个体被排除于雌雄性别分析之外，但纳入其他分析。本研究采用等位酶电泳（allozyme electrophoresis）技术，对5个多态性位点（polymorphic loci）的遗传变异进行检测，涉及的多态性酶类包括葡萄糖磷酸异构酶（GPI）、己糖激酶（HK）、亮氨酸肽酶（LP）、6-磷酸葡萄糖酸脱氢酶（PGD）以及磷酸葡萄糖变位酶（PGM）。针对雌性、雄性及性别未确定个体，其遗传变量的统计指标包括：每个位点的样本量、每个位点的等位基因数、多态性位点占比、杂合度（观测杂合度Ho、期望杂合度He）、基因型多样性（观测基因型多样性Go、期望基因型多样性Ge，以及Go与Ge的比值）、样本量（Ni）、基因型数量（观测基因型数Ngo、期望基因型数Nge）。本次统计计算的指标还包括：有性繁殖个体数（N*），以及Ngo与Nge的比值、Ni与Ngo的比值、N*与Ni的比值。研究记录了雌性、雄性及性别未确定个体的平均体重（单位：克）；统计所有可辨别性别个体的性比，并计算估算的年度分裂速率。采用F统计量（F-statistics），基于原始基因型频率与克隆基因型频率，分别对雄性、雌性及所有个体的遗传变异进行分解，区分种群内遗传变异（FIS）与种群间遗传变异（FST）。采用UPGMA聚类算法（UPGMA cluster algorithm）与Nei无偏遗传距离（Nei's unbiased genetic distance），构建包含8个采样点种群的聚类树状图，以展示各群体间的遗传关系。本研究旨在通过等位酶电泳技术解析黑海参（Holothuria atra）的种群遗传结构，量化其无性繁殖的程度，并评估该分裂繁殖海参物种有性补充的重要性。