壳聚糖-HCG缓释制剂对鱼类生殖内分泌机能的影响简

以壳聚糖为原料制备壳聚糖-HCG缓释制剂,进行激素埋植实验,间隔一定时间检测雌和雄鱼性腺发育、血清主要性激素含量和繁殖内分泌相关基因表达特征。结果表明:在埋植壳聚糖-HCG缓释激素后,雌鱼性成熟系数(GSI)、血清睾酮(T)水平、血清雌二醇(E2)水平在6—30d内较对照组效果明显;雄鱼GSI仅在第6天显著高于对照组,血清T水平在第2、第14天高于对照组。血清E2水平在实验期间与对照组无显著差异。RT-PCR结果显示:性腺型P450芳香化酶(CYP19A)在性腺中表达丰富,心脏中最少。雌鱼性腺P450芳香化酶(CYP19A)基因mRNA相对表达量在第2、第6天显著高于对照组,雄鱼在第6天显著高于对照,雌鱼性腺雌激素受体(ERα)基因mRNA相对表达量在14—30d显著高于对照组,雄鱼在第6至第21天显著高于对照组。研究表明,壳聚糖-HCG缓释制剂一次埋植后可在21d内稳定持久地释放激素,对调节鱼类的生殖机能具有良好的促进效果。     

雄烯二酮对赤点石斑鱼内分泌及性腺发育的影响

采用腹部埋植法,应用连续组织切片技术、放射免疫测定方法和半定量RT-PCR技术,检测了间隔30d3次埋植不同剂量（高剂量H：100μg／g体重,低剂量L：10μg／g体重）的雄烯二酮（4-androstene-3,17-dine,ADSD）诱导2龄赤点石斑鱼性腺发育过程中血清性类固醇激素含量、芳香化酶表达和活性的变化。结果显示：第1次埋植ADSD后30d即可诱导雌性赤点石斑鱼发生不同程度的性逆转,性腺成熟指数明显下降,性腺中卵细胞退化,精原细胞增殖,出现大量精母细胞和精子细胞,第1次埋植ADSD后30d经轻微挤压腹部有10％（低剂量组）和15％（高剂量组）的鱼可排精;第2次埋植后30d明显提高性逆转效果,排精率分别达到80％和86．7％,性逆转雄鱼的精巢在组织结构上与正常雄鱼精巢一样,精子活力与正常雄鱼相同,对照组鱼卵巢发育良好;第3次埋植后30d排精率分别为90％和80％,埋植ADSD后性腺芳香化酶（P450aromA）表达和活性显著降低,脑芳香化酶（P450aromB）表达显著升高,前脑芳香化酶活性显著升高,血清T,11-KT水平显著高于对照组,而血清E2水平先升高,然后逐渐降低。说明血清T,11-KT水平增加,性腺芳香化酶表达及活性降低在控制雌雄同体的赤点石斑鱼的性转化和精子发生中起重要作用。     

芳香化酶抑制剂letrozole对赤点石斑鱼(Epinephelus akaara)性逆转的作用

在繁殖季节,采用腹部埋植方式,用非类固醇型芳香化酶抑制剂（aromatase inhibitor,AI）letrozole以5mg／kg体重的剂量处理2龄雌性赤点石斑鱼（每4周埋植1次,共埋植2次）,检查埋植后性腺组织结构、血清性类固醇激素以及脑和性腺芳香化酶活性的变化。结果显示：一次埋植AI即可有效诱导雌性赤点石斑鱼发生不同程度的性逆转;性腺成熟指数明显下降;性腺中卵细胞退化,精原细胞增殖,出现大量精母细胞和精子细胞;性逆转雄鱼的精巢在组织结构上与正常雄鱼精巢没有明显差异,部分鱼成为功能性雄鱼。第一次埋植AI后4周轻微挤压腹部有14．3％的鱼可排精,精子活力与正常雄鱼相同。第二次埋植后明显提高性逆转效果,排精率在第6、8周分别达到35．3％和48．4％。此外,埋植AI后性腺芳香化酶活性显著降低,化脑郝芳香化酶活性的变化不明显;血清11-酮基睾酮（11-ketotestosterone,11-KT）浓度显著增加,雌二醇（estradiol-17β,E2）水平显著降低,而睾酮（testosterone,T）含量无明显变化。这些结果表明,AI主要通过抑制内源性E2的产生并提高11-KT水平,从而诱导赤点石斑鱼由雌性转变为雄性。     

注射LHRHa对泥鳅血清性类固醇激素变化研究

研究了注射促黄体激素类似物(LHRHa)后,泥鳅(Misgurnus anguillicaudatus)血清性类同醇激素的变化规律,并探讨在泥鳅繁殖季节时.孕酮(P)、睾酮(T)和雌二醇(E2)对性腺发育的作用及调节机制.实验共分两组,对照组和实验组;对照组只注射生理盐水;实验组注射LHRHa,雌鱼0.2 μg/g,雄鱼减半.注射前尾静脉采血,作为血液样本分析基础水平(Oh),注射药物后分别在7、24、48、72和96h尾静脉采血.测定雌鱼睾酮、雌二醇和孕酮,雄鱼孕酮和睾酬血清浓度.实验结果表明:注射LHRHa,雌鱼血清睾酮和雌二醇浓度显著高于对照组,雄鱼血清睾酮和孕酮显著高于对照组(P<0.05);24h浓度较高.雌鱼孕酬、睾酮和雌二醇分别为(0.710±0.082)ng/mL、(9.00±0.57)ng/mL和(696.4±26.2)pg/mL,雄鱼孕酮和睾酮分别为(0.527±0.121)ng/mL和(9.62±0.62)ng/mL.实验组雌鱼孕酮变化基本规律为,基础水平(0-7h)-逐渐升至最高(7-24h)-逐渐降至基础水平(24-48h)-维持基础水平(48-96h).实验组雌鱼睾酮和雌二醇与雄鱼孕酮和睾酮变化规律基本相似,其规律为,逐渐上升至最高(0-24h)-逐渐降至基础水平(24-72h)-维持基础水(72-96h).24h对照组雌鱼睾酮和雌二醇显著升高,浓度分别为:睾酮(2.20±0.18)ng/mL,雌二醇(269.1±36.6)pg/mL.对照组雄鱼血清孕酮和睾酮浓度实验期间均无显著变化.研究认为:LHRHa能够刺激泥鳅性类同醇激素分泌,特别是睾酮的分泌,显著提高雌鱼性腺指数(GSI).但刺激P的分泌调控能力有限,实验期间处于较低水平,诱导排卵效果差,泥鳅的性类固醇激素可能有特殊的调节机制.雌二醇和睾酮对性腺成熟有重要作用,孕酮对介导卵细胞最终成熟和排卵可能起重要作用,而雌二醇和睾酮无明显效果.     

雄烯二酮和甲基睾酮诱导雄性日本鳗鲡性腺发育的作用

多次埋植雄激素雄烯二酮（ADSD）或甲基睾酮（MT）均可促进雄性日本鳗鲡（Anguilla japonica)性腺发育成熟,明显提高脑和垂体mGnRH,垂体GtH含量,埋植3次后,MT处理组的GSI及垂体GtH含量显著高于ADSD处理组,MT处理组血清GtH含量在第1次埋植后显著升高,而ADSD处理组在第4次埋植后才显贰高于对照组。这些结果表明：埋植ADSD和MT可反馈作用于雄性日本鳗鲡脑和垂体,促进GnRH和TtH的合成和分泌,进一步诱导精巢发育。而且MT的作用效果较ADSD快。     

不同链长脂肪酸对斑马鱼的性腺脂肪酸组成、繁殖力与仔鱼成活率的影响

将体质健壮的4月龄斑马鱼(Danio rerio)亲鱼[雄鱼(0.36±0.05) g/尾, 雌鱼(0.59±0.06) g/尾]雌雄各180尾, 随机平均分配在室内斑马鱼循环系统的18个养殖缸中。在斑马鱼基础饲料(对照组)中分别添加7 g/kg n-3HUFA (高不饱和脂肪酸, Highly unsaturated fatty acid)(HUFA组)及10 g/kg MCFA (中链脂肪酸, Medium chain fatty acid)(MCFA组), 制成3组等氮等脂饲料, 饲养90d后, 探究不同链长脂肪酸对斑马鱼的性腺脂肪酸组成、繁殖力和仔鱼成活率的影响。结果表明: (1)3组雌鱼性腺的脂肪酸组成均受到所饲喂饲料脂肪酸组成的影响, 其相关系数均在0.8以上。HUFA组雌鱼性腺中EPA和DHA的相对含量显著高于MCFA组及对照组(P<0.05), 而MCFA组与对照组之间无显著差异; HUFA组油酸的相对含量显著低于MCFA组及对照组(P<0.05), 而MCFA组与对照组之间无显著差异; HUFA组及MCFA组亚麻酸的相对含量与对照组之间均无显著差异, 但MCFA组显著高于HUFA组(P<0.05)。(2)HUFA组及MCFA组雌鱼的成熟系数、绝对繁殖力、体重与体长的相对繁殖力均显著高于对照组(P<0.05), 同时在雌鱼绝对繁殖力、相对繁殖力上, HUFA组显著高于MCFA组(P<0.05)。(3)将对照组雄鱼与各组雌鱼配对繁殖的结果显示, 分别与HUFA组及MCFA组雌鱼配对繁殖后的雌鱼的绝对产卵量、相对产卵量和仔鱼成活率均显著高于与对照组雌鱼的配对, 同时以上指标HUFA组雌鱼均显著高于MCFA组雌鱼(P<0.05)。将对照组雌鱼与各组雄鱼配对繁殖的结果表明, 与HUFA组雄鱼配对后其受精率均显著高于与MCFA组及对照组雄鱼的配对(P<0.05)。综上所述, 试验饲料显著影响斑马鱼雌鱼性腺的脂肪酸组成, HUFA及MCFA均可以促进斑马鱼雌鱼的繁殖性能和仔鱼成活率的提高, 在试验条件下, HUFA的效果更好。     

LHRH-A缓释剂对雌性赤点石斑鱼卵巢发育、性类固醇激素分泌及脑垂体GTH细胞超微结构的影响

取75尾网箱养殖的2～3龄雌性赤点石斑鱼,分为3组（高、低剂量组和对照组）,各25尾。高剂量组埋植促黄体生成索释放激素类似物（LHRH—A）缓释剂,剂量为300μg／kg体重,低剂量组为100μg／kg体重,对照组埋植不含LHRH—A的药丸。分别在实验开始和埋植药丸后0、10、20、30、40天抽血,用放射免疫测定血清类固醇激素（E2、T）;解剖鱼体测定相关指标,计算性腺成熟系数;取性腺和肝脏组织常规组织学切片并透射电镜观察。埋植后第10-30天,两种剂量处理组的排卵率均高于对照组,高剂量组显著高于低剂量组。在第10天性腺成熟系数高剂量组为1．055％,卵母细胞已有部分迅速发育到Ⅳ期末（核偏位）;低剂量组相对缓慢。第20天性腺成熟系数高剂量组达1．858％,低剂量组为0．987％;处理组卵母细胞基本发育成熟。埋植后第10天,两处理组血清E2和T水平显著升高;第20天显著下降;此后E2保持在较低的水平,T显著低于对照组水平。超微结构的观察证实脑垂体GTH细胞在LHRH—A缓释剂诱导性腺发育成熟和排卵过程中处于活跃的合成与分泌状态。LHRH—A缓释剂能有效诱导赤点石斑鱼卵巢发育成熟和排卵,而性类固醇激素（E2、T）只与卵黄生成有关,与排卵无关。     

利用鱼类性逆转技术创制黄颡鱼XX雄鱼的方法

目前黄颡鱼(Pelteobagrus fulvidraco Richardson)生产上使用的母本非常混杂, 成为影响黄颡鱼产业发展的一个突出问题。建立一个优良性状稳定的全雌家系对于全雄黄颡鱼和杂交黄颡鱼的生产尤为重要, 而将XX雌性黄颡鱼逆转为XX雄性黄颡鱼是创制全雌家系中的关键一步。黄颡鱼性染色体连锁分子标记的开发为鉴定XX雄鱼提供了技术支撑。研究使用不同浓度的17α-甲基睾酮(MT)和芳香化酶抑制剂来曲唑(LZ)处理黄颡鱼鱼苗54d (7—60日龄)。61日龄测量并统计各组鱼的存活率、体长及体重。解剖观察性腺结构, 结合性腺组织切片和性别分子标记分析各组实验鱼中XX鱼性腺发育情况。结果显示: 2种药物对黄颡鱼的存活率影响较小, 与对照组没有显著差异; MT处理的XX性腺为空腔状精小囊结构, 不能逆转为功能性精巢; 适当剂量的LZ可以将XX雌性黄颡鱼性逆转成正常的XX雄鱼, 诱导效果随着剂量的增加而增强。用完全逆转的XX生理雄鱼分别与XX雌鱼和YY雌鱼交配, 能够正常繁殖并具备较好的繁殖能力。研究建立了一种使用芳香化酶抑制剂来曲唑创制黄颡鱼XX雄鱼的方法, 为全雄黄颡鱼的品种改良及新品种的培育奠定了基础。     

半滑舌鳎脑芳香化酶基因cDNA克隆及表达分析

为研究脑型芳香化酶（P450aromB）在半滑舌鳎性别分化中的作用,采用同源克隆策略,从半滑舌鳎脑分离了2184 bp长的脑型芳香化酶的全长cDNA,该基因编码498个氨基酸。氨基酸序列和系统发育分析表明,P450aromB属于脑型P450arom,P450aromB的氨基酸序列与其他鱼类脑型Ｐ450arom的同源性较高（48.3%—66.1%）,与性腺型Ｐ450arom的同源性较低(34.2%—49.9%),与自身的性腺型芳香化酶同源性为45.1％。RT-PCR分析表明：P450aromB mRNA的表达具有明显组织特异性,P450aromB只在性腺、脑、鳃和皮肤中表达,且脑中表达量远高于性腺,而在雌雄鱼的其他组织中都不表达。经过甲基睾酮浸浴处理和高温诱导半滑舌鳎由雌性性反转为雄性后,脑中P450aromB的表达量降低,这些结果表明P450aromB参与了半滑舌鳎的性腺分化和性别决定过程。     

南方鲶性腺分化的组织学观察

用芳香化酶抑制剂(Fadrozole)、雌激素受体拮抗剂(Tamoxifen)对人工孵化的南方鲶(Silurus meridionalis)幼鱼进行雄性化诱导处理(口服),获得雄鱼。对孵化后第5—130d的南方鲶幼鱼性腺进行组织学观察,结果表明,在实验条件下,南方鲶性腺分化发生在孵化后7d左右,雌雄性分化过程差异明显。雌鱼卵巢腔在孵化后12d左右形成,生殖细胞在孵化后35d左右快速增殖,成熟分裂最早发生在孵化后55d左右;雄鱼生殖细胞在孵化后130d左右快速增殖,成熟分裂最早发生在孵化后130d左右。雌性性腺分化早于雄性。     

黄颡鱼母本的肠系膜脂肪沉积对繁育性能的影响

为探索肠系膜脂肪堆积对黄颡鱼母本繁育性能的影响,选择了4个母本群体来评价肠系膜脂肪沉积与黄颡鱼繁育性能的关联:group 1,野生黄颡鱼群体[肠系膜脂肪指数(0.56±0.17)%]; group 2,肠系膜脂肪较少的黄颡鱼养殖群体[肠系膜脂肪指数(1.97±0.40)%]; group 3和4,肠系膜脂肪较多的2个黄颡鱼养殖群体[肠系膜脂肪指数(5.92±1.85)%和(9.62±1.01)%]。研究结果显示肠系膜脂肪体重比与性腺指数总体上呈负相关。人工繁殖结果表明, group 1和group 2的产卵率、受精率和单尾母鱼出苗量无显著性差异,但显著高于group 3和group4; group 1和2的苗种畸形率显著低于group 3和4。黄颡鱼雌鱼血清中促黄体生成素(LH)和卵黄蛋白原(VTG)的水平随着肠系膜脂肪沉积量的增大而降低;相反,肝脏和卵子中的油脂和糖原含量随着肠系膜脂肪沉积量的增大而升高。研究发现,肠系膜脂肪过度沉积的黄颡鱼母本的生理指标和繁殖性能明显下降,生产出来的苗种质量欠佳;建立完善的亲本培育模式并减少肠系膜脂肪沉积能够显著改善繁殖性能,文章为提高鱼类苗种质量提...     

Sex differentiation and mRNA expression of P450c17, P450arom and AMH in gonads of the chicken

The present study was conducted to reveal effects of in ovo injection of nonsteroidal aromatase inhibitor (Fadrozole) or estradiol at day 3 of incubation on mRNA levels of P45017alphahydroxylase (P450c17), P450 aromatase (P450arom) and anti-Müllerian hormone (AMH) in the chicken gonads. The mRNA levels in the gonads at days 4-8 of incubation were assessed by in situ hybridization analysis using digoxigenin labeling method. The in situ hybridization data were analyzed by relative expression of specific hybridizable signals of each mRNA corrected by the non-specific background by employing an image analyzer. P450c17 mRNA expression increased rapidly at day 6 of incubation in the male but decreased thereafter. In contrast to the transient expression in the male, the expression was gradually increased in the female. P450arom mRNA was not expressed in the male but was detectable in the female as early as day 6 and increased subsequently with days of incubation. AMH mRNA was expressed as early as day 5 of incubation followed by a sharp increase on day 6, which was maintained in the male thereafter. In contrast, the female showed very little expression. The injection of Fadrozole caused no effect on P450c17 mRNA expression, while it suppressed P450arom mRNA expression but increased AMH mRNA expression in the female. In contrast, the injection of estradiol induced P450arom mRNA expression significantly but suppressed AMH mRNA expression in the male. These results indicate that expression of P450arom and AMH is sexually dimorphic and is reciprocally regulated during early ontogenic life in chicken gonads.     

Molecular cloning and expression of TRα and TRβ in the protandrous cinnamon clownfish,Amphiprion melanopus during sex reversal

We divided the process of sex reversal from immature male to mature female in the protandrous cinnamon clownfish (Amphiprion melanopus) into six developmental stages as follows: I, immature male; II, mature male; III, male at 60 days after female removal; IV, male at 90 days after female removal; V, male at 120 days after female removal; and VI, mature female. Thyroid hormone receptors α (TRα) and β (TRβ) cDNAs were cloned from the ovary and mRNA expression levels were compared during the sex-reversal process. The nucleotide sequences of the TRα and TRβ cDNA were 1230 and 1188 bp in length with open reading frames encoding peptides of 409 and 395 amino acids, respectively. We observed that TRα mRNA and protein levels were high in all stages except the immature gonad, while TRβ mRNA levels were higher in the mature ovary than in any other gonadal stage. We injected gonadotropin-releasing hormone analogue to identify its effects on TRs mRNA in immature fish. The mRNA levels of TRs increased significantly. We therefore propose that TRs are related to testicular development as well as ovarian development in cinnamon clownfish. The present study also provides basic data on the role of TRs during sex reversal in fish.     

Effects of cadmium on the reproductive axis of Japanese medaka (Oryzias latipes)

Cadmium (Cd) is a ubquitous element and a significant inorganic pollutant that has previously been found to bioaccumulate in reproductive organs of fish and disrupt important endocrine processes, especially those involved in synthesis, release and metabolism of hormones. Clearly, there is potential for reproductive effects in fish populations exposed to Cd, however, few studies have investigated the non-lethal consequences of Cd in fish. To this extent, adult male and female Japanese medaka were exposed to 0-10 ppb Cd for 7 weeks. Reproductive endpoints were monitored during weeks 6 and 7 of exposure and compared to physiological responses along the hypothalamus-pituitary-gonadal (HPG) axis, including plasma vitellogenin (VTG), hepatic estrogen receptor (ER), plasma steroids, gonadal-somatic indices (GSI), and gonadal steroid release. There were no observed effects on VTG and ER by long-term Cd exposure. However, gonadal steroid release was significantly decreased in males and females at all exposure concentrations and female plasma estradiol levels were significantly altered at concentrations higher than 5 ppb Cd. Overall, responses along the HPG axis were more sensitive to Cd exposure than the reproductive and developmental endpoints, which were not affected in this study, indicating that higher level impairment in fish might be relatively protected.     

Effects of gonadotropin-releasing hormone analog (GnRHa) on steroidogenic factor-1 (SF-1) and estrogen receptor beta (ERbeta) gene expression in the black porgy (Acanthopagrus schlegeli)

We examined effects of GnRHa on expression of steroidogenic factor-1 (SF-1) and estrogen receptor beta (ERbeta) in the pituitary and gonad of protandrous black porgy (Acanthopagrus schlegeli). Fish were intraperitoneally injected with 0.2 microg GnRHa/g fish and then pituitary, gonad and plasma were sampled at 0, 6, 12, 24 and 48 h after injection. In gonad, the mRNA levels of the SF-1 were high at 6 h post injection, and then continuously decreased until 24 h; high expression of ERbeta mRNA levels was only observed at 12 h. In contrast, pituitary SF-1 mRNA levels were very low during the experimental period. GnRHa stimulation caused a significant increase of plasma testosterone (T) and estradiol-17beta (E(2)) after 24 h. We suggest that SF-1 and ERbeta play an important role in the development of gonad and these genes are involved with sex change in fish.     

Immunohistochemical detection and biological activities of CYP17 (P450c17) in the indifferent gonad of the frog Rana rugosa

Sex steroids play a crucial role in the gonad differentiation in various species of vertebrates. However, little is known regarding the localization and biological activity of steroid-metabolizing enzymes during gonadal sex differentiation in amphibians. In the present study, we showed by real-time RT-PCR analysis that the expression of CYP17, one of the key steroidogenic enzymes, was higher in the indifferent gonad during sex differentiation in male than in female tadpoles of Rana rugosa but that there was no difference detected in the 3betaHSD mRNA level between the male and female gonads. We next examined the localization of CYP17, 3betaHSD and 17betaHSD in the indifferent and differentiating gonads by using three kinds of antibodies specific for CYP17, 3betaHSD and 17betaHSD, respectively. Positive signals for CYP17, 3betaHSD and 17betaHSD were observed in somatic cells of the indifferent gonad of males and in the interstitial cell of the testis. The enzymatic activity of CYP17 was also examined in the gonad during sex differentiation in this species. [(3)H]Progesterone (Prog) was converted to [(3)H]androstenedione (AE) in the indifferent gonad in males and females, but the rate of its conversion was higher in males than in females. Moreover, fluorescence in situ hybridization (FISH) analysis revealed that the CYP17 gene was located on the q arm of chromosome 9, indicating that CYP17 was autosomal in R. rugosa. Taken together, the results demonstrate that the CYP17 protein is synthesized in somatic cells of the indifferent gonad during gonadal sex differentiation in R. rugosa and that it is more active in converting Prog to AE in males than in females. The data suggest that CYP17 may be involved in testicular formation during sex differentiation in this species.     

The evolution of gonad expenditure and gonadosomatic index (GSI) in male and female broadcast‐spawning invertebrates

Sedentary broadcast‐spawning marine invertebrates, which release both eggs and sperm into the water for fertilization, are of special interest for sexual selection studies. They provide unique insight into the early stages of the evolutionary succession leading to the often‐intense operation of both pre‐ and post‐mating sexual selection in mobile gonochorists. Since they are sessile or only weakly mobile, adults can interact only to a limited extent with other adults and with their own fertilized offspring. They are consequently subject mainly to selection on gamete production and gamete success, and so high gonad expenditure is expected in both sexes. We review literature on gonadosomatic index (GSI; the proportion of body tissue devoted to gamete production) of gonochoristic broadcast spawners, which we use as a proxy for gonad expenditure. We show that such taxa most often have a high GSI that is approximately equal in both sexes. When GSI is asymmetric, female GSI usually exceeds male GSI, at least in echinoderms (the majority of species recorded). Intriguingly, though, higher male GSI also occurs in some species and appears more common than female‐biased GSI in certain orders of gastropod molluscs. Our limited data also suggest that higher male GSI may be the prevalent pattern in sperm casters (where only males release gametes). We explore how selection might have shaped these patterns using game theoretic models for gonad expenditure that consider possible trade‐offs with (i) somatic maintenance or (ii) growth, while also considering sperm competition, sperm limitation, and polyspermy. Our models of the trade‐off between somatic tissue (which increases survival) and gonad (which increases reproductive success) predict that GSI should be equal for the two sexes when sperm competition is intense, as is probably common in broadcast spawners due to synchronous spawning in aggregations. Higher female GSI occurs under low sperm competition. Sperm limitation appears unlikely to alter these conclusions qualitatively, but can also act as a force to keep male GSI high, and close to that of females. Polyspermy can act to reduce male GSI. Higher male than female GSI is predicted to be less common (as observed in the data), but can occur when ova/ovaries are sufficiently more resource‐intensive to produce than sperm/testes, for which some evidence exists. We also show that sex‐specific trade‐offs between gonads and growth can generate different life‐history strategies for males and females, with males beginning reproduction earlier. This could lead to apparently higher male GSI in empirical studies if immature females are included in calculations of mean GSI. The existence of higher male GSI nonetheless remains somewhat problematic and requires further investigation. When sperm limitation is low, we suggest that the natural logarithm of the male/female GSI ratio may be a suitable index for sperm competition level in broadcast spawners, and that this may also be considered as an index for internally fertilizing taxa.