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Photoperiod and genetics of growth and maturity in cod (Gadus morhua)

Author(s):

Guðmundur Óli Hreggviðsson, Ólafur H. Friðjónsson, Þorleifur Ágústsson, Sigríður Hjörleifsdóttir, Kjell Hellman, Filipe Figueiredo, Helgi Thorarensen

Funded by:

Technology Development Fund, The Icelandic Centre for Research

Contact

Guðmundur Óli Hreggviðsson

Strategic Researcher

gudmundo@matis.is

Photoperiod and genetics of growth and maturity in cod (Gadus morhua)

Eitt af megin vandamálum þorskeldis er ótímabær kynþroski sem leiðir af sér hægari vöxt og mun lengri eldistíma. Forrannsóknir sem framkvæmdar voru í kerjum á landi bentu til þess að ný tegund ljósa, svonefndra CC ljósa, sem varpa ljósi af einni bylgjulengd (grænt ljós) hefði   mun meiri áhrif á vöxt (allt að 60% vaxtaraukning) og tímasetningu kynþroska en hefðbundin halógen ljós. Í þessari rannsókn var kannaður hver væri þáttur arfgerðar og tjáningu ákveðinna lykilgena í svörun við ljóslotu, ákvörðun ótímabærs kynþroska og tengsl við aukinn vöxt þorsks í eldi. Kannað var hvort, líkamsstærð, þyngd og ótímabær kynþroski væru fjölskyldulægir eiginleikar.      Að loknu sex mánaða innanhúss foreldi frá klaki við tvenns konar lýsingu, annars vegar við hvítt halógen ljós og hins vegar við samfellt CC‐ljós var þorskurinn stríðalinn í sjókvíum. Í sjókvíunum var hann einnig undir mismunandi ljóslotumeðferð annars vegar við samfellt CC‐ljós og hins vegar við náttúrlegt ljós. Að loknu tveggja ára sjóeldi var fisknum slátrað, svipgerðir vaxtar og kynþroska ákvarðaðar og hann greindur í systkinahópa, alls u.þ.b. 2000 fiskar úr hvorri meðferð fyrir sig. Í ljós kom að CCL ljós hafði greinleg áhrif til seinkunar kynþroska og var það mun meira áberandi hjá hængum. Kynkyrtlar þroskuðust verulega hægar og alls ekki til fulls miðað við þorsk sem var alinn við náttúrulega ljóslotu á sama tíma. Þá var nokkur breytileiki milli systkinahópa hvað varðaði áhrif ljóss á þroskun kynkyrtla. Á hinn bóginn gætti þessarar seinkunar kynþroska ekki í auknum vaxtarhraða svo nokkru næmi. Fjölskyldulægni vaxtarsvipgerða var einnig könnuð og kom fram greinlegur munur á meðalþyngd milli systkinahópa. Svörun við CCL meðferðar var hins vegar mjög misvísandi og virtist breytileg milli systkinahópa. Virtist CCL meðferð auka vaxtarhraða í sumum hópum en hægja á vaxtarhraða í öðrum. Þetta gat svo verið breytilegt eftir kynjum. Þá var kannað hvort formeðhöndlun seyða með CCL ljósi á strandeldiskeiði gerði þorskinn næmari fyrir áhrifum CCL ljóss á sjókvíastigi. Svo reyndist ekki vera.   Einangruð voru ákveðin lykilgen í vaxtar‐ og kynþroskastýringu ásamt innröðum þeirra að nokkru eða öllu leyti. Þetta voru gen fyrir vaxtarhormón (GH), viðtaka vaxtarhormóns, viðtaka fyrir vakningarþátt vaxtarhormóns (Growth hormone releasing factor (GHRF)) og viðataka insúlínlíks vaxtarþáttar2 (IGF2). Í innröðum þriggja þessara gena, GH, GHR og IGF2, fundust breytilegar stuttraðir og var þróuð aðferð til erfðamarkagreiningar sem byggði á þessum röðum. Áhrif ljóslotu á vaxtarsvipgerð voru einnig metin með mælingum á tjáningu tveggja þessara gena, GH og GHR. Hlutfallsleg tjáning GHR reyndist ekki vera meiri í stærri fiski og ekki fannst neitt samband milli seinkunnar á kynþroska af völdum CCL og tjáningar á GHR. Tjáning GH var einnig könnuð í fiski á sjókvíastiginu. Marktækur munur í tjáningu GH mældist einungis í einum sýnatökupunkti, snemma á fyrsta ári í sjókvíunum í hópnum sem hlaut CCL meðferð. Þessi aukning átti sér ekki stað í tilsvarandi breytingum í þeim sviðgerðareinginleikum vaxtar og kynþroska sem mældir voru á sama tíma.

Light and photoperiod is a powerful environmental regulator of growth and sexual maturation in fish. Initial studies had indicated that a new type of lights, Cold Cathode‐lights (CCL), had much greater effect on growth and sexual development than white halogen light traditionally used in fish farming. In this study we investigated selected candidate gene expression in cod in response to CC‐light photoperiod treatment and possible genetic contribution to this response. This effect was evaluated by quantitatively comparing phenotypic traits under the different treatments. On December 2008 (“*”), significant differences were found fish farmed under natural light, in combination with gene expression studies and genotypic family assignments.   After hatching the fish was reared from 6 months in indoor tanks under two different light regimes, white halogen light and CC‐light. The fish was then transported to sea cages and divided into two groups, one that received continuous CC‐light and another that received only natural light treatment. The fish was reared in these sea cages for additional two years until harvesting. Approximately 2000 individuals from each treatment were genotyped and assigned to different full sib groups. Total body weight, length and gonadal weight were also measured for all individuals. Physiologically, the fish responded clearly to CCL treatment regarding maturity related traits, with less gonadal development in the CCL treated fish. The difference was substantial and the same trend could be observed in all families. The degree of response however differed somewhat between families to some which may signify underlying genetic differences. The effect of CCL treatment on growth related traits was less clear. Apparent growth responses to CCL treatment varied greatly between families and they appeared to be either negative or positive, depending on family and sex. Opposite effect were even observed within families on different sexes. In this project genes associated with growth and maturity were retrieved partly or completely from cod (Gadus morhua). These genes were: Growth hormone (GH), growth hormone receptor (GHR), growth hormone releasing factor (GHR F) and insulin like growth factor 2 receptor (IGF2R). A number of their introns were also obtained and variable microsatellite regions could be identified in intron regions of three of these genes, GH, GHR F, and IGF2R. A method was developed based on the GH and GHR gene sequences to amplify and evaluate expression of these genes in different tissues of cod. GHR expression levels were measured at different sampling points both during the indoor stage where different size groups and treatments were compared and at the outdoor stage where different light treatments were mainly compared. Differences in expression levels between different size groups and between different light treatment groups were insignificant. The light influence is on the GH gene expression, was only observed in the beginning of March early at the sea cage stage and could not be associated with increased growth or delayed reproductive development. The CCL (Cold‐Cathode Light) has a single green wavelength that diffuses more effectively throughout the water column than white light. It may therefore mask natural light more efficiently. Still it may be necessary to train fish for the CCL lights and at the indoor stage one half of the juveniles received CCL treatment before transportation to the sea cages. When imprinted and not imprinted were compared negligible difference in gonadal development were, however, observed strongly indicating that prior imprinting to sea cage rearing had no effect.  

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