Hybridization Between Crassostrea Ariakensis And C.gigas, C.angulata

C. angulata is the subspecies of C. gigas in taxonomy. C.gigas mainlydistributed in the northern coastal, while C. angulata mainly distributed in thesouthern coastal, the two species are important economic shellfish in our country. Theproduction of C. angulata was the biggest one, while C. gigas was the third one afterC. hongkongensis. C. ariakensis can be seen normally in China’s north and southcoastal, its production was only less than C. gigas. These three species all belonged toCrassostrea oyster, they are traditional breeding oyster of our country (include C.hongkongensis). They were breeding by intraspecific in recent years, many badphenomenon had appeared, like growing slowly, shell height miniaturization, lowerresistance, restrict the development of oyster industry seriously. The hybridizationbetween C. gigas and C. ariakensis besides the hybridization between C. angulataand C. ariakensis had been studied, include many related experiment. Many detailedand reliable data had been attained, mainly result as follows:(1) The hybridization between C. gigas and C. ariakensis had been conducted.Symmetrical fertilization was attained in this experiment, but according to the badgametes compatibility, the fertilization of the two species was very low. Thefertilization of GA was0.16%and AG was7.31%at the same time, they weresignificantly less than intraspecific cross. All intrespecies cross grew fast in larvalstage because of maternal effects, but they had hybrid weakness in spat stage. Hybridprogeny had a higher death rate in larval stage, the survival rate was steady in spatstage though it was lower than intraspecific cross. Many replication experiments hadbeen conducted and they showed the same result. Reason for this phenomenon wasmainly because the juvenile had adapted to the chromosome compatibility. Many purespecies oyster died in breeding season, the same phenomenon appeared on hybridprogeny, but it delayed half to one month. It would be the next study to determinewhether we can obtain new specie and the specie had a higher survival rate in hot summer, if we do selective breeding to hybrid progeny.（2）We conducted2×2factorial crosses between C. angulata and C. ariakensisto determine whether the two species can fertilize with each other and the hybridprogeny can hatch, survive and get heterosis. In this study, fertilization wassymmetrical and no delay was observed. Fertilization success of C. angulata×C.ariakensis was lower than that of C. ariakensis×C. angulata, and the successrate of both of these crosses was lower than that of the two intraspecific crosses.Progeny from the two hybrid crosses had a lower survival rate during the planktonicperiod compared to progeny of the two intraspecific crosses. At day360, mean shellheight of the progeny was highest in C. angulata×C. angulata, followed by C.angulata×C. ariakensis, C. ariakensis×C. ariakensis, and C. ariakensis×C. angulata.(3) The oysters lived on rock with any shape; they can’t be identified by externalmorphology. Meanwhile hybridization usually has lower fertilization, survival rate,they could easily contaminated by other oyster, so they must be identifide bymolecular genetics. ITS1was used to identify the hybrid progeny of C. gigas and C.ariakensis, ITS2to the hybrid progeny of C. angulata and C.ariakensis. It got oneband in pure oyster, two bands in hybrid progeny through agarose gel electrophoresis,so they were amphimixis hybrid by preliminary inspection. But the band of hybridcorresponded to the parent were not in the same location, so the band was cloned andsequenced. After compared with the gene bank, it was found that all parents were pureoyster and the hybrids were amphimixis hybrid progeny. All hybrids progeny similarto C. gigas or C. angulata in360day in phenotype, it also reflected in the sequenceby the similarity to their parents.(4) The effect of salinity on two hybrid progenies (GA and AG) and two inbredgroups (GG and AA) was investigated using an experimental design incorporatingseven salinities (10,15,20,25,30,35and40ppt). The results showed that theoptimum salinities for survival and optimal growth for the various experimentalcrosses were as follows:15-35and10-25ppt, respectively, for GG;10-25and20-25,respectively, for AA;15-30and15-30respectively, for GA;20-30and20-25, respectively, for AG. The GG inbred group was sensitive to low salinity, as were theAA inbred progeny. Heterosis was positive for the AG hybrid progeny at the highersalinity, but negative for both the GA and AG hybrids for growth as measured by shellheight. The heterosis of HG×Awas13.32at a salinity of30ppt, while the single parentheterosis of HGAand HAGwas1.89and27.88, respectively. The single parent heterosisof HAGincreased to400at a salinity of40ppt. The adaptability of juvenile hybrid wasshared between parents but showed a certain degree of paternal inheritance. Thisstudy examined the effects of various salinities on the growth and survival ofCrassostrea gigas, C. ariakensis and juvenile hybrids. Through hybridization,trait-specific germplasm can be identified and traits can be improved, but notnecessarily growth performance. This study has the potential to be the basis forincreased aquaculture production of oysters, and to advance our knowledge ofinterspecies cross breeding, reproductive isolating mechanisms and heterosisutilization.(5) Hybrid progeny’s gonads of C. gigas and C. ariakensis was dissected afterone year’s culture. It was found that the hybrid progeny had the mature gonad, andhad no difference with pure oyster. Female gonads, male gonads, even hermaphroditescan be seen under microscope. So we conducted backcross between hybrid and theirparents include C. gigas and C. ariakensis to determine whether they had growth andsurvival heterosis. On the backcrossing experiment of AG and their parents, thefertilization and hatching rate of AgAg were in high level, but lower growth andsurvival rate can be seen in spat, showed a recession phenomenon. The externalmorphology was similar, no separate phenomenon happened. All backcrossing groupshad high fertilization and hatching rate and had growth and survival heterosis in larvaland early spat stage (except the survival rate of AAg). GG> GAg> AAg> AA>AgG> AgA> AgAg in shell height at day360, it reflected the strongly maternaleffects. The shell height of GAg was smaller than GG, but they have no significantdifference, and GAg had survival heterosis. On the backcrossing experiment of GAand their parents, all groups reflected a high fertilization and hatching rate. Thebackcrossing group of GaG had growth and survival heterosis. On the day90GaGa> GaG> GGa> GG> GaA> AA> Aga in the shell height, GaGa had a notable growthand survival heterosis.(6) We use AG hybrid with C. angulata to find whether this cross can have agood growth effect and whether AG had a gene introgression with other species in thewild. It can be found that they had symmetrical fertilization, even high fertilizationand hatching rate can be obtained in this experiment. In this study AG can cross withC. angulata, so it was a potential threat to biodiversity. C. angulata is the subspeciesof C. gigas, maybe they can cross easily. Whether AG can cross with other species, itshould do a further research, to estimate the risk of gene introgression.