Selective And Cross Breeding Of Bay Scallop. Arsopecten Irradians Irradiatis (Lamarck)

The northern subspecies bay scallop, Argopecten irradians irradians, isdistributed along the North of Atlantic of the United States. Since it was firstintroduced into the north of China in1982, it has become one of the most importantmariculture mollusks in China and one of the most wonderful industries based onintroduced species was founded in the word. But lots of problems like inbreedingdepression, decline of commercial traits and so on which were probably caused byfounder effect, easiness of self-fertilization or some mismanagements in the bayscallop breeding industry emerged. As known re-introduction is an effective way toovercome the inbreeding effect and achieve genetic improvement in the short run, thesubspecies was re-introduced in1998and1999to restock the Chinese bay scalloppopulation and became the main culture object of the contry. To avoid genetic declineonce again, a selective breeding program for Chinese bay scallop was launched byInstitute of Oceanology, Chinese Academy of Sciences and one of wonderfulperformance new bay scallop strain named “Zhongkehong” was successfullycultivated, and this effectively changed the situation of lack of new culture strain inthe whole industry.Based on the former work, this study carried out the further selective breedingprogram and a new multi-stage in one generation selective breeding model which wasconducted in different stage was advanced and new strain has been cultivatedaccording to the model. Genetic analysis was conducted for strains in this selectiveprogram. Hypothesis and preferential fertilization were preliminary examinedthrough inter-specific hybridization, intra-specific hybridization, gradienthybridization and the hybrid proportion in the mixed families and so on. The majorresults and conclusions are as follows:1. New selective breeding model in which selection were conducted in differentstage in one generation was advanced and new strain has been cultivated. Tendays after fertilization (shell length about150-190μm)，the bay scallop larvaestarted to appear eye-spot and successive sieving enabled the truncation selectionwhich the largest10%larvae in shell length were selected. Around top10%culling in shell length at spat stage (three months after fertilization, shell lengthabout4-9mm), juvenile stage (five months after fertilization, shell length about25-35mm) and adult stage (seven months after fertilization, shell length about45-70mm)were separately conducted by some appropriate sieves. Theaccumulative selective pressure was about0.01%. Our result showed that everysieving group at the stage we mentioned above all had a significant advantage onthe control group (P<0.05) in the rest of the life stage. Here we first advanced themulti-stage selective breeding model based on enhancing the base population sizeand selective pressure which achieved by selecting in four life stage of bayscallop.According the multi-stage in one generation selective breeding model, after three years work, a new strain named “zhongke2” was successfully cultivated forgood performance in commercial traits from a base population which wasconstructed by pure purple shell color families in the former work.2. Genetic analysis was conducted for strains in this selective program.“Zhongkehong” strain,“Zhongke2” strain and one common commercial strain intwo generations, in total, six populations were examined at ten polymorphicmicrosatellite loci to assess the level of allelic diversity, heterozygosity, andgenetic variation among the populations. All populations showed restricted geneticdiversity. The mean number of alleles per locus ranged from3.10-3.50, and41different alleles were found over all the loci. The average observed heterozygosity(Ho) ranged from0.38to0.56, whereas the polymorphic information content (PIC)ranged from0.0739to0.7518. Genetic differences among the six populationswere detected based on the number of alleles per locus, the effective number ofalleles, Shannon index (I), Fisvalues, P values, Genetic distance and pairwise Fstvalues. Our result showed that there was significant genetic differentiation amongthe six populations (either among the strains or between the generations) as thetime under selection increased. According to the neighbor-joining tree topologyconstructed using the method of unbiased genetic distances among six populationsand almost all populations were closely clustered with their corresponding strain.Bottleneck effects might be occurring in the Chinese bay scallop as inferred fromthe lower levels of observed heterozygosity in the six populations.3. Interspecific hybridization and intraspecific hybridization were carried out toabtain heterosis in the cross breeding. Complete reciprocal interspecifichybridization between Argopecten purpuratus Lamarck and A. irradians irradiansLamarck was conducted under laboratory conditions. Bidirectional fertilizationsand successful development were observed for all crosses. Fertilization rate,development success, larval survival, and growth for whole life stages (larvae,spat and adult) were compared among hybrids and pure populations. Nosignificant differences in the fertilization rate and development success werefound among the experimental groups (P>0.05). Notable heterosis (i.e., greaterthan1%of the mid-parent values) was detected for growth throughout the wholelife span and for survival during the larval stage. The influence of maternal effectand mating type (intra-vs. interspecific crosses) on growth for all life stagesexamined (except day30and day90) was significant (P <0.05), but only matingtype had a significant effect (P <0.05) on larval survival. Genetic analysis usinginternal transcribed spacer1revealed that the survivor spat from cross groupscontained DNA from both parental species, which confirmed the hybridizationbetween the two species. H. E. staining paraffin slice for the gonad of maturehybrids was performed in the study. Male gonad was found obsolescence withoutany active sperm while female gonad was developed with abundant normalappearance eggs. But the hybrids eggs could not back-cross with the parentalspecies what expressed typical postzygotic isolation-hybrid sterile.Complete reciprocal hybridization between Chinese bay scallop and Canadianbay scallop was conducted under laboratory conditions. Significant heterosis (P<0.05) were examined in larval survival and growth for the hybridizationgroups. We inferred from the result that the cross between different geographicalpopulations might still be an effective way to achieve genetic improvement.4. Hypothesis and preferential fertilization were preliminary examined throughgradient hybridization and the hybrid proportion in the mixed families.Self-fertilization family, pairwise crosses in family, pairwise crosses betweenfamilies were set up by families with same genetic background. And the heterosisfor larval survival and growth rate were found positive along with gradient geneticbackground hybrids, which in accord with Dominance Hypothesis.Five pair-crossed mixed families were constructed by Chinese cultured bayscallop (Argopecten irradians). Preferential fertilization between self-fertilizationand hybridization and the trend of the hybrid proportion in the mixed familieswere revealed by five microsatellites. Significant heterozygote excess was foundin6-Hour-Old Embryos in two of three mixed families. The hybrid proportionswere found rising on1d D-shell larval,20d spat,40day spat,3months spat andadult under natural selection, and also the shell length heterosis. Hybridproportions were detected significant positively related with the shell lengthheterosis among all the life.