| The oyster Crassostrea hongkongensis is the main aquaculture species along south China coast and has significant economic value. Maowei Sea, an inland sea in northern Beibu Bay, is not only the "natural pasture" for this oyster but also the largest natural seedling field in China. However, due to the instability of natural seedlings and the change of natural environment, since 2008. Rapidly decreasing in the amount of oyster seeds and massive death of oysters have occurred frequently in spring. Some farmers then introduced the spats from Zhuhai (Guangdong). In order to monitor and evaluate the changes of the population genetic pattern of aboriginal oysters in Maowei Sea, we used microsatellite markers to analyze the genetic structure, and estimate the Ne of C. hongkongensis populations in Maowei Sea. Meanwhile, in order to explore the changes of population genetic structure from the fundamental way of breeding, we also studied the spawning characters of C. hongkongensis.Sixteen microsatellite markers were used to screen Zhuhai (ZH, n=32, sampled in 2013) and two Maowei Sea wild populations (13MW and 15MW, collected in 2013 and 2015, n=32, respectively), three natural spat populations [settled in 2012 (12L, n=48),2013 (13L) and 2014(14L), respectively]. The results showed that, based on 8 loci in HWE, medium levels of genetic diversities were observed in all six populations, with similar average allele number (allele richness) 8.1 (8.0) for ZH,8.4 (8.2) for 13MW,9.1 (8.8) for 15MW, and 9.1 (8.7) for 12L,9.4 (8.6) for 13L,9.8 (9.0) for 14L. The average observed/expected heterozygosities were also closed ranging from 0.60/0.81 (12L) to 0.73/0.77 (13MW). The inbreeding coefficient F1S were generally higher in larval populations than that in adult populations, with the minimal and maximum values occured in 13MW (0.02) and 12L (0.21), respectively. Three spat populations showed higher genetic divergences from ZH (FST,0.044-0.061) than that from MWs (FST,0.008-0.021), and significantly genetic divergences were found between ZH and MWs (0.037-0.053), suggesting so far the impact of introduced Zhuhai oysters might be limited.Ne estimates were based on six neutral loci. Samples were collected from the Maowei Sea in different years, including one adult population (13MW) and three spat populations (12L,13L,14L). The results showed that the average allele number/allele richness were ranging from 8.1/7.99 (ZH) to 9.8/9.03 (14L). The average observed/expected heterozygosities were 0.64/0.56 for 2013MW,0.68/0.53 for 12L,0.64/0.55 for 13L, and 0.69/0.57 for 14L, respectively. The medium levels of genetic diversities were observed in all four populations. Ne estimates were variable and generally small (19-233). But there is no significant change in allele richness among adult and spat collections, which may due to the reproductive characters of the oyster.With the combination of naked-eye observation, biopsy and histological analysis, we studied the reproductive cycle of oyster gonad development from July 2013 to June 2014, and developed a method for rapid identifying the developmental stage of oyster gonad. Samples, one-year old oysters, were collected from the estuary of Dafeng River (HH). The results showed that the oyster C. hongkongensis had only one breeding cycle each year, and the breeding cycle could be divided into 4 gonad developmental stages: undifferentiated stage (from January to March), differentiation stage (April, when the gender became distinguishable), mature and discharge stage (from May to October), and resting stage (from November to December). During the mature and discharge stage, the individual of oyster spawned many times with an interval of about 4-7 days. Thus, although a batch of survival larvae were only produced by a few adults, resulting in genetic draft, lots of adults’seedlings might survive throughout the reproductive season (six months), which overall maintained Maowei Sea oyster population’s structure temporally stable.Gonad development of two-year old oysters were compared, from estuary outside of Dafeng River (XF), estuary inside of Dafeng River (HH), estuary outside of Maowei Sea (QCT), estuary inside of Maowei Sea (LM), and Maowei Sea (SJ). The gonad development of one-year old oysters was synchronous with that of two-year old oysters. In April, the gonad of XF oysters developed into mature stage. However, gonad development of SJ still remained in the undifferentiated stage. Gonad development of XF oysters started 30 days earlier than that of SJ oysters, QCT’s 20 days earlier than SJ’s, HH’s 15 days earlier than SJ’s, and LM’s 7 days earlier than SJ’s. The variation might due to the differences in temperature, salinity and food abundance. |
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