Lin Zhihua

Karyotypes of Diploid and Triploid Mercenaria mercenaria (Linnaeus)

Abstract Northern quahogs (=hard clam; Mercenaria mercenaria) is a commercially important bivalve naturally distributed along the American Atlantic coast. Because the hard clam was first introduced into China as a candidate for aquaculture in 1997, research has been conducted on its ecologic habits, ecological physiology, metabolism, growth, reproduction, artificial propagation, and seed culture. Little information about their chromosomes and karyotype is available. The present study is designed to study the karyotype of the diploids and triploids to provide a reference to genetic research on introduced M. mercenaria. Triploids were produced by inhibiting the release of second polar body (PBII) with a treatment of 450 μmol/mL 6-deimethylaminopurine (6-DMAP), with untreated fertilized eggs developing as normal diploid controls. Chromosome preparations were made from trochophore larvae with drop-splash method. Chromosomes were measured and classified according to Leven criteria. Statistical analysis of relative length and arm ratio revealed that, this species had a normal diploid chromosome number of 2n = 38, and the karyotype consists of 15 pairs of metacentric (M) and 4 pairs of submetacentric (SM) chromosomes, NF = 76. The triploid karyotype formula was 45 M + 12 SM, NF = 114. The heterochromosomes and satellite chromosomes were not observed in all chromosomes in the complement.

The growth of F 1 from matings and crosses of different geographical populations of Meretrix meretrix : The growth of F 1 from matings and crosses of different geographical populations of Meretrix meretrix

Matings and crosses within and among two different geographic stocks from Shandong and Jiangsu populations in Meretrix meretrix were produced in order to breed new strains for farming. Two populations,designated S♀×S♂ and J♀×J♂,were from matings of wild populations of Shangdong and Jiangsu. Two populations,that was S♀×J♂ and J♀×S♂,were from crosses between Shangdong and Jiangsu respectively. The traits of fertilizing rate,hatching rate,growth of D-larvae and growth of juveniles (1-12 months age) were analyzed by ANOVA. The results showed that the fertilizing rate and hatching rate of matings and crosses were not significantly different at the end of second month. There were great changes of growth of juveniles in early-term (3-12 months). Growth of S♀×J♂,S♀×S♂ and J♀×S♂ had no significant difference from each other,but all had significant difference with J♀×J♂ at the early-term (3-12 months). The results indicated that there was different extent heterosis (6%-68%) for growth in F1 crosses. The uptrend among traits was wet weight being greater than other three traits of shell length,shell width and shell height respectively. There were obviously higher growth rate and lower variation coefficicent in S♀×J♂,which indicated the Shandong population would be a good parental stock as maternal for genetic breeding. Water temperature could affect growth of Meretrix meretrix significantly,and the growth rate reached a peak in October and November,but reached a nadir after December. Four populations gained were considered as the basic populations to breed new strains. Increased variation parameters in first filial generation(F1) of cross could be a very good potential for selection of new strains.