Population variation in the mitochondrial-DNA of two marine organisms: The hard shell clam Mercenaria spp. and the killifish, Fundulus heteroclitus.

摘要

The two major topics are covered in this dissertation: the integration of molecular genetic tools with applied aquacultural research and short-term evolutionary dynamics. The first study investigated the extent of geographic differentiation of native clam stocks along the U.S. east and Gulf coasts. Clam mitochondrial DNA (mtDNA) demonstrated size polymorphism (16.5-19.0 kb) and restriction site heteroplasmy. Size heteroplasmic individuals occurred at a frequencies of 0-89% in clam populations. Restriction site heteroplasmy occurred in 12% of the 317 individuals assayed. Results of the study indicated that although harbouring a wealth of genetic variation, clams from northern regions were similar and lacked evidence of geographic differentiation. Northern populations exhibited high probabilities of gene identity (average I = 0.882), low percent nucleotide sequence divergence (average {dollar}{bsol}delta{dollar} = 0.003), and high levels of gene flow (average N{dollar}{bsol}sb{lcub}{bsol}rm e{rcub}{dollar}m = 3.6). All other populations were geographically differentiated.; Phylogenetic analysis of the clam taxa Mercenaria mercenaria, M. campechiensis and M. mercenaria texana detected similar degrees of divergence between all three taxa (ranging from {dollar}{bsol}delta{dollar} = 0.053 to 0.020), indicated that the texana group may be of multiple maternal origin, and concluded that in all probability texana deserves species distinction separate from M. mercenaria.; In the second section, the effect of restricted gene flow was evaluated as a possible explanation for maintenance of morphological and gene frequency clines in killifish. Effective migration rate was concluded using mtDNA haplotype frequencies for five partially isolated populations. From F{dollar}{bsol}sb{lcub}{bsol}rm st{rcub}{dollar}, N{dollar}{bsol}sb{lcub}{bsol}rm e{rcub}{dollar}m was estimated to be 24.09. The corresponding value from a private alleles-type analysis was N{dollar}{bsol}sb{lcub}{bsol}rm e{rcub}{dollar}m = 18.47. These estimates indicated a very large potential for gene flow necessitating that substantial selection pressures be invoked to account for the present-day clinal distributions in F. heteroclitus.