Title:Genetic structure of round scad mackerel Decapterus macrosoma (Carangidae) in the Indo-Malay archipelagoGenetic structure of round scad mackerel Decapterus macrosoma (Carangidae) in the Indo-Malay archipelago
Abstract:Round scad mackerel sampled in 1995-1998 were analysed for genetic variation using mitochondrial and nuclear-DNA markers. Sequence variation for a fragment of the cytochrome b gene (mitochondrial), amplified by polymerase chain reaction, was screened across individuals using single strand conformation polymorphism (SSCP). Sequence analysis of all SSCP haplotypes indicated two mitochondrial clades separated by 2.3% nucleotide divergence on the average. The geographic distribution of haplotypes was homogeneous (Weir and Cockerham's ^θ=-0.002). Also, no geographical heterogeneity was detected for length polymorphism of Intron 1 of the gene encoding aldolase B (^θ=0.005). While homogeneity in allele frequencies throughout the Indo-Malay archipelago conformed to the expectations for a widely distributed pelagic fish in a highly connected habitat, this was at variance with the sharp geographic structure previously uncovered in Indian scad mackerel, D. russelli, a fish with life-history characteristics similar to D. macrosoma. A remarkable similarity however was the occurrence of two similarly distinct clades within each species, suggesting a common history of geographic isolation. Low sea levels in the Pleistocene might have caused the separation and vicariance of populations within both D. macrosoma and D. russelli. Subsequent genetic exchange between populations would then have erased allele-frequency differences at the cytochrome b and aldolase B loci in D. macrosoma while some barrier to gene flow was maintained in D. russelli.