A comparison of genetic diversity between cultured and wild populations, and a test for genetic introgression in the New Zealand greenshell mussel Perna canaliculus (Gmelin 1791)

Abstract

We evaluated whether genetic material from offspring of greenshell mussels, Perna canaliculus (Gmelin 1791), obtained from Kaitaia, northern New Zealand, and transferred to mussel farms around the country, had introgressed into wild (naturally occurring) mussel stocks. The potential extent of genetic introgression of this northern mussel DNA was assessed by analysis of allozyme, mitochondrial SSCP and RFLP, and nuclear RAPD markers. Cultured and wild populations throughout New Zealand showed an absence of biochemical genetic differentiation at seven protein-encoding loci, meaning that allozyme analysis was not a suitable tool to address the question of genetic introgression. For the DNA marker techniques, a haplotype specific to mussels originating from Kaitaia was identified only using mitochondrial composite markers. However, this composite marker occurred at low frequency in the Kaitaia population and not in any other populations and, therefore, could not be used to measure introgression of Kaitaia genes into wild populations. MtDNA and RAPD analyses revealed that North Island and Greater Cook Strait mussels were genetically different from lower South Island, particularly South Island west coast mussels, and the wild Stewart Island population. Furthermore, the two marker types showed that cultured mussels were significantly differentiated from wild mussels. These two findings clearly indicate that at the molecular level, population subdivision exists in P. canaliculus. The SSCP and RFLP markers did not show any evidence of genetic introgression of spat moved from the north into southern mussel populations. The RAPD markers, however, showed that the wild Stewart Island population (HSB) fell outside the northern and the southern clades of populations. The HSB population is located near an aquaculture site that has been seeded with mussels from Kaitaia for over a decade. Our RAPD data are consistent with the possibility of genetic introgression of genes from northern populations into this southern population. We recommend that genetic analysis of wild mussel populations in the vicinity of stocked areas be carried out regularly.