CYTONUCLEAR INTROGRESSIVE SWAMPING AND SPECIES TURNOVER OF BASS AFTERAN INTRODUCTION

Species-specific RFLP markers from mitochondrial DNA (mtDNA) were iden tified and employed in conjunction with previously reported data for n uclear allozyme markers to examine the genetic consequences of an arti ficial introduction of spotted bass (Micropterus punctulatus) into a n orth Georgia reservoir originally occupied by native smallmouth bass ( M. dolomieui). The cytonuclear genetic data indicate that within 10-15 years following the unauthorized introduction, a reversal in these sp ecies' abundances has occurred and that more than 99% of the populatio n sample analyzed here consists of spotted bass or products of intersp ecific hybridization. This demographic shift, perhaps ecologically or environmentally mediated, has been accompanied by introgressive swampi ng; more than 95% of the remaining smallmouth bass nuclear and cytopla smic alleles are present in individuals of hybrid ancestry. Dilocus cy tonuclear disequilibria were significantly different from zero, with p atterns indicative of an excess of homospecific genetic combinations ( relative to expectations from single-locus allelic frequencies) and a disproportionate contribution of smallmouth bass mothers to the hybrid gene pool, Results document dramatic genetic and demographic changes following the human-mediated introduction of a nonnative species.