Molecular Population Genetic Studies On Introgressive Hybridization Between Silver Carp and Bighead Carp in the Mississippi and the Yangtze Rivers

Silver carp and bighead carp are native to China, and have been introduced into many countries for food production and/or plankton control. Hybridization between these two species has been documented in the US based on morphological and biochemical evidence. Based on genetic analysis of 167 Asian carp (24 of which were identified morphologically as hybrids) using 20 silver carp or bighead carp-specific microsatellite markers and 3 mtDNA markers, we found that extensive introgressive hybridization has occurred in the Illinois River and Mississippi River, with many backcrosses among fish identified morphologically as “purebred”. Specifically, 61.4% of samples from the Illinois River and 30% from the Mississippi River were introgressive hybrids. Using the same set of diagnostic markers, we analyzed 504 silver carp and 496 bighead carp samples from the Yangtze basin including a few cultured populations, and we found that 7.3% of silver carp and 3% of bighead carp were introgressive hybrids. A much more intensive introgressive hybridization has occurred in the Illinois and Mississippi Rivers than in the Yangtze River basin. It seems that this trend will continue in the wild populations. Ecological and societal effects of introgressive hybrids ecosystems of US and China should be further studied.