Genetic patterns across an invasion's history: a test of change versus stasis for the Eurasian round goby in North America.

Biological invasions comprise accidental evolutionary experiments, whose genetic compositions underlie relative success, spread and persistence in new habitats. However, little is known about whether, or how, their population genetic patterns change temporally and/or spatially across the invasion's history. Theory predicts that most would undergo founder effect, exhibit low genetic divergence across the new range and gain variation over time via new arriving propagules. To test these predictions, we analyse population genetic diversity and divergence patterns of the Eurasian round goby Neogobius melanostomus across the two decades of its North American invasion in the Laurentian Great Lakes, comparing results from 13 nuclear DNA microsatellite loci and mitochondrial DNA cytochrome b sequences. We test whether 'genetic stasis', 'genetic replacement' and/or 'genetic supplement' scenarios have occurred at the invasion's core and expansion sites, in comparison with its primary native source population in the Dnieper River, Black Sea. Results reveal pronounced genetic divergence across the exotic range, with population areas remaining genetically distinct and statistically consistent across two decades, supporting 'genetic stasis' and 'founder takes most'. The original genotypes continue to predominate, whose high population growth likely outpaced the relative success of later arrivals. The original invasion core has stayed the most similar to the native source. Secondary expansion sites indicate slight allelic composition convergence towards the core population over time, attributable to some early 'genetic supplementation'. The geographic and temporal coverage of this investigation offers a rare opportunity to discern population dynamics over time and space in context of invasion genetic theory vs. reality.

Invasion genetics of the silver carp Hypophthalmichthys molitrix across North America: Differentiation of fronts, introgression, and eDNA metabarcode detection.

In the 1970s, the introduced silver carp Hypophthalmichthys molitrix (which is indigenous to eastern Asia) escaped from southern U.S. aquaculture to spread throughout the Mississippi River basin, and since has steadily moved northward. This large, prolific filter-feeder reduces food availability for other fishes. It now has reached the threshold of the Laurentian Great Lakes, where it likely will significantly impact food chains and fisheries. Our study evaluates population genetic variability and differentiation of the silver carp using 10 nuclear DNA microsatellite loci, and sequences of two mitochondrial genes-cytochrome b and cytochrome c oxidase subunit 1, along with the nuclear ribosomal protein S7 gene intron 1. We analyze population samples from: two primary Great Lakes' invasion fronts (at the Illinois River outside of Chicago, IL in Lake Michigan and in the Wabash River, which leads into the Maumee River and western Lake Erie), the original establishment "core" in the Lower Mississippi River, and expansion areas in the Upper Mississippi and Missouri rivers. We analyze and compare our results with bighead and other invasive carps, and cyprinid relatives. Results reveal that the silver carp invasion possesses moderate levels of genetic diversity, with more mtDNA haplotypes and unique microsatellite alleles in the "core" Lower Mississippi River population, which also diverges the most. The two invasion fronts also significantly genetically differ. About 3% of individuals (including all populations except the Illinois River) contain a unique and very divergent mtDNA haplotype, which likely stems from historic introgression in Asia with female largescale silver carp H. harmandi. The nuclear microsatellites and S7 sequences of the introgressed individuals do not differ from silver carp and are very distant from bighead carp. These sequence variation data are employed to design and evaluate a targeted high-throughput metabarcoding sequence assay that identifies and distinguishes among species of invasive carps (i.e., silver, bighead, grass, black, and common carps, along with goldfish), as well as native cyprinids, using cytochrome b. Our assay further differentiates among selected silver carp haplotypes (including between H. molitrix and H. harmandi), for use in population genetics and future analyses of spread pathways. We test and evaluate this assay on environmental (e)DNA water samples from 48 bait shops in the Great Lakes' region (along the Lake Erie, Lake St. Clair, and Wabash River watersheds), using positive and negative controls and custom bioinformatic processing. Test results discern silver carp eDNA in four of the shops-three in Lake Erie and one in the Wabash River watershed-and bighead carp from one of the same Lake Erie venues, suggesting that retailers (who often source from established southerly populations) comprise another introduction vector. Our overall findings thus provide key population genetic and phylogenetic data for understanding and tracing introductions, vectors, and spread pathways for silver carp, their variants, and their relatives.