Restoration-mediated secondary contact leads to introgression of alewife ecotypes separated by a colonial-era dam.

Kerry Reid,John Carlos Garza,Adalgisa Caccone,Steven R Gephard,David M Post,Eric P Palkovacs

doi:10.1111/eva.12890

Abstract

Secondary contact may have important implications for ecological and evolutionary processes; however, few studies have tracked the outcomes of secondary contact from its onset in natural ecosystems. We evaluated an anadromous alewife (Alosa pseudoharengus ) reintroduction project in Rogers Lake (Connecticut, USA), which contains a landlocked alewife population that was isolated as a result of colonial‐era damming. After access to the ocean was restored, adult anadromous alewife were stocked into the lake. We assessed anadromous juvenile production, the magnitude and direction of introgression, and the potential for competition between ecotypes. We obtained fin clips from all adult alewife stocked into the lake during the restoration and a sample of juveniles produced in the lake two years after the stocking began. We assessed the ancestry of juveniles using categorical assignment and pedigree reconstruction with newly developed microhaplotype genetic markers. Anadromous alewives successfully spawned in the lake and hybridized with the landlocked population. Parentage assignments revealed that male and female anadromous fish contributed equally to juvenile F1 hybrids. The presence of landlocked backcrosses shows that some hybrids were produced within the first two years of secondary contact, matured in the lake, and reproduced. Therefore, introgression appears directional, from anadromous into landlocked, in the lake environment. Differences in estimated abundance of juveniles of different ecotypes in different habitats were also detected, which may reduce competition between ecotypes as the restoration continues. Our results illustrate the utility of restoration projects to study the outcomes of secondary contact in real ecosystems.

Highlights

Secondary contact occurs when two populations that have been isolated for a period of time reestablish the ability to interact
Our results provide several important insights into the ecological and evolutionary dynamics during the initial stages of secondary contact
We found directional introgression of alleles from the re-introduced anadromous fish into the larger resident landlocked population within 3 years of initial secondary contact

Summary

| INTRODUCTION

Secondary contact occurs when two populations that have been isolated for a period of time reestablish the ability to interact. The construction of colonial-era dams led to the isolation of alewife populations in several coastal lakes in Connecticut, USA (Palkovacs et al, 2008) These populations show parallel shifts in phenotype, including life history (migratory behavior, fecundity, generation time, spawning time), morphology (body size, gape width, gill-raker spacing), prey selectivity, salinity tolerance, and osmoregulation (Jones et al, 2013; Palkovacs et al, 2008; Palkovacs, Mandeville, & Post, 2014; Velotta et al, 2017). In 2017, we sampled alewife juveniles produced in the lake following this stocking This restoration project provided a rare opportunity to assess the abundance of anadromous, landlocked, and hybrid offspring produced immediately following secondary contact. Our study provides insight into the magnitude and direction of introgression and the potential for differences in habitat use to mediate competition between alewife ecotypes during the initial stages of secondary contact

| METHODS

Findings

| DISCUSSION

| CONCLUSIONS