Strong population structure but no equilibrium yet: Genetic connectivity and phylogeography in the kelp Saccharina latissima (Laminariales, Phaeophyta).

Pieternella C Luttikhuizen,Harry J Witte,Céline Rebours,Freek H M Van Den Heuvel,Klaas Timmermans,Judith D L Van Bleijswijk

doi:10.1002/ece3.3968

Abstract

Kelp aquaculture is globally developing steadily as human food source, along with other applications. One of the newer crop species is Saccharina latissima, a northern hemisphere kelp inhabiting temperate to arctic rocky shores. To protect and document its natural genetic variation at the onset of this novel aquaculture, as well as increase knowledge on its taxonomy and phylogeography, we collected new genetic data, both nuclear and mitochondrial, and combined it with previous knowledge to estimate genetic connectivity and infer colonization history. Isolation‐with‐migration coalescent analyses demonstrate that gene flow among the sampled locations is virtually nonexistent. An updated scenario for the origin and colonization history of S. latissima is developed as follows: We propose that the species (or species complex) originated in the northwest Pacific, crossed to the northeast Pacific in the Miocene, and then crossed the Bering Strait after its opening ~5.5 Ma into the Arctic and northeast Atlantic. It subsequently crossed the Atlantic from east to west. During the Pleistocene, it was compressed in the south with evidence for northern refugia in Europe. Postglacial recolonization led to secondary contact in the Canadian Arctic. Saccharina cichorioides is shown to probably belong to the S. latissima species complex and to derive from ancestral populations in the Asian North Pacific. Our novel approach of comparing inferred gene flow based on coalescent analysis versus Wright's island model suggests that equilibrium levels of differentiation have not yet been reached in Europe and, hence, that genetic differentiation is expected to increase further if populations are left undisturbed.

Highlights

Aquaculture is a growing portion of the world’s seafood production (FAO 2016)
Genetic connectivity was estimated in two different ways: One assumes that equilibrium between isolation and migration has been reached since population subdivision (Wright’s island model); the TA B L E 3 Maximum posterior probability estimates in demographic units for isolation-with-migration simulations: (A) large-scale comparisons across the Atlantic Ocean, (B) small-scale comparisons among European samples
Saccharina latissima populations at the European locations studied here are virtually unconnected by ongoing gene flow (Figure 4, Table 3B), which confirms results from earlier studies (Guzinski et al, 2016; Nielsen et al, 2016)

Summary

| INTRODUCTION

Aquaculture is a growing portion of the world’s seafood production (FAO 2016). Global production stemming from aquaculture was 40% in 2005 (Asche, 2008) and surpassed capture fisheries in 2013 (FAO 2016). The classical Wright’s island model (Wright, 1931) evaluates the genetic differentiation measure FST in terms of dispersal in the hypothetical case of “islands” of equal population sizes and equal migration rates for all possible island pairs It is not the unrealistic landscape model which is the main problem for inferring genetic connectivity in many species, but rather the assumption that equilibrium FST has been reached. We sequenced a portion of the mitochondrial cytochrome-c-oxidase I gene (for compatibility with earlier studies) and genotyped novel EST (expressed sequence tag)- derived microsatellites For both phylogeographic inference and genetic connectivity estimation, we took a coalescent approach to simultaneously estimate migration and timing of differentiation, and to allow for the possibility that equilibrium between isolation and migration may not yet have been reached (Hey & Nielsen, 2004; Nielsen & Wakeley, 2001)

| METHODS

May 2014

Findings

| DISCUSSION