Landlocking of diadromous fish in freshwater systems can have significant genomic consequences. For instance, the loss of the migratory life stage can dramatically reduce gene flow across populations, leading to increased genetic structuring, and stronger effects of local adaptation. These genomic consequences have been well-studied in some mainland systems, but the evolutionary impacts of landlocking in island ecosystems are largely unknown. In this study, we used a genotyping-by-sequencing (GBS) approach to examine the evolutionary history of landlocking in common smelt (Retropinna retropinna) on Chatham Island, a small isolated oceanic island 650 km southeast of mainland New Zealand. We examined the relationship among the Chatham Island and mainland smelt, and used coalescent analyses to test the number and timing of landlocking events on Chatham Island. Our genomic analysis, based on 21,135 SNPs across 169 individuals, revealed that the Chatham Island smelt were genomically distinct from the mainland New Zealand fish, consistent with a single ancestral colonisation event of Chatham Island in the Pleistocene. Significant genetic structure was also evident within the Chatham Island smelt, with a diadromous Chatham Island smelt group, along with three geographically structured landlocked groups. Coalescent demographic analysis supported three independent landlocking events, with this loss of diadromy significantly pre-dating human colonisation. Our results illustrate how landlocking of diadromous fish can occur repeatedly across a narrow spatial scale, and highlight a unique system to study the genomic basis of repeated adaptation.

Landlocking is a process whereby a population of normally diadromous fish becomes limited to freshwater, potentially leading to behavioural, morphological, and genetic changes, and occasionally speciation. The study of recently landlocked populations can shed light on how populations adapt to environmental change, and how such life-history shifts affect population-genetic structure. Kōaro (Galaxias brevipinnis) is a facultatively diadromous Southern Hemisphere galaxiid fish that frequently becomes landlocked in inland lakes. This study compares seven landlocked kōaro populations to diadromous populations from main and offshore islands of New Zealand. Genotyping-by-sequencing was used to obtain genotypes at 18,813 single nucleotide polymorphism sites for each population. Analyses of population structure revealed that most landlocked populations were genetically highly distinct from one another, as well as from diadromous populations. A few particularly isolated island and lake populations were particularly strongly genetically differentiated. Landscape characteristics were measured to test whether lake elevation, size, or distance from the sea predicted genetic diversity or differentiation from diadromous kōaro. While there were no significant relationships indicating isolation-by-distance or isolation-by-environment, we detected a trend toward lower genetic diversity in lakes at higher elevations. Our findings illustrate the critical role that landlocking can play in the structure of intraspecific genetic diversity within and between populations.

The elimination of lower trophic levels following severe habitat disturbance can trigger new community assembly processes. However, little is known about how past habitat disturbances have affected codependent evolution of trophically-linked and closely interacting taxa. Using genome-wide analysis of a macroalgal community affected by ancient catastrophic coastal uplift, we track the ecological dynamics of past co-dispersal and co-diversification among obligate interacting taxa. Our study reveals rapid and concerted reassembly of an intertidal community following disturbance. Specifically, hierarchical co-demographic analyses of multispecies genomic data support synchronous expansions of four strictly intertidal species in the wake of tectonic disturbance. These data show that tight algal-epifaunal links underpin parallel demographic responses across distinct trophic levels. These results highlight that high-resolution comparative genomic data can elucidate the strength of obligate ecological interactions, and the evolutionary dynamics of past co-dispersal and co-diversification in post-disturbance communities.

Roe deer (Capreolus spp.) are a little odd. They are one of only a few placental mammals — and the only genus among even-toed ungulates — capable of putting embryonic development “on ice”, also known as embryonic diapause (Fig. 1). It would seem such an unusual trait is likely the product of natural selection, but a big question is, how does selection for important traits, such as diapause, interact with the historical demography of a species? In a ‘From the Cover’ article in this issue of Molecular Ecology, de Jong et al. (2020) demonstrate that selection is acting on genes associated with reproductive biology in roe deer, despite heightened genetic drift due to reduced effective population size through the Pleistocene.