Effects of landlocking on the genome-wide divergence of Galaxias brevipinnis populations in the South Island of New Zealand

Abstract

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.

Cryptic diversity within two widespread diadromous freshwater fishes (Teleostei: Galaxiidae)

“…While our study lacked allozyme data for all these localities, we predict fixed differences given observations of their greater resolving power elsewhere in this (described above) and other studies (Adams et al., 2014; Hammer et al., 2014). Furthermore, the divergence among the subantarctic islands, Chatham Islands and the South Island New Zealand are supported by nuclear SNP evidence (Darestani et al., 2023). However, non‐diadromous South Island Lake populations also exhibit nuclear SNP distinction from each other and diadromous populations (Darestani et al., 2023), which raises the possibility that divergences observed during that study may reflect intraspecific spatial population genetic structure, rather than support for cryptic species.…”

“…Furthermore, the divergence among the subantarctic islands, Chatham Islands and the South Island New Zealand are supported by nuclear SNP evidence (Darestani et al., 2023). However, non‐diadromous South Island Lake populations also exhibit nuclear SNP distinction from each other and diadromous populations (Darestani et al., 2023), which raises the possibility that divergences observed during that study may reflect intraspecific spatial population genetic structure, rather than support for cryptic species. Nevertheless, we recommend further genetic assessments (including more vigorous sampling of the North Island) to assess the taxonomic distinction of these lineages.…”