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.