Abstract
Aim
To investigate how repeated historical connections between continental islands and the mainland have shaped the phylogeographic structure and genetic diversity of populations, using the widespread water strider
Aquarius paludum
as a model.
Location
Japan
Taxon
Water strider
Aquarius paludum
(Insecta: Hemiptera: Gerridae)
Methods
We analysed mitochondrial (
COI
and
COII
) and nuclear (ITS1) DNA sequences from populations across the Japanese archipelago and the Eurasian continent. Phylogenetic and population genetic analyses were used to reconstruct phylogeographic structure and infer demographic history.
Results
Western Japanese populations exhibited nucleotide diversity comparable to that of continental populations despite their insular distribution. Bayesian phylogenetic analyses identified two major mitochondrial haplogroups (A and B) that frequently co-occurred across the Japanese archipelago. Haplogroup A, which is rare on the Eurasian continent, occurred at high frequency in Japan and showed pronounced isolation by distance, whereas haplogroup B showed only weak but significant isolation by distance and was significantly more frequent in populations closer to the coast. These contrasting spatial patterns within species indicate distinct demographic histories for the two mitochondrial lineages.
Main conclusion
Repeated colonization from the Eurasian continent and the long-term coexistence of mitochondrial lineages with different demographic histories have shaped the pattern of genetic diversity of
A. paludum
in the Japanese archipelago. Our findings exemplify how continental islands can accumulate genetic diversity through lineage accumulation and secondary contact, which produces an intraspecific phylogeographic pattern comparable to those observed during an early phase of the taxon cycle between species.